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 -- Q1 ON

• Current builds up in the primary winding• Secondary winding has the opposite polarity D1

OFF• C maintains the output voltage, supplies load

current

Mode 2 Operation -- Q1 turned OFF• Polarity of the windings reverses• Diode D1 conducts, charging C and providing

current to the load RL

• 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 D2 forward biased when Q1 is ON – no energy stored in the primary

• Operates in continuous mode

Mode 1 Operation -- Q1 ON

• Current builds up in the primary winding• Energy transferred to the load

Mode 2 Operation -- Q1 turned OFF

• Polarity of transformer voltages reverses

• D2 turns OFF, D1 and D3 turn ON

Waveform Summary

Vo

D3

Current Components in the Primary

Double-ended Forward Converter

Push-Pull Converter

Push-Pull Operation• Q1 ON, Vs across the lower primary winding

• Q2 ON, Vs across the upper primary winding

Half-Bridge Converter

Mode 1 Operation

• Q1 ON, D1 conducting

• Energy transferred to the load

Mode 2 Operation

• Both transistors are OFF

• D1 continues to conduct due to current in L1

Mode 3 Operation

• Q2 ON, D2 conducting

• Energy transferred to the load

Mode 4 Operation

• Both transistors OFF

• D2 continues to conduct due to current in L1

Waveform Summary – k<0.5

Full-Bridge Converter

Mode 1 Operation

• Q1,Q4 ON, Q2,Q3 OFF

• D1 conducting, energy transferred to the load

Mode 2 Operation

• All transistors are OFF

• D1 continues to conduct due to current in L1

Mode 3 Operation

• Q2,Q3 are ON, Q1,Q4 OFF

• D2 conducting, energy transferred to the load

Mode 4 Operation

• All transistors are OFF

• D2 continues to conduct due to current in L1

Waveform Summary – k<0.5

Half-Bridge Resonant Inverter

Full-Bridge Resonant Inverter

Bidirectional Power Supply