CN112997585B

CN112997585B - LED lighting device driver and driving method

Info

Publication number: CN112997585B
Application number: CN201980071502.4A
Authority: CN
Inventors: 金高先; R·德赫尔加利斯特奥; 刘静; P·R·维尔德曼
Original assignee: Signify Holding BV
Current assignee: Signify Holding BV
Priority date: 2018-10-29
Filing date: 2019-10-16
Publication date: 2024-03-22
Anticipated expiration: 2039-10-16
Also published as: WO2020088934A1; CN112997585A; EP3874910A1

Abstract

The driver is for driving the lighting load. The switched-mode power converter is controlled in dependence on the input voltage, in particular such 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 a voltage preset when two drivers are connected in series across the power input, and the second operating voltage corresponds to a voltage preset when one driver is connected.

Description

LED lighting device driver and driving method

Technical Field

The present invention relates to LED lighting devices, and more particularly to an LED lighting device driver.

Background

Solid state lighting units, in particular LED-based (retrofit) lamps, are increasingly used in home buildings and offices. In addition to their high efficiency, they are attractive to consumers due to novel design features, different color temperatures, dimming capabilities, etc.

In order to mount the LED lighting device on an existing mains lighting fixture, each LED lighting unit uses one converter circuit for converting AC mains to a DC drive signal and also for reducing the voltage level.

The converter circuit typically includes a rectifier and a switch mode power converter.

There are a number of possible designs for the switch mode power converter. The low cost switch 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 transfer of the stored energy to the load. The main power switch controls the supply of energy from the input to the main inductor. The timing of the operation of the main power switch, in particular the duty cycle, controls the transfer of energy.

The timing can be controlled by feedback control signals, in particular signals representing the current through the LED load. To this end, many converters sense current through a current sense resistor that is placed in the current path to be sensed.

The current flowing through the current sense resistor generates a voltage that, once it reaches a desired value, closes the main power switch. This is the peak current control mode. The time the main power switch is on is until the peak current is reached. This type of current-mode control switch depends on the dc bus voltage, since the higher the bus voltage, the faster the peak current is reached. Alternatively, the current sense resistor may be used to sense the average current to the load and control the duty cycle of the main power switch according to the average current. This is the average current control mode. These modes are well known in the art.

If the output load of the converter is constant, there is a closed loop feedback control, where the on-time of the main power switch is controlled by the sensed current.

When two converters, e.g. buck converters, are connected in series to a single supply voltage, the system input current is the same and thus the input voltage of each buck converter must be the same. In a closed loop feedback converter, there is an inverse relationship between the input current and the input voltage (to define a constant power curve). If the input voltages on the two closed loop converters are different, the higher input voltage tends to increase and the lower input voltage tends to decrease, which will lead to instability.

Thus, it should be ensured that each buck converter receives half of the total system voltage.

WO 2016/008943 discloses a driver having two modes of operation; one for a single driver and one for a series connection of two drivers. There are preset drive modes (for a driver in series with another driver) and feedback control modes (for a single driver). However, this prior art implements a preset drive mode by a dedicated generator and implements a feedback control mode by another comparator circuit. US2018/235043A1 discloses a similar technique in which the driver is in open loop control for tandem (series) mounting of two lamps and the driver is in closed loop control for single socket mounting.

Further studies have shown that in some prior art, different output currents occur in different modes, resulting in different light outputs. This is due to misalignment during transitions between different modes.

Disclosure of Invention

It is an object of the invention to provide a driver for driving a lighting load, wherein a switched mode power converter is controlled in dependence of an input voltage, in particular such that the same current is delivered by a preset (fixed) switched mode at a first operating voltage (Vr/2) and by a feedback (dynamic) switched mode at a second operating voltage (Vr). The first operating voltage corresponds to a voltage preset when two drivers are connected in series across the power input, and the second operating voltage corresponds to a voltage preset when one driver is connected.

The invention is defined by the claims.

According to an example of one aspect of the present invention, there is provided a driver for driving a lighting load, the driver comprising:

a switch mode power converter adapted to receive an input voltage at a power input and to provide energy to a lighting load at a power output;

a voltage sensing device adapted to sense an input voltage;

a current sensing device adapted to sense a current flowing through the lighting load and provide a current sense signal, and

A control circuit adapted to configure the switch mode power converter in the following modes:

a preset switching pattern when the input voltage is below a threshold level, wherein the preset switching pattern is adapted to deliver a first output current at a first nominal operating voltage below the threshold level, wherein the preset switching pattern is adapted to increase the 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 pattern when the input voltage is above the threshold level, wherein the feedback switching pattern is adapted to deliver substantially the same first output current at a second nominal operating voltage at or above the threshold level without allowing the output current to be above the first output current.

