A: We often refer to quasi-resonant power supplies. So what exactly is a quasi-resonant switching power supply? As we all know, the loss of switching power supply mainly comes from the switching process of the switching tube. Since the switching tube is not an ideal switching device, the switching process is not instantaneous, and there is a certain transition time. As shown in Figure 1, the traditional square wave switching power supply is During this transitional transition, the voltage and current are zero, there is an overlapping area, and thus the loss of the switch will occur. As the frequency increases, the loss will gradually increase and limit the frequency of the switching power supply. The sharp changes in voltage and current during the short period of the conversion process also generate large switching noise, forming electromagnetic interference EMT. In order to overcome this shortcoming of square wave switching power supply, people have been working on low-power soft-switching power supply technology for more than 20 years, adding small inductors or capacitors to the circuit, using the principle of resonance to make the two ends of the switch The change of voltage or current is a sinusoidal change. The basic idea is to find a way to make the switching tube complete the switching when the voltage zero crossing ZVS (ZeroVoltage Switching) or current zero crossing ZCS (Zero Current Switching) to eliminate the voltage. The overlap with the current achieves the purpose of eliminating or reducing power consumption.
The switching loss of the resonant power supply (Resonant Switching Power) can be reduced, but the circuit is relatively complicated. A quasi-resonant mode is widely used in flyback switching power supplies. The so-called flyback type means that the primary side main power switch tube and the secondary side rectifier tube have opposite switching states. When the switch tube is turned on, the secondary side rectifier diode is turned off, and the flyback converter only stores energy when the primary side switch tube is turned on. When it is turned off, the energy is released to the load, so the high-frequency transformer is both a transformer isolation function and an inductor energy storage component during the switching process. The flyback switching power supply is widely used in color TVs because of its simple circuit and easy implementation of multiple outputs. Different from the process in which the resonance process of the resonant switching power supply actively participates in the whole energy change (oscillation >1} is a sine wave), the quasi-resonant mode is that the resonance is only completed in one phase of the entire power energy conversion—switching is completed (the waveform is still close to Square wave), through the resonance, the switching tube completes the switching at zero voltage (or minimum voltage) or zero current, and at the same time maintains the high-energy transmission mode of the square-wave switching power supply, so it is called quasi-resonance (quasi-resanent) QR.
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