WO2006062484A1

WO2006062484A1 - Led driver circuit and method of operation

Info

Publication number: WO2006062484A1
Application number: PCT/SG2004/000400
Authority: WO
Inventors: Choon Hong Chia
Original assignee: Photronfusion Solutions Pte Ltd
Priority date: 2004-12-08
Filing date: 2004-12-08
Publication date: 2006-06-15

Abstract

A LED driver circuit includes a pulse width modulator and at least one voltage/current control circuit. The pulse width modulator includes an input coupled to receive a DC supply signal, an output for providing a modulated DC supply signal, and a feedback input operable to receive a feedback signal, the feedback signal operable to indicate the input load impedance of the LED device. The pulse width modulator is operable to control the pulse width of modulation applied to the DC supply signal depending upon the LED device input load impedance as it is detected by the feedback signal. The at least one voltage/current control circuit includes an input coupled to receive the modulated DC supply signal and an output for providing a regulated LED supply signal to the LED device.

Description

LED Driver Circuit And Method Of Operation Background

The present invention relates to electronic circuits, and more particularly to light emitting diode driver circuits and methods of operation.

As known in the art, light emitting diodes (LEDs) are semiconductor devices operable to produce light when powered appropriately. Due to its relatively cool operation, long lifespan, good colour rendering, wide flexibility in colours, and low energy usage, LEDs represent an attractive alternative to traditional light sources, such as incandescent and halogen light sources. However, in order for LED light sources to provide the aforementioned advantages, they must be controlled and driven properly. These tasks are primarily provided by the LED driver circuit.

Conventional LED driver circuits are typically constant current devices which often are designed as general purpose circuits for use with a wide variety of LED devices. However, each LED device has its particular load characteristics, and the input loading conditions of different LEDs may vary substantially. Driving an LED device with a general purpose LED driver which is not particularly well matched to the LED device reduces the operating efficiency of LEDs. Excessive heat build-up, and consequently reduced LED lifetime can occur when an improperly matched driving circuit is used. In further complication, the input load characteristic of an LED does not remain constant over the device's lifetime, but instead changes with age and environmental conditions. Accordingly, a driver circuit which was previously well matched to the LED's load conditions may not be well matched later and/or under different environmental conditions.

Summary

The present invention provides an improved LED driving circuit and method of operation that can be modified to operate more efficiently with LEDs having different loading conditions. In a first embodiment, a LED driver circuit is presented which includes a frequency modulator and at least one voltage/current control circuit. The frequency modulator includes an input coupled to receive a DC supply signal, an output for providing a modulated DC supply signal, and a feedback input operable to receive a feedback signal, the feedback signal operable to indicate the input load impedance of the LED device. The frequency modulator is operable to control the frequency of modulation applied to the DC supply signal depending upon the LED device input load impedance as it is detected by the feedback signal. The at least one voltage/current control circuit includes an input coupled to receive the modulated DC supply signal and an output for providing a regulated LED supply signal to the LED device.

These and other features of the invention will be better understood when viewed in light of the following drawings and detailed description.

Brief Description of the Drawings

Fig. 1 illustrates a circuit block diagram a LED driver circuit in accordance with an embodiment of the present invention.

Fig. 2 illustrates one embodiment of the feedback loop employed in the LED driver circuit shown in Fig. 1

Fig. 3 illustrates a method for operating the LED driver circuit in accordance with one embodiment of the present invention.

For clarity, previously identified features retain their reference indicia in subsequent drawings.

Detailed Description of Specific Embodiments

Fig. 1 illustrates a circuit block diagram of a LED driver circuit in accordance with an embodiment of the present invention. As shown, the LED driver circuit 100 includes a power supply section 110, supply modulation section 120, and a regulator section 130. The power supply section 110 includes a transformer 111, a rectifying circuit 112, an input resettable fuse 113, and AC decoupling capacitors 114 and 114a.

