Exploring quasi-resonant converters for power supplies

Quasi-resonant conversion is a well established technology which has been extensively used in power supplies in the consumer arena. The new Green FPS family brings the working range of this versatile technology down to the low power range. This design article will describe quasi-resonant flyback and buck converters showing how they improve power supply efficiency.

The principle of quasi-resonant conversion is to reduce the turn on losses of the power switch in a topology. A resonant converter [1] minimizes the turn on losses and works in a very different way. One way of explaining quasi-resonant operation is to consider it as an extension of discontinuous conduction mode operation.

Single pulsed flyback converter drain voltage and current Click on image to enlarge.

The figure above shows the drain waveforms in a current mode flyback converter operating in discontinuous conduction mode. Only one gate pulse has been applied. During the first time interval, the drain current ramps up until the desired current level is reached. The power switch is then turned off. The leakage inductance in the flyback transformer rings with the node capacitance. This causes the leakage inductance spike which is limited by a clamp circuit. After the inductive spike has diminished, the drain voltage returns to the input voltage plus the reflected output voltage. When the current in the output diode drops to zero, the drain voltage would immediately drop to the bus voltage, if the effect of the primary inductance and the node capacitance were ignored. However, the drain voltage rings down to this level as shown in the figure.

The primary inductance and node capacitance form a resonant circuit. Taking a value of 1.4mH for the inductance and 73pF for the node capacitance gives a resonant frequency of 500kHz using the equation 4 π²f²LC = 1. The resonant circuit is lightly damped. We note that the resonant frequency using this approximation is independent of the input voltage and load currents.