Quasi-Resonant Discontinuous Flyback

An example of this type of power supply is a zero-current switching (ZCS) quasi-resonant power supply utilizing the STR6600 hybrid IC. STR6600 contains both the power MOSFET and the control circuitry for implementing this type of power supply. In a ZCS-type circuit, the current through the power switch is forced to be sinusoidal and the transistor is switched when this current is at or near zero. 

The following circuit (Fig. 4.72) shows the implementation of a quasi-resonant flyback converter featuring the STR6600. 

A switching cycle begins with the turn-on of the power MOSFET, which is internal to STR-F6524. The current in the MOSFET rises, starting at approximately 0 A and increasing at a rate determined by the input voltage and the primary inductance of the power transformer. 

The current generates a proportional voltage across the resistor connected from the source pin to the input return. A control signal is added to the current signal via a resistor connected from the source pin to the OOP pin, allowing a voltage or current injection into the OCP pin. The MOSFET will be turned off when the sum of the source current and the control signal reaches 0.73 V. 

After the MOSFET turns off, the drain voltage rises and the primary peak current is delivered to the load. The current then falls at a rate determined by the output voltage and the primary inductance. Once the energy in the primary inductance is depleted, the drain voltage falls, in accordance with the resonant characteristics of the primary inductance and the resonant capacitance (plus the primary transformer capacitance and the MOSFET output capacitance). 

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