In an exemplary embodiment, the transformer 111 is a toroid transformer operable to transform standard AC line signal 141 (e.g., 110 VAC/60 Hz, or 230 VAC/50 Hz) to an AC supply signal 142, the AC supply signal being in the range of 5-10 VAC as an example. Further particularly, the transformer 111 includes a high permeability core and has shielded primary and secondary cores to minimize EMI intrusion. The rectifying circuit 112 may be a full wave bridge rectifier, an exemplary rating being of 100 V @ 1.5DCA. An exemplary embodiment of the resettable fuse 113 includes polymer-based diodes which are operable to increase their effective series resistance when a predefined current limit is reached, and reset to minimum series resistance when the sensed current is returns of continues below the predefined limit. AC decoupling capacitors 114 and 114a are operable to remove AC signal levels riding on the DC supply signal 143, the presence of which could contribute to ripples in the regulated LED supply signal 146. Ripples in the regulated LED supply signal 146 are disadvantageous as it can lead to excessive heating of the LED device. In a particular embodiment, AC decoupling capacitors 114 are ultra low equivalent series resistance (ESR) capacitors which have extremely low equivalent series resistance with their capacitive elements, and AC decoupling capacitor 114a is an ultra low ESR tantalum capacitor. Optionally, the power supply section 130 includes one or more temperature sensors operable to activate the resettable fuse 113 to limit or reduce the output current upon sensing a temperature exceeding a predefined limit. Alternatively or in addition, one or more temperature sensors may be integrated into the transformer 111 or bridge rectifier 113, those sensors either in communication with the resettable fuse or with the integrated component to limit or reduce the current supplied upon detecting an excessive temperature. The DC supply signal 143 in a specific embodiment ranges from 4-20 VDC, and in a more specific embodiment ranges from 4-12 VDC.

The supply modulation section 120 includes a frequency modulator 122, a limiting diode 124, and a storage inductor 126. The frequency modulator 122 has an input 122a coupled to receive the DC supply signal 143, an output 122b for providing a modulated DC supply signal 144, and a feedback input 122c for receiving a feedback signal 145, further described below. The limiting diode 124 has an anode coupled to a predefined potential (shown as AC and DC ground) and a cathode coupled to the output of the frequency modulator 122. The limiting diode is operable to limit the voltage output from the frequency modulator 122 to within a predefined range, the breakdown voltage of the limiting diode as shown. In a particular embodiment, the limiting diode Dl is an ultra fast recovery diode having an exemplary rating of 20 VAC @ IDCA. The storage inductor 126 has a first port coupled to the output of the frequency modulator 122 and an second port coupled to the regulator section 130 and the feedback input 122c, the storage inductor 126 operable to store current during positive half cycles of the modulated DC supply signal 144, and to supply current to the regulator section 130 during negative half cycles of the modulated DC supply signal.

The frequency modulator 122 is operable to apply a modulation frequency to the DC supply signal 143 supplied thereto, the applied modulation frequency being controlled as a function of the input load impedance of the LED device. As will be further explained and illustrated below, the feedback (FB) signal 145 communicates the input load impedance of the LED device to the frequency modulator 122, which then applies to the DC supply signal 143 a modulation frequency associated with the detected load impedance. Further specifically, the process is dynamic, and thus after the frequency of modulation is initially set responsive to an initially sensed input load impedance of the LED device, the frequency modulator is further operable to change the frequency of modulation responsive to a detected change in the input load impedance of the LED device.

In an exemplary embodiment, the frequency modulator 122 is operable to apply a modulation frequency in the range of 10 KHz to 100 KHz corresponding to a LED input impedance ranging from short circuit to an open circuit, and in a more specific embodiment a modulation frequency ranging from 10.4 KHz to 52 KHz for the aforementioned short circuit to open circuit conditions. In addition, the lowest frequency of modulation may be selected such that the LED device 150, when powered by a regulated LED supply signal 146 operating at this frequency, does not produce perceptible stroboscopic effects (e.g., perceptible flickering). Further specifically, the association between the detected LED input load impedance and the applied modulation frequency corresponding thereto is determined by a feedback loop within the frequency modulator 122, an example of which illustrated below. Alternatively or in addition, the mapping between the detected input load impedance and applied modulation frequency may be made via a look-up table, a software algorithm, or other similar deterministic means. Of course, another range of frequencies, either wider or narrower, maybe applied to the DC supply signal 143, the applied modulation frequency being a function of the LED device input load impedance. The reader will appreciate that rather than the applied modulation frequency being a function of the input load impedance of the LED devicel50, the applied modulation frequency may be made dependent upon the output load impedance of the LED driver itself. Indeed, in a typical installation in which the LED device is connected to the output of the LED driver circuit, the input load impedance of the LED device will be equivalent to the output load impedance of the LED driver circuit.

