EP3874910A1 - Led lighting driver and drive method - Google Patents

Led lighting driver and drive method

Info

Publication number
EP3874910A1
EP3874910A1 EP19790181.2A EP19790181A EP3874910A1 EP 3874910 A1 EP3874910 A1 EP 3874910A1 EP 19790181 A EP19790181 A EP 19790181A EP 3874910 A1 EP3874910 A1 EP 3874910A1
Authority
EP
European Patent Office
Prior art keywords
current
relationship
voltage
threshold level
driver
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP19790181.2A
Other languages
German (de)
French (fr)
Inventor
Gaoxian JIN
Raimundo DE HEER GALISTEO
Jing Liu
Paul Robert Veldman
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Signify Holding BV
Original Assignee
Signify Holding BV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Signify Holding BV filed Critical Signify Holding BV
Publication of EP3874910A1 publication Critical patent/EP3874910A1/en
Pending legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/10—Controlling the intensity of the light
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/30—Driver circuits
    • H05B45/37—Converter circuits
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/0074—Plural converter units whose inputs are connected in series
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B20/00—Energy efficient lighting technologies, e.g. halogen lamps or gas discharge lamps
    • Y02B20/30—Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]

Definitions

  • This invention relates to LED lighting, and in particular to a LED lighting driver.
  • Solid state lighting units and in particular LED-based (retrofit) lamps, are used more and more in home buildings and offices. Besides their high efficiency they also attract consumers due to new design features, different color temperatures, dimming ability etc.
  • each LED light unit makes use of a converter circuit, for converting the AC mains into a DC drive signal, and also for reducing the voltage level.
  • the converter circuit typically comprises a rectifier and a switched mode power converter.
  • a low cost switched mode power converter is a single stage converter, such as a buck converter or a buck-boost converter.
  • a main inductor which controls the storage of energy from the input and the delivery of stored energy to the load.
  • a main power switch controls the supply of energy from the input to the main inductor. The timing of operation of the main power switch, in particular the duty cycle, controls the energy transfer.
  • the timing may be controlled by a feedback control signal, in particular a signal which represents the current through the LED load.
  • a feedback control signal in particular a signal which represents the current through the LED load.
  • many converters have current sensing by means of a current sense resistor, which is placed in the path of the current to be sensed.
  • the current through the current sense resistor generates a voltage, and once the voltage has reached a desired value, the main power switch will be switched off. This is a peak current control mode.
  • the time during which the main power switch is on is the time until the peak current is reached.
  • This type of current mode control switching is dependent on the DC bus voltage in that the higher the bus voltage, the quicker it will reach the peak current.
  • the current sense resistor may be used to sense the average current to the load, and a duty cycle of the main power switch is controlled according to the average current. This is an average current control mode. These modes are well known in this field.
  • each buck converter receives half the total system voltage.
  • WO 2016/008943 discloses a driver having two operating modes; one for a single driver and one for a series connection of two drivers. There is a preset driving mode (for the driver in series with another driver) and a feedback control mode (for a single driver). However, this prior art implements the preset driving mode via a dedicated generator and the feedback control mode by another comparator circuit.
  • the first operating voltage corresponds to the voltage present when two drivers are connected in series across a power input
  • the second operating voltage corresponds to the voltage present when a single driver is connected.
  • a driver for driving a lighting load comprising:
  • a switch mode power converter adapted to receive an input voltage at a power input and to provide energy to the lighting load at a power output
  • a voltage sensing arrangement adapted to sense the input voltage; a current sensing arrangement adapted to sense a current through the lighting load and provide a current sense signal, and
  • a control circuit adapted to configure the switch mode power converter in: a preset switching mode when the input voltage is below a threshold level, wherein the preset switching mode is adapted to deliver a first output current at a first nominal operating voltage below the threshold level, wherein the present switch mode is adapted to increase output current above the first output current as the input voltage increases from the first nominal operating voltage to the threshold level; and
  • the feedback switching mode when the input voltage is above the threshold level, wherein the feedback switching mode is adapted to deliver substantially the same first output current without allowing the output current above the first output current, at a second nominal operating voltage at or above the threshold level.
  • This driver has two operating modes depending on the input voltage.
  • the input voltage will for example depend on whether the driver is connected alone to a power supply (e.g. mains) or if it is in series with another similar driver.
  • the preset switching mode involves open loop control by which the setting of the switch mode power converter is fixed, thereby avoiding any instability issues.
  • the feedback switching mode involves closed loop current control, with variable setting of the switch mode power converter (i.e. variable on time control) to achieve a desired output current.
  • the switching between modes takes place between the two input voltage levels. Each mode results in a different relationship between current and voltage.
  • the respective relationships are designed in particular so that there are two operating voltages (one for each mode) at which the same output current is delivered. Thus, the same light output can be ensured in the different modes, and the lamp operates in a more unified way despite the different installations.
  • the first nominal operating voltage is for example half the second nominal operating voltage, wherein the first nominal operating voltage is a nominal input voltage when the driver is connected in series with another driver to a voltage supply, and the second nominal operating voltage is a nominal input voltage when the driver is connected to the voltage supply alone.
  • the first nominal operating voltage is a nominal input voltage when the driver is connected in series with another driver to a voltage supply
  • the second nominal operating voltage is a nominal input voltage when the driver is connected to the voltage supply alone.
  • the control circuit may comprise:
  • an operation circuit coupled to the current sensing arrangement, and adapted to obtain the current sense signal and to control the on time of the switch mode power converter based on the current sense signal;
  • a configuring circuit to configure the current sensing arrangement with a relationship between the sensed current and the current sense signal obtained by the operation circuit
  • the configuring circuit is adapted to configure:
  • the operation circuit is for example an IC.
  • the current sensing arrangement provides a mapping between a current level and an output signal (such as a voltage). By changing this mapping, the current setting of the switch mode power converter is altered. The different mappings give rise to different on time control methods.
  • the switch mode power converter is forced into the preset switching mode, because the current sense signal indicates a low current level that never reaches the reference current, thereby resulting in the maximum on time control (as mentioned above) because the switch mode power supply is striving to deliver current as much as tolerated by the switch mode power supply.
  • the operation circuit may be adapted to:
  • a single operation circuit can be used for different modes, instead of having two different controlling means for different modes.
  • a gain control associated with the operation circuit is used to set the operation circuit into either of the two modes.
  • the operation circuit is saturated i.e. delivering a constant control signal regardless of the input voltage thereby enabling the preset mode when the first relationship is effective, and the comparator is non-saturated i.e. delivering a varying control signal depending on the output current thereby enabling the feedback mode when the second relationship is effective.
  • the configuring circuit may be adapted to configure the current sensing arrangement with the second relationship at operating voltages (immediately) above the threshold level.
  • the preset mode below that threshold and the feedback mode above the threshold.
  • the current may have a sudden change across the threshold level. However, as the operating voltage of the lamp would not dynamically change across the threshold level after the installation, this sudden change is not likely to happen in daily use.
  • the configuring circuit may be adapted to configure a varying relationship from the first relationship to the second relationship as the input voltage increases from the threshold level to a third threshold level less than the second operating voltage and the operation circuit is adapted to be non-saturated and output an on time which varies in dependence on the current sense signal, thereby entering a current feedback control mode, by receiving the current sense signal under the varying relationship when the input voltage is between the threshold level and the third threshold level.
  • the configuring circuit is adapted to configure the first relationship when the input voltage is between the threshold level and a second threshold level smaller than the second operating voltage, and the operation circuit is adapted to be non-saturated and output an on time which varies independence on the current sense signal, thereby entering a current feedback control mode, by receiving the current sense signal under the first relationship when the input voltage is between the threshold level and the second threshold level.
  • the configuring circuit may then be adapted with the above-mentioned extra feedback mode, to configure a varying relationship from the first relationship to the second relationship as the input voltage increases from the second threshold level to a third threshold level less than the second operating voltage and the operation circuit is adapted to be non-saturated and output an on time which varies in dependence on the current sense signal thereby entering a current feedback control mode by receiving the current sense signal under the varying relationship when the input voltage is between the second threshold level and the third threshold level.
