WO2015165925A1

WO2015165925A1 - Led circuit, a lighting arrangement and an led driving method

Info

Publication number: WO2015165925A1
Application number: PCT/EP2015/059260
Authority: WO
Inventors: Jie Fu; Junhu LIU; Shu Xu; Xianhui ZHANG
Original assignee: Koninklijke Philips N.V.
Priority date: 2014-04-30
Filing date: 2015-04-29
Publication date: 2015-11-05

Abstract

An LED circuit uses a rectifier (12) which has a minimum current requirement for correct operation and provides a rectified output voltage. An LED arrangement (14) is driven by the rectified output voltage. A current source (22) is used to provide a current to the LED arrangement which exceeds the minimum current output requirement. An interrupt switch (40) is used to control the LED arrangement light output.

Description

LED CIRCUIT, A LIGHTING ARRANGEMENT AND AN LED DRIVING METHOD

FIELD OF THE INVENTION

The present invention relates to an LED circuit, a lighting arrangement and an LED driving method.

BACKGROUND OF THE INVENTION

LEDs are increasingly being used to replace incandescent light bulbs. To avoid the need for expensive wiring and component changes, LED bulbs are designed to fit to existing light sockets with power supplies.

The MR16 bulb format is one of the most widely used lamps in the lighting industry. It is a low voltage spot light with a reflective cone. Typically, a local transformer is provided at each light fitting to convert from mains voltage to 12V.

LEDs are now replacing halogen as the preferred light source for an MR 16 light bulb. Due to the different performance of LEDs to the halogen bulb, the compatibility with the Electrical Transformer (ET) already mounted for the halogen bulb becomes a challenge. Also due to the small volume of the MR 16 format, the driver design for an LED bulb is difficult.

Figure 1 shows a known driver circuit for driving LEDs from a mains voltage, for example as can be used in an MR16 bulb.

The circuit comprises an electrical transformer 10 and a rectifier 12 to generate the desired low voltage DC supply for the light source 14 which is shown as a series chain of LEDs. A DC-DC converter in the form of a buck converter 16 is used to drive the light source 14. A smoothing capacitor 18 is provided at the output of the buck converter 16.

There are compatibility issues with certain transformer types. In particular, the transformer has no load when the AC input voltage is smaller than the DC output voltage, with the result that the transformer may stop oscillating. Thus, the self-oscillation of the transformer may not be sustained as a result of incompatibility between the transformer and the load presented by the LED and its driver.

In order to address this compatibility issue, a power modulation stage 20 can be added before the buck converter as shown in Figure 2. Figure 2 also shows the transformer supplied by a triac 17 to implement a dimming function. This is widely used in indoor lighting for diming the light level from a traditional lamp. The power of an LED lamp is much smaller than the traditional incandescent lamp. However, the triac dimmer 17 needs a minimum current to maintain normal operation, otherwise it can misfire resulting in flicker.

The power modulation stage 20 enables the driver to have very good compatibility with the transformer, but the cost is high and driver size is very large due to the introduction of the power modulation circuit.

These two schemes also make use of an internal electrolytic storage capacitor which reduces life time of the lamp.

WO 2012059838A1 discloses a method and a device for driving an LED string, wherein there is a current control device 45 for providing a current I running in the LEDs.

SUMMARY OF THE INVENTION

According to the invention, there is provided an LED circuit and driving method as claimed in the independent claims.

In one aspect, the invention provides an LED circuit, comprising: a rectifier for receiving an ac input voltage from a supply having a minimum current output requirement, and providing a rectified output voltage, wherein said supply comprises an electronic transformer and said minimum current output is the minimum current required by the electronic transformer in at least some periods to sustain self-oscillation of the electronic transformer, or said supply comprises a triac dimmer and said minimum current output is the minimum current to prevent flicker of the triac dimmer;

an LED arrangement coupled to the rectifier and driven by the rectified output voltage;

a current source coupled to the rectifier and the LED arrangement, for providing, from the rectified output voltage, a current to the LED arrangement which exceeds the minimum current output requirement;

an interrupt switch, wherein the LED arrangement, the interrupt switch and the current source are coupled; and

a controller coupled to the interrupt switch for controlling the interrupt switch in pulse width modulation (PWM) and to control the LED arrangement light output to a desired level.

