Ferrite Magamp on Flyback Power Supply

For a dual output flyback power supply where both outputs deliver substantial power (5 V at 2 A and 12 V at 3 A, both regulated to ± 5%), the 12 V goes to 0 load and cannot regulate within the 5% limit. One possible solution, a linear regulator, is undesirable because it is expensive and reduces efficiency. A proposed solution is to use a magamp on the 12 V output, even given the flyback topology.

A ferrite magamp is proposed in order to reduce cost. The control scheme for a ferrite magamp however, is different than for a traditional square-loop material (high permeability material). With ferrite, the control circuit (D1 and Q1) sinks current in order to bring up the output. The circuit has been tested to work on the bench. The transformer is wound for 5 V and 13 V outputs. It can even meet input power of less than 1 W with 5 V at 300 mW and 0 load on 12 V while regulating the 12 V output to ± 5%.

Figure 1.