  • a duty cycle control may be used to create a region of the input voltage for which the current sensing arrangement is adjusted in an analog manner between the first and second relationships, to provide a gradual transition, and controlled current reduction, to the standard feedback mode.
  • the curve is for example followed when the driver is initially powered up.
  • the configuring circuit is for example adapted to configure the current sensing arrangement with the second relationship when the input voltage is above the third threshold level, and the operation circuit is adapted to be non-saturated and output an on time which varies in dependence on the current sense signal thereby entering a current feedback control mode by receiving the current sense signal under the second relationship when the input voltage is above the third threshold level.
  • the current is brought back to the first current level when the input voltage has reached the second operating voltage (which is greater than the second and third threshold levels).
  • the current sensing arrangement may comprise a main current sense resistor, a bypass resistor and a bypass switch in parallel with the main current sense resistor, wherein the configuring circuit is adapted to control the bypass switch to configure the relationship.
  • the bypass switch is used to divert current away from the main current sense resistor, so that a given current causes a different voltage to be generated. This provides a simple way to implement the required reconfiguration and thereby force the switch mode power supply into its preset, peak control, mode.
  • the configuring circuit is for example adapted to:
  • the controller may comprise:
  • an operation circuit coupled to the current sensing arrangement, and adapted to obtain the current sense signal and to control the switch mode power converter based on the current sense signal at a negative comparing input of the operation circuit and a reference signal at a positive comparing input of the operation circuit;
  • a configurable circuit to configure the reference signal, wherein the configurable circuit is adapted to configure a first reference signal to set the operation circuit to provide a first on time control to achieve the preset switching mode and a second reference signal to set the operation circuit to provide a second on time control to achieve the feedback switching mode.
  • a reference value is adapted instead of adapting a gain in the current sensing circuit itself. This is useful if the controller (e.g. an IC) of the switched mode power supply has a suitable interface to adjust the reference.
  • the preset switching mode may comprise a fixed on time control mode of the switch mode power converter and the feedback switching mode comprises varying the on time as the current sense signal varies so as to regulate the current sense signal as a set value.
  • the output current depends linearly (or nearly linearly) on the input voltage.
  • the first operating voltage and first current level is the first operating voltage and first current level.
  • the preset switching mode may be adapted to increase the current through the lighting load in a guard band increasing from the first operating voltage to the threshold level and/or decrease the current through the lighting load in a sub band decreasing from the first operating voltage.
  • the feedback switching mode may be adapted to decrease the current through the lighting load as the voltage increases from the threshold level to the second operating voltage.
  • This guard band is used because voltages may develop unevenly during start up especially in a dual-lamp installation. Maintaining the lamps in the preset mode, even if the voltage exceeds the operating voltage by a suitable tolerance, is better for eventually achieving a balance of the dual lamps. It also means the current level corresponding to the threshold is higher than the first current level. The feedback switching mode needs to bring the current level back after the threshold level.
  • the feedback switching mode has an output current which follows a power curve. At a point along this power curve is the second operating voltage and first current level.
  • the switch mode power converter for example comprises a buck converter.
  • the invention also provides a driver arrangement comprising two drivers in series, each for driving an associated lighting load, wherein each driver is as defined above.
  • the invention also provides a method of driving a lighting load, comprising: receiving an input voltage;
  • a preset switching mode when the input voltage is below a threshold level Vb, wherein the preset switching mode is adapted to deliver a first output current at a first operating voltage Vr/2 below the threshold level Vb;
  • the feedback switching mode is adapted to deliver said first current output at a second operating voltage Vr above the threshold level.
  • the method may comprise configuring a relationship between the sensed current and the current sense signal to provide a first relationship for the preset switching mode and a second relationship at the second operating voltage.
  • Fig. 1 shows an example of a low power factor buck converter circuit
  • Fig. 2 shows an alternative buck converter circuit
  • Fig. 3 shows two buck converters each of the type shown in Fig. 1 connected in series across the system voltage V 1 ;
  • Fig. 4 shows two buck converters each of the type shown in Fig. 2 connected in series across the system voltage V 1 ;
  • Figs. 5A-C show the connection options which would be desirable for a lamp design
  • Fig. 6 shows the current (y-axis) versus voltage (x-axis) relationship for a lamp which can be switched between open loop control (with preset switching function) and closed loop control (with switching based on current sensing and feedback);
  • Figs. 7A-C show three examples of possible current versus voltage
  • Fig. 8 which shows a first circuit example to implement the characteristics of Figs. 7A-C, based on the buck converter of Fig. 1;
  • Fig. 9 shows a second circuit example to implement the characteristics of Figs. 7A-C, based on the buck converter of Fig. 2.
  • the invention provides a driver for driving a lighting load which makes use of a switch mode power converter, which is controlled in dependence on the input voltage.
  • the same current is delivered by a preset (fixed) switching mode at a first operating voltage (Vr/2) and by a feedback (dynamic) switching mode at a second operating voltage (Vr).
  • Vr/2 first operating voltage
  • Vr/2 second operating voltage
  • Vr second operating voltage
  • FIG. 1 shows an example of a low power factor buck converter circuit.
  • a DC bus supplies a load R2 which could be a linear or non-linear load, an inductor Ll, a main power switch Ml and a current sense resistor Rl which are all in series.
  • a flyback diode Dl is connected across the load and inductor. The current sensing takes place at the bottom side of the main power switch in this example.
  • the duty cycle of the main power switch Ml is controlled to adjust the power transfer ratio of the circuit from the input to the inductor and from the inductor to the load. This power transfer is for example controlled in dependence on the sensed current.
  • FIG. 2 shows an alternative buck converter circuit with high side switching and sensing.
  • the DC bus supplies the main power switch Ml, the current sense resistor Rl, the inductor Ll, and the load R2 which are all in series.
  • a flyback diode Dl is connected across the load, inductor and sense resistor, and there is a smoothing capacitor C2 in parallel with the load R2.
  • each time the main switch Ml is switched on the current through the current sense resistor Rl is sensed.
  • the current through the current sense resistor Rl generates a voltage across the resistor Rl and once the voltage across Rl has reached a desired value, the main switch will be switched off.
  • This is known as a peak current mode control switching and is used in the most basic buck converters.
  • the switching frequency is dependent on the DC bus voltage.
  • FIG 3 shows two buck converters 30, 32 each of the type shown in Figure 1 connected in series across the system voltage V 1. Each converter additionally has a diode bridge rectifier and an input capacitor.
  • FIG 4 shows two buck converters 40, 42 each of the type shown in Figure 2 connected in series across the system voltage V 1. Each converter again additionally has a diode bridge rectifier.
  • FIGs 5A-C show the connection options which would be desirable for a lamp design.
  • the lamp 50 is connected directly to the mains.
  • the lamp 50 is connected to the mains via an electromagnetic ballast 52.
  • two of the lamps 50 are connected in series to the mains, and in series with an electromagnetic ballast 52.
  • a switch 54 is shown for bypassing the ballast.
  • Figure 6 shows the current (y-axis) versus voltage (x-axis) relationship for a lamp which can be switched between open loop control (with preset switching function) and closed loop control (with switching based on current sensing and feedback).
  • the mode switching takes place at an input voltage of 140V, with open loop preset switching when the input voltage is below 140V and closed loop feedback switching when the input voltage is above 140V.
  • the converter operates the power switch with a fixed on duration i.e. a fixed duty cycle, and the output current increases when the input voltage increases, because the duty cycle is constant (and hence independent of the output current).
  • the open loop control is used for lamps placed in series.
  • An ideal threshold to distinguish between a single lamp and two serial lamps could be half of the mains voltage.
  • a guard band is preferably used such that any disturbance of the mains voltage or the operation of the lamp does not create a false trigger to the lamp to enter the closed loop control.
  • the RMS voltage is 230V, so assuming a 10% disturbance the maximum RMS voltage could be 254V, and an ideal voltage to differentiate between a single lamp installation or a series/dual lamp installation could be 127V.
  • the lamps have tolerances in their components, such as inductors, ICs, etc., and there may be a variance of 10% in the output current. This may lead to voltage variances between the two lamps even if they work in an open loop control mode.