This circuit uses a current source to provide a minimum current, i.e. to present a minimum output load to the supply, to enable a supply to be correctly operated. This for example enables the LED circuit to be supplied by a low voltage ac signal as generated by an electronic transformer. Such a transformer for example requires at least some periods when the current exceeds a minimum to be able to sustain self-oscillation.

Similarly, a triac dimmer can have a minimum output current requirement to prevent flicker. The invention may thus also be used in mains voltage systems, for example providing a minimum load to a triac dimmer. The invention may also be provided in systems which combine the use of an electronic transformer and a triac dimmer, in which case the minimum current output requirement may result from either the triac dimmer or the electronic transformer. By providing the desired minimum current, a deeper dimming level may be possible from the triac dimmer without the adverse effect of increased flicker. As a further example, a motion detector (for automatic control of lighting) may also have a minimum current requirement for correct operation.

The minimum current requirement is of course equivalent to a minimum power requirement (since for example the transformer delivers a known output voltage). Thus, the supply can be considered to have a minimum power output or a minimum current output for correct operation; the two are equivalent.

The switch and controller enable the overall light output to be controlled to a desired level, in case for example the current corresponding to the minimum load

requirement is more than the desired output of the LED arrangement. The interrupt switch is for example used to implement pulse width modulation (PWM) control.

The invention enables a low cost, small size and reliable LED driver solution for low voltage LED bulbs, such as replacements for MR16 halogen bulbs. Meanwhile, good compatibility with an existing transformer can be achieved. The LED driver and LEDs can be integrated on one board, possibly even in one chip.

The current source output can for example be in the range 1 to 1.5A. This is suitable for an MR16 application. Other current ranges will be of interest for other lighting applications.

The LED arrangement may comprise:

at least one set of LEDs in series, each LED in the set having a respective parallel control switch.

This arrangement enables the number of LEDs in the series chain to be controlled, for example as a function of the rectified output voltage level. This provides a tapped linear driver architecture. Further, since the LED load is purely resistive, the circuit comprising the linear current source, the LEDs and the interrupt switch is linear, and can keep the phase of the load current in consistence with the phase of the AC input signal. This has an advantage of easy synchronization between the behavior of the LED circuit with the behavior of the transformer.

The controller is then further for controlling the parallel control switches.

The controller thus controls the series connection(s) of LEDs to maintain an appropriate number in circuit as well as controlling the interrupt switch to deliver a desired light output.

The controller preferably comprises a first input for receiving a signal representing the output voltage provided by the rectifier, wherein the controller is adapted to compare the output voltage provided by the rectifier with a set of thresholds to derive control signals for the control switches in a linear manner.

In this way, the number of LEDs which are bypassed by the parallel control switches (and therefore the number which remain in circuit) is controlled based on the available rectified voltage level. In this way, the number of LEDs in series is matched to the drive voltage at a given time. This avoids the need for electrolytic smoothing capacitors and thus extends the circuit lifetime.

It should be noted that this is not the only topology and way of driving the LEDs. For example, the rectified high frequency power signal can further be converted by using an additional DC-DC converter which eliminates the high order harmonics of the rectified high frequency power and outputs more stable DC voltage. In this case, the LEDs can be constantly connected in series or in parallel with the DC voltage and there is no need for the bypass switches.

The controller preferably comprises:

a second input for receiving a signal representing a current flowing through the LED arrangement;

a third input for receiving a signal representing the rectified output voltage as applied to the LED arrangement;

a first calculating unit for calculating the maximum output power of the circuit during one period of the ac input voltage; and

a second calculating unit for calculating a duty cycle of the interrupt switch according to the maximum output power and a desired output power.

For example, the current flowing and rectified voltage as actually present across the LEDs can be used as feedback parameters to enable the maximum output power to be derived. This can then be compared with a desired output power setting to enable a duty cycle control of the interrupt switch to be implemented. A current sense resistor can be used in series with the LED arrangement for implementing the generation of a signal representing the current flowing.