The regulator section includes at least one voltage/current control circuit 132, at least one AC decoupling capacitor 134, and at least one resettable fuse 136. As shown, two sets of these components are employed to provide the needed current for driving the exemplary LED device. Of course, fewer of additional sets may be used in alternative embodiments under the present invention.

The voltage/current control circuit 132 includes an input coupled to receive the modulated DC supply signal 144 and an output, the voltage/current control circuit 132 operable to regulate the received modulated DC supply signal 144 to within a predefined range. In a particular embodiment, the voltage/current control circuit 132 operates over an input range of 5-20 VDC to produce a regulated LED supply signal 146, 3.3 VDC @ 500 niA (± 4 %) being an exemplary output rating.

The regulator section 130 further includes resettable fuses 136 which are operable to increase their effective series resistance when a predefined current limit is reached, and reset to minimum series resistance when the sensed current is returns of continues below the predefined limit. Similar to the power supply section, the regulator section 130 may optionally include one or more temperature sensors operable to activate the resettable fuses 136 or other regulator section components (e.g., the voltage/current control circuit 132) to limit or reduce the output current upon sensing a temperature exceeding a predefined limit. Further optionally, the power supply components 110, the supply modulator components 120, and the regulator components 130 may be coupled via a trace 115 which is constructed using a size and/material to provide low ohmic loss. In a particular embodiment the LED driver circuit 100 is fabricated on a printed circuit board (PCB); in an alternative embodiment, two or more components of the LED driver circuit 100 are monolithically fabricated in an integrated circuit. The LED device 150 may be separately fabricated, or integrally formed with the driver circuit 100.

Fig. 2 illustrates a feedback circuit included within the frequency modulator 122 in accordance with one embodiment of the present invention. As shown, the feedback circuit includes a bootstrap charger 202, internal regulator 204, an oscillator 206, a thermal shutdown control module 208, a current limiter 210, comparator 212,NOR logical gate 214, driver amplifier 216, transistors 218 and 220, AND logical gate 222, reset module 224, switch 226, and error amplifier 228. Optional components external to the frequency modulator 122 include a bootstrap capacitor 230 and a resettable fuse 232, with previously described component retaining their original reference indicia.

During operation, the error amplifier 228 compares the FB signal 145 with a bandgap voltage reference, that voltage corresponding to a reference load impedance. The error amplifier 228 subsequently produces an error output voltage which is compared to the output of the oscillator 206, which operates between 10.4 KHz and 52 KHz. The compared signal 213 is output from the comparator 212, and represents a voltage- controlled duty cycle signal. In particular, when a FB signal 145 having a voltage higher than the band gap reference is received, the output voltage of the error amplifier 228 is increased, and the comparator produces a lower duty cycle output signal 213 (nearer to or at 10.4 KHz). When the voltage of the FB signal 145 is lower than the bandgap reference, the output voltage of the error amplifier 228 is reduced, and the comparator 212 produces a higher duty cycle output (nearer to or at 52 KHz).

The duty cycle output signal 213 is supplied to the driver amplifier 216, whose operation is controlled by a thermal shutdown control module 208, current limiter 210, and a rectified portion of the DC supply signal 143. In particular, the thermal shutdown module 208 (which includes resettable fuses in a particular embodiment) is operable to shutdown the driver amplifier's operation when a predefined temperature is exceeded. The thermal shutdown module may be additionally connected to the voltage/current control circuits 132 and/or resettable fuses 136 to disable current supply to the load. The current limiter 210 is used to prevent excessive current draw, and accordingly excessive amplifier gain. In a particular embodiment, precision resistors Rl and R2 (which may be integrated within the voltage/current control circuits 132) are selected based upon the expected normal load conditions, such.that under normal operating conditions, the load sinks current flow over a particular range. When the load begins to sink more current, perhaps due to age or wear and tear, excessive current will be drawn and heat dissipation will increase. Temperature sensors distributed throughout the LED driver circuit will detect the increased operating temperature, and will control the driver circuit to bring the supplied current down until normal operating conditions return.

