AN3276

Existing solutions

Doc ID 17975 Rev 1

5/22

This soft switching solution is well known, but it requires that several technical criteria be

met:

●

Reset the current in the inductor L at each switching period, whatever the variations of

the current, and input and output voltages.

●

Recover the saved inductive energy without losses.

●

Limit any overvoltage and overcurrent stress in the semiconductor devices.

●

Keep cost down when adding any device.

●

Maintain a similar power supply density.

There are many circuits that are classified in two families of recovery circuits:

●

active

●

passive

1.3

Active recovery circuit

In the active recovery circuit family, the zero voltage transition (ZVT) [see Section 5:

References, 3.] shown in Figure 4 is well known by designers. This circuit allows both

switch-on and switch-off power losses to be removed.

Figure 4.

Zero voltage transition (ZVT) active recovery circuit

A theoretical study indicates that ZVT is an excellent topology for the PFC application, since

all the switch losses are removed. In addition, this circuit can work whatever the input and

output power variations. Nevertheless, in practice, the recovery current from the boost diode

and minimum duty cycle. During the reset current in the small inductor L, the recovery

dynamic behavior of the PN diode affects the global ZVT efficiency because conduction

times in the transistor should increase and a dissipative snubber is mandatory to reduce the

electrical stress across the semiconductors.

In terms of cost the ZVT circuit requires an additional power MOSFET and a specific PWM

controller. Several derivative circuits of the ZVT circuit have the same technical issue and

their higher price makes these circuits less than ideal for mass market applications.

Therefore, the passive recovery circuit can be more attractive.

VOUT

Vmains

DB

D2

D1

L

TZVT

TR

LB

Rdamping

Ddamping