Constant current output power adapter characteristics

　　The constant voltage/constant current output monolithic power adapter can be referred to as a constant voltage/constant current source for short. Its output characteristics are shown in Figure 9-48. Its characteristic is that it has two control loops: one is the voltage control loop, and the other One is the current control loop. When the output current is small, the voltage control loop works and has voltage stabilization characteristics. It is equivalent to a constant voltage source; when the output current is close to or reaches the rated value, the current control loop keeps I. Constant, It becomes a constant current source again. This power source is especially suitable for battery chargers and special motor drivers. The following introduces a low-cost constant voltage/constant current output power adapter. The current control loop is composed of transistors and the circuit is simple , Low cost and easy to make

　　The circuit of the 7.5V, 1A constant voltage/constant current output power adapter is shown in Figure 9-49. It uses a TP200Y power adapter (IC1), and is equipped with a PC817A linear optocoupler (IC2). 85~256V AC The input voltage U passes through the EMI filter (L2､C6), the rectifier bridge (BR) and the input filter capacitor (C1) to obtain a DC high voltage U1 of about 82~375V, and then connects to the drain of TOP200Y through the primary winding. VDZ1 and The drain clamp protection circuit composed of VD1 limits the peak voltage formed by the leakage inductance of the high-frequency transformer within a safe range. VDZ1 uses BZY97-C200 transient voltage suppressor, and its clamp voltage Ub=200V. VD1 uses UF4005 Type ultra-fast recovery diode. After the secondary voltage is rectified and filtered by VD2 and C2, it is filtered through L1 and C3 to obtain +7.5V output voltage. VD2 uses a 3A/70V Schottky diode. The output voltage of the feedback winding is passed through VD3 ､C4 is rectified and filtered, and the feedback voltage Um (26V) is obtained to provide bias to the phototransistor. C3 is a bypass capacitor, which also serves as a frequency compensation capacitor and determines the automatic restart frequency. R2 is the dummy load of the feedback winding, which can be used when there is no load. Limit the feedback voltage UB not to increase.

　　The power supply has two control loops. The voltage control loop is composed of 1N5234B type 6.2V voltage regulator tube (VDZ2) and optocoupler PC817A (IC2). Its function is to make the power adapter work at a constant voltage output when the output current is small. In the mode, at this time, there is current on VDZ2, and the output voltage is determined by the voltage regulation value of VDZ2 (UZ2) and the forward voltage drop (UF) of the LED in the optocoupler. The current control loop is composed of transistors VT1 and VT2, and current detection resistors. R3, optocoupler IC2, resistors R4~R7, capacitor C8. Among them, R3 is dedicated to detecting the output current value. VT1 uses 2N4401 NPN silicon tube, and the domestic substitute model is 3DK4C; VT2 chooses 2N4403 PNP silicon tube, which can be made in China 3DK9C substitution. R6 and R5 are used to set the collector current value Ic and Ie2 of VT2. R5 also determines the DC gain of the current control loop. C8 is a frequency compensation capacitor to prevent self-excited oscillation in the loop. Or when restarting automatically, the transient peak voltage can make VT1 turn on, and use R7 to limit its transmitting junction current. The function of R4 is to bypass the turn-on current of VT1 through VT2 so that it does not pass through R1. The startup process of the current control loop is as follows: With the increase of I｡, when I｡ is close to 1A, UR3↑→VT1 turns on→Ug↑→VT2 turns on, and the collector of VT2 provides current to the optocoupler, forcing U｡ drops. Because U｡ decreases, VDZ2 cannot be reversed breakdown, and no current flows through it. Therefore, the voltage control loop opens, and the power adapter automatically switches to constant current mode. C7 is a safety capacitor that can filter out Common mode interference caused by primary and secondary coupling capacitors.

　　The power supply can work in the state of 7.5V regulated output and 1A controlled current. When the ambient temperature range is 0℃~50℃, the accuracy of constant current output is about ±8%

　　The output voltage-output current (U｡-I｡) characteristics of the power supply are shown in Figure 9-49. As can be seen from the figure, it has the following salient features

　　①When U1=85V or 265V, the characteristic curve changes very little, which shows that the output characteristics are basically not affected by the change of AC input voltage

　　②When I｡<0.90A, it is in the constant pressure zone; when I｡≈0.98A, it is in the constant current zone, and U｡ decreases rapidly with the slight increase of I｡

　　③When U｡≤2V, VT1 and VT2 can no longer provide sufficient working current to the optocoupler. At this time, the current control loop does not work, but the primary current is still limited by the maximum limit current ILIMIT(max) of TOP200Y. This When UB6 rises, the operating current of the optocoupler is rapidly reduced through VT1 and VT2, forcing TOP200Y to enter the automatic restart state. This shows that once the current control loop is out of control, it will immediately switch from the constant current mode to the automatic restart state, and pull l. Down, protect the chip.

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