  • the threshold may be set higher, such as at 140V. If the input voltage reaches or exceeds the 140V threshold the lamps enter the close loop control mode which keeps the current constant at the same value as was present at 140V. This causes a problem that the lamp working in the open loop control mode at a nominal half mains voltage (operating point 60) has a different output current from that with the closed loop control (operating point 62). For some applications with both of the different installations, the lamp may emit different output lumen and not give a uniform output appearance.
  • the invention provides a driver which is also switchable between preset switching and feedback switching operating modes of a switch mode power converter (such as a buck converter) but in which the same output current is provided at first and second operating voltages.
  • a switch mode power converter such as a buck converter
  • Figures 7A-C show three examples of possible current versus voltage characteristics.
  • the switch mode power converter is operable in a preset switching mode when the input voltage is below a threshold level Vb, and delivers a first output current Io rated at a first operating voltage Vr/2 below the threshold level Vb.
  • the switch mode power converter is also operable in a feedback switching mode when the input voltage is above the threshold level Vb (either immediately above Vb, or starting at a voltage somewhat higher than Vb).
  • the feedback switching mode delivers substantially same said first current output Io rated at a second operating voltage Vr at or above the threshold level Vb.
  • the first operating voltage Vr/2 is half the second operating voltage Vr and is a nominal input voltage when the driver is connected in series with another driver to a voltage supply.
  • the second operating voltage Vr is a nominal input voltage when the driver is connected to the voltage supply alone.
  • Figure 7A shows a first possible characteristic
  • the current increases linearly with voltage.
  • the desired output current Io rated results.
  • the switching threshold Vb is above the input voltage Vr/2 so that the current has increased above the desired value to Io_h.
  • Figure 7B shows a second possible characteristic.
  • the current increases linearly with voltage.
  • the desired output current Io rated results.
  • the switching threshold Vb is above the input voltage Vr/2 so that the current has increased above the desired value Io rated.
  • the current does not drop instantly to the desired value Io_h, but slowly slews to the desired value.
  • the current may for example follow the power curve of the switch mode power converter, by which the product of the current and voltage remains constant. As a result, the current does not have a step decrease, but follows a curve while the voltage increases to a third threshold level Vc which is still less than the second operating voltage Vr.
  • the input voltage is between the thresholds Vb and Vc, the current is controlled in feedback manner with a variable current setting. The output current has dropped to the desired current Io rated after the voltage reaches the third threshold Vc and it is then held constant.
  • Figure 7C shows a third possible characteristic.
  • Figure 8 shows a first circuit example, based on the buck converter of Figure 1.
  • the driver includes a control circuit 80 which is adapted to configure the switch mode power converter in the different operating modes.
  • a control circuit 80 which is adapted to configure the switch mode power converter in the different operating modes.
  • a current sensing arrangement which can be configured in different ways.
  • the control circuit comprises an operation circuit 82, coupled to the current sensing arrangement, and adapted to obtain the current sense signal and to control the on time of the switch mode power converter based on the current sense signal.
  • This may for example comprise a standard controller IC of the driver. It comprises a comparator 83 at its input which compares the current sense signal (at the negative input) with a reference (at the positive input). In principle, if the signal on the negative input exceeds the signal on the positive input, the output of the comparator becomes low and turns off the main switch Ml; otherwise it is high and the main switch Ml still allows the input current to ramp up. The main switch of the switch mode power converter is controlled depending on the comparison result.
  • a configuring circuit 84 is used to configure the current sensing arrangement with a relationship between the sensed current and the current sense signal provided to the operation circuit 82.
  • the signal provided to the operation circuit 82 will be different for the same current flowing when the current sensing arrangement is configured differently.
  • the operation of the driver will depend on the configuration of the current sensing arrangement.
  • the current sensing arrangement comprises the conventional (main) current sense resistor Rl, a bypass resistor R4 and a bypass switch M2 in parallel with the main current sense resistor Rl .
  • the configuring circuit is adapted to control the bypass switch M2 to configure the relationship between the current flowing and the current sense signal provided to the operation circuit 82.
  • the sense resistors Rl and R4 are connected in parallel hence reducing the effective resistance of the current sense resistor and thus reducing the sensed voltage provided to the operation circuit 82.
  • the current sensing arrangement has a relatively low gain.
  • the operation circuit is made to believe that a low current is flowing and accordingly it keeps output a high voltage to turn on the main switch Ml, namely the on time is long.
  • the switched mode power converter or the IC has a mechanism to turn off the main switch when the on time reaches a maximum value. Thus, the maximum value is used in every switching.
  • the first relationship is used to set the operation circuit 82 to provide a first on time control to provide the first current in the preset switching mode at the first operating voltage Vr/2.
  • the operation circuit may thus be considered to be saturated (because the comparator 82 is always delivering a high signal i.e. a saturated output) and a fixed maximum on time is applied, thereby entering an open loop mode as the preset switching mode when the input voltage is below the threshold level Vb.
  • a second relationship is established, in which just the main current sensor resistor Rl operates.
  • the second relationship different from the first relationship, is used to set the operation circuit to provide a second on time control to provide substantially the same first current in the feedback switching mode at the second operating voltage Vr as shown in Figures 7A-C.
  • This is the conventional closed loop feedback control.
  • the current sensing arrangement has a relatively large gain (a larger gain than that of the first relationship).
  • the signal on the negative input of the comparator is able to reach the voltage on the positive input, and the comparator can output a low level to turn off the main switch Ml .
  • the operation circuit (in particular its comparator) may then be considered to be non-saturated and output an on time which varies in dependence on the current sense signal, thereby entering a current feedback control mode when the input voltage is at or above the second operating voltage Vr.
  • the operation circuit obtains the current sense signal at a negative comparing input, the low current sense signal received during the first relationship translates to a larger on time, by the comparison function.
  • the circuit of Figure 8 comprises a voltage divider of resistors R5, R6 and R7, which follows the DC bus voltage.
  • a capacitor C2 is in parallel with the resistor R7 and it needs to be charged to provide a deliberate delay/buffer to the voltage behavior across resistor R7 as a function of the average DC bus voltage.
  • the voltage across resistor R7 is a first operating voltage.
  • a Zener diode D2 is used to detect the voltage level across the resistor R7. If the DC bus voltage is too low, the voltage across R7 does not reach the Zener voltage so that transistor Ql will be switched off.
  • a second voltage divider R3 and R8 defines a second operating voltage across the resistor R3.
  • this second operating voltage turns on the transistor M2.
  • the second Zener diode D3 is used to ensure the voltage on R3 is high enough.
  • the second sense resistor R4 is placed in parallel with the main sense resistor Rl .
  • the circuit operates in response to a low DC bus voltage to turn on the transistor M2 and place both current sense resistors in parallel, defining a first relationship between current and the current sense signal.
  • the second relationship is used to provide a fixed current immediately when voltage Vb is reached in Figure 7A, or to provide a gradual drop in current between Vb and Vc in Figure 7B, or to provide a constant current from Vb to Va then a gradual drop in current between Va and Vc in Figure 7C.
  • Vr/2 is the mean value of the series installation.
  • a guard band from Vr/2 to Vb is provided because the voltage may develop unevenly during the start up. The lamp will thus work in open loop as long as the voltage is less than Vb. Thus, compared with the output at Vr/2, the output at Vb is larger.
  • M2 is on, and the current is still sensed by main sense resistor Rl in parallel with the bypass resistor and resistor R4. Since the input voltage increases, the input current also increases and the voltage across the parallel connection of Rl and R4 can reach the reference value on the positive input of the comparator 82.
  • the operation circuit leaves the maximum Ton mode and the on time is instead adjusted in closed loop to maintain the current.
  • Va ⁇ Vin ⁇ Vc the circuit operates in peak current control mode (closed loop mode).
  • M2 operates based on a chopping method (pulse width modulation, PWM).
  • Ql is turned on and off alternatively in a high frequency and turns off and on M2 in turn alternately.
  • the effective current sensed is based on the main sense resistor Rl in parallel with R4 multiplied by D, which is a duty cycle from 0 to 1.
  • the circuit While Vin>Vc, the circuit operates in peak current control mode (closed loop mode), Ql is on and M2 is off all the time, and current sensing is by the main current sense resistor Rl alone. This is to bring the larger current at Vb/Va back to the same current at Vr/2.
