User`s guide

SMPS AC/DC Reference Design User’s Guide

2.2 FULL-BRIDGE ZVT CONVERTER

The main power circuit for a ZVT Full-Bridge converter is shown in Figure 2-5. It is a

standard Full-Bridge converter, but with additional series resonant inductance, which

limits the rise rate of current at switching transitions and can eliminate turn-off switching

power dissipation in the MOSFETs. The stray leakage inductance of the transformer

forms part of the series resonant inductor and, in this particular design, is large enough

to ensure quasi-resonant operation over 80% of the operating power range without the

need for an additional inductor. The secondary-side high-frequency rectification is

normally done by using ultrafast recovery rectifiers or Schottky diodes. Alternatively,

lower loss rectification can be achieved by using MOSFETs operating as synchronous

rectifiers with primary-side commutation control, and this is the preferred solution in this

reference design.

ZVT operation occurs when the stored energy in the inductor is transferred to the

capacitor in parallel with the MOSFET. In this design, the stray output capacitance of

the MOSFET is large enough to not require additional capacitors in parallel. From

Reference 3 (see Appendix C. “References”), the equation relating energy in the

MOSFET output capacitance and the series inductance for ZVT operation is given by

Equation 2-9.

EQUATION 2-9:

This ensures that there is more than enough energy to charge the MOSFET output

capacitance and maintain ZVT operation. Note that at low output power, there will be

far less energy stored in the resonant inductance so ZVT operation will be lost. The

inductor is therefore selected based on the minimum operating output power for ZVT

switching.

The modulation control scheme required for ZVT operation of a Full-Bridge converter

is phase-shifted PWM. The ideal power stage waveforms for the circuit are shown in

Figure 2-6. The ZVT transition in the switch is short in comparison with the primary

current transition time. This time, Δt, is dictated by the resonant inductance, L

, which

is given by Equation 2-10.

EQUATION 2-10:

The control duty cycle is limited in a ZVT due to the time taken for the current to rise/fall

during switching transitions. The maximum duty cycle, D

max

, is achievable under the

ZVT operating conditions given by Equation 2-11.

EQUATION 2-11:

Rpri Rin

ICV

≥

Rpri

Δ=

max

=−