The driver has two modes of operation depending on the input voltage. The input voltage will for example depend on whether the driver is connected to a power supply (e.g. mains) alone or in series with another similar driver. The preset switching pattern involves an open loop control by which the setting of the switching pattern power converter is fixed, thereby avoiding any instability problems. Feedback switching mode involves closed loop current control, utilizing a variable setting (i.e., variable on-time control) of the switching mode power converter to achieve a desired output current. The switching between modes is between two input voltage levels. Each mode results in a different relationship between current and voltage. The respective relationships are designed such that there are two operating voltages (one for each mode) at which the same output current is output. Thus, the same light output can be ensured in different modes, and the lamp operates in a more uniform manner despite the different mounting.

The first nominal operating voltage is, for example, half of the second nominal operating voltage, wherein the first nominal operating voltage is the nominal input voltage when the driver is connected in series with the other driver to the power supply, and the second nominal operating voltage is the nominal input voltage when said drivers are individually connected to the power supply.

Thus, there is an equal current output for a single driver and a driver that is one of a pair of drivers in a series connection.

The control circuit may include:

an operating circuit coupled to the current sensing device and adapted to acquire a current sense signal and control an on-time of the switch-mode power converter based on the current sense signal; and

a configuration circuit for configuring the current sensing device using a relationship between a sensed current and a current sensing signal obtained by the operation circuit;

wherein the configuration circuit is adapted to configure:

a first relationship for setting the operating circuit to provide a first on-time control to provide the first current in the preset switch mode at the first operating voltage; and

a second relationship, different from the first relationship, for setting the operating circuit to provide a second on-time control to provide substantially the same first current in the feedback switch mode at a second operating voltage.

The operating circuit is, for example, an IC. The current sensing means provides a mapping between the level of the voltage and the output signal (e.g. voltage). By changing this mapping, the current setting of the switch mode power converter can be changed. Different mappings result in different time control methods.

In particular, by using the configuration circuit to alter the mapping, the switch-mode power converter is forced into a preset switch-mode, because the current sense signal indicates that the low current level of the reference current is never reached, thereby achieving maximum on-time control (as described above), because the switch-mode power supply is striving to provide the maximum current that the switch-mode power supply can withstand.

The operating circuit may be adapted to:

when the input voltage is lower than a threshold level, the input voltage is saturated by receiving a current sensing signal in a first relationship and outputting a fixed maximum on-time, thereby entering an open loop mode as a preset switching mode; and

when the input voltage is at or above the second operating voltage, entering a current feedback control mode by receiving the current sense signal in a second relationship without being saturated and outputting an on-time that varies depending on the current sense signal,

Wherein the second relationship has a greater gain than the first relationship and the operating circuit obtains the current sense signal at the negative comparison input.

Thus, a single operating circuit may be used for different modes instead of having two different control components for different modes. Gain control associated with the operating circuit is used to set the operating circuit to either of two modes. The operating circuit is saturated, i.e. a constant control signal is delivered irrespective of the input voltage, so that the preset mode is enabled when the first relationship is active, whereas the comparator is unsaturated, i.e. a control signal varying in dependence on the output current is delivered, so that the feedback mode is enabled when the second relationship is active.

In one example, the configuration circuit may be adapted to configure the current sensing means in the second relationship (immediately) at an operating voltage above said threshold level.

In this example, there is a single threshold below which the preset mode is below and above which the feedback mode is above. This is the simplest implementation. The current may change abruptly across the threshold level. However, since the operating voltage of the lamp does not dynamically change across the threshold level after installation, such abrupt changes are unlikely to occur in everyday use.

In another example, the configuration circuit is adapted to: configuring a change relationship from the first relationship to the second relationship as the input voltage increases from a threshold level to a third threshold level that is less than the second operating voltage; and the operating circuit is adapted to: when the input voltage is between the threshold level and the third threshold level, the current feedback control mode is entered by receiving the current sense signal under a varying relationship without being saturated and outputting an on-time that varies depending on the current sense signal.

In this example, there is an additional feedback mode between the threshold level and the second operating voltage. This additional feedback mode has a reference that varies with voltage variations. 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 configuration circuit is adapted to: the first relationship is configured when the input voltage is between a threshold level and a second threshold level that is less than the second operating voltage, and the operating circuit is adapted to: when the input voltage is between the threshold level and the second threshold level, the current feedback control mode is entered by receiving the current sense signal under the first relationship without being saturated and outputting an on-time that varies depending on the current sense signal.

