Operation of Phase-shift full-bridge converter (PSFB)

The PSFB circuit with the phase-shift PWM control method can be divided into 10 operating modes. The typical operating signals and waveforms are given below.

Mode-1 (t₀ -t₁): Duty cycle loss mode

• G₁ and G₂ gating signals are existed –> Positive inverter output voltage, V_AB = + V_s
• Negative inductor current period –> Body diodes should conduct –> D_S1, D_S2
• Positive inductor current period –> Active switches should conduct –> S₁, S₂
• Inductor current lags behind the voltage and it stores the energy and reaches refelected output current (I_{Lo_min}) with the slope of
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• The output inductor still is in discharge mode through both the diodes D₁ and D₂, hence secondry side voltage is zero (short circuited) and then primary side voltage is zero.
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Mode-2 (t₁ – t₂): Power Delivery Mode

• G₁ and G₂ gating signals are existed –> Positive inverter output voltage, V_AB = + V_s
• Positive inductor current –> Active switches should conduct –> S₁, S₂
• Inductor charges with change in slope due to filter inductor adds up to the primary leakage inductor
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• Output inductor starts charging through diode D₁ and power transfer to the load
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Mode-3 (t₂ – t₃): Dead Zone Mode

• Only G₂ gating signals is existed –> No inverter output voltage, V_AB = 0
• Positive inductor current –> Switch should conduct –> S₂
• The switch S₁ capacitor starts charging and at the same time, switch S₄ capacitor starts discharging
• Slope of resonant inductor current is same as output inductor current as it is discharging through D₁ and D₂
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• Dead time should be enough to completely charge and discharge the switch capacitors

After capacitors fully charge and discharge, the discharged capacitor switch provides the freewheeling path for the inductor current through body diode D_S4 (as shown in next mode). This ensure that switch turns-on with a zero-voltage.

Mode-4 (t₃ – t₄): Freewheeling Mode

• G₂ and G₄ gating signals are existed –> No inverter output voltage, V_AB = 0
• Positive inductor current –> Switch S₂ and body diode of S₄ should conduct –> S₂, D_S4
• Body diode of S₄ provides the freewheeling path for the resonant inductor current with reduced slope since output indcutor comes into series
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• In freewheeling operation, energy will not transfer to secondary since the output indcutor is in discharge throgh both diodes and hence secondary voltage is zero.

Mode-5 (t₄ – t₅): Dead Zone Mode

• Only G₄ gating signal is existed –> No inverter output voltage, V_AB = 0
• Positive inductor current –> Body diode should conduct –> D_S4
• The turned-off switch S₂ capacitor starts charging and at the same time, switch S₃ capacitor starts discharging
• Slope of resonant inductor current is same as output inductor current as it is discharging through D₁ and D₂
  • 
• Dead time should be enough to completely charge and discharge the switch capacitors

After capacitors fully charge and discharge, the discharged capacitor switch provides the freewheeling path for the inductor current through body diode D_S3 (as shown in next mode). This ensure that switch turns-on with a zero-voltage

Mode-6 (t₅ – t₆): Duty Cycle loss Mode

• G₄ and G₃ gating signal is existed –> Inverter output voltage, V_AB = – V_s
• Positive inductor current –> Body diode should conduct –> D_S4 , D_S3
• Negative inductor current –> Active switches should conduct –> S₄ , S₃
• Inductor current lags behind the voltage and it release the energy rapidly with the slope of
  • 
• The output inductor still is in discharge mode through both the diodes D₁ and D₂, hence secondry side voltage is zero (short circuited) and then primary side voltage is zero.

Mode-7 (t₆ – t₇): Power Transfer Mode

• G₃ and G₄ gating signals are existed –> Negative inverter output voltage, V_AB = – V_s
• negative inductor current –> Switches should conduct –> S₃, S₄
• Inductor charges with change in slope due to filter inductor adds up to the primary leakage inductor
  • 
• Output inductor starts charging through diode D₂ and power transfer to the load
  • 

Mode-8 (t₇ – t₈): Dead Zone Mode

• Only G₃ gating signal is existed –> No inverter output voltage, V_AB = 0
• Negative inductor current –> Switch should conduct –> S₃
• The turned-off switch S₄ capacitor starts charge and at the same time, switch S₁ capacitor starts discharging
• Slope of resonant inductor current is same as output inductor current as it is discharging through D₁ and D₂
  • 
• Dead time should be enough to completely charge and discharge the switch capacitors

Mode-9 (t₈ – t₉): Freewheeling Mode

• G₁ and G₃ gating signal is existed –> No inverter output voltage, V_AB = 0
• Negative inductor current –> Switch S₃ and body diode of S₁ should conduct –> S₃ , D_S1
• Body diode of S₁ provides the freewheeling path for the resonant inductor current with reduced slope since output indcutor comes into series
  • 
• In freewheeling operation, energy will not transfer to secondary since the output indcutor is in discharge throgh both diodes and hence secondary voltage is zero.

Mode-10 (t₉ – T_s): Dead Zone Mode

• Only G₁ gating signal is existed –> No inverter output voltage, V_AB = 0
• Negative inductor current –> Body diode should conduct –> D_S1
• The turned-off switch S₃ capacitor starts charge and at the same time, switch S₂ capacitor starts discharging
• Slope of resonant inductor current is same as output inductor current as it is discharging through D₁ and D₂
  • 
• Dead time should be enough to completely charge and discharge the switch capacitors

The summarization of these operation modes of PSFB resonant converter is given in operation summary