TWI771039B - LED drive system with high light conversion efficiency - Google Patents

Abstract

An LED driving system with high light conversion efficiency, which has: a full-bridge switch circuit, a capacitor-inductor series circuit, a transformer, a first diode, a second diode, a first inductor, a first diode Two inductors, an output capacitor, an LED load, a voltage feedback circuit, a control unit and a gate driver, wherein the control unit is used to perform a dimming operation according to a dimming command current to make the LED load An average current of the sink current is equal to the dimming command current, and the dimming operation includes: in five current intervals (0, I ₁ ], (I ₁ , I ₂ ], (I ₂ , I ₃ ], (I ₃ ,I ₄ ] and (I ₄ ,I ₅ ] to find out the interval to which one of the dimming command currents belongs, so as to enable five PWM signal combinations (PWM1), (PWM1, PWM2), (PWM2, PWM3), (PWM3 , PWM4), (PWM4, PWM5), select a PWM signal combination, and perform a negative feedback control calculation program according to the difference between a current feedback signal and the dimming command current to generate a duty cycle D to determine the The pulse width of each PWM signal in the selected PWM signal combination, D is a non-negative real number not greater than 1.

Description

LED drive system with high light conversion efficiency

The present invention relates to an LED driving system, in particular to an LED driving system with high light conversion efficiency.

In the past 100 years, lighting sources have been continuously improved and developed, from early carbon filament lamps to tungsten filament lamps to fluorescent lamps. The emergence of solid-state lighting (SSL) has brought a new light source concept to the world. Compared with the past light sources, LEDs have the advantages of energy saving, long life, high color rendering, and fast startup. With the development of science and technology, the luminous efficiency of LEDs has gradually improved, and the cost has become more affordable. Therefore, light sources such as street lamps, screen displays, and indoor lighting are gradually being converted from traditional light sources to LED light sources. According to the "Energy Savings Forecast of Solid-State Lighting in General Illumination Applications" (Energy Savings Forecast of Solid-State Lighting in General Illumination Applications) updated by the U.S. Department of Energy at the end of 2019, in 2017, the lighting equipment for the appearance of buildings in the United States, LED Accounting for 31%, while metal halide (Metal Halide) and high pressure sodium lamps together account for 25%, linear fluorescent lamps account for 27%, and the rest of halogen lamps and incandescent lamps account for 15%, and it is predicted that in 2025, LEDs will reach a 92% installation rate, while LED lighting will reach a 98% installation rate in 2035.

The illuminance-current curve of most LEDs is not linear. Taking CREE CXA2540 as an example, its illuminance-current curve is shown in Figure 1. It can be seen from Figure 1 that when the forward conduction current is smaller, the ratio of illuminance to current will be larger, and it will gradually decrease as the current rises. Since the digital dimming uses the length of the current conduction time to change the average current size, Thus, for dimming, the current through the LED will be converted between zero and the rated value, rather than changing the maximum value of its forward current, so the luminous efficiency is not optimal. Taking the characteristic curve at 25°C as an example, when the current is 2100mA, the corresponding relative illuminance (Relative Luminous Flux) is about 162%, but if the average current is controlled to 1080mA using digital PWM dimming, it will get 83.3%. Relative illuminance, rather than the relative illuminance of about 110% of the original characteristic.

Regarding the technology of LED driver, there have been many studies before: some literatures propose to connect a linear current regulator circuit (Linear Current Regulator) to each group of LEDs, and add a feedforward stabilizer The efficiency of the circuit is improved by means of feed-forward voltage regulation and feedback, but the efficiency improvement effect is not good. The efficiency of this technology is greatly affected by the consistency of characteristics between LEDs in the same string; control the output voltage at the highest forward voltage value, make the voltage difference of one group of LEDs approach zero to reduce losses, and adopt phase shift pulse width modulation (Phase Shift Pulse Width Modulation) , PSPWM) to limit the change of output current between one group of LED current and the maximum current to reduce the electromagnetic interference of high pulsating current caused by general pulse width modulation control; there are also literatures that digital PWM dimming, dual After the dimming technologies such as pulse width dimming and threshold voltage dimming are implemented, they can be compared. After the comparison, it can be seen that the threshold voltage dimming has better efficiency, but the linearity of the dimming curve is caused by the slow response speed. Poor; some literatures propose to use an analog circuit to detect the voltage between the gate and the drain to realize the adaptive adjustment of the output voltage, thereby improving the efficiency; some literatures propose to change the current detection resistance and reference voltage of the linear current-stabilizing circuit value and set the forward current of the seven-segment LED, and use digital PWM dimming to operate the LED at seven different forward current values. When low-light dimming, operate at low forward current, and when high-light dimming, operate at high Forward current, thereby improving the luminous efficiency of LEDs; there are also literatures that propose Multilevel Pulse Width Modulation (MPWM) dimming method to improve the problem of low luminous efficiency of digital PWM dimming; there are also literatures using Single Inductor Multiple Floating Output (SIMFO) improves the low dimming frequency problem of Single Inductor Multiple Output (SIMO), and proposes Average Current Correetion (ACC) to improve multi-string LEDs The current deviation caused by different forward conduction voltages; there are also literatures that use a half-bridge non-resonant converter as an LED drive circuit, so that the circuit switch can achieve soft switching to improve circuit efficiency. There are also literatures that use the synchronous integration method to achieve output voltage adaptation to improve efficiency, and propose to change the dimming frequency to solve the problem of poor linearity of the circuit's low-illuminance dimming curve; more literature uses adaptive current control to forward multiple strings of LEDs. The current is controlled at the maximum forward current to reduce the conduction loss of the switch.

