Introduction to the principle of quasi-resonant flyback and calculation of application parameters

If a fixed clock is not used to initialize the on-time, but a detection circuit is used to effectively "sense" the first minimum or valley of the MOSFET (VDS) drain-source voltage and only start the MOSFET on-time at this time, the result will be that the current spike at turn-on will be minimized due to the parasitic capacitance being charged to the minimum voltage. This situation is often called valley switching or quasi-resonant switching. The purpose of this article is to share with you the knowledge about the principle, application and parameter calculation of quasi-resonant flyback.

Quasi-resonant QR

Q

R( resonant)

The main purpose is to reduce the switching loss of mosfet, and the switching loss of mos mainly comes from its own output capacitance.

From the above figure, we can discuss which parts of the parasitic capacitance generally cause the switching loss. During the dead time of the traditional discontinuous mode flyback (DCM), the parasitic capacitance will oscillate with the main inductance around VDC. The voltage on the parasitic capacitance will change with the oscillation, but it always has a considerable value. When the next cycle MOSFET conduction time begins, the parasitic capacitance will discharge through the MOSFET, generating a large current spike. Since there is a large voltage on the MOSFET when this current appears, the current spike will cause switching loss.

As can be seen from the figure above, the quasi-resonance principle is basically the same as the general traditional flyback principle.

Lleak is the primary leakage inductance, Rp is the primary resistance, and Cp is the resonant capacitance;

When the energy in the secondary winding is released (i.e. the transformer flux is completely reset), a sinusoidal oscillating voltage appears at the drain of the switch tube. The oscillation frequency is determined by LP and CP, and the attenuation factor is determined by RP.

For a traditional flyback converter, the operating frequency is fixed, so the switch tube may be turned on again at any position of the oscillation voltage (including the peak and the valley).