Design considerations for magnetic feedback using amplitude modulation

In off-line ac-dc and high-voltage dc-dc power supplies, galvanic isolation between the input and output is often implemented with optocoupler feedback. However, several disadvantages exist when implementing optocoupler feedback, such as a variable loop gain due to optocoupler tolerance and sensitivity to temperature, as well as a relatively high cost. An alternative to optocoupler feedback is to use magnetic feedback, which can be designed to have insensitivity to component tolerance, and good temperature stability. Although magnetic feedback has been in use for many years, a detailed analysis and clear design procedure has not been presented in the literature. This paper presents a thorough analysis of a magnetic feedback implementation, and provides a comprehensive design procedure which is verified on a 7-V/15-W experimental prototype.