In this example, there are again two thresholds. There is yet another additional feedback mode having 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 shown by the first relationship at the threshold level. Thus, a portion of the current versus voltage curve has a constant regulated current.

In addition to the further additional feedback mode, the configuration circuit is adapted to: configuring a changing relationship from the first relationship to the second relationship as the input voltage increases from the second threshold level to a third threshold level that is less than the second operating voltage; and the operating circuit is adapted to: when the input voltage is between the second threshold level and the third threshold level, the current feedback control mode is entered by receiving the current sense signal under a varying relationship without being saturated and outputting an on-time that varies depending on the current sense signal.

In these examples of adjusting the relationship between the first and second relationships, duty cycle control may be used to create a region of the input voltage for which the current sensing means is adjusted in an analog manner between the first and second relationships to provide a gradual transition to the standard feedback mode and a controlled current reduction. For example, the curve is followed when the driver is initially powered on.

For two examples using the third threshold, the configuration circuit is adapted to: configuring a current sensing device in a second relationship when the input voltage is above the third threshold level, and the operating circuit is adapted to: when the input voltage is higher than the third threshold level, the current feedback control mode is entered by receiving the current sense signal in the second relationship without being saturated and outputting an on-time that varies depending on the current sense signal.

Those embodiments with varying relationships provide gradual transitions that may provide better output characteristics for the user.

When the input voltage reaches the second operating voltage (which is greater than the second and third threshold levels), the current returns to the first current level.

The current sensing means may comprise a main current sensing resistor, a bypass resistor and a bypass switch connected in parallel with the main current sensing resistor, wherein the configuration circuit is adapted to control the bypass switch in a configuration 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 achieve the required reconfiguration, forcing the switched mode power supply into its preset peak control mode.

The configuration circuit is for example adapted to:

closing the bypass switch to configure the first relationship; and

the bypass switch is turned off to configure the second relationship.

In an alternative arrangement, instead of configuring current sensing, the controller may comprise:

an operating circuit coupled to the current sensing means and adapted to obtain a current sense signal and to control the switch mode power converter based on the current sense signal at a negative comparison input of the operating circuit and a reference signal at a proportional comparison input of the operating circuit; and

a configurable circuit for configuring a reference signal, wherein the configurable circuit is adapted to configure: a first reference signal to set an operating circuit for providing a first on-time control to achieve a preset switching pattern; and a second reference signal to set the operating circuit for providing a second on-time control to implement the feedback switch mode.

This is another way of achieving adjustable current sensing. The reference value is adapted instead of adapting the gain of the current sensing circuit itself. This is useful if the controller (e.g. IC) of the switched mode power supply has a suitable interface to adjust the reference value.

In all examples, the preset switching pattern may include a fixed on-time control pattern of the switching pattern power converter, and the feedback switching pattern includes an on-time variation as a function of the current sense signal to adjust the current sense signal to a set value.

In the fixed on-time control mode, the output current depends linearly (or almost linearly) on the input voltage. Thus, the first operating voltage and the first current level are at a point along the linear curve.

The preset switching pattern may be adapted to increase the current flowing through the lighting load in the guard band, thereby increasing from the first operating voltage to the threshold level; and/or reducing the current flowing through the lighting load in the sub-band, thereby reducing from the first operating voltage. The feedback switch mode may be adapted to reduce the current flowing through the lighting load as the voltage increases from the threshold level to the second operating voltage.

This guard band is used because the voltage may rise unevenly during starting, especially in the case of double lamps. The lamp is kept in a preset mode even if the voltage exceeds the operating voltage by a suitable tolerance, which is better for achieving a balanced double lamp in the end. This also means that the current level corresponding to the threshold value is higher than the first current level. The feedback switch mode requires returning the current level to the threshold level. The output current of the feedback switch mode follows a power curve. The second operating voltage and the first current level are at a point along the power curve.

The switch mode power converter comprises, for example, a buck converter.

The invention also provides a driver arrangement comprising two drivers in series, each driver for driving an associated lighting load, wherein each driver is as defined above.

The present invention also provides a method of driving a lighting load, comprising:

receiving an input voltage;

sensing an input voltage;

converting an input voltage to an output voltage using a switch mode power converter, thereby delivering a current to a lighting load;

sensing a current through the lighting load to deliver a current sense signal;

configuring a switch-mode power converter to a mode:

a preset switching mode adapted to deliver a first output current at a first operating voltage Vr/2 below a threshold level Vb when the input voltage is below the threshold level Vb; and

and a feedback switch mode when the input voltage is above the threshold level Vb, wherein the feedback switch mode is adapted to pass the first current output at the second operating voltage Vr above the threshold level.

