AN2626

Topology description

5/13

●

energy equal to:

Equation 1

current continues to flow, with a like-linear shape (since the current is forced by the output

inductor) using the charge stored in the switch output capacitance C2. Simultaneously, the

primary current will charge the output capacitance C1 of Q1 from essentially 0 V to the

transition finishes when the Q2 source voltage exceeds the Q2 drain voltage sufficiently to

directly bias the internal body diode D2.

●

Now D3 is directly biased and the output inductor freewheels. The voltage across the switch

Q2 is equal to the drop on its internal body diode D2 hence the ZVS condition is verified.

The switch Q3 is turned on and the current, flowing through the primary side, now is shared

between the body diode D2 and the channel of MOSFET Q2. During the freewheeling state

both output rectifier diodes are forward biased, and hence the reflected voltage to the

primary is null.

Figure 3.

Typical waveforms in a P-S ZVS FB converter DC/DC

●

The switch Q3 is turned off, the energy available to complete the transition is:

Equation 2

It is much lower than it was in the lead leg, since the magnetizing and output inductance do

not contribute [4]. For this reason it is easier to miss the ZVS condition. If the energy stored

E

t

0

t

1

–

E

mag

E

output

reflected

–

–

1

2

--- L

leak

L

res

+

()

I

() NI

()

+

()

⋅

+

=

t

t

E

t

2

t

4

–

1

2

--- L

leak

L

res

+

()

I

() NI

()

+

()

⋅

=