  • the maximum on time and the main current sense resistor Rl are selected such that the output current at half of the rated voltage Vr is set to be same as the current at the rated voltage, so the same lumen output per lamp can be achieved with one lamp and with dual lamps in series.
  • Vb is for example set to between 140V to 150V for a 230V mains
  • Va is set to be larger than Vb
  • Vc is set to below 200V.
  • Figure 9 shows a second circuit example, based on the buck converter of Figure 2. This is a high-side buck circuit.
  • an auxiliary winding L1B is used to induce a voltage across the main inductor Ll, namely sense the input voltage.
  • L1B is the auxiliary winding of a transformer, which behaves linearly with the DC bus voltage.
  • the sensed voltage is provided to a resistor divider R5, R6 which provides the base voltage for transistor Ql .
  • a capacitor C2 is charged to give some delay to the voltage behavior of R5 and R6. If the DC bus voltage is too low, the voltage on R6 does not reach the Zener voltage of Zener diode ZD1 needed for transistor Ql to switch on. When transistor Ql is on, the transistor M2 can be switched on, and this again means the second sense resistor R4 is placed in parallel with the main sense resistor Rl .
  • the peak current during the charging phase of the buck converter flows through Ml, a parallel connection of Rl and R4, the inductor Ll and the load R2. This peak current is sensed as the feedback line 90.
  • the invention also provides a method of driving a lighting load, comprising: receiving an input voltage;
  • a preset switching mode when the input voltage is below a threshold level Vb, wherein the preset switching mode is adapted to deliver a first output current at a first operating voltage Vr/2 below the threshold level Vb; and a feedback switching mode when the input voltage is above the threshold level Vb, wherein the feedback switching mode is adapted to deliver said first current output at a second operating voltage Vr above the threshold level.
  • the method may comprise configuring a relationship between the sensed current and the current sense signal to provide a first relationship for the preset switching mode and a second relationship at the second operating voltage.
  • the current sensing is made configurable.
  • the way a non-configurable current signal may be interpreted within the operation circuit may be made configurable, for example by adapting a reference value (with which the sensed current is compared) in dependence on the input voltage.
  • the transfer function of the current sensing circuit is adjustable, or else a reference which with the current sensing signal is compared is adjustable, based on the prevailing input voltage.
  • a reference which with the current sensing signal is compared is adjustable, based on the prevailing input voltage.

Abstract

A driver is for driving a lighting load. A switch mode power converter is controlled in dependence on the input voltage, in particular so that the same current is delivered by a preset (fixed) switching mode at a first nominal operating voltage (Vr/2) and by a feedback (dynamic) switching mode at a second nominal operating voltage (Vr). The first operating voltage corresponds to the voltage present when two drivers are connected in series across a power input, and the second operating voltage corresponds to the voltage present when a single driver is connected.

Description

LED lighting driver and drive method
FIELD OF THE INVENTION
This invention relates to LED lighting, and in particular to a LED lighting driver.
BACKGROUND OF THE INVENTION
Solid state lighting units, and in particular LED-based (retrofit) lamps, are used more and more in home buildings and offices. Besides their high efficiency they also attract consumers due to new design features, different color temperatures, dimming ability etc.
To fit LED lighting to existing mains lighting fixtures, each LED light unit makes use of a converter circuit, for converting the AC mains into a DC drive signal, and also for reducing the voltage level.
The converter circuit typically comprises a rectifier and a switched mode power converter.
There are various possible designs of switched mode power converter. A low cost switched mode power converter is a single stage converter, such as a buck converter or a buck-boost converter. In both cases, there is a main inductor which controls the storage of energy from the input and the delivery of stored energy to the load. A main power switch controls the supply of energy from the input to the main inductor. The timing of operation of the main power switch, in particular the duty cycle, controls the energy transfer.
The timing may be controlled by a feedback control signal, in particular a signal which represents the current through the LED load. For this purpose, many converters have current sensing by means of a current sense resistor, which is placed in the path of the current to be sensed.
The current through the current sense resistor generates a voltage, and once the voltage has reached a desired value, the main power switch will be switched off. This is a peak current control mode. The time during which the main power switch is on is the time until the peak current is reached. This type of current mode control switching is dependent on the DC bus voltage in that the higher the bus voltage, the quicker it will reach the peak current. Alternatively, the current sense resistor may be used to sense the average current to the load, and a duty cycle of the main power switch is controlled according to the average current. This is an average current control mode. These modes are well known in this field.
If the output load of the converter is constant, there is a closed loop feedback control in which the switch on time of the main power switch is controlled by the sensed current.
When two converters, such as buck converters, are connected in series to a single supply voltage, the system input current is the same, and accordingly the input voltage for each buck converter needs to be the same. There is an inverse relationship between the input current and the input voltage (to define a constant power curve) in a closed loop feedback converter. If the input voltages on the two closed loop converters are not the same, a higher input voltage tends to increase while the lower input voltage tends to decrease, and instability will result.
Thus, it should be ensured that each buck converter receives half the total system voltage.
WO 2016/008943 discloses a driver having two operating modes; one for a single driver and one for a series connection of two drivers. There is a preset driving mode (for the driver in series with another driver) and a feedback control mode (for a single driver). However, this prior art implements the preset driving mode via a dedicated generator and the feedback control mode by another comparator circuit.
A further study shows that in some prior art, different output currents arise in the different modes, giving rise to different light outputs. This is due to misalignment during the transition between the different modes.
SUMMARY OF THE INVENTION
It is a concept of the invention to provide a driver for driving a lighting load in which a switch mode power converter is controlled in dependence on the input voltage, in particular so that the same current is delivered by a preset (fixed) switching mode at a first operating voltage (Vr/2) and by a feedback (dynamic) switching mode at a second operating voltage (Vr). The first operating voltage corresponds to the voltage present when two drivers are connected in series across a power input, and the second operating voltage corresponds to the voltage present when a single driver is connected.
The invention is defined by the claims. According to examples in accordance with an aspect of the invention, there is provided a driver for driving a lighting load, comprising:
a switch mode power converter adapted to receive an input voltage at a power input and to provide energy to the lighting load at a power output;
a voltage sensing arrangement adapted to sense the input voltage; a current sensing arrangement adapted to sense a current through the lighting load and provide a current sense signal, and
a control circuit adapted to configure the switch mode power converter in: a preset switching mode when the input voltage is below a threshold level, wherein the preset switching mode is adapted to deliver a first output current at a first nominal operating voltage below the threshold level, wherein the present switch mode is adapted to increase output current above the first output current as the input voltage increases from the first nominal operating voltage to the threshold level; and
a feedback switching mode when the input voltage is above the threshold level, wherein the feedback switching mode is adapted to deliver substantially the same first output current without allowing the output current above the first output current, at a second nominal operating voltage at or above the threshold level.
This driver has two operating modes depending on the input voltage. The input voltage will for example depend on whether the driver is connected alone to a power supply (e.g. mains) or if it is in series with another similar driver. The preset switching mode involves open loop control by which the setting of the switch mode power converter is fixed, thereby avoiding any instability issues. The feedback switching mode involves closed loop current control, with variable setting of the switch mode power converter (i.e. variable on time control) to achieve a desired output current. The switching between modes takes place between the two input voltage levels. Each mode results in a different relationship between current and voltage. The respective relationships are designed in particular so that there are two operating voltages (one for each mode) at which the same output current is delivered. Thus, the same light output can be ensured in the different modes, and the lamp operates in a more unified way despite the different installations.
The first nominal operating voltage is for example half the second nominal operating voltage, wherein the first nominal operating voltage is a nominal input voltage when the driver is connected in series with another driver to a voltage supply, and the second nominal operating voltage is a nominal input voltage when the driver is connected to the voltage supply alone. Thus, there are equal current outputs for a single driver and for a driver which is one of a pair of drivers in a series connection.
The control circuit may comprise:
an operation circuit, coupled to the current sensing arrangement, and adapted to obtain the current sense signal and to control the on time of the switch mode power converter based on the current sense signal; and
a configuring circuit to configure the current sensing arrangement with a relationship between the sensed current and the current sense signal obtained by the operation circuit,
wherein the configuring circuit is adapted to configure:
a first relationship to set the operation circuit to provide a first on time control to provide the first current in the preset switching mode at the first operating voltage; and
a second relationship, different from the first relationship, to set the operation circuit to provide a second on time control to provide substantially the same first current in the feedback switching mode at the second operating voltage.
