Design principle of constant voltage/constant current output single chip switching power supply
Its characteristic is that it has 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 functions and has a voltage 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, which in turn becomes a constant current source. 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.
The working principle of a constant voltage/constant current output switching power supply
The circuit of a 7-5V, 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 85V~256V AC input voltage u is filtered through EMI filters L2, C6, rectifier bridge (BR), and input filter capacitor (C1) to obtain a DC high voltage UI of approximately 82V~375V. It 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 the high-frequency transformer to a range of * *. VDZ1 adopts a BZY97 C200 transient voltage suppressor, with a clamping voltage UB=200V. VD1 adopts UF4005 ultra fast recovery diode. The secondary voltage is rectified and filtered by VD2 and C2, and then filtered by L1 and C3 to obtain+7.5V output. VD2 adopts a 3A/70V Schottky diode. The output voltage of the feedback winding is filtered by VD3 and C4 rectification to obtain a feedback voltage of UFB=26V, providing bias voltage for the photosensitive transistor. C5 is the bypass capacitor, which also acts as the Frequency compensation capacitor and determines the automatic restart frequency. R2 is the false load of the feedback winding, which can limit the feedback voltage UFB to not increase during no-load.
This power supply has two control loops. The voltage control loop is composed of 1N5234B type 6.2V voltage regulator (VDZ2) and optocoupler PC817A (IC2). Its function is to make the switching power supply operate in constant voltage output mode when the output current is small. At this time, there is current passing through VDZ2, and the output voltage is determined by the voltage stabilization value (UZ2) of VDZ2 and the forward voltage drop (UF) of the LED in the optocoupler. The current control loop is composed of transistors VT1 and VT2, current detection resistor R3, optocoupler IC2, resistors R4-R7, and capacitor C8. Among them, R3 is dedicated to detecting the output current value. VT1 adopts 2N4401 NPN silicon tube, and the domestic substitute model is 3DK4C; For VT2, choose 2N4403 PNP silicon tube, which can be replaced by domestic 3DK9C. R6 and R5 are used to set the collector current values IC1 and IC2 for VT1 and VT2, respectively. R5 also determines the DC gain of the current control loop. C8 is a Frequency compensation capacitor to prevent the loop from Self-oscillation. When just powered on or automatically restarted