The rectified portion of the DC supply voltage 143 is used as negative feedback to further prevent excessive gain supplied by the driver amplifier 216. The amplified signal is subsequently supplied to darlington coupled transistors 218 and 220, the resulting signal supplied to switch 226. The state of the switch 226 is controlled by the output of the AND logical gate 222, the AND gate 222 having a first input coupled to receive the DC supply signal 143, and a second input coupled to the external resettable fuse 232. During normal operation, the resettable fuse 232 conducts the DC supply signal 143, and the AND gate outputs a high state signal to close the switch 226. If the DC supply signal 143 provides excessive current, the resettable fuse 232 begins applying increased series resistance, thereby providing a low state to the second AND gate input, resulting in the AND gate outputting a low state signal and opening the switch 226. In a particular embodiment, the output of an error signal from the error amplifier 228 results in a "safe" mode of operation, whereby operation (e.g., effective amplification) of the driver 216 is reduced to a predefined level. Further particularly, when an error signal is output from amplifier 228 and a short circuit condition is sensed, operation of the driver 216 is terminated completely. Resettable fuse 232 may be used, for example, to detect the later condition of when a short circuit condition exists, and thereby control the driver 216 to discontinue operation.

Additional components include a bootstrap charger 202, internal regulator 204, reset actuator 224, and bootstrap capacitor 230. The bootstrap charger 202 and bootstrap capacitor 230 together operate to boost the input voltage applied to the drain line of transistor 218. The bootstrap capacitor 230 couples to node SW which sees the dv/dt of the switching action as an AC signal. The AC voltage is rectified and used to provide additional drive to the drain terminal of driver 216 and transistor 218.

The internal regulator 204 is configured to shut down the LED driver when a control signal (either a high or low logic state signal) is received. The control signal may originate from a load condition (e.g., when a short circuit is sensed), or it may be generated from a temperature sensor when the operating temperature of the LED driver circuit exceeds a predefined limit. The reset actuator 224 is coupled via logic to disable the duty cycle output signal 213 when an error occurs in the error amplifier 228 or the oscillator 206. In such instances, a reset signal may be sent multiple times in order to allow sufficient time for the error amplifier 228 and/or the oscillator 206 to completely reset.

Fig. 3 illustrates a method of operating a LED driver circuit in accordance with one embodiment of the present invention. Intially at 305, a DC signal is provided to the LED driver circuit. An exemplary embodiment of this process is performed by the transformer 111 transforming an AC signal to an AC supply signal, the rectifying circuit 112 rectifying the AC supply signal to a DC supply signal and the resettable fuse 113 reducing or limiting the current based upon the sensed current and/or temperature of the power supply section. Next at 310, the supplied DC signal 143 is modulated at a first frequency, producing the modulated DC supply signal 144, the applied frequency of modulation being determined as a function of an input load impedance of the LED device 150. An exemplary embodiment of this process is performed by the feedback circuit 200 included within the frequency modulator 122, as described above in Fig. 2. Next at 315, the modulation frequency of the modulated DC supply signal 144 is varied from the first frequency to a second frequency responsive to a change in the input load impedance of the LED device. An exemplary embodiment of this process is also performed by the feedback circuit 200, described above in Fig. 2.

The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the disclosed teaching. The described embodiments were chosen in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

CLAIMSWhat is claimed is:

1. An LED driver circuit for supplying power to a LED device, comprising: a frequency modulator having an input coupled to receive a DC supply signal, an output for providing a modulated DC supply signal, and a feedback input operable to receive a feedback signal, the feedback signal operable to indicate the input load impedance of the LED device, the frequency modulator configured to control the frequency of modulation applied to the DC supply signal responsive to the input load impedance of the LED device as detected by the feedback signal; and at least one voltage/current control circuit, the voltage/current control circuit having an input coupled to receive the modulated DC supply signal and an output for providing a regulated LED supply signal.

