Design of LED lighting drive circuit based on SA7527

With the development of society, people are increasingly advocating green lighting. As one of them, LED fluorescent lamps are being widely used. Compared with ordinary fluorescent lamps, LED fluorescent lamps have the characteristics of energy saving, long life, and good applicability. Because the size of a single LED is small, it can be made into any shape, has the advantages of short response time, environmental protection, no harmful metals, easy recycling of waste gas, bright colors, and pure luminous colors. This article designs an LED fluorescent lamp drive circuit through SA7527, which has good stability and reliability. It can not only reduce the cost of fluorescent lamps and improve its conversion efficiency, but also achieve constant current and constant voltage output, and can drive LEDs of different powers at the same time .

1. Circuit Design

1. Circuit composition

The entire circuit consists of surge protection, EMI filtering, full-bridge rectification, flyback converter, PWM LED driver controller , and closed-loop feedback circuit, as shown in Figure 1.

Figure 1 LED driver circuit based on SA7527 Block diagram

2. Main circuit analysis

The main circuit is shown in Figure 2. From the perspective of AC220V, the AC mains inlet is connected to a fuse F1 and a surge-resistant varistor RV1. The fuse plays the role of overcurrent protection for the line input circuit, and the varistor RV1 is used to suppress the instantaneous high voltage from the power grid to protect the safety of the input line. Then there is an EMI filter. L1, L2, and C1 are common-mode filters, L3, L4, and C2 are differential-mode filters, DB107 is a full-bridge rectifier circuit, and C13 is a capacitor filter. The voltage (current) after rectification is still a pulsed DC. In order to reduce fluctuations, a filter is usually added. The RCD snubber circuit composed of R19, C8, and D5 is to prevent the power tube Q1 from being subjected to a large reverse voltage during the shutdown process. The diode of the snubber circuit is generally a fast recovery diode.

The output filter C10, C11, C12 are connected in parallel to reduce voltage ripple .

Features of this circuit: (1) Wide voltage input range; (2) Constant current/constant voltage characteristics; (3) Output feedback sampling and constant current/constant voltage control circuit composed of LM358, low cost, high control accuracy, and simple debugging; (4) This circuit can drive LEDs of different power.

3. Design of starting circuit

The startup circuit is shown in Figure 2. In order to start the circuit normally, a startup resistor R20 should be connected between the primary coil of the transformer after rectification by the rectifier bridge and the supply voltage terminal 8 pin of SA7527, and a startup capacitor C9 should be connected between the 8 pin and the ground. When the power is turned on , the current flowing through the startup resistor R20 charges the startup capacitor C9. When the charging voltage of C9 reaches the startup threshold voltage (typical value is 11.5V), SA7527 is turned on and drives the power tube Q1 to start working. The maximum and minimum values ​​of the rectified voltage are represented by U imax and U imin respectively, I STmax is the maximum startup current, V th (st) max is the maximum startup threshold voltage, and the startup resistor R20 is determined by the following formulas (1) and (2). The resistor should be a power resistor, and the maximum power consumption cannot exceed 1W.

Figure 2 Main circuit and starting circuit

The starting capacitor C9 should be determined by the following formula:

Where, I dcc is the dynamic working current; f ac is the AC grid frequency; HY (ST) is the undervoltage lockout hysteresis voltage.

4. Design of control circuit

4.1 Chip Introduction

SA7527 is a simple and efficient power factor correction chip. This circuit is suitable for electronic ballasts and high-density power supplies that require small size, low power consumption, and few peripheral devices.

4.2 Analysis of control methods

The control circuit is shown in Figure 3. This control circuit is a peak current control mode. When the power tube Q1 is turned on, the diodes D6 and D7 are turned off, and the primary inductor current of the transformer T1 rises linearly. When the current rises to the multiplier output current reference, the power tube Q1 is turned off; when the power tube Q1 is turned off, the diodes D6 and D7 are turned on, and the inductor current starts to decrease linearly from the peak value. Once the inductor current drops to zero, it is detected by the zero current detection resistor, and the power tube Q1 is turned on again, starting a new switching cycle, and repeating this process.