However, the light conversion efficiency of the prior art solutions is still insufficient. Therefore, there is an urgent need for a novel LED driving system in the art.

The main purpose of the present invention is to disclose an LED driving system with high light conversion efficiency, It can improve the luminous efficiency of the LED module through a multi-stage PWM current modulation scheme.

In order to achieve the aforementioned purpose, an LED driving system with high light conversion efficiency is proposed. is coupled with a first switch control signal, a second switch control signal, a third switch control signal and a fourth switch control signal; a first output terminal is connected to the positive terminal when the first switch presents an active potential and the second switch is coupled to the negative terminal when the second switch presents an active potential; and a second output terminal is coupled to the positive terminal when the third switch presents an active potential and the fourth switch presents an active potential is coupled to the negative terminal; a capacitor-inductor series circuit, one end of which is coupled to the first output terminal of the full-bridge switching circuit; a transformer, which has a main coil and a secondary coil, the main coil is connected in parallel with a magnetizing inductor, one end of which is coupled to the other end of the capacitor-inductor series circuit, and the other end is coupled to the second output end of the full-bridge switch circuit, the secondary coil has a a first output terminal and a second output terminal; a first diode with a first anode and a first cathode, the first anode is coupled to a voltage reference terminal, the first cathode is connected to the secondary The first output terminal of the stage coil is coupled; a second diode has a second anode and a second cathode, the second anode is coupled to the voltage reference terminal, and the second cathode is connected to the secondary The second output terminal of the primary coil is coupled; a first inductor is coupled between the first output terminal of the secondary coil and a voltage output terminal; a second inductor is coupled to the secondary coil between the second output terminal and the voltage output terminal; an output capacitor coupled between the voltage output terminal and the voltage reference terminal; an LED load coupled between the voltage output terminal and the voltage reference terminal , is composed of an LED module, a voltage regulator circuit, a set of current sources and a current sensor connected in series, wherein the voltage regulator circuit is used to generate a fixed voltage to bias the set of current sources, the set of The current source has five gated constant current sources I ₁ , I ₂ , I ₃ , I ₄ and I ₅ from small to large, the five gated constant current sources are controlled by five gating signals. a current output, and the collection current of the group of current sources flows through the current sensor to generate a current feedback signal; a voltage feedback circuit is used for generating a voltage feedback signal according to a voltage across the output capacitor; a The control unit is used for executing a firmware program to perform a dimming operation according to a dimming command current so that an average current of the sink current is equal to the dimming command current, and the dimming operation includes: according to the voltage feedback signal and the A first negative feedback control calculation procedure is performed on the difference between the reference voltages to generate a first switch signal, a second switch signal, a third switch signal and a fourth switch signal; and in five current intervals (0 , I ₁ ], (I ₁ , I ₂ ], (I ₂ , I ₃ ], (I ₃ , I ₄ ] and (I ₄ , I ₅ ], find the interval to which one of the dimming command currents belongs, so that Can combine five PWM signals (PWM1), (PWM1, One of PWM2), (PWM2, PWM3), (PWM3, PWM4), (PWM4, PWM5) selects a combination of PWM signals, and performs a second negation according to the difference between the current feedback signal and the dimming command current. The feedback control calculation program generates a duty cycle D to determine the pulse width of each PWM signal in the selected PWM signal combination, D is a non-negative real number not greater than 1; and a gate driver for the first switch signal, the second switch signal, the third switch signal and the fourth switch signal generate the first switch control signal, the second switch control signal, the third switch control signal and the fourth switch control signal, and according to Five of the PWM signals generate five of the gating signals.

In one embodiment, the voltage feedback circuit includes a voltage divider circuit and an optical coupling circuit.

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

In one embodiment, the control unit includes a filter operation function module to perform 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 correspondingly .

In one embodiment, the control unit includes a proportional-integral-derivative operation function module for executing the first negative feedback control calculation program on the third input digital signal, and executing the first negative feedback control calculation program on the fourth input digital signal. Two negative feedback control calculation procedures.

In one embodiment, the control unit includes a pulse width modulation module to generate the first switch signal, the second switch signal, the third switch signal, the fourth switch signal and five of the PWM signals .

