TWI760068B - A LED driver with multiplex current balance control - Google Patents

Abstract

An LED driver with multiplex current balance control, comprising: a half-bridge switch circuit, a capacitor-inductor series circuit, a transformer, a first diode, a second diode, an output capacitor, and an LED load circuit , a feedback circuit, a control unit and a drive circuit, wherein the LED load circuit has N LED circuits connected in parallel, N is an integer greater than 1, and each of the LED circuits has a multiplexer and a series connection with it. An LED light string, and each multiplexer is controlled by a multiplexer control signal to turn on or off its channels; the control unit is used for performing a proportional- The integral control program generates a first PWM signal and a second PWM signal; the driving circuit is used for generating N sequential pulse signals to provide N multiplexer control signals, and according to the first PWM signal and the The second PWM signal generates a first driving signal and a second driving signal to drive the half-bridge switch circuit.

Description

A LED driver with multiplex current balance control

The present invention relates to an LED driver, in particular to an LED driver with multiplexed current balance control.

At the 21st United Nations Climate Change Conference (COP21) held in Paris, France in 2015, through the historic inclusive and legally binding carbon reduction agreement - the Paris Agreement, participating countries agreed to control greenhouse gas emissions in order to achieve pre-industrialization to The global average temperature increase in 2100 will not exceed 2℃, and efforts are made to control it within the target of 1.5℃. This shows that the deterioration of the global environment, climate, and ecology caused by greenhouse gas emissions has been fully valued by the world, and issues such as green environmental protection and energy conservation and carbon reduction have truly received attention from countries around the world.

The human demand for light sources is not only to provide lighting, but also to have an excellent lighting environment. In the context of global promotion of energy saving and carbon reduction, it is more important to choose a lighting source that conforms to the concept of environmental protection. At present, common artificial light sources include incandescent lamps, fluorescent lamps, halogen lamps, multi-metal lamps and light-emitting diodes (LEDs), etc., each of which has a different driving method. Among them, the developed light-emitting diode has the advantages of low power consumption, fast on-off response, small size, long life and no mercury pollution. With the advancement of materials and manufacturing processes, there are more and more related applications. LEDs can be seen everywhere in life. Many applications require lighting equipment to have dimming functions, which are used in outdoor lighting such as gas stations and parking lots. The light source is therefore gradually converted from traditional high pressure sodium lamps to LED light sources.

LEDs have the advantages of high energy efficiency, long life, high color rendering and fast start-up, etc., and the light color is also shifted to a warm color temperature to improve the health effects of potential glare, light pollution and night lighting. The energy conversion efficiency and lifespan of LEDs are also better than those of traditional light sources. Although their cost is high, LEDs are highly malleable and suitable for various lighting designs. The lighting industry regards them as rising stars. Characteristics such as longevity as the main target.

Using DC-DC converter to drive LED can make LED lamp have better photoelectric conversion efficiency. Most high-wattage LED lamps used in outdoor use mains power supply. If the LED lamp is driven by a type converter, there will be a certain voltage difference between the LED lamp and the shell used for heat dissipation. Once the insulation fails, the shell and the mains will be short-circuited.

Furthermore, today's high brightness light emitting diode (LED) drivers use a pulse width modulation (PWM) dimming method that changes the average current by switching between maximum current and zero current, which maintains color quality at the same time. However, the luminous efficiency (lm/W) of driving the LED with the maximum current is still lower than that of the analog dimming method, and adding a current regulating circuit to each LED string will also increase its cost and power loss.

Some literatures propose a drive architecture combining feedforward and linear current regulators. The voltage drop of the current regulators of each group of LED strings is sensed through diodes, so that the voltage drop of one of the current regulators approaches zero. The individual differences of the diodes and the influence of temperature characteristics cause the detection accuracy and stability to decrease; some literatures also propose the use of multi-level PWM dimming method. Since the brightness-current curve of LEDs is not linear, a lower current can be used. Compared with traditional PWM dimming, it has higher photoelectric conversion efficiency to drive LEDs, and uses multiple current sensing resistors and dimming signal levels to achieve dimming control; ) and a single-inductance multi-string output LED driver realized by a multiplex switch, which has the effect of automatic current sharing and can adapt to LED strings with different on-voltage voltages; some literatures propose to make the circuit operate in step-down conversion under the condition of rated voltage. The converter mode transfers energy to the output, and in the case of critical voltage, the circuit operates in a boost converter mode, so that the energy in the output capacitor can be recharged to the input terminal, which can speed up the voltage transient response from the output to the LED. , can also improve the overall efficiency.

However, the circuit control of the above-mentioned documents is still complicated, the parameter design is not easy, and the conversion efficiency is not ideal. Therefore, a novel LED driver is urgently needed in the art.