In this embodiment, the real output power of the LED circuit is measured and compared with the desired power, and the comparison result is used in a feedback manner to control the output power. Thus more accurate power control can be achieved.

The controller may further be for determining the timing of an oscillation of the supply and controlling the interrupt switch according to said timing.

This is of particular interest if the circuit is supplied by a self-oscillating electronic transformer. This typically outputs an ac signal which has a self-oscillation frequency which is higher than the mains frequency. The envelope of the high frequency self-oscillating signal has a frequency corresponding to the mains frequency.

The controller is then preferably further adapted to synchronize the timing of the control of the interrupt switch with respect to the self-oscillation of the electronic transformer, wherein the controller is adapted to control the interrupt switch to deliver power to the LED arrangement at a time instant suitable for starting a self-oscillation of the electronic transformer.

There are many different types of electronic transformers. Different types may have the same or different starting points for their oscillation. In order to enable self-oscillation, the starting time could be at any phase of input AC line voltage. For example, in one embodiment, the electronic transformer has a starting point of oscillation at the zero-crossing point of the AC line voltage. The starting time can be controlled to be at the desired phase of the ac line voltage in order to enable self-oscillation of the particular type of electronic transformer used.

Given the linear LED circuit as discussed above, the interrupt switch may for example be controlled at the same instant as the starting point of the AC mains signal, and in this way it is simple to guarantee the self-oscillation. For example, if the starting point of the electronic transformer is at the zero crossing point of the AC mains signal, the interrupt switch may be closed at the zero crossing of the AC mains cycle. However, other timing solutions may be employed with aim of ensuring that self-oscillation of the electronic transformer can start, and the closing time of the interrupt switch can be adapted accordingly.

The invention also provides a lighting arrangement comprising a circuit as described above and an electronic self-oscillating transformer as the supply for supplying the ac input signal to the circuit. In another aspect, the invention provides a method of driving an LED arrangement, comprising:

receiving an ac input from a supply having a minimum current output requirement and providing a rectified output voltage;

operating a current source to provide, from the rectified output voltage, a current to the LED arrangement which exceeds the minimum current output requirement; and

controlling coupling between the LED arrangement and the current source thereby to control the LED arrangement output.

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:

Figure 1 shows a known LED driver circuit;

Figure 2 shows a known modification to the circuit of Figure 1 for improved compatibility with an electronic transformer;

Figure 3 shows a known tapped linear driver circuit for driving a string of LEDs, and also shows timing signals;

Figure 4 shows an example of LED driver using a modified linear tapped driver circuit;

Figure 5 shows timing diagrams for the circuit of Figure 4;

Figure 6 is a flow chart to explain the operation of the circuit of Figure 4; and Figure 7 shows more detailed timing diagrams for the circuit of Figure 4.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides an LED circuit which connects to a supply having a minimum current requirement for correct operation. The LED circuit rectifies the voltage from the supply. A current source in the LED circuit is used to provide a current to an LED arrangement which exceeds the minimum current output requirement. The LED arrangement is driven by the rectified output voltage and the current. An interrupt switch is used to control the LED arrangement light output. This arrangement ensures a desired minimum load, and uses an interrupt switch to reduce the power output if it exceeds a desired level. LED lamps typically use approximately 5 times less power than halogen lamps. For low voltage applications using a transformer, or for applications using a dimmer, this means that the loading of the dimmer and the transformer is 5 times lower.

The under-loading of the transformer poses a challenge especially for low-cost electronic transformers which are often designed to work with a minimum load of around 20W.

Most commonly used electronic-transformers make use of a self-oscillating circuit. A minimum load is required to operate, otherwise the operation ceases. Some self-oscillating transformers can start oscillating only under certain phases. Both of these conditions can give rise to transformer compatibility issues. Thus, there are two

requirements for LED driver electronics to enable compatibility with existing low voltage transformers, for example as may be used with existing MR16 installations:

(i) the transformer should be loaded at all times when it may need to start operation.

(ii) the load of the transformer should be large enough to maintain self-oscillation.

The invention makes use of current injection to provide an essentially linear LED driver which presents a load to the input source which approximates a resistive load. This means that during a sinusoidal AC input, at almost every moment (when the output is being driven) the transformer can have a load, which can meet requirement (i) above.

