EP2189041A1

EP2189041A1 - Led driver

Info

Publication number: EP2189041A1
Application number: EP08788345A
Authority: EP
Inventors: Stephen Puddefoot
Original assignee: Enfis Ltd
Current assignee: Enfis Ltd
Priority date: 2007-08-15
Filing date: 2008-08-15
Publication date: 2010-05-26
Also published as: GB0715846D0; CN101843169A; WO2009022153A1

Abstract

A method and apparatus suitable for driving one or more LEDs (20) from an AC mains power supply (2b) is described. An input circuit (10) receives AC mains power (2b) having a sinusoidal waveform and converts, for example by means of a suitable driver (14), said AC mains power into a pulsed LED drive current (2d) having a cyclically varying current, for example in the form of a modified sinusoidal waveform. There may be provided a circuit (12) for monitoring and detecting the instantaneous level of a rectified waveform (2c), thus facilitating regulation of the drive current (2d) by the driver (14).

Description

LED DRIVER

The present invention concerns an LED driver and a method of driving one or more LEDs.

Background of the Invention

It is known that, for energy-efficient operation of an LED (light emitting diode) an electronic driver is required. A known apparatus is diagrammatically illustrated in Figures Ia to Ie of the attached drawings, which shows an input circuit 110 for receiving an AC input waveform connected via a rectifier circuit 111 and an electrolytic reservoir capacitor 113 to the LED driver 114 which supplies current to one or more LEDs 120. The circuit maintains a constant average current supply by varying the timing of pulses to the LED 120 to give a consistent light level with minimal power losses. Drivers can be designed to work from a range of voltages such as 90V to 240V AC typical household mains supply to low voltage DC supplies such as batteries. In the case of AC mains, the circuit requires a relatively large electrolytic reservoir capacitor at the input stage to act as a current reservoir to smooth the varying voltage supply. Figures Ib to Ie show waveforms at points Ib to Ie in the circuit shown in Figure Ia.

The electrolytic capacitor 113 has a number of disadvantages such as:

1. It has a shorter working life than the LED and this lifetime is reduced in a high temperature environment .

2. For a household mains driver the capacitor is a large device owing to the high voltage and low frequency power supply requirements and usually necessitates the drivers being in a housing separate from the LED(s) which can cause difficulties if the capacitor needs to be concealed, for example in a ceiling void. The cost associated with separately housing the capacitor and ensuring its longevity through derating can make this method unsuitable for general lighting applications.

3. Electrolytic capacitors also generate heat with applied current ripple, can be a costly component and are hazardous in a fault condition.

There are some drivers claimed to be electrolytic capacitor free but they use large inductors to replace the capacitors. This resolves the lifetime limitations but increases the size and EMC (electromagnetic compatibility) emissions and is usually only suitable for low current LEDs, i.e. low light levels.

The output stage of an existing LED driver is generally supplied from a stable DC power source. This provides enough current capability supplied by an electrolytic reservoir capacitor to illuminate the LED during the switching cycle of the driver.

If a rectified AC voltage source were used without the capacitor, the current to the LED may be switched when the voltage source is near zero. If the current to the LED were to be switched on when the voltage supplied from the mains is near zero then the driver chip would not effectively control the current to the LED circuitry. As the driver is a power converter stage this would cause excessive current to be drawn from the supply at these low voltages and since the LED circuitry utilizes reactive components this issue can result in unacceptable mains harmonics and inconsistent light levels from the LED, such as flickering. This method will also lead to transients being transferred back into the main supply which is unacceptable in general use. Furthermore, simply removing the reservoir capacitor would result in power being applied to any inductor provided at the drive stage of the LED drive circuitry that varies with the AC mains input frequency resulting in excessive current drain at the low voltage quadrants of the AC waveform. This may magnetically saturate the inductor reducing the LED light output and produce harmonics into the AC mains source which may exceed current certification regulations and reduce the overall efficiency. It may also alter the biasing of the drive transistor in the drive stage causing heat generation and possible damage to the device . The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved LED driver and/or an improved method of driving one or more LEDs.

