Used for: High power voltage conversion with isolation


  • Basic building block for all high power converters
  • Small transformer
  • Low input and output ripple
  • Outpu ripple can be arbiraritly low, set by output inductor
  • relatively easy to implement synchronous rectification for high efficiency


  • Requires output choke
  • Can be complex to stabilise optimally under all load and line conditions
  • Transformer must be 'reset' each cycle which can limit duty cycle


  • Q1 switches on and transfers current to the output directly in the turns ratio
  • Output inductor charges during this period and transformer magnetising current rises
  • Q1 switches off and third winding on transformer resets the magnetising current
  • Other reset mechanisms are possible including resonant reset and active reset
  • In off period Df conducts keeping current circulating through output inductor
  • Regulation achieved by feddback controlling duty cycle
  • Vo = Vin.D. Ns/Npri