Figure 3 Control circuit

4.3 Design of zero current detection resistor

The zero current detection terminal peripheral circuit is shown in Figure 4. The MOSFET power tube is turned on using the zero current detector and is turned off when the peak inductor current reaches the threshold level set by the multiplier output.

Figure 4 Peripheral circuit of zero current detection terminal

Once the inductor current drops to zero level along the downward slope, the zero current detector of SA7527 detects by reversing the polarity of the transformer secondary winding voltage connected to pin 5, and pin 7 of SA7527 generates an output to drive the MOSFET power tube to start conducting again. When the inductor current increases from zero to the peak along the upward slope, the MOSFET power tube begins to turn off. The MOSFET power tube is cut off until the inductor current drops to zero. According to the chip introduction, the maximum current at the zero current detection end cannot exceed 3mA, so the zero current detection resistor R25 is determined by the following formula.

Where Vcc is the chip supply voltage.

4.4 Design of input voltage detection resistor

The peripheral circuit of the multiplier is shown in Figure 5. After the AC input is rectified, a half-wave sine voltage waveform is obtained. In order to make the input current track the input voltage waveform better, we need to sample the voltage after the AC input is rectified. After the voltage is divided by resistors R21 and R22, the voltage is reduced by about 100 times and input to pin 3 of SA7527. A capacitor C15 is connected in parallel with resistor R2 to remove the voltage ripple after rectification. From the internal structure of the chip, it can be seen that the voltage at pin 3 of the multiplier input terminal is below 3.8V to ensure a good power factor correction effect.

Figure 5 Multiplier peripheral circuit

Therefore, the maximum input voltage of pin 3 should not exceed 3.8V, that is:

4.5 Design of Current Sensing Resistor

The current detection peripheral circuit is shown in Figure 6.

Figure 6 Current detection peripheral circuit

The circuit adopts the peak current detection method, so a current sensing resistor R24 ​​is connected between the source of the MOSFET power tube and the ground, and the source terminal of the MOSFET power tube is connected to the CS terminal of the current sensing terminal 4 of the SA7527. In general application circuits, an RC filter circuit is connected after the current sensing resistor to filter out the peak of the switching current. Because the SA7527 chip already has an RC filter circuit inside, there is no need to add an external RC filter circuit here, thereby reducing the number of external components of the SA7527. The current sensing comparator adopts an RS latch structure, which can ensure that only one signal pulse appears at the drive output within a given cycle. When the induced voltage across the current sensing resistor exceeds the threshold voltage of the output terminal of the multiplier, the current sensing comparator will turn off the MOSFET power tube and reset the PWM latch. Under normal circumstances, the peak value of the inductor current is controlled by the output Vmo of the multiplier, but when the input voltage is too high or there is a problem with the output voltage error amplifier detection, the threshold value of the current sensing terminal will be clamped internally at 1.8V. This is because the inverting input of the current sensing comparator inside the chip is connected to a 1.8V voltage regulator diode, so the value of the current sensing resistor must meet the two conditions of formula (6) and formula (7).

in

K is the multiplier gain, ΔVm2 =Vm2 -Vref, which is the difference between the output of the voltage error amplifier and the internal reference voltage of the chip.

4.6 Design of Closed-Loop Feedback Circuit

The closed-loop feedback circuit is shown in Figure 7. This circuit is a constant current and constant voltage output circuit. It is a current control loop and a voltage control loop composed of dual op amps LM358 and TL431. Constant current is achieved first and then constant voltage. First, current sampling is performed, D2 is turned on, D1 is turned off, and constant current is achieved. Then, voltage sampling is performed, D1 is turned on, D2 is turned off, and constant voltage is achieved.