Summary of the Invention

According to one aspect of the present invention, there is provided electronic apparatus comprising a device arranged to supply a substantially purely positive current waveform, an LED driver connected to the output of said device and arranged to convert said purely positive current waveform to a series of pulses having a modified sinusoidal waveform, and an LED connected to the output of said LED driver. Having a device arranged to supply a substantially purely positive waveform may in certain embodiments of the invention mean that a substantially purely positive waveform is only provided at certain components of the electronic apparatus. For example, the LED itself which being a diode is only supposed to be driven forwards and only emits substantial light levels in a forward biased manner. In¹ such a case, it will be understood that the substantially purely positive waveform is positive in the sense that the LED diode is biased only in the forward direction.

According to another aspect of the present invention, there is provided a method of operating an LED, comprising supplying thereto current of a modified sinusoidal waveform. According to a further aspect of the invention there is provided a method of driving one or more LEDs as set forth in claim 1 below.

According to yet another aspect of the invention there is provided an LED drive circuit as set forth in claim 8 below. Owing to the invention, it is possible for an LED to be efficiently driven from an AC source such as household mains, aircraft AC supply, or a generator, without using an electrolytic capacitor, or similar charge storage device. An advantage of certain embodiments of the present invention is the ability to reduce the power demand in line with the power available at any instant in time from the AC mains input.

The principles of the present invention can be applied to other drive circuits, for example using other integrated circuits, micro-controller or discrete components. Preferred embodiments of the present invention do not include an electrolytic capacitor used to smooth out the cycles of the input AC power.

A particular advantage of the present invention is that an LED light with driver and heat sink can be manufactured small enough to be a direct replacement for household and office lighting with equivalent light level. An existing filament bulb could be removed and an equivalent, high- efficiency LED unit fitted by anyone capable of changing a light bulb.

The frequency of the AC source may be greater than 30Hz, and will typically be of the order of 50-60Hz. The frequency of the pulses driving the LED(s) is therefore preferably at a frequency that is at least 5 times greater than the frequency of the input power. The frequency of the pulses may be greater than 200Hz. The frequency of the pulses may be greater than 10kHz. Further preferred (but optional) features of the invention are set out in the dependent claims.

It will be appreciated that features of the present invention described or claimed in relation to one aspect of the present invention are equally applicable to other aspects of the present invention. For example, features of the present invention described in relation to the apparatus of the present invention are equally applicable to the method of the present invention and vice versa.

Description of the Drawings

Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

Figures Ia to Ie show a prior art arrangement; Figures 2a to 2d show a first embodiment of the invention and include a flow diagram illustrating the function of the first embodiment; and

Figure 3 shows a circuit in accordance with a second embodiment of the invention. Detailed Description

A first embodiment of the invention is illustrated schematically by Figure 2a. An input AC voltage (input 2b, the waveform for which being illustrated by Figure 2b) is passed through a bridge rectifier (input circuit 10), which in the present embodiment is in the form of a full-wave bridge rectifier, to give two positive going waves (output 2c, the waveform for which being illustrated by Figure 2c) per input cycle. This waveform is then monitored by an electronic circuit (monitoring circuit 12) that detects the instantaneous level of the waveform. This circuit 12 then uses the information to regulate the maximum current drawn by the LED 20. During low voltage supply, the driver 14 uses low current and, during high voltage, supply the driver 14 uses high current, thus ensuring that the voltage supplied and current drawn are in phase enabling the circuit to have low noise and efficient operation without flicker. When the input voltage is below an acceptable level the drive circuit 14 is switched off until the voltage has risen above the acceptable level.

The LED 20 is thus supplied with a current waveform (output 2d, the waveform for which being illustrated by Figure 2d) , in the form of a modified sinusoidal waveform, that increases the current pulses applied to the LED in phase with the input voltage, resulting in less shock to the LED for a given brightness than with fixed amplitude pulses.