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: Full bridge switch circuit

110: Capacitor-Inductor Series Circuit

120: Transformer

130: First diode

140: Second diode

150: first inductance

160: The second inductor

170: output capacitor

180: LED load

181: LED module

182: Amplifier

183: NMOS transistor

184: Current source

184a1: NMOS transistor

184a2: Resistor

184b1: NMOS transistor

184b2: Resistor

184c1: NMOS transistor

184c2: Resistor

184d1: NMOS transistor

184d2: Resistor

184e1: NMOS transistor

184e2: Resistor

185: Current sensor

190: Voltage feedback circuit

191: Voltage divider circuit

192: Optical coupling circuit

200: Control Unit

201: Analog to Digital Converters

202: Filter operation function module

203: Proportional-Integral-Derivative Operation Function Module

204: PWM module

210: Gate driver

FIG. 1 shows the relative illuminance, current and temperature curves of a conventional LED.

FIG. 2 is a block diagram illustrating an embodiment of the high light conversion efficiency LED driving system of the present invention.

FIG. 3 shows the topology of a phase-shifted full-bridge power stage circuit.

FIG. 4 illustrates the main operating point waveforms of the phase-shifted full-bridge power stage circuit of FIG. 3 .

FIG. 5 illustrates a linear current-stabilizing structure adopted by the present invention.

FIG. 6 shows the forward current of a conventional digital PWM dimming LED.

7a-7b respectively illustrate the first stage dimming and the second stage dimming of the forward current of the multi-stage current dimming LED used in the present invention.

FIG. 8 is a comparison diagram of the illuminance-current curve of a conventional digital PWM dimming and two-stage current dimming.

FIG. 9 shows a dimming curve of the five-stage current dimming method of the present invention.

FIG. 10 shows a linear current stabilizing circuit structure adopted by the five-stage current dimming method of the present invention.

FIG. 11 is a flowchart illustrating an embodiment of the system firmware of the present invention.

FIG. 12 is a flowchart of an ADC interrupt subroutine of the system firmware of FIG. 11 .

FIG. 13 is a flowchart of the five-stage current dimming of the system firmware of FIG. 11 .

FIG. 14 is a flowchart of the incremental PID control of the ADC interrupt subroutine of FIG. 12 .

15a-15c illustrate different output current waveforms of a conventional digital PWM dimming scheme under different duty cycles (10%, 50%, 100%).

16a-16c illustrate different output current waveforms of the LED driving system with high light conversion efficiency of the present invention under different duty cycles (10%, 50%, 100%).

FIG. 17 is a graph showing the comparison of the photoelectric conversion efficiency between the conventional digital PWM dimming scheme and the five-stage current dimming scheme of the present invention.

FIG. 18 is a schematic diagram showing the illuminance-current curve diagram and the calculation method of the improvement effect of the conventional digital PWM dimming scheme and the five-stage current dimming scheme of the present invention.

Please refer to FIG. 2 , which shows a block diagram of an embodiment of the LED driving system with high light conversion efficiency of the present invention.

As shown in FIG. 2 , the LED driving system with high light conversion efficiency of the present application includes a full-bridge switching circuit 100 , a capacitor-inductor series circuit 110 , a transformer 120 , a first diode 130 , and a second diode 140 , a first inductor 150 , a second inductor 160 , an output capacitor 170 , an LED load 180 , a voltage feedback circuit 190 , a control unit 200 and a gate driver 210 .

The full-bridge switch circuit 100 has two input terminals A and B coupled to the positive and negative terminals of an input voltage _Vin , and four control terminals respectively connected to a first switch control signal S ₁ and a second switch control signal S _2. A _third switch control signal S3 and a _fourth switch control signal S4 are coupled, a first output terminal C is coupled to the positive terminal when the _first switch S1 exhibits an active potential, and the second The switch S2 is coupled to the negative terminal when the switch S2 presents an active potential, and a _second output terminal D is coupled to the positive terminal when the _third switch S3 presents an active potential and the _fourth switch S4 presents an active It is coupled to the negative terminal when the potential is present.

Among them, the full-bridge phase-shift converter has two more main power switches S ₃ and S ₄ than the half-bridge phase-shift converter, which can improve the output power capability.

One end of a capacitor-inductor series circuit 110 is coupled to the first output end C of the full-bridge switch circuit 100 .

A transformer 120 has a main coil and a secondary coil, the main coil is connected in parallel with a magnetizing inductance, one end of which is coupled to the other end of the capacitor-inductor series circuit 110, and the other end is connected to the full-bridge switch The second output terminal of the circuit 100 is coupled to D, and the secondary coil has a first output terminal E and a second output terminal F.

A first diode 130 has a first anode and a first cathode, the first anode is coupled to a voltage reference terminal, and the first cathode is coupled to the first output terminal E.

A second diode 140 has a second anode and a second cathode, the second anode is coupled to the voltage reference terminal, and the second cathode is coupled to the second output terminal F.

A first inductor 150 is coupled between the first output terminal E and a voltage output terminal O.

