High efficiency 70W universal switching power supply circuit

The power supply uses 3 integrated circuits:

TOP249Y type 6-terminal monolithic switching power supply (IC1); linear optocoupler PC817A (IC2); adjustable precision shunt regulator TL431 (IC3).

Resistors R2 and R3 set the drain limit current of the power switch tube externally (slightly higher than the drain peak current when fully loaded or input undervoltage). When the input DC voltage is overvoltage, R2 and R3 can also automatically reduce the maximum duty cycle to limit the maximum load power. R1 is an undervoltage or overvoltage detection resistor and can provide a feed-forward voltage to the line to reduce the fluctuation of the switching frequency. When R1 is equal to 2MΩ, the power supply can only start when the input voltage reaches ACIOOV.

The drain clamp circuit composed of VDZ1 and VD1 can absorb the peak voltage generated by the primary leakage inductance of the high-frequency transformer when the MOSFET is turned off, avoiding overvoltage damage to the MOSFET. VDZI uses a p6KE2OO transient voltage suppressor with a clamping voltage of 200V, and VD1 uses a UF4006 ultra-fast recovery diode with a reverse withstand voltage of 800V. Capacitor Cl6 and VDZ1 are connected in parallel to reduce clamping losses. When the full-frequency working mode is selected, the switching frequency is set to 132kHz.

In order to reduce the loss of the secondary winding and the rectifier diode, the secondary winding is divided into two paths, each of which uses a MER20100 type 20A/1OOV common cathode Schottky pair tube (VD2, VD3), and then works in parallel.

The output filter circuit is composed of C8, C9, L3, Cl5 and CIO. When unloaded, TOP249Y can automatically reduce the switching frequency, so that the power loss is only 520mW when the AC230V input is used. C5 is connected between +300V and the cold ground to filter out the common-mode interference generated by the primary and secondary coupling capacitors, and C1 is connected to the AC power supply line to filter out the differential-mode interference between the power grid lines.

The voltage stabilization circuit is composed of IC2, IC3 and other components. The output voltage UO obtains the sampling voltage through the resistors R8~R10, and generates an error voltage after comparing it with the 2.50V reference voltage of IC3R pole, and then changes the current flowing into the control terminal of TOP249Y through IC2, so that the duty cycle of the switch tube drive pulse changes.