It will be seen that the drive current shown in Figure 2d has a waveform having two sets of pulses per cycle. The pulses of the LED drive current have peak current levels that follow the shape of a rectified sine wave. Thus, each set of pulses has a first pulse at a first current, a second pulse at a second higher current, a pulse in the middle of the set having a peak, even higher, current, and then subsequent pulses having progressively lower currents. Each set of pulses thus comprises at least five pulses per half cycle (i.e. per set), there thus being more than ten pulses per cycle. The pulses in each cycle are separated by a time gap which is substantially constant at a given desired illumination intensity. Each set of pulses is separated from the adjacent set of pulses by a time gap, which is longer than the time gap separating adjacent pulses within each set. The reason for this relatively long time gap between pulse sets is because the circuit effectively switches off the LED drive current when the voltage of the input waveform drops below a threshold level.

The width of the pulses of the drive current may be decreased or increased to provide adjustment of the perceived intensity of light emitted by the LEDs. As such the gaps between pulses may be varied during use of the LED drive circuit to provide a dimming function, by means of pulse-width modulation.

Figure 3 shows a circuit in accordance with a second embodiment that provides a function similar to that of the flow diagram shown in Figure 2a. Thus, an input AC voltage (240V AC mains at 50Hz) is applied at input terminals 32. The AC waveform passes through a full-wave bridge rectifier 34 to give a fully rectified waveform having two positive waves per input cycle. A small value ceramic capacitor 36 (10OnF) is used at the rectified waveform to remove noise spikes and slightly increase the minimum voltage of the rectified waveform and help prevent any negative voltages. The capacitor is not however used to smooth out the cycles of the input AC power, because the general shape of the waveform is not affected by the capacitor 36. The components in box 38 may thus be considered as an input circuit, connectable to an AC mains power supply that simply converts AC power into a fully rectified cyclically varying waveform (similar to that shown in Figure 2c) .

The fully rectified waveform is then received by the block of components in box 40, which includes the LED driver circuit. The driver circuit includes a common switch mode controller 42 with analogue gate control, which in this embodiment is in the form of an 8-pin HV9910 LED driver chip available from Supertex, Inc, California, US. The function of the chip is publicly available, but a brief description of the pins, counting clockwise from pin 8 at the top right-hand side of the chip as shown in Fig. 3 is as follows: pin 8 - pulse width modulation control; pin 4 - driver output for external MOSFET device 44 which powers the external LED device (s) (not shown) connectable at output terminals 46; pin 2 - LED current sensor; pin 3 - ground; pin 6 - output voltage supply from chip; pin 5 - low frequency pulse-width modulation/enable input; pin 7 - linear dimming (i.e. proportional to the LED drive current) ; pin 1 - input voltage, of the power supply used to power the LED(s) . The chip 42 requires a voltage between OV and 25OmV on the analogue gate (pin 7) to control the switching point of the MOSFET device 44 and hence the current pulses to the LED(s) at output 46. The chip 42 is designed to provide a constant current output at pin 4 (to provide constant light intensity output) , the output current being proportional to the voltage applied at the analogue gate (input pin 7) . As such, in the prior art configurations of the HV9910 LED driver chip, the input pin 7 is held at a substantially constant voltage. (The input pin 7 of the HV9910 LED driver chip could alternatively be used to provide user-controllable linear dimming, but for various reasons dimming of LEDs is typically effected by means of pulse-width modulation, primarily controllable by means of the input at pin 8.) In contrast, to prior art configurations of the HV9910 LED driver chip, a cyclically varying voltage waveform is applied at the analogue gate, pin 7, of the chip 42. The circuit includes a potential divider network of resistors 48 with resistances selected to give 25OmV to pin 7, when the voltage of the AC waveform is at its maximum point during each cycle. The input voltage at pin 7 thus varies between OmV and 25OmV in phase with the rectified AC waveform. The peak of the AC waveform corresponds to maximum output (or "fully on" point) for the LED drive output at pin 4, when "on" according to the duty cycle controlled by pin 8. A Zener diode 49 is used to prevent the voltage on the analogue gate (pin 7) exceeding its safe limit of operation. When the input voltage at pin 7 is below a threshold level, the output at pin 4 drops to zero, so that the output power at terminals 46 is effectively switched off. This ensures that the circuit does not attempt to drive LEDs at a voltage below the bias voltage required for effective operation, thus leading to more power efficient driving of the LED(s) and/or extending the life-time of the LED(s) by means of more appropriate driving regime.