A second inductor 160 is coupled between the second output terminal F and the voltage output terminal O.

An output capacitor 170 is coupled between the voltage output terminal O and the voltage reference terminal.

An LED load 180 is coupled between the voltage output terminal O and the voltage reference terminal, and is composed of an LED module 181 and a voltage regulator circuit (a negative feedback circuit composed of an amplifier 182 and an NMOS transistor 183 ) , a group of current sources 184 and a current sensor 185 are connected in series, wherein the voltage regulator circuit is used to generate a fixed voltage V _r to bias the group of current sources 184, the group of current sources 184 has five Small to large gated constant current sources I ₁ (formed by NMOS transistor 184a1 and resistor 184a2 in series), I ₂ (formed by NMOS transistor 184b1 and resistor 184b2 in series), I ₃ (formed by NMOS transistor 184c1 and resistor 184c2 in series), I ₄ (composed of NMOS transistor 184d1 and resistor 184d2 in series) and I ₅ (composed of NMOS transistor 184e1 and resistor 184e2 in series), the five gated constant current sources The current output is controlled according to five gating signals VG ₁ -VG ₅ , and the sink current i _out of the group of current sources flows through the current sensor 185 to generate a current feedback signal I _FB .

A voltage feedback circuit 190 includes a voltage dividing circuit 191 and an optical coupling circuit 192 for generating a voltage feedback signal V _FB according to a voltage V _out of the output capacitor 170 .

A control unit 200 stores a firmware program for executing the firmware program to perform a dimming operation according to a dimming command current C _DIM so that an average current of the sink current i _out is equal to the dimming command current C _DIM , The dimming operation includes: performing a first negative feedback control calculation program according to the difference between the voltage feedback signal V _FB and a reference voltage (not shown in the figure) to generate a first switch signal and a second switch signal, a third switch signal and a fourth switch signal; and in five current intervals (0, I ₁ ], (I ₁ , I ₂ ], (I ₂ , I ₃ ], (I ₃ , I ₄ ] and (I ₄ , I ₅ ] to find out the interval to which one of the dimming command currents C _DIM belongs, so as to enable five PWM signal combinations (PWM1), (PWM1, PWM2), (PWM2, PWM3), (PWM3, PWM4 ), (PWM4, PWM5), select a combination of PWM signals, and perform a second negative feedback control calculation program according to the difference between the current feedback signal I _FB and the dimming command current C _DIM to generate a duty The period D is used to determine the pulse width of each PWM signal in the selected PWM signal combination, and D is a non-negative real number not greater than 1.

The control unit 200 includes an analog-to-digital converter 201, a filter operation function module 202, a proportional-integral-derivative operation function module 203 and a pulse width modulation module 204, wherein the analog-to-digital conversion The device 201 is used to perform an analog-to-digital conversion operation on the voltage feedback signal V _FB and the current feedback signal I _FB to correspondingly generate a first input digital signal and a second input digital signal; the filtering operation function module 202 is used for filtering the first input digital signal and the second input digital signal to generate a third input digital signal and a fourth input digital signal correspondingly; the proportional-integral-derivative operation function module 203 is for executing the first negative feedback control calculation program on the third input digital signal, and executing the second negative feedback control calculation program on the fourth input digital signal; and the pulse width modulation module 204 is for generating the first switch signal, the second switch signal, the third switch signal, the fourth switch signal and the five PWM signals.

The gate driver 210 is used for generating the first switch control signal S ₁ , the second switch control signal S 2 , the second switch control signal S ₂ , and the The third switch control signal S ₃ and the fourth switch control signal S ₄ , and the five gate control signals VG ₁ -VG ₅ are generated according to the five PWM signals.

The principle of the present invention will be described below:

1. LED driver system architecture

In this embodiment, the LED module 181 is formed by 10 groups of LED lamps in parallel, and the control unit 200 is realized by a TMS320E280049C digital signal processor introduced by Texas Instruments.

The power stage circuit topology of the LED driver adopts a phase-shift full-bridge converter (Phase Shift Eull Bridge Converter) with soft switching characteristics. As shown in Figure 3, the main components on the primary side are power switches S ₁ , S ₂ , S ₃ , S ₄ and resonant inductance L _r and blocking capacitor C _b for blocking DC to avoid magnetic flux imbalance of transformer core, the secondary side is the multiple of output diodes D ₁ , D ₂ and output inductances L ₁ , L ₂ Stream output architecture and output capacitor C _o . Figure 4 shows the control signals of the four power switches of the phase-shifted full-bridge converter and the corresponding theoretical waveforms of the primary side current i _p and the transformer primary side voltage V _AB , from Figure 4 it can be seen that the upper and lower bridge switching signals of the same arm There is obviously a dead time interval, and this dead time interval can make the parasitic capacitance and resonant inductance of the power switch complete the resonance to achieve zero-voltage switching. It can be seen from the corresponding current waveforms that there are two nonlinear resonance intervals t ₁ ~t ₂ and t ₃ ~t ₄ before the zero-voltage switching of S ₁ and S ₄ . Table 1 is the circuit design specification of the phase-shift full-bridge converter used in the present invention. The output voltage is based on the voltage-current relationship of the selected CREE CXA2540 LED. When the temperature is 25°C and the current is 1100mA, the cross-voltage is about 37V, plus the resistance of the linear current-stabilizing circuit and the voltage drop of the power switch drain-source, so the output voltage is set to 40V, and the input voltage is set to 200V. In this case, ten CXA2540 LED modules are connected in parallel, and the maximum output current is 11A .