One object of the present invention is to disclose an LED driver with multiplexed current balance control, which can use an LLC resonant converter to be based on a gallium nitride high electron mobility transistor (GaN HEMT), and the output is time-division multiplexed at equal intervals. Connect to each LED string channel to keep the current of each channel equal, and switch the conduction sequence through the multiplexer to continuously time-division multiplexing, so as to achieve the purpose of greatly reducing the current imbalance.

Another object of the present invention is to disclose an LED driver with multiple current balance control, since the current of each LED string channel can be controlled to be very close under different illumination requirements, even if the LED light string is aging due to aging The turn-on voltage drop changes, and each LED string can still be controlled to maintain a current-sharing state.

Another object of the present invention is to disclose an LED driver with multiplexed current balance control, which can achieve a switching frequency of 1MHz, avoid the change of the switching frequency through the burst mode, the maximum power conversion efficiency reaches 93.8%, and the voltage ripple The factor is less than 0.3%, and the current ripple factor is less than 0.8%.

Another object of the present invention is to disclose an LED driver with multiple current balance control, the photoelectric conversion efficiency before the equivalent illuminance of 90% can be improved by up to 19.8% compared to the conventional digital PWM dimming mode light. %, and on the premise that there is no need to sense the current of each channel, the current error of each light string can be less than 10%.

In order to achieve the aforementioned purpose, an LED driver with multiplexed current balance control is proposed, which has: a half-bridge switch circuit, which has two input terminals to be coupled to the positive and negative terminals of an input voltage, and two control terminals to be respectively connected with the A first driving signal and a second driving signal are coupled, and an output terminal is coupled to the positive terminal when the first driving signal exhibits an active potential and is coupled to the negative terminal when the second driving signal presents an active potential a capacitor-inductor series circuit, one end of which is coupled to the output end of the half-bridge switch circuit; a transformer, which has a main coil and a secondary coil, and one end of the main coil is connected to the capacitor- The other end of the inductor series circuit is coupled, the other end of the primary coil is coupled to the negative end of the input voltage, the secondary coil has a first output end, a second output end, and a center tap connection point; a first diode with a first anode and a first cathode, the first cathode is coupled to the first output terminal, the first anode is coupled to a voltage output terminal; a second The diode has a second anode and a second cathode, the second cathode is coupled to the second output terminal, the second anode is coupled to the voltage output terminal; an output capacitor is coupled to the voltage output terminal between the voltage output terminal and the center tap contact; an LED load circuit, coupled between the voltage output terminal and the center tap contact, has N LED circuits connected in parallel, N is an integer greater than 1, and each Each of the LED circuits has a multiplexer and an LED light string connected in series, wherein each multiplexer is controlled by a multiplexer control signal to turn on or off its channels; a feedback circuit is connected with the multiplexer. The LED load circuit is coupled to generate a voltage feedback signal and a current feedback signal input; a control unit storing a firmware program for executing a proportional-integral control program according to the voltage feedback signal and the current feedback signal to generate a first PWM signal, a second PWM signal; and a driver The circuit is used for generating N sequential pulse signals to provide N multiplexer control signals, and generating the first driving signal and the second driving signal according to the first PWM signal and the second PWM signal.

In one embodiment, the voltage feedback circuit includes an optical coupling circuit, and the current feedback circuit includes a Hall sensor.

In one embodiment, the half-bridge switch circuit includes two power switches.

In one embodiment, the power switch is a gallium nitride metal oxide semiconductor field effect transistor.

In one embodiment, the control unit includes an analog-to-digital converter for performing an analog-to-digital conversion operation on the voltage feedback signal and the current feedback signal to generate a first input digital signal and a second input, respectively digital signal.

In one embodiment, the control unit includes a filter operation function module to perform a filter operation on the first input digital signal and the second input digital signal to generate a third input digital signal and a fourth input digital signal, respectively signal, and the control unit performs the proportional-integral operation on the third input digital signal and the fourth input digital signal according to a predetermined voltage value to determine the first PWM signal and the second PWM signal responsibility cycle.

In one embodiment, the control unit includes a pulse width modulation module to generate the first PWM signal and the second PWM signal according to the duty cycle.

In order to enable your examiners to further understand the structure, features and purposes of the present invention, the accompanying drawings and detailed descriptions of preferred embodiments are as follows.

100: LED driver

110: Half-bridge switch circuit

120: Capacitor-Inductor Series Circuit

130: Transformer

140: First diode

150: Second diode

160: output capacitor

170: LED load circuit

171: Multiplexer

172: LED String Lights

181: Optical coupling circuit

182: Hall sensor

190: Control Unit

191: Analog to Digital Converters

192: Filter operation function module

193: Proportional-Integral Operation Module

194: PWM Module

200: drive circuit

FIG. 1 is a block diagram illustrating an embodiment of the LED driver with multiplexed current balance control of the present invention.

