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A: We frequently refer to quasi-resonant power supplies. But what exactly is a quasi-resonant switching power supply? As we all know, the main losses in a switching power supply come from the switching process of the switching transistor. Since the transistor is not an ideal switching device, the switching process isn't instantaneous, and there's always a transition period where both voltage and current exist simultaneously, causing losses in the switch. As the frequency increases, these losses grow, limiting the maximum frequency of the power supply. These rapid changes in voltage and current during the conversion process also generate significant switching noise, leading to electromagnetic interference (EMI). To address this issue with conventional square wave switching power supplies, researchers have been working on low-power soft-switching technology for over two decades, introducing small inductors or capacitors into the circuit. By leveraging the principles of resonance, they aim to make the voltage or current at the switch terminals vary sinusoidally. The fundamental idea is to find a way to ensure the switching transistor completes its operation at zero voltage (ZVS - Zero Voltage Switching) or zero current (ZCS - Zero Current Switching), thereby eliminating the overlap between voltage and current and minimizing power loss.
Power circuit
While resonant power supplies (Resonant Switching Power Supplies) reduce switching losses, their circuits tend to be more complex. A quasi-resonant mode is commonly used in flyback switching power supplies. The term "flyback" refers to the fact that the primary-side main power switch and the secondary-side rectifier operate in opposite states. When the switch is turned on, the secondary-side rectifier is off, and the flyback converter only stores energy while the primary-side switch is on. When it turns off, the stored energy is released to the load. Thus, the high-frequency transformer serves both as an isolation transformer and an inductor during the switching process. Flyback switching power supplies are widely used in televisions due to their simplicity and ability to provide multiple outputs easily. Unlike resonant switching power supplies, where the resonance actively participates throughout the entire energy conversion process (resulting in a sine wave), the quasi-resonant mode completes resonance only during one phase of the power conversion—when the switch is completing its operation (the waveform remains close to a square wave). Through resonance, the switching transistor operates at zero voltage (or minimal voltage) or zero current, maintaining the high-efficiency energy transfer characteristic of square-wave switching power supplies. This approach is therefore referred to as quasi-resonance (QR). Quasi-resonant power supplies offer several advantages, including reduced switching losses, lower EMI, and improved efficiency compared to traditional square wave designs. However, achieving this requires careful design and control, ensuring that the resonance conditions are met precisely during each switching cycle. These systems are particularly beneficial in applications where high-frequency operation and compact size are critical, such as in consumer electronics and portable devices.

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