65W-105W electronic transformer with overcurrent protection

(1) The core of the circuit is a double-ended half-bridge self-excited switch conversion circuit consisting of C2, C3, T3, L3 and the trigger circuit R2, C4, D7, L2. DB1 is a bidirectional trigger diode with a trigger voltage of 30V. T-L1-3 is a small high-frequency magnetic ring (diameter 7mm×29mm×12mm). The primary is wound with 48 turns of 0.50mm diameter enameled wire, with an inductance of 9.5mH. The secondary is wound with 30 enameled wires with a diameter of 0.23mm and 5 turns. The primary and secondary are insulated with suitable nylon sleeves to meet CE safety standards.

(2) Q1, C4, C5, R3, R4, D8, R8 form an overcurrent protection circuit. When the output stage is short-circuited or overloaded, the voltage on resistor R2 will increase greatly. After the integral voltage division of D3, R3, R7, and C8, the transistor Q1 is triggered to turn on, so that the switch tube cannot be triggered to turn on and play a protective role. C4 plays a state retention role in the protection circuit.

(3) C1 and L1 form an LC single-arm filter to filter out the spike pulses generated by the switch circuit. L1 is made of 0.37mm diameter enameled wire wound 200 times on the EE-20 type high-frequency magnetic core. A gap must be reserved between the high-frequency magnetic cores to prevent the core from saturation. The inductance of L1 after winding should be about 6.9mH.

(4) F1 is an 800mA fast-melting fuse, R1 is a varistor, and F2 is a thermistor. When installed, it should be connected to the heat sink of the switch tubes T2 and T3. The above components can play a protective role when the input circuit is overcurrent, overvoltage and the transistor is overheated.

(5) D5, D6, D8, D9, C6 and C7 all play the role of discharging the reverse peak voltage of the output transformer L3.