The operation circuit is for example an IC. The current sensing arrangement provides a mapping between a current level and an output signal (such as a voltage). By changing this mapping, the current setting of the switch mode power converter is altered. The different mappings give rise to different on time control methods.
In particular, by altering the mapping using the configuring circuit, the switch mode power converter is forced into the preset switching mode, because the current sense signal indicates a low current level that never reaches the reference current, thereby resulting in the maximum on time control (as mentioned above) because the switch mode power supply is striving to deliver current as much as tolerated by the switch mode power supply.
The operation circuit may be adapted to:
be saturated and output a fixed maximum on time, thereby entering an open loop mode as the preset switching mode, by receiving the current sense signal under the first relationship when the input voltage is below the threshold level; and
be non-saturated and output an on time which varies in dependence on the current sense signal, thereby entering a current feedback control mode, by receiving the current sense signal under the second relationship when the input voltage is at or above the second operating voltage, wherein the second relationship has a larger gain than that of the first relationship, and the operation circuit obtains the current sense signal at a negative comparing input.
Thus, a single operation circuit can be used for different modes, instead of having two different controlling means for different modes. A gain control associated with the operation circuit is used to set the operation circuit into either of the two modes. The operation circuit is saturated i.e. delivering a constant control signal regardless of the input voltage thereby enabling the preset mode when the first relationship is effective, and the comparator is non-saturated i.e. delivering a varying control signal depending on the output current thereby enabling the feedback mode when the second relationship is effective.
In one example, the configuring circuit may be adapted to configure the current sensing arrangement with the second relationship at operating voltages (immediately) above the threshold level.
In this example, there is a single threshold with the preset mode below that threshold and the feedback mode above the threshold. This is a most simple implementation. The current may have a sudden change across the threshold level. However, as the operating voltage of the lamp would not dynamically change across the threshold level after the installation, this sudden change is not likely to happen in daily use.
In another example, the configuring circuit may be adapted to configure a varying relationship from the first relationship to the second relationship as the input voltage increases from the threshold level to a third threshold level less than the second operating voltage and the operation circuit is adapted to be non-saturated and output an on time which varies in dependence on the current sense signal, thereby entering a current feedback control mode, by receiving the current sense signal under the varying relationship when the input voltage is between the threshold level and the third threshold level.
In this example, there is an extra feedback mode between the threshold level and the second operating voltage. This extra feedback mode has a varying reference as the voltage changes. Thus, there are two threshold levels and between these levels there is a gradual and feedback-controlled transition between the preset mode and the final feedback mode.
In another example, the configuring circuit is adapted to configure the first relationship when the input voltage is between the threshold level and a second threshold level smaller than the second operating voltage, and the operation circuit is adapted to be non-saturated and output an on time which varies independence on the current sense signal, thereby entering a current feedback control mode, by receiving the current sense signal under the first relationship when the input voltage is between the threshold level and the second threshold level.
In this example, there are again two thresholds. There is a further extra feedback mode which has a first region (between the threshold level and the second threshold level) for which peak current control is implemented but with a higher current (than the first current level) as represented by the first relationship at the threshold level. Thus, there is a portion of the current versus voltage curve with a constant regulated current.
In addition to the further extra feedback mode, the configuring circuit may then be adapted with the above-mentioned extra feedback mode, to configure a varying relationship from the first relationship to the second relationship as the input voltage increases from the second threshold level to a third threshold level less than the second operating voltage and the operation circuit is adapted to be non-saturated and output an on time which varies in dependence on the current sense signal thereby entering a current feedback control mode by receiving the current sense signal under the varying relationship when the input voltage is between the second threshold level and the third threshold level.
In these examples of adjusting the relationship between the first and the second relationships, a duty cycle control may be used to create a region of the input voltage for which the current sensing arrangement is adjusted in an analog manner between the first and second relationships, to provide a gradual transition, and controlled current reduction, to the standard feedback mode. The curve is for example followed when the driver is initially powered up.
For the two examples using a third threshold, the configuring circuit is for example adapted to configure the current sensing arrangement with the second relationship when the input voltage is above the third threshold level, and the operation circuit is adapted to be non-saturated and output an on time which varies in dependence on the current sense signal thereby entering a current feedback control mode by receiving the current sense signal under the second relationship when the input voltage is above the third threshold level.
Those embodiments with varying relationship provide a gradual transition which may give better output characteristics for users.
The current is brought back to the first current level when the input voltage has reached the second operating voltage (which is greater than the second and third threshold levels). The current sensing arrangement may comprise a main current sense resistor, a bypass resistor and a bypass switch in parallel with the main current sense resistor, wherein the configuring circuit is adapted to control the bypass switch to configure the relationship.
The bypass switch is used to divert current away from the main current sense resistor, so that a given current causes a different voltage to be generated. This provides a simple way to implement the required reconfiguration and thereby force the switch mode power supply into its preset, peak control, mode.
The configuring circuit is for example adapted to:
close the bypass switch to configure the first relationship; and open the bypass switch to configure the second relationship.
In an alternative arrangement, instead of configuring the current sensing, the controller may comprise:
an operation circuit coupled to the current sensing arrangement, and adapted to obtain the current sense signal and to control the switch mode power converter based on the current sense signal at a negative comparing input of the operation circuit and a reference signal at a positive comparing input of the operation circuit;
a configurable circuit to configure the reference signal, wherein the configurable circuit is adapted to configure a first reference signal to set the operation circuit to provide a first on time control to achieve the preset switching mode and a second reference signal to set the operation circuit to provide a second on time control to achieve the feedback switching mode.
This is an alternative way to implement the adjustable current sensing. A reference value is adapted instead of adapting a gain in the current sensing circuit itself. This is useful if the controller (e.g. an IC) of the switched mode power supply has a suitable interface to adjust the reference.
In all examples, the preset switching mode may comprise a fixed on time control mode of the switch mode power converter and the feedback switching mode comprises varying the on time as the current sense signal varies so as to regulate the current sense signal as a set value.
In the fixed on time control mode, the output current depends linearly (or nearly linearly) on the input voltage. Thus, at a point along this linear curve is the first operating voltage and first current level.
The preset switching mode may be adapted to increase the current through the lighting load in a guard band increasing from the first operating voltage to the threshold level and/or decrease the current through the lighting load in a sub band decreasing from the first operating voltage. The feedback switching mode may be adapted to decrease the current through the lighting load as the voltage increases from the threshold level to the second operating voltage.
This guard band is used because voltages may develop unevenly during start up especially in a dual-lamp installation. Maintaining the lamps in the preset mode, even if the voltage exceeds the operating voltage by a suitable tolerance, is better for eventually achieving a balance of the dual lamps. It also means the current level corresponding to the threshold is higher than the first current level. The feedback switching mode needs to bring the current level back after the threshold level. The feedback switching mode has an output current which follows a power curve. At a point along this power curve is the second operating voltage and first current level.
The switch mode power converter for example comprises a buck converter. The invention also provides a driver arrangement comprising two drivers in series, each for driving an associated lighting load, wherein each driver is as defined above.
The invention also provides a method of driving a lighting load, comprising: receiving an input voltage;
sensing the input voltage;
converting the input voltage to an output voltage using a switch mode power converter and thereby delivering a current to the lighting load;
sensing a current through the lighting load to deliver a current sense signal; configuring the switch mode power converter is in:
a preset switching mode when the input voltage is below a threshold level Vb, wherein the preset switching mode is adapted to deliver a first output current at a first operating voltage Vr/2 below the threshold level Vb; and
a feedback switching mode when the input voltage is above the threshold level Vb, wherein the feedback switching mode is adapted to deliver said first current output at a second operating voltage Vr above the threshold level.