One implementation of the invention can have an architecture which is based on a modified tapped linear driver. The basic tapped linear driver architecture is shown in Figure 3.

The AC input is rectified by a rectifier 12. A current source 22 drives a current through a series chain 14 of LEDs 14. Each LED 30i to 30₄ has a parallel bypass switch 32i to 32₄ so that the number of LEDs driven in the circuit can be controlled.

The timing diagram in Figure 3 shows how the bypass switches are controlled in dependence on the level of the rectified voltage. The top plot shows the rectified AC voltage provided by the rectifier 12.

The lower plots show the times when the LEDs are in circuit, by showing when a current flows through the four LEDs. I_LEDII is the current flowing through the LED 30n (where n takes values 1 to 4).

When the rectified voltage reaches a first threshold VI, the first LED 30i is switched into circuit. When the rectified voltage reaches a second threshold V2, the second LED 30₂ is switched into circuit so that the current is driven through both LEDs 30i and 30₂.

When the rectified voltage reaches a third threshold V3, the third LED 30₃ is switched into circuit so that the current is driven through the LEDs 30i to 30₃.

When the rectified voltage reaches a fourth threshold V4, the fourth LED 30₄ is switched into circuit so that the current is driven through all the LEDs 30i to 30₄.

The LEDs are bypassed in sequence in the same way during the falling part of the rectified voltage. Thus, when the rectified input is high, all LEDs are switched into circuit, because the voltage supply can provide the cumulative voltage drop across all the LEDs. When the rectified input is lower, some LEDs need to be switched out of the circuit. In the above embodiment, LED 30i is the most often driven LED during this cycle, and in the long term, this LED is probably the one earliest to fail. In order to increase usage ratio of all LEDs and prolong the overall lifetime of the LED circuit, the most driven LED in one cycle can be switched to another LED during the operating of the LED circuit.

This circuit selects a number of the LEDs such that the desired current can be driven through those LEDs based on the rectified voltage supply at any particular time.

This arrangement meets the requirement (i) in that a load is always present. In order to meet requirement (ii) above, a minimum load is needed to maintain the normal operation of the transformer. With the linear driver circuit of Figure 3, the current level is selected according to the required light output. It can happen that the selected value of the current is too small, so that the self-oscillation of the transformer is not maintained.

Figure 4 shows one example of a driver circuit in accordance with an example of the invention. The circuit is based on the architecture of Figure 3, with the addition of a feedback mechanism and an interrupt switch as described below, so that a larger current source can be used.

The rectifier 12 receives an ac input voltage from the transformer 10. This transformer functions as the supply to the LED circuit, and it has a minimum current output requirement, for example to sustain self-oscillation.

The current source 22 provides a current to the LED arrangement 14 which exceeds the minimum current output requirement or the minimum output power requirement to enable self-oscillation. In this example the LED arrangement is shown as five LEDs 30i to 30₅ each in series with a respective bypass switch (a MOSFET) 32i to 32₅. Of course, five LEDs is only an example, and there may be more or less LEDs. The LEDs may also be arranged as multiple parallel branches of LEDs instead of the single branch shown. An interrupt switch 40 is provided, with the LED arrangement 14, the interrupt switch 40 and the current source 22 in series in this example. In this way, the interrupt switch is used to prevent the current source current flowing through the LED arrangement 14.

A current sense resistor 41 is provided in series with the LED arrangement, so that the LED arrangement current passes through the current sense resistor 41.

The current source, interrupt switch and LED arrangement can be in any order in the series circuit. Furthermore, the interrupt switch can be anywhere which breaks electrical the circuit. It may be part of the rectifier circuit for example.

A controller 42 is used to control the interrupt switch 40 to control the LED arrangement light output. The same controller is also used to control the control switches 32i to 32₅. However, an alternative implementation may use different controllers for the interrupt switch 40 and for the control switches 32₁ to 32s.

The current source provides a predetermined minimum current, so that a minimum output load (i.e. a current load or power load) is presented to the supply. The transformer for example requires at least some periods when the current exceeds a minimum to be able to sustain self-oscillation. The detailed way of implementing a current source with a certain output current is known for those skilled in the art, and the description will not give unnecessary details.

