In recent years, with the wide application of microelectronic technology and computer technology in communication equipment, all kinds of advanced equipment have higher and higher requirements on power supply devices. AC/DC power adapter and DC/DC converter, as the main devices in the power system to provide DC power for equipment, are faced with many requirements such as smaller volume, lighter quality, higher efficiency and better reliability. To meet the above requirements, the power converter must realize the conversion of operating frequency from low frequency to high frequency. When the converter operates in a hard switching mode, increasing the operating frequency of the converter will cause the following problems:
1.High switching loss.
When the switch is on, the current of the switch device goes up and the voltage goes up at the same time. When the switch is off, the voltage rises and the current drops simultaneously. Switching losses are caused by the overlapping of voltage and current waveforms, which increase rapidly with the increase of switching frequency.
2. High peak of inductive turn-off voltage.
When the device is turned off, the inductive element in the circuit senses a peak voltage. The higher the switching frequency, the faster the turn-off, the higher the induced voltage. This voltage is applied to both ends of the switch device, which may easily cause device breakdown.
3. Large peak of capacitive opening current.
When a switching device is switched on at a high voltage, all the energy stored in the junction capacitance of the switching device will be dissipated in the device in the form of current. The higher the frequency is, the larger the switching current peak will be, which will cause the device overheating damage. In addition, there is a reverse recovery period when the diode changes from conduction to cut-off, during which the switching tube is open, it is easy to produce a large impulse current. The higher the frequency is, the greater the impulse current will be, which will harm the safe operation of the device.
Serious electromagnetic interference. With the increase of frequency, di/ DT and DU/DT in the circuit increase, which leads to the increase of EMI electromagnetic interference and affects the work of the rectifier and the surrounding electronic equipment.
The above problems have seriously hindered the improvement of the working frequency of switching devices. In recent years, the research of soft switch technology provides an effective way to overcome the above defects. The working mode of soft switch and hard switch, the ideal zero-current soft switch process is that the current first drops to zero, then the voltage slowly rises to the off-state value, so the turn-off loss is approximately zero. The inductive turn-off problem is solved because the current of the device has been reduced to zero before the turn-off. The ideal zero-voltage soft opening process is that the voltage first drops to zero, and then the current slowly rises to the on-state value, so the opening loss is approximately zero and the voltage on the junction capacitance of the device is also zero, thus solving the capacitive opening problem. At the same time, the diode reverse recovery process has been completed by the time the device is turned on, so the diode reverse recovery problem does not exist. The EMI problem was solved by the reduction of Di/dt and DU/DT.
Soft switching technology actually USES capacitor and inductance resonance to make the current or voltage in the switching device change according to the law of sine or quasi-sine. When the current is zero, the device is turned off. When the voltage is over zero, make the device open, so as to achieve zero switching loss. According to its control mode, soft switch technology can be divided into pulse frequency modulation mode, pulse width modulation mode and pulse phase shift control mode.