2. Dimming principle and illuminance measurement environment setting

2.1 Digital PWM dimming

The traditional digital PWM dimming 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. According to previous literature research, the working frequency is generally set between 100 and 400 Hz. In this case, 200 Hz is used as the PWM dimming frequency. When driving the LED, it will be driven with a constant current, which can not only stabilize the on-current of the LED, but also improve the internal resistance of the LED due to the negative temperature coefficient. situation. The linear current stabilization structure is composed of an operational amplifier and a field effect transistor. As shown in Figure 5, the field effect transistor is operated in the linear region, and 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 dimming function.

2.2 The proposed multi-stage current dimming method

I ₁)時，如同數位PWM調光，LED順向電流會在I₁與零之間作切換，並藉由改變其責任週期進行調光，如圖7a所示，D₁越大照度越高；第二段(I ₁<I

I ₂)時，LED順向電流會在I₂與I₁之間作切換，利用改變I₂與I₁之間的責任週期比例進行調光，如圖7b所示，其中D_dim1=1-D_dim2，D_dim2越大照度越高。數位PWM調光與二段式電流調光之照度-電流曲線如圖8所示，由圖8可知二段式電流調光之發光效率較佳。 The LED forward current of traditional digital PWM dimming will switch between the rated forward current and zero, and by changing its PWM duty cycle to obtain different average current values for dimming, as shown in Figure 6, where D _dim The bigger the illumination, the higher the illumination. The dimming of the proposed multi-stage current dimming method will be divided into multiple stages. Taking the two-stage dimming as an example, the first stage (0<I

I ₁ ), like digital PWM dimming, the LED forward current will switch between I ₁ and zero, and the dimming will be performed by changing its duty cycle, as shown in Figure 7a, the greater the D ₁ , the higher the illuminance ; second segment ( I ₁ < I

When I ₂ ), the LED forward current will switch between I ₂ and I ₁ , and the dimming is performed by changing the duty cycle ratio between I ₂ and I ₁ , as shown in Figure 7b, where D _dim1 =1- D _dim2 , the larger the D _dim2 , the higher the illuminance. The illuminance-current curves of the digital PWM dimming and the two-stage current dimming are shown in FIG. 8 . It can be seen from FIG. 8 that the luminous efficiency of the two-stage current dimming is better.

2.2.1 The derivation of the current value of each stage of multi-stage dimming

As shown in FIG. 9 , the present invention actually uses five-stage current dimming, and defines the LED forward current as I ₁ to I ₅ from small to large. Before determining the LED forward current, it is assumed that the LED illuminance curve is a The quadratic curve of the zero-crossing point and define the illuminance L _n as

If the area enclosed by the five forward currents is defined as A, it can be seen from Figure 9 that A can be divided into one triangle and four trapezoids, then the area A can be calculated as

And by calculating L ₁ to L ₅ according to formula (1) and substituting them into formula (2), the area A can be obtained as

Then make partial differentiation of I ₁ to I ₅ by formula (3), and set the result of partial differentiation equal to zero to find out the value of each segmental current that maximizes the area A. The relationship between LED forward current is

As shown in Figure 10, the structure of the linear current stabilizing circuit in this case is that 10 CXA2540 LED modules share 5 current detection resistors. Table 2 shows the electrical specifications of CXA2540. The total current value of a CXA2540 is 11A, so in this case, the maximum current value I5 is set to 11A, and substituting into equation (4), the required _five -stage currents _I1 to I5 are 2.2A, _4.4A , 6.6A, 8.8A and 11A.

2.2.2 Design of Current Sense Resistor

In Fig. 10, the reference voltage V _r is originally set to be 3.3V, and the required five-stage currents I ₁ to I ₅ derived from the previous section are 2.2A, 4.4A, 6.6A, 8.8A, and 11A, respectively. R ₅ =0.3Ω, R ₄ =0.375Ω, R ₃ =0.5Ω, R ₂ =0.75Ω, R ₁ =1.5Ω, because the current flowing through the above resistors is very large, so the current detection resistor in this case It is realized by using cement resistors with a large error range in series and parallel, and this also causes the actual five-stage forward current value in this case to be deviated. The actual value of R ₅ =0.383Ω, R ₄ = 0.422Ω, R ₃ =0.523Ω, R ₂ =0.78Ω, R ₁ =1.41Ω, in order to adjust the maximum forward current value I ₅ to be close to 11A, V _r is adjusted to 4V, and the actual measured corrections The subsequent forward current values I ₁ to I ₅ are 2.9A, 5.3A, 7.5A, 9.3A, and 10.5A, respectively.

