ZVS Forward Converter - assumptions for analysis

1
ZVS Forward Converter- assumptions for analysis

• Assume ideal devices
• Assume iL smooth so that iL IO

• VO mean value of VAB

2
ZVS Forward ConverterMode 1

• Before Mode 1 starts Q has been on for some time
  (see Mode 5)
• Mode 1 starts when Q is turned OFF
• Conditions at start of Mode 1 VC 0, i IO

• IO charges C linearly ? VC rises linearly
• VAB E VC ? when VC reaches E, VAB tries to go
  negative and D turns ON ? this ends MODE 1
• Duration for Mode 1 is given by

3
ZVS Forward ConverterMode 2

• Conditions at start of Mode 2 VC E, i IO, D
  is ON

• L and C form a resonant circuit (see separate
  analysis sheet)
• VAB 0, VC follows an offset sine, i follows a
  cosine

4
ZVS Forward ConverterMode 2 continued
D2

• Mode 2 finishes when VC tries to go negative and
  D2 turns ON
• Duration for Mode 2 (?2) can be found from

• Value of i (I) at the end of Mode 2 is given by

• I is negative

5
ZVS Forward ConverterMode 3

• Conditions at start of Mode 3 VC 0, i I
  (negative), D is ON, D2 is ON

• Resonance stops since C is shorted by D2
• VAB 0, VC 0
• Voltage across L E ? i increases linearly
  (towards 0)
• Q has a drive (Gate) signal applied during Mode
  3, but doesnt conduct immediately since i is
  negative
• Mode 3 finishes when i reaches 0 and Q takes over
  i from D2
• Duration for Mode 3 is given by

6
ZVS Forward ConverterMode 4

• Mode 4 starts when Q takes over i from D2 as it
  passes through 0
• Conditions at start of Mode 4 VC 0, i 0, D
  is ON, Q is ON

• No resonance since C is shorted by Q
• VAB 0, VC 0
• Voltage across L E ? i increases linearly
  (towards IO)
• Mode 4 finishes when i reaches IO and all of the
  load current is now flowing through Q ? D turns
  OFF
• Duration for Mode 4 is given by

7
ZVS Forward ConverterMode 5

• Mode 5 starts when D turns OFF since Q has all
  the load current
• Conditions at start of Mode 5 VC 0, i IO, D
  is OFF, Q is ON, D2 is OFF

• No resonance since C is shorted by Q
• Circuit is in steady state VAB E, VC 0, i
  IO
• Mode 5 finishes when Q is turned OFF by the
  control circuit ? back to Mode 1
• Duration for Mode 5 is controllable and provides
  the means for controlling the output voltage Vo
  ( mean value of VAB)