Since the minimum current may generate output lumen higher than desired, the interrupt switch may be controlled using pulse width modulation (PWM) to adjust the total output lumen. The controller 42 is for example a digital circuit, which then controls a driver circuit 44 which generates the required switch control signals.

The output voltage and output current of the transformer are shown in Figure

5.

The top plot shows the rectified output voltage V_L of the transformer 10 and the rectifier 12, and shows the transformer self-oscillation cycles, the envelope of which correspond to the (rectified) mains voltage input to the electronic transformer. It should be noted that the wavelength of the oscillations in the signal V_L are shown exaggerated to show most clearly the high frequency harmonics with respect to the low frequency AC mains. In practice, the frequency of the oscillations in the signal V_L can be as high as several KHz which is of course difficult to illustrate clearly.

The voltage V_L supplies the current source in the example shown. The middle plot shows the voltage Vo across the bank of LEDs, and the bottom plot shows the series current flowing, for example the current Is6 through the interrupt switch 40. The voltage difference between the continuous function V_L and the discrete function Vo corresponds to a voltage drop across the current source 22. The voltage Vo has discrete values because it essentially corresponds to the sum of the forward voltage drops across the LEDs, and these forward voltage drops are constant for a given current flowing. Note that Vo can be used (or processed) to represent the voltage across the bank of LEDs because the voltage drop across the sense resistor is constant for a given constant current.

The current Is6 remains constant while the interrupt switch is conducting. The period with no current corresponds to the interrupt switch open to implement control of the output illumination.

The output voltage Vo depends on how many of the LEDs are conducting and how many are bypassed. The bypass switches are controlled so that the LEDs which are in the circuit can be driven by the available supply voltage at the time. For example, when the supply voltage drops below the required drive voltage of a single LED, all switches are closed. This corresponds to the gaps between the pulses in the Vo waveform. The constant current then flows through the sense resistor.

Both requirements, of a transformer which should be loaded at all times during driving of the load and with a load large enough to maintain self-oscillation, can be met. The circuit of Figure 4 also avoids the need for inductors or capacitors so that the driver circuit can be fully integrated in one IC chip to achieve a low cost, compact and fully integrated solution.

The circuit provides segmented linear control, i.e. the number of LEDs in circuit is controlled. There is also PWM control for each LED using the bypass switches. The bypass switches varying the output total lumen of each individual LED within each cycle. The interrupt switch instead controls the global output of all LEDs.

The phase of the gate driver signal of the interrupt switch can be adjusted according to the self-oscillation time of the transformer to keep driver operating normally. The method disclosed in US2013/0049619 can be adopted to detect self-oscillation time of the transformer and to make the operation of the switch 40 synchronous with it. In one embodiment, the specific cycle length of the AC main can be detected and the LED circuit controls the interrupt switch according to said cycle. Alternatively, the cycle length is not necessarily detected but a phase lock loop (PLL) can be used to synchronize the interrupt switch 40 with the input AC mains, and a phase offset can be used for determining in what phase of the AC mains the interrupt switch 40 should be closed. For example, if the phase offset is zero, the interrupt switch 40 will close at the zero crossing of the AC mains. Those skilled in the art are familiar with the existing methods of time synchronization and control, thus the present specification will not give unnecessary details.

The current source current flows through each LED to provide linear control. The minimum value of the current source output for example can be larger than 1.3 A to meet compatibility requirements. The maximum value of the current source output can depend on the current stress limit of the electronic transformer and the maximum power of the lamp. It should be noted that for other kind of electronic transformers, such minimum value may vary.

Vo and Vsense are detected and used to determine a maximum output power available.

V_L IS used to determine how many LEDs can be connected in the circuit and still be supplied by the available voltage at that time. Namely, the voltage V_L is detected and used to control the PWM signal of the switches 32i to 32₅.

The control algorithm is shown in Figure 6.

The process starts in step 60.

In step 62, the switches are all closed (i.e. ON) to define a closed conducting path for the current source current to flow. A desired (e.g. user-defined) output power value Po ref is set.

