Design aspects of a switch-mode transformer under wide input voltage variation

Proposed is a discrete switch-mode fly-back power supply that combines part of its resonance topology into its switch-mode transformer (SMT). The resonant action is generated by the snubber circuit, and an expensive tank circuit is avoided. The "minimum switch on" circuit from the proposed SMT primary and lower auxiliary winding is designed in such a way that it provides a delay of half-resonant cycle before the driver circuit switches on the biasing circuit. This paper presents some practical design aspects of a series of SMTs intended to be used in developed and developing countries where the ac mains voltage is expected to fluctuate from very narrow to wide ranges, respectively.