2.2.3 Selection of Current Stages

In this case, the luminous efficiency improvement percentage [η _n (%)] is defined as

Among them, An _is the area enclosed by the illuminance-current curve of the multi-stage current dimming method, A _PWM is the area enclosed by the illuminance-current curve of the digital PWM dimming method, and A _LED is the LED illuminance-current curve. The area enclosed, and the actual value of the 25 ℃ illuminance-current curve in Figure 1 is substituted into the formula (1), and the simultaneous equations are solved to obtain the 25 ℃ illuminance-current curve equation:

Then, the formula (6) is integrated from 0 to the maximum current of 1100mA to obtain A _LED of 66.7, and the triangle area A _PWM surrounded by the illuminance-current curve of the digital PWM dimming method can be calculated as 61.33, then according to the segmented current derivation method mentioned above, deduce the current value of each segment of the two-segment to six-segment current dimming method, and substitute it into the formula (3) to get the current of each segment. The corresponding illuminance values, and then calculate the areas A ₂ to A ₆ enclosed by the illuminance-current curves of the two-stage to six-stage current dimming methods are 65.36, 66.1, 66.36, 66.49, 66.55, and finally A _n , A _PWM and A _LED are substituted into formula (5), the luminous efficiency improvement percentages η ₂ to η ₆ of the two-stage to six-stage current dimming methods are 6%, 7.15%, 7.54%, 7.74%, 7.83%. From these data, it can be seen that the higher the number of segments, the higher the percentage of luminous efficiency improvement, but as the number of segments increases, the improvement effect will become less obvious, and the light from the fifth segment to the sixth segment will emit light. The efficiency improvement percentage is less than 0.1%, so the five-stage current dimming method is used in this case.

2.3 Illumination measurement environment settings

When measuring illuminance, in order to avoid being affected by other ambient light sources, a wooden box with a length of 55 cm, a width of 35 cm, and a height of 30 cm is selected as the test space, and there is a 4 cm square hole on the side for wiring, and the measurement When measuring, seal the gap with tape to make it opaque. due to 10 The maximum brightness of CXA2540 exceeds 50830 lumens, so the illuminance meter is set on the side of the box (the illuminance meter used in the present invention is TES-1339R) to prevent the illuminance value from exceeding the measuring range of this illuminance meter.

3. Controller firmware design

In this case, the TMS320F280049C digital signal processor introduced by Texas Instruments is used as the digital controller for the LED driver and the five-stage current dimming control. The system architecture diagram is shown in Figure 2. The output voltage and current are sampled by the sampling circuit, the signal is sent to a digital filter for filtering through analog/digital conversion, and then the filtering result is calculated by PID compensation. Finally, the PWM module outputs the PWM signal to the power switch to control the phase shift A full-bridge converter and a linear current-stabilizing circuit can achieve digital control. The overall firmware program flow is shown in Figure 11. The program can be divided into two parts: the main program and the analog-to-digital converter interrupt. When the program starts, first declare the variables required by the program, initialize the core modules such as the system CLK, set the ADC and GPIO modules, then set and enable the PWM module, and then enter an infinite loop to execute dimming subroutine and wait for the ADC interrupt to occur.

ADC interrupt sub-program flow: The ADC interrupt program flow is shown in Figure 12. After entering the ADC interrupt, the analog-to-digital converter is first used to sample V _out , and the sampled signal is sent to the digital filter for processing to prevent Influenced by high-frequency noise, after entering the phase shift mode, input the error amount to the PID controller to calculate the PWM phase shift amount, and then limit the upper and lower limits of the phase shift amount calculated by the PID control, so that the phase shift amount will not exceed the setting. range, and then update the phase shift amount of the PWM module, and clear the ADC interrupt flag and return to the main program to enter an infinite loop, wait for the next ADC interrupt to occur, and repeat the above actions to achieve the stability of the phase-shift full-bridge converter. pressure control.

Five-stage current dimming program flow: use the PWM module to generate five pulse signals with a frequency of 200Hz, and the pulse signals used to control I ₄ and I ₂ must be controlled with the pulse waves of I ₁ , I ₃ and I ₅ The signals are complementary, and the reference voltage V _r is provided by the power supply. The five control currents composed of five current detection resistors are defined as I ₁ to I ₅ from small to large, and their corresponding duty cycles are defined as D _dim1 to D _{dim5 respectively} , and then returned through the current sensor The reading value of the ADC module is used to judge the current size, so as to determine how to adjust D _dim1 to D _dim5 . The control flow chart of the dimming program is shown in Figure 13.

