Soft switching applications in switching power supplies
At present, switching power supply is widely used in almost all electronic equipment with the characteristics of small size, light weight and high efficiency, which is an indispensable power supply method for the rapid development of today's electronic information industry. It is an indispensable power supply for the rapid development of today's electronic information industry.
Hard switching and soft switching in switching power supplies are for switching transistors. Hard switching is to turn on or off the switching transistor forcibly regardless of the voltage or current on the switching transistor. When the switching tube (between drain and source, or between collector and emitter) voltage and current is large, switching the switching tube, due to the switching between switching tube state switching (from conduction to cutoff, or from cutoff to conduction) takes a certain amount of time, which will cause the switching tube state switching for a certain period of time, the voltage and current there is a cross over the region, the cross over caused by the switching tube loss (switching tube switching loss) with the switching frequency, the switching tube switching loss. Switching loss (switching loss of the switching tube) increases rapidly with the increase in switching frequency.
In the case of inductive loads, a spike voltage is induced when the switching transistor is switched off. The higher the switching frequency, the faster the turn-off, and the higher the induced voltage. This voltage is added to both ends of the switching device, which can easily cause the device to break down.
In the case of capacitive loads, the spike current at the moment of switching transistor conduction is high. Therefore, when the switching transistor is turned on at a very high voltage, all the energy stored in the junction capacitance of the switching transistor will be dissipated in the device in the form of current. The higher the frequency, the larger the turn-on current spike, which can cause overheating damage to the switching tube.
In addition, the diode in the secondary high-frequency rectifier circuit, from the conduction to cut-off, there is a reverse recovery period, the switching transistor in the period on, it is easy to produce a large inrush current. Obviously, the higher the frequency, the larger the inrush current, which is harmful to the safe operation of the switching transistor.
Finally, in the switching power supply used for hard switching, the switching transistor generates serious electromagnetic nuisance. As the frequency increases and the di/dt and du/dt in the circuit increase, the electromagnetic nuisance generated also increases. As the frequency increases and the di/dt and du/dt in the circuit increase, the EMI generated also increases, affecting the normal operation of the switching power supply itself and the surrounding electronic equipment.
The above problems seriously hinder the improvement of the operating frequency of switching devices (switching transistors and high-frequency rectifier diodes). In recent years, the research of soft switching technology has provided an effective way to overcome the above defects. The soft switching technology research conducted in recent years provides an effective way to overcome the above defects. Unlike the hard switching principle, the ideal soft turn-off process is that the current drops to zero first, and the voltage rises slowly to the off-state value, so that the turn-off loss is reduced. The turn-off loss is approximately zero because the current has already dropped to zero before the device is turned off. Since the current has already dropped to zero before the device is turned off, the inductive turn-off problem is solved. The ideal soft turn-on process is one in which the voltage first drops to zero and the current rises slowly to the off-state value. Ideal soft turn-on process is the voltage first dropped to zero, the current slowly rising to the on-state value, so the turn-on loss is approximately zero, the device junction capacitance voltage is also zero, solving the capacitive turn-on problem. At the same time, the diode reverse recovery process is over at turn-on, so the diode reverse recovery problem does not exist.
Soft-switching technology also contributes to the reduction of electromagnetic nuisance levels due to the fact that the switching transistor conducts at zero voltage and switches off at zero current, while the fast recovery diode is also soft-off.
At the same time the fast recovery diode is also soft switched off, which can significantly reduce the di/dt and du/dt of the power device, and thus the level of EMI can be reduced.