FIG. 2a is a schematic diagram of the structure of the half-bridge LLC resonant converter.

FIG. 2b is a waveform diagram of FIG. 2a operating at the resonant frequency.

FIG. 2c shows a waveform diagram of FIG. 2a operating above the resonant frequency.

FIG. 2d shows a waveform diagram of FIG. 2a operating below the resonant frequency.

FIG. 3 is a schematic diagram of the structure of the linear steady current control used in PWM dimming.

FIG. 4a is a schematic diagram showing the structure of the multiplexer used in the present invention.

FIG. 4b is a graph showing the relationship between the switching sequence of the multiplexer and the resonant current of FIG. 4a.

FIG. 4c is a schematic diagram illustrating the opening sequence of each step in FIG. 4a.

FIG. 4d is a schematic diagram of the waveform timing of the resonant converter combined with the multiplexer.

FIG. 5 is a schematic diagram of the circuit wiring of the second and fourth LED channels with MOSFETs added to simulate forward voltage rise.

FIG. 6 is a schematic diagram showing the setting of the illumination measurement environment of this implementation.

FIG. 7 is a schematic flowchart of the firmware program of this implementation.

FIG. 8a shows the waveform measurement of this implementation at full load.

FIG. 8b shows the waveforms of the output voltage and current ripple of this implementation at full load.

FIG. 8c shows the measured efficiency curve of this implementation.

FIG. 9a shows the dimming command voltage and the LED current waveform of channel 1 under 10% duty cycle of the control group (PWM dimming control).

FIG. 9b shows the dimming command voltage and the LED current waveform of channel one under 50% duty cycle of the control group.

FIG. 9c shows the dimming command voltage and the LED current waveform of channel one under 100% duty cycle of the control group.

FIG. 10a shows the measurement waveform of this implementation at the equivalent luminance of 10% duty cycle.

FIG. 10b shows the measurement waveform of this implementation under the equivalent luminance of 50% duty cycle.

FIG. 10c shows the measurement waveform of this implementation under the equivalent luminance of 100% duty cycle.

FIG. 11 is a waveform diagram of current versus forward voltage drop measurement when the forward voltage drop of the simulated LED string increases.

FIG. 12a is a graph showing the duty cycle and illuminance of the prior art digital PWM dimming.

FIG. 12b shows a graph of the LED average current versus luminance of this implementation.

FIG. 12c is a comparison diagram of the photoelectric conversion efficiency of the digital PWM dimming of the prior art and the present invention.

FIG. 13a is a graph showing the current curve of each channel LED of the multiplexer.

13b is a graph showing the currents of each channel of the multiplexer simulating the increase in forward voltage of the second and fourth strings of LEDs.

Please refer to FIG. 1 , which shows a block diagram of an embodiment of an LED driver with multiplexed current balance control of the present invention.

As shown in the figure, the LED driver 100 with multiplexed current balance control includes: a half-bridge switch circuit 110, a capacitor-inductor series circuit 120, a transformer 130, a first diode 140, and a second diode 150 , an output capacitor 160 , an LED load circuit 170 , a feedback circuit ( 181 , 182 ), a control unit 190 , and a drive circuit 200 .

The half-bridge switch circuit 110 has two input terminals A and B coupled to the positive and negative terminals of an input voltage _Vin , and two control terminals coupled to a _first driving signal S1 and a _second driving signal S2 respectively and an output terminal C is coupled to the positive terminal when the _first driving signal S1 exhibits an active potential and is coupled to the negative terminal when the _second driving signal S2 presents an active potential.

One end of the capacitor-inductor series circuit 120 is coupled to the output end C of the half-bridge switch circuit 110 .

The transformer 130 has a main coil and a secondary coil, one end of the main coil is coupled to the other end of the capacitor-inductor series circuit 120, and the other end of the main coil is coupled to the negative end of the input voltage , the secondary coil has a first output end D, a second output end E, and a center tap contact F.

The first diode 140 has a first anode and a first cathode, the first cathode is coupled to the first output terminal D, and the first anode is coupled to a voltage output terminal O.

The second diode 150 has a second anode and a second cathode, the second cathode is coupled to the second output terminal E, and the second anode is coupled to the voltage output terminal O.

The output capacitor 160 is coupled between the voltage output terminal O and the center tap contact F.

The LED load circuit 170 is coupled between the voltage output terminal O and the center tap contact F, and has N LED circuits connected in parallel, where N is an integer greater than 1, and each of the LED circuits Each has a multiplexer 171 and an LED light string 172 connected in series, wherein each multiplexer 171 is controlled by a multiplexer control signal to turn on or off its channels.

