Main products: power adapter, medical power, LED power, charger and so on.
Shenzhen Lianyunda Electronics Co., Ltd. is committed to doing a good job in power supply products, and sincerely welcomes customers to contact us by phone or email.
First, the power adapter principle
At power-on, the startup voltage is applied to pin 7 of ICl (UC3842AN) through resistors R4 and R5 (68kΩ/1W). When the capacitor voltage C3 (47μF/50V) reaches 16V, the chip starts and the switching power adapter starts to work. After that, The 7-pin voltage is provided by the switching transformer L1 feedback winding 1-2. When the voltage of the 7-pin is lower than 10V, the 6-pin PWM pulse output is turned off, and the switching power supply stops working.
The output DCC9V voltage is connected to the pins 1 and 2 of the optocoupler IC2 (PC817) via the voltage divider resistors R15 (15kΩ), R11 (2.2kΩ), precision regulator IC3 (KA431LZ), etc., and passes through the 4 and 3 pins. The phototransistor is fed back to pin 1 of ICl and controlled by the internal circuit to control the output voltage.
When the overcurrent protection detection resistor R3 (0.26Ω/2W) voltage, and the resistance R1 (1MΩ) reaches the mains voltage on the IC13 pin or C3 voltage, the Zener diode D2 (1N4747A) is turned on, When the ICl3 pin is greater than 1V, the chip implements overcurrent or overvoltage protection and stops output. Similarly, when the output voltage rises, the Zener diode D6 (1N4747A) also breaks through and clamps the output voltage to 20V for overvoltage protection.
ICl4 pin charge and discharge resistor R9 (39kΩ), capacitor C4 (102J/2A) determines the chip oscillation frequency, and the DC5V reference voltage output from the ICl8 pin provides the charging power adapter. The set and emitter of Q2 (2SC945P) are connected to R9. When the voltage of ICl3 pin turns on Q2, the parallel resistance becomes smaller, the charging is faster than the discharge, and the maximum duty ratio becomes smaller, which facilitates the overload protection adjustment of the circuit.
Second, the power adapter troubleshooting
The power-on indicator is off and there is no output.
After observation, it was found that the BXG fuse of the fuse tube was gasified, the resistor R2 and the regulator tube D2 were blackened, the parameters were unrecognizable, the power MOS tube Q1 had cracks on the outside, the overcurrent detection resistor R3 had black spots, and the estimated circuit was severely short-circuited. Then, I found that the switching power adapter chip UC3842AN, the output rectifier diode D8, and the regulator D6 are also broken.
Replace the bad components, R2, D2 can not be identified due to parameters, try 1kΩ resistor, IN4747A voltage regulator tube, power test machine, output voltage is stable, troubleshooting.