The method may comprise configuring a relationship between the sensed current and the current sense signal to provide a first relationship for the preset switching mode and a second relationship at the second operating voltage.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS
Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:
Fig. 1 shows an example of a low power factor buck converter circuit;
Fig. 2 shows an alternative buck converter circuit;
Fig. 3 shows two buck converters each of the type shown in Fig. 1 connected in series across the system voltage V 1 ;
Fig. 4 shows two buck converters each of the type shown in Fig. 2 connected in series across the system voltage V 1 ;
Figs. 5A-C show the connection options which would be desirable for a lamp design;
Fig. 6 shows the current (y-axis) versus voltage (x-axis) relationship for a lamp which can be switched between open loop control (with preset switching function) and closed loop control (with switching based on current sensing and feedback);
Figs. 7A-C show three examples of possible current versus voltage
characteristics;
Fig. 8 which shows a first circuit example to implement the characteristics of Figs. 7A-C, based on the buck converter of Fig. 1;
Fig. 9 shows a second circuit example to implement the characteristics of Figs. 7A-C, based on the buck converter of Fig. 2.
DETAIFED DESCRIPTION OF THE EMBODIMENTS
The invention will be described with reference to the Figures.
It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.
The invention provides a driver for driving a lighting load which makes use of a switch mode power converter, which is controlled in dependence on the input voltage. The same current is delivered by a preset (fixed) switching mode at a first operating voltage (Vr/2) and by a feedback (dynamic) switching mode at a second operating voltage (Vr). This means the drive can be used as a single driver or one of a pair of drivers in series.
Figure 1 shows an example of a low power factor buck converter circuit. A DC bus supplies a load R2 which could be a linear or non-linear load, an inductor Ll, a main power switch Ml and a current sense resistor Rl which are all in series. A flyback diode Dl is connected across the load and inductor. The current sensing takes place at the bottom side of the main power switch in this example.
The duty cycle of the main power switch Ml is controlled to adjust the power transfer ratio of the circuit from the input to the inductor and from the inductor to the load. This power transfer is for example controlled in dependence on the sensed current.
Figure 2 shows an alternative buck converter circuit with high side switching and sensing. The DC bus supplies the main power switch Ml, the current sense resistor Rl, the inductor Ll, and the load R2 which are all in series. A flyback diode Dl is connected across the load, inductor and sense resistor, and there is a smoothing capacitor C2 in parallel with the load R2.
In these circuits, each time the main switch Ml is switched on, the current through the current sense resistor Rl is sensed. The current through the current sense resistor Rl generates a voltage across the resistor Rl and once the voltage across Rl has reached a desired value, the main switch will be switched off. This is known as a peak current mode control switching and is used in the most basic buck converters. The switching frequency is dependent on the DC bus voltage.
As long the output load R2 of the buck converter does not change, there is a closed loop control of the system, where the switch on time of the main switch Ml is controlled by the sensed current independent of the DC bus voltage.
Figure 3 shows two buck converters 30, 32 each of the type shown in Figure 1 connected in series across the system voltage V 1. Each converter additionally has a diode bridge rectifier and an input capacitor.
Figure 4 shows two buck converters 40, 42 each of the type shown in Figure 2 connected in series across the system voltage V 1. Each converter again additionally has a diode bridge rectifier.
In this series configuration, the input voltage of each buck converter should be at the same level to prevent imbalance and instability. However, if the buck converter works in the above-mentioned feedback mode, it is hard to ensure the input voltages are equal. There is a need for a lamp that can be placed either alone, or in series combination with another lamp. If a lamp operating with closed loop control is operating in a series connection, the voltage may develop unevenly. As explained above, WO2016/008943 discloses a technology that operates a lamp in closed loop control mode when it is a single lamp and in open loop control mode when it is in series with another lamp.
Figures 5A-C show the connection options which would be desirable for a lamp design. In Figure 5A, the lamp 50 is connected directly to the mains. In Figure 5B, the lamp 50 is connected to the mains via an electromagnetic ballast 52. In Figure 5C, two of the lamps 50 are connected in series to the mains, and in series with an electromagnetic ballast 52. Optionally, a switch 54 is shown for bypassing the ballast.
Figure 6 shows the current (y-axis) versus voltage (x-axis) relationship for a lamp which can be switched between open loop control (with preset switching function) and closed loop control (with switching based on current sensing and feedback). The mode switching takes place at an input voltage of 140V, with open loop preset switching when the input voltage is below 140V and closed loop feedback switching when the input voltage is above 140V.
During open loop control, the converter operates the power switch with a fixed on duration i.e. a fixed duty cycle, and the output current increases when the input voltage increases, because the duty cycle is constant (and hence independent of the output current). The open loop control is used for lamps placed in series.
An ideal threshold to distinguish between a single lamp and two serial lamps could be half of the mains voltage. However, a guard band is preferably used such that any disturbance of the mains voltage or the operation of the lamp does not create a false trigger to the lamp to enter the closed loop control. For example, in Europe, the RMS voltage is 230V, so assuming a 10% disturbance the maximum RMS voltage could be 254V, and an ideal voltage to differentiate between a single lamp installation or a series/dual lamp installation could be 127V.
The lamps have tolerances in their components, such as inductors, ICs, etc., and there may be a variance of 10% in the output current. This may lead to voltage variances between the two lamps even if they work in an open loop control mode. Thus, the threshold may be set higher, such as at 140V. If the input voltage reaches or exceeds the 140V threshold the lamps enter the close loop control mode which keeps the current constant at the same value as was present at 140V. This causes a problem that the lamp working in the open loop control mode at a nominal half mains voltage (operating point 60) has a different output current from that with the closed loop control (operating point 62). For some applications with both of the different installations, the lamp may emit different output lumen and not give a uniform output appearance.
The invention provides a driver which is also switchable between preset switching and feedback switching operating modes of a switch mode power converter (such as a buck converter) but in which the same output current is provided at first and second operating voltages.
Figures 7A-C show three examples of possible current versus voltage characteristics.
In each case, the switch mode power converter is operable in a preset switching mode when the input voltage is below a threshold level Vb, and delivers a first output current Io rated at a first operating voltage Vr/2 below the threshold level Vb. The switch mode power converter is also operable in a feedback switching mode when the input voltage is above the threshold level Vb (either immediately above Vb, or starting at a voltage somewhat higher than Vb). The feedback switching mode delivers substantially same said first current output Io rated at a second operating voltage Vr at or above the threshold level Vb.
The first operating voltage Vr/2 is half the second operating voltage Vr and is a nominal input voltage when the driver is connected in series with another driver to a voltage supply. The second operating voltage Vr is a nominal input voltage when the driver is connected to the voltage supply alone.
Figure 7A shows a first possible characteristic.
During the open loop control, the current increases linearly with voltage. At the input voltage Vr/2, the desired output current Io rated results. The switching threshold Vb is above the input voltage Vr/2 so that the current has increased above the desired value to Io_h. There is a step at the input voltage Vb to the closed loop feedback control and the output current drops to the desired current Io rated, more or less instantly.
Figure 7B shows a second possible characteristic.
During the open loop control, the current increases linearly with voltage. At the input voltage Vr/2, the desired output current Io rated results. The switching threshold Vb is above the input voltage Vr/2 so that the current has increased above the desired value Io rated. There is change at the input voltage Vb to the closed loop feedback control. Different to the embodiment in Figure 7A, the current does not drop instantly to the desired value Io_h, but slowly slews to the desired value. The current may for example follow the power curve of the switch mode power converter, by which the product of the current and voltage remains constant. As a result, the current does not have a step decrease, but follows a curve while the voltage increases to a third threshold level Vc which is still less than the second operating voltage Vr. While the input voltage is between the thresholds Vb and Vc, the current is controlled in feedback manner with a variable current setting. The output current has dropped to the desired current Io rated after the voltage reaches the third threshold Vc and it is then held constant.
Figure 7C shows a third possible characteristic.
In this case, there is a second threshold level Va larger than the first threshold level Vb and smaller than the second operating voltage.
There is a change at the input voltage Vb to the closed loop feedback control. The current is kept constant for an initial period of increase of the input voltage from Vb to Va. This part is similar to the solution shown in Figure 6. There is then the power curve while the voltage increases from the second threshold Va to the third threshold level Vc which is still less than the second operating voltage Vr. While the input voltage is between the thresholds Vb and Vc, the current is controlled in feedback manner initially with a fixed current setting and then with a variable current setting. The output current again has dropped to the desired current Io rated once the voltage reaches the third threshold Vc and it is then held constant.
The way these current versus voltage characteristics may be obtained will be explained with reference to Figure 8 which shows a first circuit example, based on the buck converter of Figure 1.
