The low cost, simplicity of design and intrinsic efficiency of flyback transformers have made them a popular solution for power supply designs of below 100W to 150W. Other advantages of the flyback transformer over circuits with similar topology include isolation between primary and secondary and the ability to provide multiple outputs and a choice of positive or negative voltage for the output.
This article discusses design parameters for the flyback or swinging choke type of transformer used in flyback converters. The latter has been used for many years and its topology is unique within the transformer-isolated family of regulators.
Flyback transformer function
When the switch is turned on, energy is stored in the primary (within the core material). As shown in Figure 1, the polarity dots on the transformer and the diode are arranged such that there is no energy transferred to the load when the switch is on. When the switch is turned off, the polarity of the transformer winding reverses due to the collapsing magnetic field, the output rectifier conducts and the energy stored in the core material is transferred to the load. This activity continues until the core is depleted of energy or the power switch is once again turned on.
Figure 1. Typical flyback transformer circuit
The flyback regulator can operate in either discontinuous or continuous mode. In the discontinuous mode (see Figure 2), the energy stored in the core when the FET is on/off is completely emptied from the core during the flyback period. In the continuous mode, (see Figure 3) the FET is turned on before the core empties of flyback energy. A typical flyback transformer may operate in both modes depending on the load and input voltage.
Designers should consider the maximum load at low voltage, including all conditions within the operating range of the flyback, as it will simply shut down (discontinuous mode) between cycles and wait for the load demand to catch up with the power-delivery capability. This is one of the most dynamic characteristics of the flyback, regulated over a wide range of input voltage and load.
Figure 2. Flyback transformer in discontinuous mode
Figure 3. Flyback transformer in continuous mode