The feedback circuit (181, 182) is coupled to the LED load circuit 170 to generate a voltage feedback signal V _FB and a current feedback signal I _FB .

The control unit 190 stores a firmware program for executing a proportional-integral control program according to the voltage feedback signal V _FB and the current feedback signal I _FB to generate a first PWM signal and a second PWM signal; the driving The circuit 200 is used for generating N sequential pulse signals to provide N multiplexer control signals, and generating the _first driving signal S1 and the second driving signal S according to the first PWM signal and the second PWM signal ₂ ; wherein, the voltage feedback circuit includes an optical coupling circuit 181, and the current feedback circuit includes a Hall sensor 182.

The half-bridge switch circuit 110 includes, for example, but not limited to, two power switches, such as but not limited to GaN MOSFETs.

The control unit 190 further includes an analog-to-digital converter 191 , a filtering operation module 192 , a proportional-integral operation module 193 and a pulse width modulation module 194 .

The analog-to-digital converter 191 performs an analog-to-digital conversion operation on the voltage feedback signal V _FB and the current feedback signal I _FB to generate a first input digital signal and a second input digital signal, respectively.

The filtering operation function module 192 performs a filtering operation on the first input digital signal and the second input digital signal to generate a third input digital signal and a fourth input digital signal respectively, and the proportional-integral operation modulo The group 193 performs the proportional-integral operation on the third input digital signal and the fourth input digital signal according to a predetermined voltage value to determine the duty cycle of the first PWM signal and the second PWM signal.

The PWM module 194 generates the first PWM signal and the second PWM signal according to the duty cycle.

The present invention adopts a half-bridge LLC resonant converter, because it is suitable for medium power applications and has the advantages of soft switching characteristics. By using a gallium nitride element with a wide energy gap to increase the operating frequency, the operating frequency can be increased from the common 100kHz to 1MHz, so that the resonant tank, main transformer and output capacitors and other components can be reduced. The following will introduce the operation principle and component design of the half-bridge LLC resonant converter.

Please refer to FIGS. 2a to 2d together, wherein FIG. 2a shows a schematic diagram of the structure of a half-bridge LLC resonant converter, FIG. 2b shows a waveform diagram of FIG. 2a operating at the resonant frequency, and FIG. 2c shows that FIG. 2a operates at A waveform diagram above the resonant frequency, FIG. 2d shows the waveform diagram of FIG. 2a operating below the resonant frequency.

As shown in Figure 2a, the half-bridge LLC resonant converter is an LLC resonant tank composed of GaN MOSFET half-bridge power switches S ₁ , S ₂ , a resonant inductor L _r and a resonant capacitor Cr , an isolation transformer with an excitation inductor L _{m ,} It is composed of output rectifier diodes D _{o 1} , D _{o 2} and filter capacitor C _o .

The circuit action mechanism of the half-bridge LLC resonant converter is that the half-bridge circuit composed of S ₁ and S ₂ will generate a square wave and resonate through the LLC resonant tank, so that it generates a current similar to a sine wave, and transmits it to the transformer according to the turns ratio. On the secondary side, the secondary side winding with the center tap cooperates with the diode to form a full-wave rectifier circuit, and the output capacitor filters the rectified AC current and outputs the DC voltage.

Taking the _{resonant frequency fr} as the separation point, the LLC resonant converter has three interval operation modes, which are described as follows:

1. Operate at the resonant frequency ( f _{= fr} )

As shown in Fig. 2b, each half switching cycle includes a complete power transfer operation. When the half switching cycle ends, the resonant tank current i _Lr is exactly equal to the excitation current i _Lm , and the secondary side output current is also zero, At this time, the diode on the secondary side belongs to the zero current cut-off, which will help to improve the efficiency.

2. Operation above the resonant frequency ( f > f _r )

As shown in Fig. 2c, each half switching cycle has only a partial power transfer operation. When the half switching cycle ends, the resonant tank current is not equal to the excitation current, and the turn-off loss of the power switch on the primary side will increase at this time, and The diode on the secondary side is a hard turn-off, and the loss will increase at this time.

3. The frequency f operates above the resonant frequency ( f > f _r )

As shown in Figure 2d, each half switching cycle includes a complete power transfer operation, but the resonant current is equal to the excitation current in advance. At this time, the switch on the primary side will generate a circulating current loss, and the rectifier diode on the secondary side will be turned off early. , so that the diode on the secondary side achieves zero current cut-off. If synchronous rectification is used, the switch must be turned off in advance to avoid energy backflow to the primary side, and the efficiency of the primary side will also decrease slightly due to the loss of circulating current.