The driver includes a control circuit 80 which is adapted to configure the switch mode power converter in the different operating modes. Instead of a single current sense resistor Rl, there is a current sensing arrangement which can be configured in different ways.
The control circuit comprises an operation circuit 82, coupled to the current sensing arrangement, and adapted to obtain the current sense signal and to control the on time of the switch mode power converter based on the current sense signal. This may for example comprise a standard controller IC of the driver. It comprises a comparator 83 at its input which compares the current sense signal (at the negative input) with a reference (at the positive input). In principle, if the signal on the negative input exceeds the signal on the positive input, the output of the comparator becomes low and turns off the main switch Ml; otherwise it is high and the main switch Ml still allows the input current to ramp up. The main switch of the switch mode power converter is controlled depending on the comparison result.
A configuring circuit 84 is used to configure the current sensing arrangement with a relationship between the sensed current and the current sense signal provided to the operation circuit 82.
In other words, the signal provided to the operation circuit 82 will be different for the same current flowing when the current sensing arrangement is configured differently. Thus, the operation of the driver will depend on the configuration of the current sensing arrangement.
The current sensing arrangement comprises the conventional (main) current sense resistor Rl, a bypass resistor R4 and a bypass switch M2 in parallel with the main current sense resistor Rl . The configuring circuit is adapted to control the bypass switch M2 to configure the relationship between the current flowing and the current sense signal provided to the operation circuit 82.
When the bypass switch M2 is closed, a first relationship is established. The sense resistors Rl and R4 are connected in parallel hence reducing the effective resistance of the current sense resistor and thus reducing the sensed voltage provided to the operation circuit 82. The current sensing arrangement has a relatively low gain. Thus, the operation circuit is made to believe that a low current is flowing and accordingly it keeps output a high voltage to turn on the main switch Ml, namely the on time is long. Normally the switched mode power converter or the IC has a mechanism to turn off the main switch when the on time reaches a maximum value. Thus, the maximum value is used in every switching. The first relationship is used to set the operation circuit 82 to provide a first on time control to provide the first current in the preset switching mode at the first operating voltage Vr/2.
The operation circuit may thus be considered to be saturated (because the comparator 82 is always delivering a high signal i.e. a saturated output) and a fixed maximum on time is applied, thereby entering an open loop mode as the preset switching mode when the input voltage is below the threshold level Vb.
When the bypass switch M2 is open, a second relationship is established, in which just the main current sensor resistor Rl operates. The second relationship, different from the first relationship, is used to set the operation circuit to provide a second on time control to provide substantially the same first current in the feedback switching mode at the second operating voltage Vr as shown in Figures 7A-C. This is the conventional closed loop feedback control. The current sensing arrangement has a relatively large gain (a larger gain than that of the first relationship). The signal on the negative input of the comparator is able to reach the voltage on the positive input, and the comparator can output a low level to turn off the main switch Ml .
The operation circuit (in particular its comparator) may then be considered to be non-saturated and output an on time which varies in dependence on the current sense signal, thereby entering a current feedback control mode when the input voltage is at or above the second operating voltage Vr.
Because the operation circuit obtains the current sense signal at a negative comparing input, the low current sense signal received during the first relationship translates to a larger on time, by the comparison function.
The circuit of Figure 8 comprises a voltage divider of resistors R5, R6 and R7, which follows the DC bus voltage. A capacitor C2 is in parallel with the resistor R7 and it needs to be charged to provide a deliberate delay/buffer to the voltage behavior across resistor R7 as a function of the average DC bus voltage. The voltage across resistor R7 is a first operating voltage.
A Zener diode D2 is used to detect the voltage level across the resistor R7. If the DC bus voltage is too low, the voltage across R7 does not reach the Zener voltage so that transistor Ql will be switched off.
A second voltage divider R3 and R8 defines a second operating voltage across the resistor R3. When the transistor Ql is off, this second operating voltage turns on the transistor M2. The second Zener diode D3 is used to ensure the voltage on R3 is high enough. When the transistor M2 is on, the second sense resistor R4 is placed in parallel with the main sense resistor Rl . Thus, the circuit operates in response to a low DC bus voltage to turn on the transistor M2 and place both current sense resistors in parallel, defining a first relationship between current and the current sense signal.
If the DC bus voltage is high enough to reach the Zener voltage of Zener diode D2, the transistor Ql will switch on. This shorts the resistor R3 and hence pulls down the second operating voltage. The voltage is insufficient to reach the threshold of the Zener diode D3 and the gate voltage to transistor M2 is kept low. Thus, transistor M2 is off and the current sensing arrangement is only the main current sense resistor Rl . This defines a second relationship between current and the current sense signal. In the examples of Figures 7A-C, the first relationship is present for input voltages below Vb and the second relationship is present for input voltages above Vb.
The second relationship is used to provide a fixed current immediately when voltage Vb is reached in Figure 7A, or to provide a gradual drop in current between Vb and Vc in Figure 7B, or to provide a constant current from Vb to Va then a gradual drop in current between Va and Vc in Figure 7C.
The transition between the first and second relationships is gradual in Figures 7B and 7C (i.e. there is a varying relationship). This may be achieved by operating the transistor M2 with duty cycle control.
The way the circuit of Figure 8 is used to implement the profile of Figure 7C will now be explained:
While Vin<Vb, the circuit operates in a maximum on time control mode, and M2 is on. The sensed current is small due to the parallel connection of Rl and R4, so the operation circuit enters the maximum Ton mode automatically. The voltage Vr/2 is the mean value of the series installation. A guard band from Vr/2 to Vb is provided because the voltage may develop unevenly during the start up. The lamp will thus work in open loop as long as the voltage is less than Vb. Thus, compared with the output at Vr/2, the output at Vb is larger.
While Vb<Vin<Va, the circuit operates in peak current control mode (but a closed loop control mode rather than the previous open loop maximum current control mode). M2 is on, and the current is still sensed by main sense resistor Rl in parallel with the bypass resistor and resistor R4. Since the input voltage increases, the input current also increases and the voltage across the parallel connection of Rl and R4 can reach the reference value on the positive input of the comparator 82. The operation circuit leaves the maximum Ton mode and the on time is instead adjusted in closed loop to maintain the current.
While Va<Vin<Vc, the circuit operates in peak current control mode (closed loop mode). M2 operates based on a chopping method (pulse width modulation, PWM).
More specifically, Ql is turned on and off alternatively in a high frequency and turns off and on M2 in turn alternately. The effective current sensed is based on the main sense resistor Rl in parallel with R4 multiplied by D, which is a duty cycle from 0 to 1.
While Vin>Vc, the circuit operates in peak current control mode (closed loop mode), Ql is on and M2 is off all the time, and current sensing is by the main current sense resistor Rl alone. This is to bring the larger current at Vb/Va back to the same current at Vr/2. Normally, the maximum on time and the main current sense resistor Rl are selected such that the output current at half of the rated voltage Vr is set to be same as the current at the rated voltage, so the same lumen output per lamp can be achieved with one lamp and with dual lamps in series. Vb is for example set to between 140V to 150V for a 230V mains, Va is set to be larger than Vb, Vc is set to below 200V.
Figure 9 shows a second circuit example, based on the buck converter of Figure 2. This is a high-side buck circuit.
Instead of using a voltage detector that directly connects to the DC bus, an auxiliary winding L1B is used to induce a voltage across the main inductor Ll, namely sense the input voltage. L1B is the auxiliary winding of a transformer, which behaves linearly with the DC bus voltage.
Similar to the circuit of Figure 8, the sensed voltage is provided to a resistor divider R5, R6 which provides the base voltage for transistor Ql .
A capacitor C2 is charged to give some delay to the voltage behavior of R5 and R6. If the DC bus voltage is too low, the voltage on R6 does not reach the Zener voltage of Zener diode ZD1 needed for transistor Ql to switch on. When transistor Ql is on, the transistor M2 can be switched on, and this again means the second sense resistor R4 is placed in parallel with the main sense resistor Rl . The peak current during the charging phase of the buck converter flows through Ml, a parallel connection of Rl and R4, the inductor Ll and the load R2. This peak current is sensed as the feedback line 90.
