A Quasi-Resonant Converter (QRC) is a type of power converter that utilizes resonant switching to optimize power conversion efficiency. Resonant switching is a technique used in power converters to minimize switching losses. The basic idea of a quasi-resonator circuit is to utilize a resonant circuit (parasitic elements through a tank circuit) to achieve zero-voltage or zero-current conditions for the power switch during turn-on or turn-off, respectively. This approach minimizes switching losses by allowing the switching element to naturally reach a zero current state, enabling zero-current switching (ZCS) when turning off the element.

In traditional converters, power dissipation mainly occurs during switching. Quasi-resonant converters reduce turn-on switching losses by dynamically connecting the power switch in parallel to the tank circuit for Zero-Voltage Switching (ZVS) or in series for Zero-Current Switching (ZCS). These converters optimize the power transfer process and enhance efficiency by leveraging resonant circuits to control the switching behavior effectively.

Working Principle: 

Quasi-Resonant Converter 

The working of a quasi-resonant converter is based on a resonant switching element. When the FET (Field-Effect Transistor) is turned on in a quasi-resonant converter, it generates a resonant pulse which is then filtered by the output LC circuit, similar to a traditional switching converter. The key aspects of the working of a quasi-resonant switch are as follows:

• Resonant Pulse Generation: When the FET is turned on, it generates a resonant pulse that is filtered by the output LC circuit. This resonant pulse has a fixed width and amplitude, and the converter operates at a variable frequency by sending more pulses per second to provide more power.
• Zero-Current Switching (ZCS): The resonant pulse allows the switching element to naturally reach a zero current state, enabling zero-current switching (ZCS) when turning off the FET. This ZCS operation minimizes switching losses and enhances the efficiency of the converter.
• Operation in Four Sub-Intervals: The operation of a quasi-resonant converter is divided into four sub-intervals for better understanding. 
  • Interval 1: As soon as the FET is turned ON, the current through the resonant inductor, L_r starts to increase linearly. During this period, D₁ is conducting and the resonant node is connected to the ground. This initial state continues till the time the current through L_r increases to load current, causing D₁ to switch off. Equivalent circuit of interval 1
  • Interval 2: Once D₁ switches off, the resonant capacitor, C_r starts charging. The current in the circuit charges the capacitor as well as provides the load current. The current and voltage oscillate in their natural frequency the current through the resonant inductor diminishes to zero. At this point, D₂ turns off, enabling the zero current switching (ZCS) of the FET.
  • Interval 3: During this interval, the voltage across the resonant switch falls linearly due to the discharge of C_r. Current flows through the load. Once the resonant node reaches zero, D₁ starts conducting and pins it to the ground.  
  • Interval 4: During this interval, the time between the pulses is adjusted based on the resonant pulse length, desired output voltage, and the load current.

Key Features of Quasi-Resonant Converter

• Resonant Tank Circuit: A QRC typically includes a resonant tank circuit composed of inductors and capacitors. This circuit creates a sinusoidal voltage or current waveform, allowing for soft switching (either zero-voltage switching (ZVS) or zero-current switching (ZCS)).
• Soft Switching: By switching when the voltage across the switch is zero (ZVS) or the current through the switch is zero (ZCS), the converter reduces the switching losses and the stress on the switching devices, enhancing efficiency and reliability.
• Variable Frequency Operation: Unlike traditional pulse-width modulation (PWM) converters that operate at a fixed frequency, QRCs typically operate at a variable frequency. The switching frequency changes in response to load conditions to maintain resonance.
• Reduced EMI: The smooth waveforms generated by the resonant circuit result in lower high-frequency noise, reducing electromagnetic interference.

Quasi-resonant converters are used in various applications where efficiency and EMI performance are critical. Common applications include - switch-mode power supplies (SMPS) that are used in computers, televisions, and other consumer electronics, LED drivers, battery chargers, and telecommunications equipment.

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