In step 64 a start up process is followed for example for at least 20ms (one full mains cycle for a 50Hz mains) so that the initial values of V_L, V_O and Vsense can be detected. From the V_L waveform, the self-oscillation time of the transformer can be derived. The phase of the interrupt switch can then be based on the timing of the V_L cycle. During the at least 20ms, the interrupt switch is always closed so as to calculate the maximum output of the LED circuit. This is not shown in Figure 5 which instead shows the operation after the start up process. In Figure 5, the interrupt switch 40 is open in the later part of each half-cycle for the purposes of obtaining a desired output. As discussed below, the duty cycle of the interrupt switch is derived from the maximum output measured during the start up process.

In step 66, the setting of the bypass switches and interrupt switch is provided. The forward voltage drops of the five LEDs 30i to 30₅ are defined asVLEDl to VLED5. These are fixed values rather than values that need to be measured, and represent the forward voltage drop across each LED when the current source current is driven through the LED. If the LEDs are all the same then the five values will be equal, but the LEDs may be different (for example with different colour output). In Figure 6, the switch signals are identified as SI to S6. Switch signals SI to S5 relate to the switches 32i to 32₅ and switch signal S6 relates to the bypass switch 40 (hence the reference to current Is6 above).

If the rectified transformer voltage V_L is below the forward voltage drop for the first LED 30i (i.e. VLEDl), then all switches are closed so that no LEDs are in the circuit.

If the rectified transformer voltage V_L is more than the forward voltage drop VLEDl for the first LED 30i and less than the combined forward voltage drop for the first and second LEDs (this is expressed as VLED 1+2 in Figure 6) then then the switch 32i in parallel with the first LED is open so that the first LED is in the circuit.

If the rectified transformer voltage V_L is more than the forward voltage drop for the first two LEDs 30i,30₂ and less than the forward voltage drop for the first three LEDs 30i,30₂,30₃ then then the switch signals SI, S2 (i.e. switches 32i,30₂) in parallel with the first two LEDs are zero (open switches) so that the first two LEDs are in the circuit.

If the rectified transformer voltage V_L is more than the forward voltage drop for the first three LEDs 30i,30₂,30₃ and less than the forward voltage drop for the first four LEDs 30i,30₂,30₃,30₄ then then the switches 32i,30₂,30₃ in parallel with the first three LEDs are open so that the first three LEDs are in the circuit.

If the rectified transformer voltage V_L is more than the forward voltage drop for the first four LEDs 30i,30₂,30₃,30₄ and less than the forward voltage drop for all five LEDs then then the switches 32i,30₂,30₃,30₄ in parallel with the first four LEDs are open so that the first four LEDs are in the circuit.

If the rectified transformer voltage V_L is more than the forward voltage drop for all five LEDs then all five switches in parallel with the LEDs are open so that all five LEDs are in the circuit.

This is the operation carried out in step 66. It should be noted the switch S6 namely the interrupt switch 40 is always closed, as discussed above.

Essentially, the controller compares the rectified output voltage (before the current source) V_L with a set of thresholds to derive control signals for the control switches.

The number of LEDs which are bypassed by the control switches (and therefore the number which remain in circuit) is controlled based on the rectified voltage level. The number of LEDs in series is matched to the drive voltage at a given time. This avoids the need for electrolytic smoothing capacitors and thus extends the circuit lifetime. In step 68, the period of the supply voltage is calculated (value T). This calculation can be obtained by timing between zero crossings. The output power of the LED driver is calculated as the integral of V*I over time:

This integral takes account of the changing value of Vo (as shown in Figure 5) during the full cycle so that the overall power output can be derived assuming no interruption (namely the interrupt switch 40 is always closed). By using the voltage Vo actually applied to the LEDs rather than the voltage V_L, an accurate power output is derived.

A desired output lumen has been set as Po ref in step 62 (Po ref must be less than or equal to Po) and this is used to determine the duty cycle of the interrupt switch.

Step 70 involves determining the duty cycle of the interrupt switch.

As shown, a sawtooth waveform is defined which ramps down from the maximum output power Po to zero during each time period T.