If the DC bus voltage is high enough to reach the Zener voltage for Zener diode ZD1, transistor Ql will switch off and the gate on of transistor M2 is kept low, so only Rl is used as the current sense resistor.
The circuit thus operates generally in the same way as Figure 8.
The invention also provides a method of driving a lighting load, comprising: receiving an input voltage;
sensing the input voltage;
converting the input voltage to an output voltage using a switch mode power converter and thereby delivering a current to the lighting load;
sensing a current through the lighting load to deliver a current sense signal; configuring the switch mode power converter is in:
a preset switching mode when the input voltage is below a threshold level Vb, wherein the preset switching mode is adapted to deliver a first output current at a first operating voltage Vr/2 below the threshold level Vb; and a feedback switching mode when the input voltage is above the threshold level Vb, wherein the feedback switching mode is adapted to deliver said first current output at a second operating voltage Vr above the threshold level.
The method may comprise configuring a relationship between the sensed current and the current sense signal to provide a first relationship for the preset switching mode and a second relationship at the second operating voltage.
In the examples above, the current sensing is made configurable. Instead, the way a non-configurable current signal may be interpreted within the operation circuit may be made configurable, for example by adapting a reference value (with which the sensed current is compared) in dependence on the input voltage.
Two circuit examples have been shown above. However, other circuits may be used. Basically, the transfer function of the current sensing circuit is adjustable, or else a reference which with the current sensing signal is compared is adjustable, based on the prevailing input voltage. Thus, there is basically input voltage sensing, and configuration of the current sensing function. These aims can be achieved in many different ways.
Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a” or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:
1. A driver for driving a lighting load, comprising:
a switch mode power converter adapted to receive an input voltage at a power input and to provide energy to the lighting load at a power output;
a voltage sensing arrangement adapted to sense the input voltage; a current sensing arrangement adapted to sense a current through the lighting load and provide a current sense signal, and
a control circuit (80) adapted to configure the switch mode power converter in: a preset switching mode when the input voltage is below a threshold level (Vb), wherein the preset switching mode is adapted to deliver a first output current at a first nominal operating voltage (Vr/2) below the threshold level (Vb), wherein the present switch mode is adapted to increase output current above the first output current as the input voltage increases from the first nominal operating voltage (Vr/2) to the threshold level (Vb); and
a feedback switching mode when the input voltage is above the threshold level (Vb), wherein the feedback switching mode is adapted to deliver substantially same said first output current without allowing the output current above the first output current, at a second nominal operating voltage (Vr) at or above the threshold level (Vb).
2. A driver as claimed in claim 1, wherein the first nominal operating voltage (Vr/2) is half the second nominal operating voltage (Vr), wherein:
the first nominal operating voltage (Vr/2) is a nominal input voltage when the driver is connected in series with another driver to a voltage supply; and
the second nominal operating voltage is a nominal input voltage when the driver is connected to the voltage supply alone.
3. A driver as claimed in claim 1 or 2, wherein the control circuit (80) comprises:
an operation circuit (82), coupled to the current sensing arrangement, and adapted to obtain the current sense signal and to control the on time of the switch mode power converter based on the current sense signal; and a configuring circuit (84) to configure the current sensing arrangement with a relationship between the sensed current and the current sense signal obtained by the operation circuit,
wherein the configuring circuit is adapted to configure:
a first relationship to set the operation circuit to provide a first on time control to provide the first current in the preset switching mode at the first nominal operating voltage; and
a second relationship, different from the first relationship, to set the operation circuit to provide a second on time control to provide substantially the same first current in the feedback switching mode at the second nominal operating voltage.
4. A driver as claimed in claim 3, wherein the operation circuit is adapted to:
be saturated and output a fixed maximum on time, thereby entering an open loop mode as the preset switching mode, by receiving the current sense signal under the first relationship when the input voltage is below the threshold level (Vb); and
be non-saturated and output an on time which varies in dependence on the current sense signal, thereby entering a current feedback control mode, by receiving the current sense signal under the second relationship when the input voltage is at or above the second nominal operating voltage (Vr),
wherein the second relationship has a larger gain than that of the first relationship, and the operation circuit obtains the current sense signal at a negative comparing input.
5. A driver as claimed in claim 4, wherein the configuring circuit is adapted to configure the current sensing arrangement with the second relationship at operating voltages above the threshold level (Vb).
6. A driver as claimed in claim 4, wherein:
the configuring circuit is adapted to configure a varying relationship from the first relationship to the second relationship as the input voltage increases from the threshold level (Vb) to a third threshold level (Vc) less than the second nominal operating voltage (Vr); and
the operation circuit is adapted to be non-saturated and output an on time which varies in dependence on the current sense signal, thereby entering a current feedback control mode, by receiving the current sense signal under the varying relationship when the input voltage is between the threshold level (Vb) and the third threshold level (Vc).
7. A driver as claimed in claim 4, wherein:
the configuring circuit is adapted to configure the first relationship when the input voltage is between the threshold level (Vb) and a second threshold level (Va) smaller than the second nominal operating voltage (Vr); and
the operation circuit is adapted to be non-saturated and output an on time which varies independence on the current sense signal, thereby entering a current feedback control mode, by receiving the current sense signal under the first relationship when the input voltage is between the threshold level (Vb) and the second threshold level (Va).
8. A driver as claimed in claim 7 wherein:
the configuring circuit is adapted to configure a varying relationship from the first relationship to the second relationship as the input voltage increases from the second threshold level (Va) to a third threshold level (Vc) less than the second nominal operating voltage (Vr); and
the operation circuit is adapted to be non-saturated and output an on time which varies in dependence on the current sense signal thereby entering a current feedback control mode by receiving the current sense signal under the varying relationship when the input voltage is between the second threshold level (Va) and the third threshold level (Vc).
9. A driver as claimed in claim 6 or 8, wherein:
the configuring circuit is adapted to configure the current sensing arrangement with the second relationship when the input voltage is above the third threshold level (Vc); and
the operation circuit is adapted to be non-saturated and output an on time which varies in dependence on the current sense signal thereby entering a current feedback control mode by receiving the current sense signal under the second relationship when the input voltage is above the third threshold level (Vc).
10. A driver as claimed in any one of claims 3 to 9, wherein the current sensing arrangement comprises a main current sense resistor (Rl), a bypass resistor (R4) and a bypass switch (M2) in parallel with the main current sense resistor (Rl), wherein the configuring circuit is adapted to control the bypass switch (M2) to configure the relationship.
11. A driver as claimed in claim 10, wherein the configuring circuit is adapted to:
close the bypass switch (M2) to configure the first relationship; and open the bypass switch (M2) to configure the second relationship.
12. A driver as claimed in claim 1 or 2, wherein the controller comprises:
an operation circuit coupled to the current sensing arrangement, and adapted to obtain the current sense signal and to control the switch mode power converter based on the current sense signal at a negative comparing input of the operation circuit and a reference signal at a positive comparing input of the operation circuit; and
a configurable circuit to configure the reference signal, wherein the configurable circuit is adapted to configure a first reference signal to set the operation circuit to provide a first on time control to achieve the preset switching mode and a second reference signal to set the operation circuit to provide a second on time control to achieve the feedback switching mode.
13. A driver as claimed in any one of claims 1 to 12, wherein:
the preset switching mode comprises a fixed on time control mode of the switch mode power converter, and
the feedback switching mode comprises on time varying as the current sense signal varies to as to regulate the current sense signal as a set value.
14. A driver as claimed in any one of claims 1 to 13, wherein:
the preset switching mode is adapted to:
increase the current through the lighting load in a guard band increasing from the first nominal operating voltage (Vr/2) to the threshold level (Vb); and/or decrease the current through the lighting load in a sub band decreasing from the first nominal operating voltage (Vr/2); and
the feedback switching mode is adapted to decrease the current through the lighting load as the voltage increases to the second nominal operating voltage (Vr).
EP19790181.2A 2018-10-29 2019-10-16 Led lighting driver and drive method Pending EP3874910A1 (en)

Applications Claiming Priority (3)

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CN2018112467 2018-10-29
EP18213961 2018-12-19
PCT/EP2019/078064 WO2020088934A1 (en) 2018-10-29 2019-10-16 Led lighting driver and drive method

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