Power supply technology basics: "soft switching" technology to reduce switching losses

With the continuous development of electronic technology, there are many kinds of electronic equipment, which are naturally closely related to people's work and life. Any electronic equipment also needs the support of power supply. With the continuous maturity and development of electronic technology, the requirements for power supply design are also constantly improving. They all need a stable power supply to provide energy for their normal operation. The switching technology that controls the power supply is a major factor affecting the stability of the power supply. The following will introduce the hard switching technology and soft switching technology in the switching technology.

　　The volume and weight of magnetic components account for a large proportion in switching power supplies , and the development direction of switching power supplies is small size, light weight and low cost. High frequency can effectively reduce the volume and weight of magnetic components, that is, the higher the operating frequency of the switching device, the smaller its volume and weight. In traditional DCOC converters, the switching device works in a hard switching state.

　　The disadvantages of hard switching are as follows:

　　(1) The loss during the opening and closing process is relatively large;

　　(2) Inductive shutdown problem:

　　When there are inductive devices in the circuit, a very high voltage spike will be induced when the switch device is turned off, which can easily damage the switch device;

　　(3) Capacitive opening problem:

　　When the switch device is turned on under high voltage, the energy in the junction capacitance of the switch device will be completely consumed by the device, which can easily cause the switch device to be damaged due to overheating;

　　(4) Diode reverse recovery problem:

　　When a diode changes from on to off, there is a reverse recovery time. During this time, the diode is still in the on-off state. If the switching device connected in series with it is turned on immediately, a large impact current will be generated, which may easily damage the switching device.

　　Due to the above disadvantages of hard switching, the increase of the operating frequency of the switching device is limited. Before the emergence of soft switching technology, the switching loss of power switching devices is very large. In order to make up for the shortcomings of hard switching, soft switching technology is proposed.

　　Principle of soft switching technology

　　The so-called "soft switch" is the opposite of "hard switch". Hard switch means that during the opening and closing process of the power switch, the voltage and current change greatly, resulting in large switching loss and noise. The switching loss increases with the increase of switching frequency, resulting in a decrease in circuit efficiency; the switching noise brings serious electromagnetic interference to the circuit, affecting the normal operation of itself and surrounding electronic equipment.

　　Soft switching is based on hard switching circuits, adding resonant devices such as inductors and capacitors, introducing resonance in the switching process, and turning on the switch when the voltage across it is zero; or turning off the switch when the current flowing through the switch device is zero, so that the switching conditions are improved, the switching loss and switching noise of the hard switch are reduced, and

　　Improves the efficiency of the circuit.

　　Figure 1 Comparison of soft switching and hard switching waveforms under ideal conditions

　　Soft switching includes two processes: soft turn-on and soft turn-off:

　　The ideal soft-turn-on process is: the voltage across the switching device first drops to zero, and then the current slowly rises to the on-state value, so no loss or noise is generated when it is turned on. The soft-turn-on switch is called a zero voltage switch.

　　The ideal soft-turnoff process is: the current at both ends of the switching device first drops to zero, and then the voltage slowly rises to the on-state value, so no loss and noise are generated when shutting down. The soft-turnoff switch is called a zero-current switch.

　　The ideal soft-switching waveform and hard-switching waveform of the power switch tube are shown in Figure 1. It can be seen from the figure that in the hard-switching state, the voltage and current at both ends of the power switch tube have an obvious overlapping area, and switching loss is generated in this overlapping area; while in the soft-switching state, the voltage and current at both ends of the power switch tube have almost no overlapping area, so no switching loss will be generated.