Figure 7 Closed-loop feedback circuit

Current control loop: TL431 is a precision voltage regulator. The cathode K and the control electrode R are directly short-circuited to form a precise 2.5V reference voltage. This voltage is sent to the 5th pin (non-inverting input terminal) of LM358 by R11. R5 directly samples the current from the output terminal, converts the current into voltage, and then sends the voltage value to the 6th pin (inverting input terminal) of LM358. The voltage at the non-inverting input terminal is compared with the voltage at the inverting input terminal, and the high and low levels are output at the 7th pin to control the conduction and shutdown of the optocoupler EL817, and then the duty cycle of the primary output of the transformer is controlled by SA7527 to achieve the result of stable output current. C1 and R3 are feedback elements of the inverting input terminal and the output terminal, and the feedback gain of the amplifier can be adjusted by adjusting their values. When the circuit is connected to the P5 port, the output current is:

，

，

The same applies to other ports.

Voltage control loop: TL431 is a precision voltage regulator. The cathode K and the control electrode R are directly short-circuited to form a precise 2.5V reference voltage. This voltage is sent to the 3rd pin (non-inverting input) of LM358 by R10. R7 directly samples the voltage from the output. R7 and R9 form a voltage divider circuit, and send the voltage divider value to the 2nd pin (inverting input) of LM358. The voltage at the non-inverting input is compared with the voltage at the inverting input, and the high and low levels are output at the 1st pin to control the conduction and shutdown of the optocoupler EL817, and then the duty cycle of the primary output of the transformer is controlled by SA7527 to achieve a stable output voltage. C3 and R8 are feedback elements of the inverting input and output, and the feedback gain of the amplifier can be adjusted by adjusting their values. When the circuit is connected to the P1 port, the output voltage of the P1 port is:

，

The same applies to other ports.

2. Modeling and Simulation of Voltage Control Loop and Current Control Loop

1. Modeling and simulation of voltage control loop

First, an important intermediate quantity is the relationship between the TL431 cathode voltage change k Δv and the output fluctuation o Δv:

in

The change in cathode voltage causes the optocoupler diode current to change:

High voltage sensing side Photoelectric Current changes:

in

Feedback Network :

The control block diagram is shown in Figure 8.

Figure 8 Voltage loop structure

The open-loop transfer function of the system is:

Substitute R2=4.7KΩ, R7=150kΩ, R8=2.2kΩ, R9=4.7kΩ, R19=1kΩ, C3=1mF, CTR=100%, 101 pwm k= L? f = into equation 16, and the Bode diagram of the voltage control loop obtained by MATLAB simulation is shown in Figure 9. The crossover frequency is 4.8KHZ and the phase margin is 100o.

Figure 9 Bode plot of voltage loop

2. Modeling and simulation of current control loop

The open-loop transfer function of the system is:

Substitute R2 = 4.7 kΩ, R3 = 2.2 kΩ, R4 = 2.2 kΩ, R5 = 0.36 Ω, R19 = 1 kΩ, C1 = 1 mF, CTR = 100%, 101 pwm k = L? f = into equation 19, and the Bode plot of the voltage control loop obtained by MATLAB simulation is shown in Figure 10. The crossover frequency is 220 kHz and the phase margin is 46°.

Figure 10 Current loop structure

3. Experimental Results Analysis

An 18W experimental circuit was built and connected to the power supply. The waveforms of the tests with various instruments are shown in Figures 11, 12, 13 and 14. As can be seen from the above waveforms, the output current and voltage can be output at a constant current and constant voltage, the circuit efficiency reaches more than 85%, and the power factor (PF) reaches about 90%.

Figure 11 Bode plot of current loop

Figure 12 Current and voltage output waveforms

Figure 13 Input voltage and efficiency curve

Figure 14 Input voltage and power factor curve

in conclusion

LED fluorescent lamp is a green light source with a very wide application prospect. Through simulation and experimental verification, this circuit can have wide voltage input, constant current and constant voltage output, the current control loop and voltage control loop are not only fast and stable in response, the output current and voltage are very stable, the efficiency of the circuit reaches more than 85%, achieving satisfactory results, the circuit also has multiple ports, can drive LEDs of different powers, and can be used in real life.