In switching power supplies, hard switching and soft switching refer to the switching transistors. Hard switching forcibly turns the transistor on or off regardless of the voltage or current across it. When the voltage and current between the drain and source, or between the collector and emitter, are high, switching the transistor requires a certain amount of time to transition between states (from on to off, or from off to on). This creates a voltage and current crossover region during the transition, and the switching losses caused by this crossover increase rapidly with the switching frequency.
If the load is inductive, a voltage spike will be induced when the switching transistor turns off. The higher the switching frequency and the faster the turn-off, the higher this induced voltage will be. This voltage applied across the switching device can easily cause it to break down.
If the load is capacitive, the peak current at the moment the switching transistor turns 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 dissipated into the device in the form of current. The higher the frequency, the larger the peak current, which can cause the switching transistor to overheat and be damaged.
Furthermore, in the secondary high-frequency rectifier circuit, the diodes undergo a reverse recovery period when switching from conduction to cutoff. During this period, when the switching transistor is turned on, a large inrush current is easily generated. Obviously, the higher the frequency, the larger this inrush current becomes, posing a threat to the safe operation of the switching transistor.
Finally, in switching power supplies used for hard switching, the switching transistors generate significant electromagnetic interference (EMI). As the frequency increases and the di/dt and du/dt values in the circuit increase, the EMI also increases, affecting the normal operation of the switching power supply itself and surrounding electronic equipment.
The aforementioned problems severely hinder the increase in the operating frequency of switching devices (switching transistors and high-frequency rectifier diodes). Recent research into soft-switching technology has provided an effective way to overcome these shortcomings. Unlike the working principle of hard switching, the ideal soft-turn-off process involves the current first decreasing to zero, and the voltage slowly rising to the off-state value, so the turn-off loss is approximately zero. Since the current has already decreased to zero before the device turns off, the inductive turn-off problem is solved. The ideal soft-turn-on process involves the voltage first decreasing to zero, and the current slowly rising to the on-state value, so the turn-on loss is approximately zero, and the voltage across the junction capacitance of the device is also zero, solving the capacitive turn-on problem. Simultaneously, the diode's reverse recovery process has already ended during turn-on, therefore the diode's reverse recovery problem does not exist.
Soft-switching technology also helps reduce electromagnetic interference levels because the switching transistor turns on at zero voltage and turns off at zero current, while the fast recovery diode is also soft-turned off. This can significantly reduce the di/dt and du/dt of power devices, thereby reducing the level of electromagnetic interference.
Generally, soft switching has higher efficiency (because there are no switching losses); it also allows for higher operating frequencies, and the size of the PFC or transformer can be reduced, thus making the switching power supply smaller. However, the cost is also relatively higher, and the design is more complex.
Basic working principle of switching power supply
As the name suggests, a switching power supply uses electronic switching devices (such as transistors, field-effect transistors, and silicon controlled thyristors) and control circuits to continuously turn the electronic switching devices on and off, allowing the electronic switching devices to pulse-modulate the input voltage, thereby achieving DC/AC and DC/DC voltage conversion, as well as adjustable and automatic voltage regulation of the output voltage.