A voltage regulation circuit (provided by the components in box 50) is provided so that a supply voltage may be applied at pin 1 of the chip 42, thus providing the power supply for powering the chip 42. The chip 42 is rated for 450V operation but at the higher voltages, it generates a considerable amount of heat and it is preferable to limit the applied voltage at pin 1. The voltage regulation circuit provides power having an alternating voltage at pin 1 of the chip 42.

A ceramic capacitor 52 is provided across the LED output terminals 46 to reduce or remove switching spikes. An inductor 53 is provided, which may act to store and deliver power to the output 46 during the off-cycle of the MOSFET device 44. The combination of inductor 53, capacitor 52 and diode 54 are selected with regard for the whole circuit efficiency and improve efficiency and reduce back EMF transient spikes ensuring current supply to the LEDs for a portion of the cycle waveform where the driver is turned off thus improving the circuit efficiency.

The circuit with one or more LEDs attached may be provided in an integrated light fitting for use as a light bulb connectable directly to a mains AC light bulb socket. Thus the components in box 40 effectively receive a fully rectified power waveform having a cyclically varying voltage, and convert it into a pulsed LED drive current having a cyclically varying current (similar to that illustrated in Figure 2d) . Each pulse of the pulsed LED drive current has a width of less than lms. The width of each pulse is typically less than 100 micro-seconds, and may for example be of the order of 10 micro-seconds. The frequency of the pulses is of the order of 100kHz. The pulsed LED drive current is in phase with the received AC waveform, such that there is a lower power demand from the LED(s) connected at output 46 at the lower voltage parts of the input AC waveform. The fact that the current delivered to the LEDs is in phase with the voltage supplied from the mains assists in attempting to provide optimum efficiency and minimum noise in the circuit.

Whilst the present invention has been described and illustrated with reference to a particular embodiment, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. Instead of providing a control voltage at pin 7 of the chip shown in Figure 3 by means of the potential divider 48 other means could be used, such as for example, using Zener diodes or transistors etc. There is no need to use a chip as used in the Figure 3 embodiment and instead the function of the circuit of Figure 3 could instead be provided by means of separate non-chip based components. Alternatively, some of the non-chip based components shown in Figure 3 could be combined with the functionality of the chip 42 of Figure 3 to in a new design of chip.

A modification of the Figure 3 embodiment could be to remove the voltage regulation circuit and instead supply a lower voltage AC mains power to the circuit, such as a mains voltage supply of 115V AC. Using AC power at such a level would heat the chip 42 within acceptable levels.

In the embodiments described full wave rectification is used. As an alternative, half wave rectification could be used, possibly in conjunction with basic voltage regulation techniques to reduce the peak voltage of the input waveform to allow the use of lower voltage components in the circuit. This would depend on the required average peak current, allowable maximum current and perceived intensity of the LED(s) connected at the output.

An isolating transformer and/or a residual current device (RCD) may be added to provide effective isolation from the mains supply to protect the user.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

1. A method of driving one or more LEDs comprising the steps of: a circuit receiving AC mains power having a sinusoidal waveform, the circuit converting said AC mains power into a pulsed LED drive current having a cyclically varying current, and driving the one or more LEDs with the drive current .

2. A method according to claim 1, wherein the drive current includes three or more pulses per cycle.

3. A method according to claim 1 or claim 2, wherein the drive current includes a first pair of adjacent pulses separated by a first time gap and a second pair of adjacent pulses separated by a second time gap, which is longer than the first time gap.