The digital PWM dimming in the prior art achieves the dimming effect by changing the current conduction time per unit time to change its average value. If the working frequency is too low, the human eye will feel flickering. Generally, the working frequency is set between 100 and 400 Hz. In the implementation of the present invention, 200 Hz is used as the PWM dimming frequency, but not limited thereto. When driving the LED, it is driven by a constant current, which can not only stabilize the LED conduction current, but also improve the constant voltage driving. The characteristics of the LED are caused by the negative temperature coefficient.

Please refer to FIG. 3 , which shows a schematic diagram of the structure of the linear steady current control used in PWM dimming.

As shown in the figure, the linear steady current control is composed of an operational amplifier and a field effect transistor. The field effect transistor is operated in the linear region. V _r is a variable amplitude dimming signal. When the non-inverting input of the operational amplifier When the terminal voltage is high, the field effect transistor is turned on, which is equivalent to a variable resistor, and the output current is fed back to the operational amplifier through the current detection resistor R _s for comparison; when the non-inverting input terminal voltage is low voltage When the FET is turned off, the average current flowing through the LED is controlled by this mechanism to achieve the PWM dimming function.

Please refer to FIGS. 4a to 4c together, wherein FIG. 4a shows a schematic diagram of the multiplexer structure used in the present invention, FIG. 4b shows the relationship between the switching sequence of the multiplexer and the resonant current in FIG. 4a, and FIG. 4c shows FIG. 4a is a schematic diagram of the turn-on sequence of each step, and FIG. 4d is a schematic diagram of the waveform timing diagram of the resonant converter combined with the multiplexer.

As shown in Figure 4a, the multiplexer uses two TPH1R306 field effect transistors to form a bidirectional switch Muxx, and uses 20 GRM31CR71H475K 1206 MLCC multilayer ceramic capacitors C _ox with a size of 1206 and a capacitance value of 4.7μF. , which is used to eliminate the peak current when the multiplexer is turned on, and to continuously provide LED current when the multiplexer is turned off.

As shown in Figure 4b, since the output current of the LLC converter is a resonant current, it is not significantly affected by the instantaneous change of the equivalent load. Adjust the operation sequence of the multiplexer once during the resonance period to achieve the effect of automatic current sharing. The multiplexer circulates in 5 steps.

As shown in Figure 4c, it can be seen that channel 1 is the first to open in the first step, and each subsequent step will move backward by one order; while channel 5 is the last to open in the first step, and the second step is the first one to open. If it is the first one to open, then the multiplexer of this channel will be opened twice in a row, which only happens when the steps are changed, and can reduce the number of times the multiplexer is opened and reduce the switching loss of the multiplexer.

As shown in Figure 4d, since the multiplexer of each channel is composed of an active switch, when the LED lamp of the channel is open or short-circuited, the faulty light string can be isolated through the multiplexer to improve the reliability of the lamp. The analog dimming method is adopted to control the brightness by adjusting the output current. Compared with the digital PWM dimming method of the prior art, it not only does not flicker but also has a higher photoelectric conversion efficiency.

In the present invention, 5 CXA2540 COB LEDs are independently controlled in parallel, and their specifications are shown in Table 1.

Please refer to FIG. 5 , which shows a schematic diagram of the circuit wiring of the second and fourth LED channels with MOSFETs added to simulate the forward voltage rise.

As shown in the figure, the rated power of each CXA2540 is 40W and a total of 200W. Multiple series and parallel connections help to improve reliability. In order to confirm the effect of automatic current sharing, the second and fourth LED channels are connected in series with two and one respectively. IRF540 field effect transistor, and short-circuit the drain and gate of IRF540, then its conduction voltage drop can be expressed as V _DS = I _D . R _{DS (on)} + V _{GS (th)} , the actual connection method is used to simulate the rise of the forward voltage of the LED, and check the effect of the constant current control of the multiplexer.

Please refer to FIG. 6 , which shows a schematic diagram of the setting of the illumination measurement environment of this implementation.

As shown in the figure, in order to avoid being affected by other ambient light sources during measurement, a wooden box with a length of 55 cm, a width of 35 cm and a height of 30 cm is used as the test space, and there is a 4 cm square hole on the side for wiring. And fill the gap during measurement to ensure that no ambient light enters. Since the maximum brightness of the LED exceeds 25800 lumens, the illuminance meter is set on the side of the box and is not directly irradiated by the LED to ensure that the illuminance is within the measurement range of the illuminance meter. The illuminometer used is TES-1339R.

In order to achieve the purpose of digital control, this implementation uses the TMS320F280049C digital signal processor from Texas Instruments as the digital controller for the LLC resonant converter and the constant current control of the multiplexer. The output voltage and total LED current are sampled by the sampling circuit, the signal is converted by an analog-to-digital converter (ADC), and then digitally filtered (Moving Average), and then the filtering result is calculated by a digital PI compensator, and the PWM module is adjusted to generate PWM After receiving the signal, the isolated gate driver drives the GaN power switch to control the LLC resonant converter to achieve digital control. This implementation uses a TMS320F280049C microcontroller to implement a Moving Average filter and an incremental proportional-integral controller, and uses a Control Law Accelerator (CLA) to process the sequential transformation of the output multiplexer switch MUXi. The CLA operates independently of the CPU, which will improve system performance.