The method may include configuring a relationship between the sensed current and the current sense signal to provide a first relationship for a preset switching mode and a second relationship at a second operating voltage.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

Drawings

Examples of the present 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 of the respective type shown in fig. 1, each connected in series across the system voltage V1.

Fig. 4 shows two buck converters of the respective type shown in fig. 2, each connected in series across the system voltage V1.

5A-C illustrate connection options desired for a lamp design;

FIG. 6 shows the current (y-axis) versus voltage (x-axis) of a lamp, which can be switched between open loop control (with preset switching functions) and closed loop control (with current sensing and feedback based switches);

figures 7A-C show three examples of possible current versus voltage characteristics;

FIG. 8 illustrates a first circuit example for implementing the features of FIGS. 7A-C based on the buck converter of FIG. 1;

fig. 9 shows a second circuit example for implementing the features of fig. 7A-C based on the buck converter of fig. 2.

Detailed Description

The present invention will be described with reference to the accompanying drawings.

It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, system, and method, 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, system, and method 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 drawings to designate the same or similar parts.

The present invention provides a driver for driving a lighting load using a switch-mode power converter controlled in dependence of an input voltage. The same current is delivered by a preset (fixed) switching pattern at a first operating voltage (Vr/2) and a feedback (dynamic) switching pattern at a second operating voltage (Vr). This means that the driver can be used as a single driver, or as one of a pair of series drivers.

Fig. 1 shows an example of a low power factor buck converter circuit. The DC bus supplies a load R2 (which may be a linear or a non-linear load), an inductor L1, a main power switch M1 and a current sense resistor R1, all in series. A freewheeling diode D1 is connected across the load and the inductor. In this example, current sensing occurs at the bottom side of the main power switch.

The duty cycle of the main power switch M1 is controlled to regulate the power transfer ratio of the circuit from the input to the inductor and from the inductor to the load. For example, the power transfer is controlled in dependence of the sensed current.

Fig. 2 shows an alternative buck converter circuit with high-side switching and sensing functions. The DC bus supplies a main power switch M1, a current sense resistor R1, an inductor L1, and a load R2 all in series. The freewheeling diode D1 is connected across the load, the inductor and the sense resistor and has a smoothing capacitor C2 in parallel with the load R2.

In these circuits, the current flowing through the current sense resistor R1 is sensed each time the main switch M1 is turned on. The current flowing through the current sense resistor R1 generates a voltage across the resistor R1, and once the voltage across R1 reaches a desired value, the main switch turns off. This is known as peak current mode control switching and is used in the most basic buck converter. The switching frequency depends on the DC bus voltage.

As long as the output load R2 of the buck converter is unchanged, there is a closed loop control of the system where the on-time of the main switch M1 is controlled by a sense current independent of the DC bus voltage.

Fig. 3 shows two buck converters 30, 32 of the respective type shown in fig. 1, each connected in series across the system voltage V1. Each converter additionally has a diode bridge rectifier and an input capacitor.

Fig. 4 shows two buck converters 40, 42 of the respective type shown in fig. 1, each connected in series across the system voltage V1. Each converter additionally has a diode bridge rectifier.

In such a 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 is operated in the feedback mode described above, it is difficult to ensure that the input voltages are equal.

There is a need for a lamp that can be placed alone or in series combination with another lamp. If lamps operated in closed loop control are operated in a series connection, the voltage may rise unevenly. As described above, WO2016/008943 discloses a technique that operates a lamp in a closed-loop control mode when the lamp is a single lamp, and in an open-loop control mode when the lamp is in series with another lamp.

Fig. 5A-C show the connection options desired for the lamp design. In fig. 5A, the lamp 50 is directly connected to the mains. In fig. 5B, lamp 50 is connected to mains via an electromagnetic ballast 52. In fig. 5C, two lamps 50 are connected in series to the mains and in series with an electromagnetic ballast 52. Optionally, switch 54 is shown bypassing the ballast.

Fig. 6 shows the current (y-axis) versus voltage (x-axis) of a lamp, which can be switched between open loop control (with preset switching functions) and closed loop control (with current sensing and feedback based switching). The mode switching is performed at an input voltage of 140V, with an open loop preset switching when the input voltage is below 140V, and a 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, since the duty cycle is constant (and thus independent of the output current), and the output current increases when the input voltage increases. Open loop control is used for lamps placed in series.

The ideal threshold for distinguishing between a single lamp and two lamps in series may be half the mains voltage. However, a guard band is preferably used so that any disturbance of the mains voltage or operation of the lamp does not cause false triggering of the lamp to enter closed loop control. For example, in europe, the RMS voltage is 230V, so assuming a 10% disturbance, the maximum RMS voltage may be 254V, while the ideal voltage to distinguish between a single lamp installation or a series/double lamp installation may be 127V.

