Design Principles of Constant Voltage/Constant Current Output Monolithic Switching Power Supplies
Its characteristic is having two control loops, one is the voltage control loop and the other is the current control loop. When the output current is small, the voltage control loop comes into effect and has a stabilizing characteristic, which is equivalent to a constant voltage source; When the output current approaches or reaches the rated value, the IO is maintained constant through the current control loop, and it becomes a constant current source again. This power supply is particularly suitable for battery chargers and special motor drivers. The following introduces a low-cost constant voltage/constant current output switching power supply, whose current control loop is composed of transistors, with a simple circuit, low cost, and easy production.
Working principle of constant voltage/constant current output switching power supply
The circuit of a 5V, 1A constant voltage/constant current output switching power supply is shown in Figure 1. It adopts a TOP200Y switching power supply (IC1) and is equipped with a PC817A linear optocoupler (IC2). The AC input voltage u from 85V to 256V passes through EMI filters L2 and C6, rectifier bridge (BR), and input filtering capacitor (C1) to obtain a DC high voltage UI of approximately 82V to 375V, which is then connected to the drain of TOP200Y through the primary winding. The drain clamp protection circuit composed of VDZ1 and VD1 limits the peak voltage formed by the leakage inductance of high-frequency transformers within the range of * *. VDZ1 adopts BZY97-C200 transient voltage suppressor, with a clamping voltage of UB=200V. VD1 uses UF4005 type ultrafast recovery diode. The secondary voltage is rectified and filtered by VD2 and C2, and then filtered by L1 and C3 to obtain a+7.5V output. VD2 uses 3A/70V Schottky diodes. The output voltage of the feedback winding is rectified and filtered by VD3 and C4 to obtain a feedback voltage UFB=26V, which provides bias voltage to the phototransistor. C5 is a bypass capacitor that also serves as a frequency compensation capacitor and determines the automatic restart frequency. R2 is a false load for the feedback winding, which can limit the feedback voltage UFB from increasing when unloaded.