Please refer to FIG. 7 , which shows a schematic flowchart of the firmware program of this implementation.

As shown in the figure, the program can be divided into three parts: the main program, the analog-to-digital converter interrupt and the CLA MUX adjustment program. When the main program starts, it first declares the variables required by the program, initializes core modules such as the system clock module, and then initializes peripheral modules such as ADC, GPIO (general-purpose output and input port), and PWM. Entering the soft-start mode, when the driver output voltage reaches 30 volts, it enters the normal LLC converter operating mode operating at the resonant frequency. Then start the CLA and CPU interrupt mode, enter the infinite loop and wait for the interrupt to appear.

ADC interrupt sub-program flow part: In normal operation mode, the controller is used to realize the automatic current sharing effect of single-group output LLC converters combined with multiplexers. The ADC interrupt of controlling LLC converters will detect the output voltage and output current to minimize the error. The value is used for PI control operation to achieve the effect of constant voltage and constant current. Since the life of the LED is greatly affected by overcurrent, once a large negative error occurs, the converter will immediately turn off the output to avoid overshoot of voltage and current, because once the converter is adjusted The operating frequency will also affect the cycle of the output multiplexer, which will be disadvantageous for the CLA to complete the operation in a very short time, so this implementation will use the burst mode (Burst Mode) for fixed frequency operation.

Multiplexer subroutine process part: Because the LLC converter will have output in the positive and negative half cycles, the higher the switching frequency of the multiplexer can reduce the current ripple caused by the insufficient capacitance of the output capacitor. The frequency-operated MLCC multilayer ceramic capacitor is used as the output filter capacitor. The control law accelerator CLA is an auxiliary processor independent of the CPU. Like the CPU, the CLA can operate the peripheral PWM and other modules, and has the same computing capability. The processor must complete the transformation of the startup sequence by the judgment within 1μs. Although the current processor can speed up the execution of instructions through the pipeline architecture, it has no effect on the judgment, so it will consume a lot of processor resources. Reduce the CPU load, so that it can perform the filtering and PI operation of the ADC interrupt, the PWM module will generate an interrupt, so that the CLA adjusts the sequence of the multiplexer once in each converter switching cycle, and completes it in every half output cycle. The switching of 5 output LED channels, that is, the multiplexer will complete 10 switching actions in each switching cycle to reduce the magnitude of the output current ripple.

In addition, in order to compare the luminous efficiency with the PWM dimming method in the prior art, the controller additionally outputs 2 sets of PWM for dimming purposes, one set is 200 Hz PWM for dimming, and the other set is 10k PWM for setting the dimming PWM amplitude. Hertz PWM, when operating in PWM dimming mode, the converter will output the maximum output voltage of the linear constant current module plus 2V as the output voltage.

Experimental results and analysis

In this practice, a prototype of a multi-string LED driver is developed and tested to verify its correctness and performance. The circuit design specifications are shown in Table 2.

In order to verify the efficiency of the prototype driver, this test uses an electronic load as the load to test the efficiency value of 10% to 100% wattage output at 40V output and the voltage and current ripple values at full load.

Please refer to FIGS. 8a to 8c together, wherein FIG. 8a shows the waveform measurement of the implementation at full load, FIG. 8b shows the waveform of the output voltage and current ripple of the implementation at full load, and FIG. 8c It shows the measured efficiency curve of this implementation.

As shown in Figure 8a, the two main switch gate driving signals ( V _{GS 1} , V _{GS 2} ), the resonant current i _Lr and the output voltage V _o waveforms at full load, as shown in Figure 8b, are measured by the oscilloscope measurement function The effective value of voltage ripple is 109.7mV, and the effective value of current ripple is 35.54mA. The calculated voltage ripple factor is 0.27%, and the current ripple factor is 0.71%. The maximum efficiency is 93.8% under load, and the efficiencies under other loads are all higher than 92%.

The control group of this implementation uses the digital PWM control mode to drive the LED, and every 10% duty cycle is the interval. Please refer to Figures 9a to 9c together, and Figure 9a shows the control group under the 10% duty cycle. The light command voltage and the LED current waveform of channel one, Fig. 9b shows the dimming command voltage and the LED current waveform of channel one under 50% duty cycle of the control group, and Fig. 9c shows the control group under 100% duty cycle The dimming command voltage and the LED current waveform of channel one.

