During the saturation and conduction period of the switch tube VT, the DC voltage Ui of the positive electrode of C1 is charged through L→VT→C2 positive electrode→C2 negative electrode. On the one hand, a DC voltage is established at both ends of C2, and on the other hand, the magnetic energy in the energy storage inductor L is continuously increased. When the switch tube VT is off, L induces a left negative and right positive voltage, and the magnetic energy in L is released to C2 and the load through the freewheeling diode VD. The voltage Uo of the output end of the power adapter is determined by the length of the saturated conduction time of VT, that is, it is determined by the base excitation pulse width. The width of the base excitation pulse is determined by the error sampling and amplifying circuit.
If there is no freewheeling diode VD in the series power adapter, when the switch tube suddenly turns from saturated conduction to cut-off, because the magnetic energy in L cannot be released, an extremely high voltage will be induced. This voltage can easily lead to VT breakdown of the switch tube. After the freewheeling diode VD is connected, when the switch tube turns from saturated conduction to cut-off, the magnetic energy in L is released to C2 and the load circuit through VD. On the one hand, the voltage at both ends of L drops, making the switch tube collector-emitting The extreme voltage drop is the input Ui value with sufficient margin; on the other hand, during the VT cut-off period, L will release energy through the freewheeling diode VD, so that the load circuit will be supplemented with energy during the switch-off period. The output voltage is smoother and the efficiency of the power adapter is higher.
