The operating principle of the surge protection circuit is as follows:
R1, C1, D1, and R2, C2, D2 constitute a spike pulse absorption circuit.
The purpose is to prevent the reverse electromotive force (polarity: upper negative and lower positive) generated on the primary side of the switching transformer from knocking out Q1 when Q1 is cut off.
Because the secondary side of the switching transformer output AC signal frequency is very high 40KHz or more, which requires the rectifier diode switching speed must be high enough to work, the general switching power supply rectifier circuit uses a fast recovery diode half-wave rectification, reduce the rectifier diode switching losses, and fast recovery diode forward voltage drop is larger, if the bridge rectifier, the voltage drop of the second stage tube will be doubled and the power consumption of the diode will increase.
References
Baidu Knowledge Introduced (2107.12.24). Baidu Knowledge[cited 2017-12-24]