The lamp components (e.g., inductors, ICs, etc.) have tolerances and the output currents may vary by 10%. This may result in a voltage change between the two lamps even if the two lamps are operated in an open loop control mode. Thus, the threshold value may be set higher, for example, at 140V. If the input voltage reaches or exceeds the 140V threshold, the lamp enters a closed loop control mode that constantly maintains the current at the same value as preset at 140V.

This causes a problem that lamps operating in an open loop control mode at a nominal half mains voltage (operating point 60) have a different output current than the closed loop control (operating point 62). For some applications with two different installations, the lamp may emit different output lumens and may not give a uniform output appearance.

The present invention provides a driver that can also switch between preset switching and feedback switching modes of operation of a switch-mode power converter (e.g., a buck converter), but wherein the same output current is provided at the first and second operating voltages.

Fig. 7A-C show three examples of possible current versus voltage characteristics.

In each case, when the input voltage is below the threshold level Vb, the switch-mode power converter may operate in a preset switch mode and pass the first output current io_rated at the first operating voltage Vr/2 below the threshold level Vb. The switch mode power converter may also operate in a feedback switch mode when the input voltage is above a threshold level Vb (either just above Vb or starting at a voltage slightly above Vb). The feedback switch mode delivers substantially the same first current output io_rated at the second operating voltage Vr at or above the threshold level Vb.

When the driver is connected in series with another driver to the power supply, the first operating voltage Vr/2 is half the second operating voltage Vr and is the nominal input voltage. The second operating voltage Vr is the nominal input voltage when the driver is individually connected to the power supply.

Fig. 7A shows a first possible feature.

During open loop control, the current increases linearly with voltage. At the input voltage Vr/2, the desired output current Io_rated is obtained. The switching threshold Vb is higher than the input voltage Vr/2, so the current has increased above the desired value until io_h. There is a step to closed loop feedback control at the input voltage Vb and the output current drops more or less immediately to the desired current io_rated.

Fig. 7B shows a second possible characteristic.

During open loop control, the current increases linearly with voltage. At the input voltage Vr/2, the desired output current Io_rated is obtained. The switching threshold Vb is higher than the input voltage Vr/2, and thus the current has increased above the desired value io_rated.

There is a change at the input voltage Vb to the closed loop feedback control. Unlike the embodiment in fig. 7A, the current does not immediately drop to the desired value io_h, but slowly slides to the desired value. The current may for example follow a power curve of a switch-mode power converter, by which curve the product of the current and the voltage is kept constant. As a result, the current does not decrease in steps, but follows a curve as the voltage increases to a third threshold level Vc that is still smaller than the second operating voltage Vr. When the input voltage is between the threshold values Vb and Vc, the current is controlled in a feedback manner with a variable current setting. After the voltage reaches the third threshold value Vc, the output current has dropped to the desired current io_rated, and then remains constant.

Fig. 7C shows a third possible characteristic.

In this case, there is a second threshold level Va that is greater than the first threshold level Vb and less than the second operating voltage.

There is a change at the input voltage Vb to the closed loop feedback control. In the initial phase of the input voltage increasing from Vb to Va, the current remains constant. This part is similar to the solution shown in fig. 6. When the voltage increases from the second threshold Va to a third threshold Vc, which is still smaller than the second operating voltage Vr, a power curve exists. When the input voltage is between the threshold values Vb and Vc, the current is first controlled in a feedback manner with a fixed current setting and then with a variable current setting. Once the voltage reaches the third threshold value Vc, the output current drops again to the desired current io_rated, and then remains constant.

The manner in which these current and voltage characteristics are obtained will be explained with reference to fig. 8, fig. 8 showing a first circuit example based on the buck converter of fig. 1.

The driver comprises a control circuit 80 adapted to configure the switch-mode power converter into different modes of operation. Instead of a single current sensing resistor R1, there are current sensing means that can be configured in different ways.

The control circuit comprises an operating circuit 82 coupled to the current sensing means and adapted to obtain a current sense signal and to control the on-time of the switch-mode power converter based on the current sense signal. This may include, for example, a standard controller IC of the driver. It comprises at its input a comparator 83 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 will go low and turn off the main switch M1; otherwise, it is high and the main switch M1 still allows the input current to ramp up. The main switch of the switching power converter is controlled depending on the comparison result.

The configuration circuit 84 is used to configure the current sensing device using the relationship between the sensed current and the current sense signal provided to the operating circuit 82.

