This paper introduces the topology structure with SG3525 as the control chip and half bridge conversion. The output is constant voltage/constant current 12V/20A. When the load output is less than 0.6W, it is in constant current mode. When the load is greater than 0.6W, it works. In constant voltage mode, the power supply is a high-power LED power supply with a maximum output power of 240W, a uniform current, a wide input voltage range, and low output ripple.
1. System structure designThe power supply designed in this paper mainly includes input protection, EMI filter circuit, full-bridge rectifier circuit, half-bridge DC-DC conversion circuit, feedback circuit and control drive circuit. The general circuit structure is shown in Figure 1. After the AC voltage is input, the electromagnetic interference generated by the power supply is filtered by EMI, and then rectified. The current and voltage of the load are sampled, fed back to the control circuit by the control circuit, and finally the precise current or voltage control is achieved by changing the duty cycle of the two MOSFET tubes.
Figure 1 Power system structure
The main topology of the power supply adopts a half-bridge converter as shown in Figure 1. It is developed on the basis of a push-pull converter. It consists of two power switch tubes to form a half bridge, two capacitors, and a high frequency transformer. And so on. The bridge circuit converts the input DC power into a high-frequency square wave AC, and then is stepped down by the high-frequency transformer and the secondary side is rectified and filtered, and then outputted to DC. It is suitable for the occasion where the output power is large, and belongs to the isolated transformer topology.
The control and drive circuit consists of a dedicated integrated chip SG3525 and its peripheral circuits. The modulation range of the chip is 100-400 kHz. When the selected frequency is too low, the output waveform pulsation is relatively large. If the output waveform is required, the frequency should be higher. 20kHz, if the frequency is too high, the switching tube is required to be high, and the cost is much higher. The switching frequency in the market is generally 50-100 kHz [1-4]. Under the comprehensive consideration, the design adopts 60 kHz.
2, SG3525 control chip introduction and settings 2.1, chip structure and characteristicsThe internal structure of the SG3525 is shown in Figure 2. It consists of a reference voltage regulator, an oscillator, an error amplifier, a comparator, a latch, an undervoltage lockout circuit, a latching control circuit, a soft start circuit, and an output circuit.
Figure 2 Internal structure of the chip SG3525
The frequency of the output control signal PWM is changed by adjusting the capacitance on pin 5 of the SG3525 and the resistor on pin 6. Adjusting the voltage of the 9-pin COMP can change the output pulse width. These functions can improve the dynamic performance of the switching power supply and simplify the design of the control circuit. .
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