Incremental PID controller: The incremental PID control law can be expressed as:

Wherein K _P is the proportional gain, the integral gain T _i =K _P /K _I , the differential gain K _D =K _P .T _d , T _d is the sampling period, e(n) is the current error of the system, e(n-1 ) is the previous error of the system, and n is the sampling signal point. The control increment △ un can be expressed as △ un = un - un -1 (8)

From equation (7), we can get △ un = K _P en - en -1+ K _I . en + K _D en -2 e n -1+ en -2 (9)

Let A = en - en -1, B = en , C=en -2 e n -1+ en -2, then equation (9) can be rewritten as △ un = A. K _P + B. K _I + C. K _D (10)

The incremental PID program flow is shown in Figure 14. The output command value is subtracted from the sampled value to obtain an error amount e(n), which is then combined with the previous error amount e(n-1) and the previous two error amounts Substitute e(n-2) into Equation (9) together for operation, A, B and C can be obtained, and then multiplied by K _P , K _I and K _D in sequence and added to obtain the output variation Δ u , PID output The result PID _out is equal to the phase shift amount minus △ u , and then compares it with the upper and lower limits of the phase shift amount. If the output result is less than the lower limit phase shift amount or greater than the upper limit phase shift amount, the output result is equal to the lower limit or upper limit value respectively, and finally The result is output to the PWM generator.

4. Experimental results and analysis

4.1 PWM dimming measurement

The control group compared in this case uses the digital PWM control mode to drive the LED, and tests the input power and illuminance values at intervals of 10% duty cycle. Figures 15a-15c show the digital PWM dimming duty cycle of 10%, 50% and 100% respectively. Waveform diagram, the upper part is the voltage waveform of the dimming command, and the lower part is the current flowing through the LED; the experimental data of each dimming command in Table 3, the subsequent measurement of the five-stage dimming method will be based on the illuminance in Table 3. for a fair comparison.

4.2 Five-stage current dimming measurement

Figures 16a-16c are waveform diagrams of the five-stage current dimming method at different equivalent dimming levels, the upper part is the voltage waveform of the dimming command, and the lower part is the LED forward current waveform. Table 4 shows the experimental data of each equivalent illuminance. The output power of the converter is tested with the equivalent illuminance of each 10% dimming command of the digital PWM dimming method as the interval. Table 5 is the experimental data of each equivalent output power. The illuminance value is tested with the output power of each 10% dimming command of the digital PWM dimming method as the interval.

4.3 Comparison of Photoelectric Conversion Efficiency

In this section, according to the results in Table 3, Table 4, and Table 5, the illuminance curves and photoelectric conversion efficiency comparison diagrams of digital PWM dimming and five-stage current dimming are drawn. FIG. 17 is a comparison diagram of photoelectric conversion efficiency between digital PWM dimming and five-stage current dimming. Figure 18 is a schematic diagram of the illuminance-current curve diagram and the calculation method of the improvement effect. At the same illuminance, the power difference between point A and point B divided by the power value of point B is the percentage of reduction (improvement) of the output power of the LED drive circuit; When the LED drive circuit outputs power, the illuminance difference between point C and point D divided by the illuminance value of point D is the illuminance improvement (improvement) percentage. Table 6 shows the improvement effect of the five-stage current dimming method in the full control section.

V. Conclusion

This case proposes a multi-stage current dimming method with high luminous efficiency, and successfully implemented it in a digital signal processor, so that the LED drive circuit can increase the illuminance through a control strategy without increasing the output power, and then based on the actual to verify its correctness and feasibility. This multi-stage dimming method uses the conversion of the current detection resistor of the linear current stabilizer circuit to set the multi-stage forward current value, and uses the duty cycle ratio of the dimming command to control the upper and lower currents to adjust the average value of the LED forward current to achieve illuminance. The multi-stage current dimming method can make the LED illuminance-current curve close to its original light source characteristics, so its luminous efficiency will be higher than that of the digital PWM dimming method that makes the LED illuminance-current curve tend to be linear.

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

1. The LED driving system with high light conversion efficiency of the present invention can improve the luminous efficiency of the LED module through a multi-stage PWM current modulation scheme.

2. In the implementation of the 480W LED driving system with high light conversion efficiency of the present invention, compared with the general digital PWM dimming method: under the condition of the same illumination, the output power of the driving circuit is reduced by 8.2% on average; With the same output power, the illuminance is increased by 8.9% on average.

What is disclosed in this case is a preferred embodiment, and any partial changes or modifications that originate from the technical ideas of this case and are easily inferred by those who are familiar with the art are within the scope of the patent right of this case.

To sum up, regardless of the purpose, means and effect of this case, it shows that it is completely different from the conventional technology, and its first invention is suitable for practical use, and it does meet the patent requirements of the invention. Society is to pray for the best.