In other words, when the current sensing means are configured differently, the signal provided to the operating circuit 82 will be different for the same current flowing. Thus, the operation of the driver will depend on the configuration of the current sensing means.

The current sensing means comprises a conventional (main) current sense resistor R1, a shunt resistor R4, and a shunt switch M2 connected in parallel with the main current sense resistor R1. The configuration circuit is adapted to control the bypass switch M2 to configure the relationship between the current flow and the current sense signal provided to the operating circuit 82.

When the bypass switch M2 is closed, a first relationship is established. The sense resistors R1 and R4 are connected in parallel, thus reducing the effective resistance of the current sense resistor and thus the sensed voltage provided to the operating circuit 82. The current sensing means has a relatively low gain. Thus, the operating circuit is made to believe that a low current is flowing, so it keeps outputting a high voltage to turn on the main switch M1, i.e., the on time is long. Typically, a switch-mode power converter or IC has a mechanism to close the main switch when the on-time reaches a maximum. Therefore, the maximum value is used in each handover. The first relationship is used to set the operating circuit 82 to provide a first on-time control to provide a first current in a preset switching mode at the first operating voltage Vr/2.

Thus, the operating circuit may be considered saturated (because the comparator 82 is always delivering a high signal, i.e. a saturated output), and a fixed maximum on-time is used, thereby entering the open loop mode as a preset switching mode when the input voltage input is below the threshold level Vb.

When the bypass switch M2 is turned off, a second relationship is established in which only the main current sensor resistor R1 is operated. Unlike the first relationship, the second relationship is used to set the operating circuit to provide a second on-time control to provide substantially the same first current in the feedback switch mode at the second operating voltage Vr, as shown in fig. 7A-C. This is a conventional closed loop feedback control. The current sensing means has a relatively large gain (a gain greater than the first relation). The signal on the negative input of the comparator can reach the voltage on the positive input and the comparator can output a low level to turn off the main switch M1.

Then, the operation circuit (particularly, the comparator thereof) may be regarded as not saturated and output an on-time that varies depending on the current sensing signal, thereby entering the current feedback control mode when the input voltage is at or above the second operation voltage Vr.

Since the operating circuit obtains the current sense signal at the negative comparison input, the low current sense signal received during the first relationship is converted to a larger on-time by the comparison function.

The circuit of fig. 8 includes a voltage divider of resistors R5, R6, and R7 that follows the DC bus voltage. Capacitor C2 is in parallel with resistor R7 and needs to be charged to provide intentional delay/buffering for the voltage characteristics across resistor R7 as a function of the average DC bus voltage. The voltage across resistor R7 is the first operating voltage.

The 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 will not reach the zener voltage and therefore transistor Q1 will be turned off.

The second voltage dividers R3 and R8 define a second operating voltage across the resistor R3. When the transistor Q1 is turned off, the second operating voltage turns on the transistor M2. The second zener diode D3 is used to ensure that the voltage across R3 is sufficiently high. When the transistor M2 is turned on, the second sense resistor R4 is placed in parallel with the main sense resistor R1. Thus, the circuit operates in response to a low DC bus voltage to turn on transistor M2 and place two current sense resistors in parallel, defining a first relationship between current and current sense signals.

If the DC bus voltage is high enough to reach the zener voltage of zener diode D2, transistor Q1 will turn on. This shorts out resistor R3, thus pulling down the second operating voltage. The voltage is insufficient to reach the threshold of the zener diode D3 and the gate voltage of the transistor M2 remains low. Thus, transistor M2 is turned off and the current sensing means is only the main current sensing resistor R1. This defines a second relationship between the current and the current sense signal.

In the example of fig. 7A-C, there is a first relationship for input voltages below Vb and a second relationship for input voltages above Vb.

The second relationship is used to provide a fixed current immediately upon reaching voltage Vb in fig. 7A, or to provide a gradual decrease in current between Vb and Vc in fig. 7B, or to provide a constant current from Vb to Va in fig. 7C, followed by a gradual decrease in current between Va and Vc.

The transition between the first and second relationships is gradual (i.e., there is a changing relationship) in fig. 7B and 7C. This can be achieved by controlling the operation transistor M2 with a duty ratio.

The manner in which the circuit of fig. 8 is used to implement the outline of fig. 7C will now be explained:

when Vin < Vb, the circuit operates in the maximum on-time control mode, and M2 is on. Because of the parallel connection of R1 and R4, the sensed current is small, and thus the operating circuit will automatically enter maximum Ton mode. The voltage Vr/2 is the average value of the series arrangement. A guard band from Vr/2 to Vb is provided because the voltage may develop unevenly during start-up. Thus, the lamp can operate in open loop as long as the voltage is less than Vb. Thus, the output at Vb is greater than the output at Vr/2.

