Application on Switching Power Supply

In the past, in the design of ordinary switching power supplies, the output voltage was usually amplified by error and directly fed back to the input terminal. This voltage control mode can also play a good role in certain applications, but with the development of technology, most of the world's power manufacturing industry has adopted a solution with a similar topology structure. The switching power supply with this type of structure has the following characteristics: the output is fed back by TL431 (controllable shunt reference) and the error is amplified. The constant current terminal of TL431 drives the emitting part of an optocoupler, and the feedback voltage obtained from the photosensitive part of the optocoupler on the high-voltage side of the power supply is used to adjust the switching time of a current mode PWM controller, thereby obtaining a stable DC voltage output. The following diagram is a practical circuit of a 4W switch type 5V DC regulated power supply. The circuit adopts this topology structure and also uses TOPSwitch technology. C1, L1, C8, and C9 in the figure form EMI filters, BR1 and C2 rectify and filter the input AC voltage, D1 and D2 are used to eliminate peak voltages caused by transformer leakage inductance, and U1 is a current mode PWM controller chip with built-in MOSFETs, which receives feedback and controls the operation of the entire circuit. D3 and C3 are secondary rectification and filtering circuits, while L2 and C4 form a low-pass filter to reduce the output ripple voltage. R2 and R3 are output sampling resistors, and their partial voltage to the output is controlled by the REF terminal of TL431 to control the shunt from the cathode to the anode of the device. This current directly drives the emitting part of optocoupler U2. So when there is a trend of change in the output voltage, Vref increases, leading to an increase in the current flowing through TL431. As a result, the luminescence of the optocoupler increases, and the feedback voltage obtained at the photosensitive end also increases. After receiving this feedback voltage change, U1 will change the switching time of the MOSFET, and the output voltage will fall back with the change. In fact, the process described above will reach equilibrium in a very short time, with Vref=2.5V at equilibrium and R2=R3, resulting in a stable output of 5V. It should be noted that the output voltage can no longer be changed simply by changing the values of the sampling resistors R2 and R3, as the parameters of each component in a switching power supply have a significant impact on the working state of the entire circuit. According to the parameters shown in the figure, the circuit can output+5V within the input range of 90VAC~264VAC (50/60Hz), with an accuracy of better than ± 3%, an output power of 4W, a maximum output current of 0.8A, and a typical conversion efficiency of 70%