The origin of soft switching technology

Since around 1970, the high-efficiency switching voltage regulator power supply replaced the previous series step-down DC power supply, and then developed rapidly, not only no longer need large heat sink, but also can replace the heavy frequency transformer with a lighter high-frequency transformer, in order to meet the needs of small and light power supply.

By around 1980, the technology of high-frequency switching frequency had a great effect on the realization of miniaturization. If the forward converter of about 100W (5V, 20A), after only 10 years, compared with the standard 2kHz in the early stage of its practical application, it has been improved to between 200^-500kHa, and the body is only 1/10 of the initial stage, and the miniaturization needs have been realized.

However, after entering the 1980s, the implementation of high-frequency upgrading resulted in a sharp increase in the conversion shock (surge) caused by the switching loss of semiconductor switching devices and the leakage inductance of transformers, and it was finally realized that it was difficult to expect to meet the needs of further small and lightweight power supplies.

When the switching tube of the basic DC converter adopts PWM control mode, because the switching tube is not an ideal device, the voltage of the switching tube is not immediately reduced to zero at the time of opening, but there is a falling time, and the current is not immediately rising to the load current, there is also a rising time, during this time, the current and voltage have an overlap area and a loss, that is, the opening loss. Similarly, when the switching tube is turned off, the voltage at both ends of the switching tube does not immediately rise from zero to the supply voltage, and the flowing current does not immediately fall to zero, resulting in a turn-off loss during the overlap time. These two losses are collectively referred to as switching losses, and this working state of the switching tube is a hard switching state.

The way to reduce the switching loss is to realize the high-frequency soft switch of the switching tube, another name of the soft switch is "quasi-resonant switch", that is, the application of resonance principle, so that the voltage and current waveform of the switching tube changes according to the law of sine or quasi-harmonic string, when the current is zero, the switching tube is turned off, or when the voltage is zero, the switching tube is opened, and the switching loss is zero. Thus the switching frequency can be increased.