2. The LED driver circuit of claim 1, further comprising an inductor having an input coupled to the output of the frequency modulator and an output coupled to the input of the at least one voltage/current control circuit, the inductor configured to provide energy to the at least one voltage/current control circuit during a low state of the modulated DC supply signal.

3. The LED driver circuit of any one of claims 1 or 2, further comprising a limiting diode having a cathode port coupled to the output of the frequency modulator and an anode port coupled to predefined potential, the limiting diode having a predefined breakdown voltage operable to prevent the output of the frequency modulator from exceeding said predefined breakdown voltage over said predefined potential coupled to the anode port.

4. The LED driver circuit of any one of claims 1-3, further comprising a respective at least one resettable fuse having an input coupled to the output of the at least one voltage/current control circuit and an output; the at least one resettable fuse operable to increase series resistance therethrough upon detecting current flow of the regulated LED supply signal above a predefined current limit.

5. The LED driver circuit of any one of claims 1-4, further comprising an input resettable fuse having an input for receiving the DC supply signal and an output coupled to the input of the frequency modulator, the input resettable fuse operable to increase series resistance therethrough upon detecting current flow of the DC supply signal above a predefined current limit.

6. The LED driver circuit of any one of claims 1-5, further comprising: a transformer having an input for receiving an AC signal and an output for providing an AC supply signal; and an AC rectifier circuit having an input coupled to receive the AC signal and an output for providing the DC supply signal, the AC rectified circuit operable to rectify the AC supply signal into the DC supply signal.

7. The LED driver circuit of any one of claims 1-6, further comprising: at least one AC decoupling capacitor having a first port coupled to the input of the frequency modulator and a second port coupled to an AC signal ground; and at least one AC decoupling capacitor having a first port coupled to the output of the voltage/current control circuit and a second port coupled to an AC signal ground.

8. The LED driver circuit of claim 1 , wherein the modulation frequency of the modulated DC supply signal is variable between 10-100 KHz.

9. The LED driver circuit of claim 2, wherein the inductor is rated at greater than 80% efficiency at an operating temperature of 90° C or less.

10. The LED driver circuit of claim 6, wherein the at least one AC decoupling capacitors and the input AC decoupling capacitors comprise ESR capacitors.

11. A method for operating an LED driver circuit to supply power to a LED device, the method comprising: providing a DC supply signal; modulating the DC supply signal at a first frequency to generate a modulated DC supply signal, wherein the first frequency of modulation is determined as a function of the input load impedance of the LED device; and varying, in response to a change in the input load impedance of the LED device, the modulation frequency applied to the DC supply signal to a second frequency.

12. The method of claim 11 , wherein varying the modulating frequency to a second frequency comprises decreasing the modulation frequency applied to the DC supply signal responsive to receiving a feedback signal indicating a decrease in the magnitude of the input load impedance of the LED device.

13. The method of claim 11 , wherein varying the modulation frequency to a second frequency comprises increasing the modulation frequency applied to the DC supply signal responsive to receiving a feedback signal indicating an increase in the magnitude of the input load impedance of the LED device.

14. The method of one of the claims 11-13, further comprising: storing current within an inductor during the positive half-cycle of the modulated DC supply signal; and outputting current from the inductor during the negative half-cycle of the modulated DC supply signal.

15. The method of one of claims 11-14, further comprising regulating, to a predefined range, the current and voltage of the modulated DC supply signal.

16. The method of one of claims 11-15, further comprising limiting, to a predefined limit, the voltage of the modulated DC supply signal.

17. The method of one of the claims 11-16, wherein providing a DC supply signal comprises: receiving an AC signal; transforming the AC signal to an AC supply signal; and rectifying the AC supply signal to the DC supply signal.

18. The method of one or the claims 11-17, further comprising: sensing the temperature of one or more components within the LED driver circuits; and decreasing the DC supply signal when the sensed temperature of the one or more components exceeds a predefined limit.

19. The method of claim 18, wherein decreasing the DC supply signal comprises decreasing the current delivered to the input of the LED device.

20. The method of claim 18, wherein decreasing the DC supply signal comprises decreasing the modulation frequency applied to the modulated DC supply signal.