A threshold value is defined:

Th = \ (P₀_ Po_ref)dt

This threshold is 0 if the desired power is full power and is Po if the desired power is zero.

The crossing point of the sawtooth waveform with the threshold Th enables the required duty cycle to be derived as shown in step 70.

The threshold testing of step 66 is repeated to track the electronic transformer output.

Figure 7 shows the resulting waveforms.

Plot 80 corresponds to the top plot in Figure 5 and shows the rectified output voltage V_L from the electronic transformer (again with an artificially low frequency to make the plots easier to see) as well as the envelope signal. Plots 82,84,86,88,90 show the five control signals for the bypass switches 32i to 32₅ (respectively). A high pulse indicates that the bypass switch is closed so that the current source current bypasses the respective LED. Thus, LED 32i (controlled by switch signal SI) is on nearly all of the time, whereas LED 32₅ is only on for short bursts. The plots 82,84,86,88,90 together give rise to the middle plot of Figure 5.

Plot 92 shows the sawtooth waveform and the threshold value Th for calculating the duty cycle of the interrupt switch. Plot 94 shows the duty cycle control signal for the interrupt switch. It can be seen that after the determined duty cycle, the interrupt switch 40 namely switch S6 is open and there is no light output. The output of the electronic transformer should be zero since the circuit is open, but practically it may generate some small fluctuations in the V_L curve as can be seen in the later part of the plot 80.

Referring back to Figure 4, the controller 42 comprises a first input for receiving the rectified output voltage V_L, a second input for receiving a signal Vsense representing a current flowing through the LED arrangement (based on the voltage across the current sense resistor 41), and a third input for receiving a another signal Vo representing the rectified output voltage and which is actually present across the LEDs. As explained above, Vo is used for power calculations, whereas V_L is used for setting the bypass switches, and also for detecting the zero according point of input AC voltage which can be used to calculate the time period and for synchronization control.

In order to obtain the maximum output power Po of the circuit during one period of the ac input voltage, a first calculating unit is provided.

In order to calculate a duty cycle of the interrupt switch according to the maximum output power and a desired output power, a second calculating unit is provided. This derives the threshold value Th in the manner explained above as well as the sawtooth function.

In practice, there is no need actually to generate a sawtooth waveform - this is purely shown for visual explanation. In practice, the necessary computations to derive the duty cycle are simply performed by a processor which calculates the required duty cycle.

The end of the interrupt signal causes the end of the self oscillation as can be seen in Figure 7.

The timing of the interrupt switch operation can be controlled to ensure that the electronic transformer is able to commence self-oscillation and to control the output power by adjusting the duty cycle. In particular, in one example, the zero-crossing of the AC mains is used as the starting point for the closure of the interrupt switch. However, alternative timings are possible. In general, the controller is adapted to synchronize the timing of the control of the interrupt switch with respect to the oscillation of the supply voltage and/or the self oscillation of the transformer.

The timing of the switching off of the interrupt switch is dependent on the determined duty cycle.

The system makes use of a controller for the control of switches and also for the required timing calculations. Thus, the calculating units mentioned above are implemented by a controller. Components that may be employed for the controller include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs).

In various implementations, a processor or controller may be associated with one or more storage media such as volatile and non-volatile computer memory such as RAM, PROM, EPROM, and EEPROM. The storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform at the required functions. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller.

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 measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An LED circuit, comprising:

a rectifier (12) for receiving an ac input voltage from a supply having a minimum current output requirement, and providing a rectified output voltage, wherein said supply comprises an electronic transformer and said minimum current output is the minimum current required by the electronic transformer in at least some periods to sustain

self-oscillation of the electronic transformer, or said supply comprise a triac dimmer and said minimum current output is the minimum current to prevent flicker of the triac dimmer;

an LED arrangement (14) coupled to the rectifier (12) and driven by the rectified output voltage;

a current source (220) coupled to the rectifier (12) and the LED arrangement (14), for providing, from the rectified output voltage, a current to the LED arrangement which exceeds the minimum current output requirement;

an interrupt switch (40), wherein the LED arrangement, the interrupt switch and the current source are coupled; and

a controller (42) coupled to the interrupt switch (40) for controlling the interrupt switch in pulse width modulation (PWM) and to control the LED arrangement light output to a desired level.

