The isolated flyback converter is a commonly-used converter for small power supplies such as battery chargers for portable devices and for auxiliary power supplies in electronic equipment. In this application example the control design of a flyback converter that provides three low-voltage DC output voltages from a single-phase mains source is explained. The flyback transformer is modeled using an ideal transformer with a magnetizing inductance in parallel. Leakage inductance is ignored because the model is intended for controller design and circuit level simulation rather than simulating the parasitic effects of components. The flyback converter uses a two-loop control scheme. The inner current loop, based on a peak current mode controller, provides a fast transient response, and the outer voltage control loop provides a reference current to the inner loop to regulate the output voltage of the 5 V winding. The voltage loop is based on a type 2 voltage controller, that is implemented using an inverting op-amp circuit. The example circuit is shown below.
Type 2 Voltage Controller Design for Flyback Converter