Application of soft switching technology in switching power supply

The hard and soft switching in the switching power supply is for the switching transistor. The hard switch is to forcibly turn on or off the switch regardless of the voltage or current on the switch. When the voltage and current of the switch (between the drain and the source, or between the collector and the emitter) are large, switch the switch, due to the switch between the states of the switch (from on to off, or from off To turn on) takes a certain amount of time, which will cause a crossover area between the voltage and current during a certain period of time when the state of the switch is switched. The frequency increases rapidly.

In the case of inductive loads, a peak voltage is induced when the switching transistor is turned off. The higher the switching frequency and the faster the turn-off, the higher the induced voltage. This voltage is applied across the switching device, which can easily cause device breakdown.

If it is a capacitive load, the peak current at the instant when the switching transistor is turned on is large. Therefore, when the switching transistor is turned on at a very high voltage, the energy stored in the junction capacitance of the switching transistor will be completely dissipated in the device in the form of current. The higher the frequency, the greater the turn-on current spike, which will cause overheating damage to the switch.

In addition, the diode in the secondary high-frequency rectifier circuit has a reverse recovery period when it is turned from on to off. When the switching transistor is turned on during this period, it is easy to generate a large inrush current. Obviously, the higher the frequency, the greater the inrush current, causing harm to the safe operation of the switching transistor.

Finally, in the switching power supply used for hard switching, the switching transistor will cause serious electromagnetic disturbance. As the frequency increases and the di / dt and du / dt in the circuit increase, the electromagnetic disturbance generated also increases, affecting the normal operation of the switching power supply itself and surrounding electronic equipment.

The above problems severely hindered the increase in the operating frequency of switching devices (switching transistors and high-frequency rectifier diodes). The soft switch technology research carried out in recent years provides an effective way to overcome the above-mentioned defects. Unlike the working principle of hard switches, the ideal soft turn-off process is that the current first decreases to zero, and the voltage slowly rises to the off-state value, so the turn-off loss is approximately zero. Since the current has dropped to zero before the device is turned off, the problem of inductive shutdown is solved. The ideal soft turn-on process is that the voltage drops to zero first, and the current slowly rises to the on-state value, so the turn-on loss is approximately zero, and the voltage at the device junction capacitance is also zero, which solves the problem of capacitive turn-on. At the same time, when it is turned on, the diode reverse recovery process has ended, so the problem of diode reverse recovery does not exist.

Soft switching technology also helps to reduce the level of electromagnetic disturbance, the reason is that the switching transistor is turned on at zero voltage and turned off at zero current, and the fast recovery diode is also soft turned off, which can be significantly reduced The di / dt and du / dt of low-power devices can reduce the level of electromagnetic interference.

Generally speaking, the efficiency of soft switching is higher (because there is no switching loss); the operating frequency is higher, and the volume of PFC or transformer can be reduced, so the volume of switching power supply can be made smaller. However, the cost is relatively high and the design is relatively complex.

Basic working principle of switching power supply

1-1. Several basic types of switching power supply

As the name suggests, the switching power supply is to use electronic switching devices (such as transistors, field effect transistors, thyristors, etc.), through the control circuit, so that the electronic switching devices are constantly turned on and off, allowing the electronic switching devices to Perform pulse modulation to achieve DC / AC, DC / DC voltage conversion, and adjustable output voltage and automatic voltage regulation.