2. A circuit as claimed in claim 1, wherein the current source output is in the range 1 to 1.5 A.

3. A circuit as claimed in claim 1 or 2, wherein the LED arrangement (14) comprises:

at least one set of LEDs (30i - 30₅) in series, each LED in the set having a respective parallel control switch (32i - 32₅).

4. A circuit as claimed in claim 3, wherein the controller (42) is further for controlling the control switches.

5. A circuit as claimed in claim 4, wherein the controller (42) comprises a first input for receiving a signal (V_L) representing the output voltage provided by the rectifier, wherein the controller is adapted to compare the output voltage (V_L) provided by the rectifier with a set of thresholds to derive control signals for the control switches in a linear manner.

6. A circuit as claimed in claim 5, wherein the controller comprises:

a second input for receiving a signal (Vsense) representing a current flowing through the LED arrangement (14);

a third input for receiving a signal (Vo) representing the rectified output voltage as applied to the LED arrangement;

a first calculating unit for calculating the maximum output power of the circuit during one period of the ac input voltage based on the current and the rectified output voltage of the LED arrangement in that period; and

a second calculating unit for calculating a duty cycle of the interrupt switch (40) according to the maximum output power and a desired output power.

7. A circuit as claimed in claim 6, comprising a current sense resistor (41) in series with the LED arrangement (14), and said third input of the controller is coupled to said current sense resistor (41).

8. A circuit as claimed in any preceding claim, wherein the controller (42) is further for determining the timing of an oscillation of the supply and controlling the interrupt switch (40) according to said timing.

9. A circuit as claimed in claim 8, wherein the supply comprises an electronic transformer (10) and/or a TRIAC (17), and the controller is further adapted to synchronize the timing of the control of the interrupt switch (40) with respect to the self-oscillation of the electronic transformer and/or the TRIAC, wherein the controller is adapted to control the interrupt switch to deliver power to the LED arrangement at a time instant suitable for starting a self-oscillation of the electronic transformer and/or the TRIAC.

10. A lighting arrangement comprising:

a circuit as claimed in any one of the preceding claims; and

an electronic self-oscillating transformer (10) and/or a TRIAC (17) as the supply for supplying the ac input signal to the circuit.

11. A method of driving an LED arrangement (14), comprising:

receiving an ac input from a supply having a minimum current output requirement and providing a rectified output voltage (V_L), wherein said supply comprises an electronic transformer and said minimum current output is the minimum current required by the electronic transformer in at least some periods to sustain self-oscillation of the electronic transformer, or said supply comprise a triac dimmer and said minimum current output is the minimum current to prevent flicker of the triac dimmer;

operating a current source (22) to provide, from the rectified output voltage, a current to the LED arrangement which exceeds the minimum current output requirement; and controlling coupling between the LED arrangement (14) and current source

(22) in pulse width modulation (PWM) thereby to control the LED arrangement output to a desired level.

12. A method as claimed in claim 11, wherein the LED arrangement comprises at least one set of LEDs (30i-30₅) in series, wherein the method comprises bypassing a selected group of the LEDs by comparing the rectified output voltage (V_L) with a set of thresholds to derive control signals for the control switches.

13. A method as claimed in claim 12, wherein the ac input is received from a self-oscillating mains electronic transformer (10) as said supply, and the method further comprises determining the timing of an oscillation of the supply and controlling the coupling according to said timing.

14. A method as claimed in claim 13, further comprising synchronizing the timing of the control of the coupling with respect to the self-oscillation of the electronic transformer, to control the current source (22) to deliver power to the LED arrangement at a time instant suitable for starting a the self-oscillation of the electronic transformer (10).

15. A method as claimed in claim 14, wherein said controlling step further comprises:

receiving a signal (Vsense) representing a current flowing through the LED arrangement (14);

receiving a signal representing the rectified output voltage (V_L) as applied to the LED arrangement (14); calculating the maximum output power of the circuit during one period of the ac input voltage; and

calculating a duty cycle of the coupling according to the maximum output power and a desired output power.