When vb.ltoreq.vin.ltoreq.Va, the circuit operates in peak current control mode (but in closed loop control mode, rather than the previous open loop maximum current control mode). M2 is on and current is still sensed by the main sense resistor R1 in parallel with the shunt resistor and resistor R4. As the input voltage increases, so does the input current, and the voltage across the parallel connection of R1 and R4 may reach the reference value on the positive input of comparator 82. The operating circuit leaves the maximum Ton mode and the on-time is instead adjusted in a closed loop to maintain the current.

When Va.ltoreq.vin.ltoreq.Vc, the circuit operates in a peak current control mode (closed loop mode). M2 operates based on a chopping method (pulse width modulation, PWM). More specifically, Q1 is alternately turned on and off at a high frequency, and M2 is alternately turned off and on in turn. The sensed effective current is based on the primary sense resistor R1 in parallel with R4 times D, which is a duty cycle from 0 to 1.

When Vin > Vc, the circuit operates in peak current control mode (closed loop mode), Q1 is always on, M2 is always off, and current sensing is performed solely through the main current sense resistor R1. This will restore the larger current at Vb/Va to the same current as at Vr/2.

Typically, the maximum on-time and the main current sense resistor R1 are chosen so as to set the output current at half the rated voltage Vr to be the same as the current at the rated voltage, so that the same lumen output per lamp can be achieved with one lamp and two double lamps in series. For example, for a 230V mains, vb is set between 140V and 150V, va is set greater than Vb, and Vc is set lower than 200V.

Fig. 9 shows a second circuit example based on the buck converter of fig. 2. This is a high side buck circuit.

Instead of using a voltage detector directly connected to the DC bus, the auxiliary winding L1B is used to sense the voltage across the main inductor L1, i.e. to sense the input voltage. L1B is the auxiliary winding of the transformer, which is in linear relation to the DC bus voltage.

Similar to the circuit of fig. 8, the sensed voltage is provided to a base voltage resistor divider R5, R6 that provides a base voltage for transistor Q1.

The capacitor C2 charges, delaying the voltage characteristics of R5 and R6. If the DC bus voltage is too low, the voltage on R6 cannot reach the zener voltage of zener diode ZD1 required for transistor Q1 to turn on. When transistor Q1 is on, transistor M2 may be turned on, which in turn means that the second sense resistor R4 is placed in parallel with the main sense resistor R1. The peak current of the buck converter charging phase flows through the parallel connection of M1, R1 and R4, inductor L1, and load R2. The peak current is sensed as feedback line 90.

If the dc bus voltage is high enough to reach the zener voltage of zener diode ZD1, transistor Q1 will turn off and the gate of transistor M2 remains low, so only R1 acts as a current sense resistor.

Thus, the circuit generally operates in the same manner as in fig. 8.

receiving an input voltage;

sensing an input voltage;

sensing current flowing through the lighting load to deliver a current sense signal;

the switch mode power converter is configured in the following modes:

In the above example, the current sensing is configurable. Instead, the way in which the non-configurable current signal can be interpreted within the operating circuit can be configured, for example by adapting the reference value (compared to the sensed current) in dependence on the input voltage.

Two circuit examples are shown above. However, other circuits may be used. Basically, the transfer function of the current sensing circuit is adjustable or the reference to which the current sensing signal is compared is adjustable based on the current input voltage. Thus, there is basically an input voltage sensing, and a configuration of current sensing functions. These objectives can be achieved in a number of 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 shall not be construed as limiting the scope.

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 device adapted to sense the input voltage;

-a control circuit (80) adapted to configure the switched mode power converter in the following modes:

a preset switching pattern when the driver and the further driver are mounted in series and the input voltage is below a threshold level (Vb), wherein the preset switching pattern is adapted to deliver a first output current (io_rated) at a first nominal operating voltage (Vr/2) for the series mounting below the threshold level (Vb), wherein the preset switching pattern is adapted to increase the 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 pattern when the drivers are individually mounted and the input voltage is above the threshold level (Vb), wherein the feedback switching pattern is adapted to deliver substantially the same first output current (io_rated) at a second nominal operating voltage (Vr) at or above the threshold level (Vb) for a series mounting.

2. Driver according to 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 power supply; and

the second nominal operating voltage is a nominal input voltage when the driver is individually connected to the power supply.