100: Full bridge switch circuit

110: Capacitor-Inductor Series Circuit

120: Transformer

130: First diode

140: Second diode

150: first inductance

160: The second inductor

170: output capacitor

180: LED load

181: LED module

182: Amplifier

183: NMOS transistor

184: Current source

184a1: NMOS transistor

184a2: Resistor

184b1: NMOS transistor

184b2: Resistor

184c1: NMOS transistor

184c2: Resistor

184d1: NMOS transistor

184d2: Resistor

184e1: NMOS transistor

184e2: Resistor

185: Current sensor

190: Voltage feedback circuit

191: Voltage divider circuit

192: Optical coupling circuit

200: Control Unit

201: Analog to Digital Converters

202: Filter operation function module

203: Proportional-Integral-Derivative Operation Function Module

204: PWM module

210: Gate driver

Claims (6)

An LED driving system with high light conversion efficiency, comprising: a full-bridge switch circuit, having: two input terminals to be coupled to positive and negative terminals of an input voltage, and four control terminals to be respectively connected to a first switch control signal, A second switch control signal, a third switch control signal and a fourth switch control signal are coupled; a first output terminal is coupled to the positive terminal when the first switch presents an active potential and the second switch presents A second output terminal is coupled to the positive terminal when the third switch presents an active potential and is coupled to the negative terminal when the fourth switch presents an active potential; A capacitor-inductor series circuit, one end of which is coupled to the first output end of the full-bridge switching circuit; a transformer, which has a main coil and a secondary coil, the main coil is connected in parallel with a magnetizing inductance and one end of which is connected is coupled to the other end of the capacitor-inductor series circuit, and the other end is coupled to the second output end of the full-bridge switch circuit, the secondary coil has a first output end and a second output end output terminal; a first diode with a first anode and a first cathode, the first anode is coupled to a voltage reference terminal, the first cathode is connected to the first output terminal of the secondary coil coupling; a second diode with a second anode and a second cathode, the second anode is coupled with the voltage reference terminal, the second cathode is with the second output terminal of the secondary coil coupling; a first inductor coupled between the first output end of the secondary coil and a voltage output end; a second inductor coupled between the second output end of the secondary coil and the voltage between the output terminals; an output capacitor coupled between the voltage output terminal and the voltage reference terminal; an LED load coupled between the voltage output terminal and the voltage reference terminal, which is composed of an LED module, A voltage-stabilizing circuit, a group of current sources and a current sensor are connected in series, wherein the voltage-stabilizing circuit is used to generate a fixed voltage to bias the group of current sources, and the group of current sources has five current sources ranging from small to Large gating constant current sources I ₁ , I ₂ , I ₃ , I ₄ and I ₅ , the five gating constant current sources control their current outputs according to five gating signals, and the group of current sources The collected current flows through the current sensor to generate a current feedback signal; a voltage feedback circuit is used to generate a voltage feedback signal according to a voltage across the output capacitor; a control unit is used to execute a flexible The system is programmed to perform a dimming operation according to a dimming command current so that an average current of the sink current is equal to the dimming command current, and the dimming operation includes: performing a dimming operation according to the difference between the voltage feedback signal and a reference voltage the first negative feedback control calculation program to generate a first switch signal, a second switch signal, a third switch signal and a fourth switch signal; and in the five current intervals (0, I ₁ ], (I ₁ , I ₂ ], (I ₂ , I ₃ ], (I ₃ , I ₄ ] and (I ₄ , I ₅ ], find the interval to which one of the dimming command currents belongs, so as to enable five PWM signal combinations (PWM1 ), (PWM1 , PWM2), (PWM2, PWM3), (PWM3, PWM4), (PWM4, PWM5) one of the selected PWM signal combination, and according to the difference between the current feedback signal and the dimming command current to perform a second Negative feedback control calculation program to generate a duty cycle D to determine the pulse width of each PWM signal in the selected PWM signal combination, D is a non-negative real number not greater than 1; and a gate driver for according to the first The switch signal, the second switch signal, the third switch signal and the fourth switch signal generate the first switch control signal, the second switch control signal, the third switch control signal and the fourth switch control signal, and The five gate control signals are generated according to the five PWM signals. The LED driving system with high light conversion efficiency as described in claim 1, wherein the voltage feedback circuit comprises a voltage divider circuit and an optical coupling circuit. The LED driving system with high light conversion efficiency as described in claim 1, wherein the control unit comprises an analog-to-digital converter for performing analog-to-digital conversion operations on the voltage feedback signal and the current feedback signal to correspond to A first input digital signal and a second input digital signal are generated. The LED driving system with high light conversion efficiency as described in claim 3, wherein the control unit includes a filter operation function module for performing filter operation on the first input digital signal and the second input digital signal to generate corresponding A third input digital signal and a fourth input digital signal. The LED driving system with high light conversion efficiency as described in claim 4, wherein the control unit comprises a proportional-integral-derivative operation function module to perform the first negative feedback control calculation on the third input digital signal a program, and executing the second negative feedback control calculation program on the fourth input digital signal. The LED driving system with high light conversion efficiency as described in claim 1, wherein the control unit comprises a pulse width modulation module to generate the first switching signal, the second switching signal, and the third switching signal , the fourth switch signal and the five PWM signals.

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