Switched-Mode DC Power Supplies Five configurations –Flyback –Forward –Push-pull –Half Bridge –Full-Bridge Operate at high frequencies –Easy to filter out harmonics

Flyback Converter

Mode 1 Operation -- Q 1 ON Current builds up in the primary winding Secondary winding has the opposite polarity D 1 OFF C maintains the output voltage, supplies load current

Mode 2 Operation -- Q 1 turned OFF Polarity of the windings reverses Diode D 1 conducts, charging C and providing current to the load R L Secondary current falls to 0 before the next cycle begins

Waveform Summary

Double-ended Flyback Converter

Forward Converter

Features Includes a “reset” winding to return energy Secondary “dot” so that D 2 forward biased when Q 1 is ON – no energy stored in the primary Operates in continuous mode

Mode 1 Operation -- Q 1 ON Current builds up in the primary winding Energy transferred to the load

Mode 2 Operation -- Q 1 turned OFF Polarity of transformer voltages reverses D 2 turns OFF, D 1 and D 3 turn ON

Waveform Summary VoVo D3D3

Current Components in the Primary

Double-ended Forward Converter

Push-Pull Converter

Push-Pull Operation Q 1 ON, V s across the lower primary winding Q 2 ON, V s across the upper primary winding

Half-Bridge Converter

Mode 1 Operation Q 1 ON, D 1 conducting Energy transferred to the load

Mode 2 Operation Both transistors are OFF D 1 continues to conduct due to current in L 1

Mode 3 Operation Q 2 ON, D 2 conducting Energy transferred to the load

Mode 4 Operation Both transistors OFF D 2 continues to conduct due to current in L 1

Waveform Summary – k<0.5

Full-Bridge Converter

Mode 1 Operation Q 1,Q 4 ON, Q 2,Q 3 OFF D 1 conducting, energy transferred to the load

Mode 2 Operation All transistors are OFF D 1 continues to conduct due to current in L 1

Mode 3 Operation Q 2,Q 3 are ON, Q 1,Q 4 OFF D 2 conducting, energy transferred to the load

Mode 4 Operation All transistors are OFF D 2 continues to conduct due to current in L 1

Waveform Summary – k<0.5

Half-Bridge Resonant Inverter

Full-Bridge Resonant Inverter

Bidirectional Power Supply