As shown in the figure, the duty cycles are 10%, 50% and 100% respectively. The upper part is the voltage waveform of the dimming command, and the lower part is the current flowing through the LED of channel one. Among them, CH1 ( V _command ): 1V/div, CH2 ( I _LED ): 1A/div, Time: 2ms/div. The experimental data of the photoelectric conversion efficiency of each illuminance are shown in Table 3. Subsequent implementations also use the illuminance in Table 3 as a benchmark for a fair comparison.

Please refer to FIGS. 10a to 10c together, wherein FIG. 10a shows the measurement waveforms of V _{GS 1} , V _{GS 2} , i _Lr , and i _out under the equivalent luminance of 10% duty cycle of the present implementation, and FIG. 10 b shows The measurement waveforms of V _{GS 1} , V _{GS 2} , i _Lr , and i _out of this implementation at the equivalent brightness of the 50% duty cycle are shown. Figure 10c shows this implementation at the equivalent brightness of the 100% duty cycle. The V _{GS 1} , V _{GS 2} , i _Lr , i _out measurement waveforms. The measurement data of the photoelectric conversion efficiency of each illuminance are shown in Table 4.

In order to verify the current sharing capability of the present invention, the measured data of the forward voltage generated by each channel LED flowing with a current of 1 ampere is shown in Table 5. It can be seen from Table 5 that the present invention can enable each LED string to flow a consistent current, to achieve the effect of equalization.

Please refer to FIG. 11 , which is a waveform diagram of current versus forward voltage drop measurement when the forward voltage drop of the simulated LED string increases.

In order to further verify the current sharing capability of the present invention, two and one IRF540 field effect transistors were connected in series with the second and fourth strings of LEDs, respectively, and the drain and gate of the IRF540 were short-circuited to simulate the forward voltage rise of the LEDs. In this case, use the power supply to drive the LED with a current rise rate of 3.2mA/μs to confirm the actual effect of the simulated forward bias voltage rise, as shown in Figure 11, where CH1 ( V _LED + 2‧IRF540 ): 10V /div, CH2 ( I _LED2 ): 500mA/div, CH R1 ( V _LED ): 10V/div, CH R2 ( V _LED + IRF540): 10V/div, Time: 100μs/div. The actual measured on-voltage drop of each channel is shown in Table 6.

Table 7 shows the illuminance and photoelectric conversion efficiency of each LED string channel under each equivalent illuminance when the on-state voltage drop changes due to aging of the LED light string.

Please refer to FIGS. 12a to 12c together, wherein FIG. 12a shows the duty cycle and illuminance curve of the digital PWM dimming in the prior art, and FIG. 12b shows the LED average current versus illuminance curve of this implementation. The line diagram, FIG. 12c, is a comparison diagram of the photoelectric conversion efficiency between the digital PWM dimming of the prior art and the present invention.

As shown in Figure 12a, the prior art digital PWM dimming operates the LED at the maximum output current and only changes the duty cycle, so the output curve is relatively linear, and its illuminance value is used as the reference value for this measurement; as shown in Figure 12b , the present invention belongs to the analog dimming method, so the output curve is slightly curved, and the LED can be lit with a lower current when the illumination is low, so a higher luminous (photoelectric conversion) efficiency can be obtained; as shown in Figure 12c, the comparison The digital PWM dimming of the prior art and the photoelectric conversion efficiency of the present implementation, the present invention has the characteristics of analog dimming when the illumination is low, and the photoelectric efficiency is higher than the digital PWM dimming of the prior art, but because of the use of traditional silicon field effect electric power. The crystal leads to high switching loss of the multiplexer, and the photoelectric efficiency is slightly lower than the digital PWM dimming method of the prior art after the equivalent brightness is 90%.

In order to understand the actual current sharing capability of the present invention, please refer to FIGS. 13a and 13b together, wherein FIG. 13a shows the current curve of each channel LED of the present invention, and FIG. 13b shows the simulation of the second and fourth strings of LEDs of the present invention. The current curve of each channel when the forward voltage increases.

As shown in the figure, the current of each LED string channel of the present invention is very close under different illumination requirements. Even if the forward conduction voltage drop of the LED light string changes due to aging, the present invention can still control each LED string. Maintain almost equalized flow state operation.

By the design disclosed above, the present invention has the following advantages:

1. An LED driver with multiplexed current balance control of the present invention can use an LLC resonant converter based on Gallium Nitride High Electron Mobility Transistor (GaN HEMT), the output of which can be connected to a time-division multiplexing method at equal intervals. For each LED string channel, the current of each channel is kept equal, and the conduction sequence is switched by the multiplexer through time-division multiplexing, so as to achieve the purpose of greatly reducing the current imbalance.

2. An LED driver with multiple current balance control of the present invention, since the current of each LED string channel can be very close under different illumination requirements, even if the forward conduction voltage drop of the LED light string changes due to aging, It is still possible to control each LED string to maintain a current sharing state.

