SG3524 is a circuit diagram of switching power supply integrated controller

Crystal oscillator

Photocoupler

The SG3524 is a switching power supply integrated controller. It adopts advanced pulse width modulation control, working frequency is higher than 100kHz; working voltage range is 6~40V, internal reference voltage is 5V, reference source load capacity is 50mA; internal open collector, emitter drive tube maximum output current is 100mA; working temperature is 0~+70°C.
The SG3524's reference source produces a +5V reference with an accuracy of 1%. It supplies power to all circuits except the NAND gate, causing the oscillator to generate a continuous asymmetric triangular wave with an amplitude of 1.2 to 3.6V, while generating a T-type flip-flop. The synchronous square wave pulse of the operation, the frequency of the oscillator output signal is determined by the external capacitors and resistors of pins 6 and 7 of the SG3524; in addition, the reference source provides a voltage divider reference for the non-inverting input of the error amplifier through the 16-pin output, and the inverting input is input. The compared sampling voltage, the output control voltage is compared with the triangular wave of the oscillator, so that the internal PWM output square wave pulse is sent to the NOR gate circuit; the square wave output of the PWM comparator is sent to the input of the two NOR gates. When the input of the NOR gate is low, the output is high, or the output of the non-gate input is high when the output is low. Therefore, when the complementary output Q or Q of the T-type flip-flop is non- When the square wave is alternately output, the two output drive tubes are alternately turned on to generate an output switch control signal. In addition, the pulse width time of the dead zone control pulse of the PWM comparator and the T flip-flop causes the duty cycle of the NOR gate output pulse to vary from 0 to 45%, and the dead time control prevents the drive tube from being simultaneously guided. through. Another error amplifier and a turn-off transistor in the chip form a protection circuit. The error amplifier generally takes over-current sampling from the outside through the positive and negative input terminals, and its output controls the PWM comparator. To turn off the circuit, it is only necessary to input a voltage of 0.7V from the 10-pin. The transistor can be saturated and turned on, so that the PWM comparator outputs a high level. Therefore, in the protection state, the PWM causes both the NOR gates to output a low level, and the output of the driving tube is turned off.