3. The driver according to claim 1 or 2, wherein the control circuit (80) comprises:

-an operating circuit (82) coupled to the current sensing means and adapted to acquire the current sensing signal and to control the on-time of the switch-mode power converter based on the current sensing signal; and

a configuration circuit (84) for configuring the current sensing device using a relationship between the sensed current and the current sensing signal obtained by the operation circuit;

wherein the configuration circuit is adapted to configure:

a first relationship for setting the operating circuit to provide a first on-time control to provide a first current in the preset switch mode at the first nominal operating voltage; and

A second relationship, different from the first relationship, for setting the operating circuit to provide a second on-time control to provide substantially the same first current in the feedback switch mode at the second nominal operating voltage.

4. A driver according to claim 3, wherein the operating circuit is adapted to:

when the input voltage is lower than the threshold level (Vb), being saturated by receiving the current sense signal in the first relationship and outputting a fixed maximum on-time, thereby entering an open loop mode as the preset switching mode; and

when the input voltage is at or above the second nominal operating voltage (Vr), entering the feedback switching mode by receiving the current sense signal in the second relationship without being saturated and outputting an on-time that varies depending on the current sense signal,

wherein the second relationship has a greater gain than the first relationship and the operating circuit obtains the current sense signal at a negative comparison input.

5. The driver of claim 4, wherein the configuration circuit is adapted to configure the current sensing device in the second relationship at an operating voltage above the threshold level (Vb).

6. The driver of claim 4, wherein:

the configuration circuit is adapted to: configuring a changing 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) that is less than the second nominal operating voltage (Vr); and

the operating circuit is adapted to: when the input voltage is between the threshold level (Vb) and the third threshold level (Vc), a current feedback control mode is entered by receiving the current sense signal under the varying relationship without saturation and outputting an on-time that varies depending on the current sense signal.

7. The driver of claim 4, wherein:

the configuration circuit is adapted to: configuring the first relationship when the input voltage is between the threshold level (Vb) and a second threshold level (Va) that is less than the second nominal operating voltage (Vr); and

the operating circuit is adapted to: when the input voltage is between the threshold level (Vb) and the second threshold level (Va), a current feedback control mode is entered by receiving the current sense signal under the first relationship without being saturated and outputting an on-time that varies depending on the current sense signal.

8. The driver of claim 7, wherein:

the configuration circuit is adapted to: configuring a changing 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 operating circuit is adapted to: when the input voltage is between the second threshold level (Va) and the third threshold level (Vc), a current feedback control mode is entered by receiving the current sense signal under the varying relationship without being saturated and outputting an on-time that varies depending on the current sense signal.

9. The driver according to claim 6 or 8, wherein:

the configuration circuit is adapted to: configuring the current sensing means in the second relationship when the input voltage is above the third threshold level (Vc); and

the operating circuit is adapted to: when the input voltage is higher than the third threshold level (Vc), a current feedback control mode is entered by receiving the current sense signal in the second relationship without being saturated and outputting an on-time that varies depending on the current sense signal.

10. A driver according to claim 3, wherein the current sensing means comprises a main current sensing resistor (R1), a bypass resistor (R4) and a bypass switch (M2), the bypass resistor and the bypass switch being connected in parallel with the main current sensing resistor (R1), wherein the configuration circuit is adapted to control the bypass switch (M2) to configure the relationship.

11. The driver of claim 10, wherein the configuration circuit is adapted to:

closing the bypass switch (M2) to configure the first relationship; and

the bypass switch (M2) is turned off to configure the second relationship.

12. The driver of claim 1 or 2, wherein the driver further comprises a controller comprising:

an operating circuit coupled to the current sensing means and adapted to obtain the current sense signal and control the switch mode power converter based on the current sense signal at a negative comparison input of the operating circuit and a reference signal at a proportional comparison input of the operating circuit; and

a configurable circuit for configuring the reference signal, wherein the configurable circuit is adapted to configure: a first reference signal to set the operating circuit to provide a first on-time control to achieve the preset switching mode, wherein the first reference signal is never reached by the current sense signal; and a second reference signal to set the operating circuit to provide a second on-time control to implement the feedback switch mode.

13. The driver according to claim 1 or 2, wherein:

the preset switching pattern includes a fixed on-time control pattern of the switching pattern power converter, and

the feedback switch mode includes an on-time that varies with a variation of the current sense signal to adjust the current sense signal to a set point.

14. The driver according to claim 1 or 2, wherein:

the preset switching pattern is adapted to:

increasing the current flowing through the lighting load in a guard band, thereby increasing from the first nominal operating voltage (Vr/2) to the threshold level (Vb); and/or

Reducing the current flowing through the lighting load in a sub-band, thereby reducing from the first nominal operating voltage (Vr/2); and

the feedback switch mode is adapted to reduce the current through the lighting load as the voltage increases to the second nominal operating voltage (Vr).