Design higher energy efficiency, very low EMI quasi-resonant adapter - Power Circuit - Circuit Diagram

Industrial Router Crystal 3.2*2.5mm 3225 26M (26.000MHZ) 12PF 10PPM 20PPM 30PPM

DFN1006 TP-RCLAMP3321P ESD Diode Low Capacitance

Brand AVX TPSE226M035R0125 Low impedance tantalum capacitor AVX 22

MOS power IC full range

Photocoupler

Quasi-square wave resonant converters, also known as quasi-resonant (QR) converters, are commonly used in modern power adapters due to their high efficiency and low electromagnetic interference (EMI). One of the key advantages of these converters is that they allow the MOSFETs to turn on when the drain-to-source voltage (V_DS) reaches its minimum value, which significantly reduces switching losses.

This technique helps improve overall system performance by minimizing energy loss during the switching process. In discontinuous conduction mode (DCM), the V_DS must drop from the sum of the input voltage (V_in) and the reflected voltage (V_reflect) down to just V_in. This voltage reduction is achieved through a resonant network formed by the primary inductance (L_p) of the transformer and the node capacitance (C_lump).

C_lump includes the parasitic capacitances from the MOSFET and the transformer, as well as other stray capacitances in the circuit. These components oscillate together, creating a resonant effect that allows for soft switching and further improves efficiency. The oscillation half-period can be calculated using the formula: [3].

By leveraging this resonant behavior, quasi-resonant converters provide an effective solution for power management in applications where efficiency and reliability are critical. They are especially useful in low-power and high-frequency designs where minimizing losses and noise is essential.