4. A method according to claim 3, wherein the method includes a step in which the circuit switches off the LED drive current when the voltage of an input waveform drops below a threshold level, the period during which the current is switched off corresponding to the second time gap.

5. A method according to any preceding claim, wherein each cycle of pulsed LED current includes a first pulse having an average current at a first level, a subsequent second pulse having an average current at a second level higher than the first level, and a subsequent third pulse having an average current at a third level lower than the second level.

6. A method according to any preceding claim, wherein the series of pulses of the pulsed LED current have a modified sinusoidal waveform.

7. A method according to any preceding claim, wherein the pulses of the pulsed LED current have peak current levels that substantially follow the shape of a rectified sine wave.

8. A method according to any preceding claim, wherein the pulsed LED drive current has a cyclically varying current, which is substantially in phase with the cyclically varying voltage of the AC mains power.

9. An LED drive circuit comprising an input adapted to receive AC mains power having a sinusoidal waveform, and a power control unit arranged to supply regulated power to one or more LEDs, wherein the power control unit is adapted to convert a power input having a cyclically varying voltage into a pulsed LED drive current having a cyclically varying current .

10. An LED drive circuit according to claim 9, wherein the circuit is arranged such that the one or more LEDs are supplied only with forward biasing current or no current.

11. An LED drive circuit according to claim 8 or claim 10, wherein the LED drive circuit includes a rectifier arranged to convert a sinusoidal waveform received at the input into a rectified waveform for producing the pulsed LED drive current.

12. An LED drive circuit according to any of claims 9 to 11, wherein the LED drive circuit is connected to one or more LEDs.

13. An LED drive circuit according to claim 12, wherein the LED drive circuit and the one or more LEDs are packaged as an integral illuminating device.

14. An LED drive circuit according to claim 13, wherein the integral illuminating device is in the form of a light fitting wherein the input adapted to receive AC mains power is adapted to connect to a light bulb socket.

15. An LED drive circuit according to any of claims 9 to 14, wherein the LED drive circuit is adapted to perform the method of any of claims 1 to 8.

16. An electronic apparatus comprising a device arranged to supply a substantially purely positive current waveform, an LED driver connected to the output of said device and arranged to convert said purely positive current waveform to a series of pulses having a modified sinusoidal waveform, and an LED connected to the output of said LED driver.

17. An electronic apparatus according to claim 16, arranged to be driven from an AC mains power source.

18. An electronic apparatus according to claim 16 or claim

17, wherein the electronic apparatus is arranged such that it is able to drive the LED without using an electrolytic capacitor .

19. An electronic apparatus according to any of claims 16 to

18, wherein the device arranged to supply a substantially purely positive current waveform includes a bridge rectifier adapted to convert an input AC voltage into a rectified waveform having two positive going waves per input cycle.

20. An electronic apparatus according to claim 19, wherein the apparatus includes a circuit adapted to monitor and detect the instantaneous level of the rectified waveform and, as a result, to regulate the maximum current drawn by the LED.

21. A method of operating an LED, comprising supplying thereto current of a modified sinusoidal waveform.

22. A method according to claim 21, wherein the method includes a step of passing an input AC voltage through a bridge rectifier to give a rectified waveform having two positive going waves per input cycle.

23. A method according to claim 22, wherein method includes a step of monitoring the instantaneous level of the rectified waveform and a step of regulating the maximum current drawn by the LED in dependence on the instantaneous level of the rectified waveform so monitored.

24. A method according to claim 23, wherein during low voltage level of the rectified waveform, the current drawn by the LED is low, whereas during high voltage level of the rectified waveform, the current drawn by the LED is high.

25. A method according to claim 23 or claim 24, wherein the method includes a step of monitoring when the voltage level of the rectified waveform is below a threshold level and a step of switching off the current drawn by the LED until the voltage level has risen above the threshold level.

26. A method according to any of claims 21 to 25, wherein the modified sinusoidal waveform comprises pulses.

27. A method according to any of claims 21 to 25, wherein the LED is supplied with a current waveform that varies the current of the pulses applied to the LED in phase with the input voltage.