3. The LED driver with multiplexed current balance control of the present invention can achieve a switching frequency of 1MHz, and avoid the change of the switching frequency through the operation in the burst mode, the maximum power conversion efficiency reaches 93.8%, and the voltage ripple factor is less than 0.3 %, the current ripple factor is less than 0.8%.

4. Compared with the digital PWM dimming method of the prior art, the LED driver with multiple current balance control of the present invention can improve the photoelectric conversion efficiency by up to 19.8% before the equivalent illuminance of 90%, and it can be improved by up to 19.8%. Under the premise of sensing the current of each channel, the current error of each light string is less than 10%.

What is disclosed in the present invention is the preferred embodiment, and any partial changes or modifications that are derived from the technical idea of the present invention and can be easily inferred by those skilled in the art do not depart from the scope of the patent right of the present invention.

To sum up, regardless of the purpose, means and effect of the present invention, it is showing its technical characteristics that are completely different from the conventional ones, and its first invention is suitable for practical use, and it also meets the patent requirements of the invention. Pray for the patent to be granted as soon as possible to benefit the society, and I truly feel the virtue.

100: LED driver

110: Half-bridge switch circuit

120: Capacitor-Inductor Series Circuit

130: Transformer

140: First diode

150: Second diode

160: output capacitor

170: LED load circuit

171: Multiplexer

172: LED String Lights

181: Optical coupling circuit

182: Hall sensor

190: Control Unit

191: Analog to Digital Converters

192: Filter operation function module

193: Proportional-Integral Operation Module

194: PWM Module

200: drive circuit

Claims (7)

An LED driver with multiplexed current balance control, which has: a half-bridge switch circuit, has two input terminals to be coupled with positive and negative terminals of an input voltage, and two control terminals to be respectively connected to a first driving signal and a first Two driving signals are coupled, and an output terminal is coupled to the positive terminal when the first driving signal exhibits an active potential and is coupled to the negative terminal when the second driving signal presents an active potential; a capacitor-inductor a series circuit, one end of which is coupled to the output end of the half-bridge switching circuit; a transformer has a main coil and a secondary coil, one end of the main coil is coupled to the other end of the capacitor-inductor series circuit , the other end of the primary coil is coupled to the negative end of the input voltage, the secondary coil has a first output end, a second output end, and a center tap contact; a first diode , has a first anode and a first cathode, the first cathode is coupled with the first output terminal, the first anode is coupled with a voltage output terminal; a second diode has a second an anode and a second cathode, the second cathode is coupled to the second output terminal, the second anode is coupled to the voltage output terminal; an output capacitor is coupled to the voltage output terminal and connected to the center tap between the points; an LED load circuit, coupled between the voltage output terminal and the center tap contact, has N LED circuits connected in parallel, where N is an integer greater than 1, and each of the LED circuits has more than one A multiplexer and an LED light string connected in series with it, wherein each multiplexer is controlled by a multiplexer control signal to turn on or off its channel; a feedback circuit is coupled to the LED load circuit to generating a voltage feedback signal and a current feedback signal; a control unit storing a firmware program for executing a proportional-integral control program according to the voltage feedback signal and the current feedback signal to generate a first PWM signal, a second PWM signal; and a driving circuit for generating N sequential pulse signals to provide N multiplexer control signals, and generating the first PWM signal according to the first PWM signal and the second PWM signal drive signal and the second drive signal. LED driver with multiple current balance control as described in item 1 of the patent application scope The actuator, wherein the feedback circuit includes an optical coupling circuit to generate the voltage feedback signal, and the feedback circuit includes a Hall sensor to generate the current feedback signal. The LED driver with multiplexed current balance control as described in claim 1, wherein the half-bridge switch circuit includes two power switches. The LED driver with multiplexed current balance control as described in claim 3, wherein the power switch is a gallium nitride metal oxide semiconductor field effect transistor. The LED driver with multiplexed current balance control as described in claim 1, wherein the control unit comprises an analog-to-digital converter for performing analog-to-digital conversion on the voltage feedback signal and the current feedback signal The operation is performed to generate a first input digital signal and a second input digital signal respectively. The LED driver with multiplexed current balance control as described in claim 5, wherein the control unit includes a filter operation function module for performing a filter operation on the first input digital signal and the second input digital signal In order to generate a third input digital signal and a fourth input digital signal respectively, and the control unit performs the said ratio- The integral operation is performed to determine the duty cycle of the first PWM signal and the second PWM signal. The LED driver with multiplexed current balance control as described in claim 6, wherein the control unit comprises a pulse width modulation module to generate the first PWM signal and the second PWM signal according to the duty cycle PWM signal.

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