Resonant Inductor

Resonant inductor selection is depends on many crucial factors

1. Power transfer capability/duty-cycle-loss
2. Dead time
3. ZVS range

{Power transfer capability, Dead time, ZVS }= function (Resonant inductor)

Power Transfer Capability

The power transfer capability of a converter is depends on the how much time the active switches are turned-on to deliver the energy to load. The mode (t4 – t7) in PSFB converter operation have the active switches turned-on position, but power is transferring only in mode (t6-t7) due to duty cycle loss in mode (t4-t6).

Duty cycle loss means no secondary voltage even after primary voltage exists. The amount of duty cycle loss depends on the slope of inductor current that changes flow direction.

Duty cycle loss = function (primary current slope)

primary current slope in this period is,

Vs/Lr

Resonant inductor increases -> current slope reduces -> time increases -> duty cycle loss increases -> power transfer capability reduces

Resonant inductor decreases -> current slope increases -> time decreases -> duty cycle loss decreases -> power transfer capability increases

Therefore,

Power transfer capability = function (resonant inductor)

Dead Time

The giving dead time should sufficiently enough to completely charge & discharge the switch output capacitances to achieve ZVS. The resonant circuit provides a sinusoidal charge/discharge with a time period of ‘t_r‘ (full-cycle).

The resonant circuit time period,

`t_r=2\pi\sqrt(L_r.C_{eq})` 

Here, the equivalent capacitance is combination of one leg switch capacitances and transformer input capacitance

`C_{eq}=C_{oss1}`  // `C_{oss4}` // `C_{tr}`

`C_{eq}=2C_{oss}+C_{tr}`

The peak of this sinusoidal capacitor voltage is after Tr/4 from switch turn-off. Dead time plays an important role on the ZVS, to obtain ZVS the condition is that:

`t_d \geq t_r/4`

Resonant inductor increases -> resonant circuit time increases -> dead time increases -> can select switch with high capacitance.

Resonant inductor decreases -> resonant circuit time decreases -> dead time decreases -> should select switch with low capacitance.

ZVS Range

For achieving ZVS at any condition is, energy stored in the inductor should be sufficiently enough to charge/discharge the switch capacitors with in dead time.

E_L > E_C

The stored Inductor energy available for both legs is different.