AN4150

Synchronous buck converter description

Doc ID 023526 Rev 1

5/20

During the deadtime and before the HS turn-on, the LS device must remove the charge

stored in the LS body-drain diode (reverse recovery charge process) before sustaining

drain-source voltage. Therefore, the body-drain characteristics, in terms of reverse recovery

current and charge, seriously impact the Power MOSFET switching behavior and converter

power losses, especially when the converter switching frequency rises up.

In a synchronous buck converter, the low-side drain is subjected to fast positive/negative

slopes and high voltage spikes, which can exceed the low-side absolute maximum voltage,

degrading the Power MOSFET reliability up unto its failure. Therefore, the right Power

MOSFET choice and system configuration, device placement on the board, and the

optimization of the stray inductances and parasitic, allow an important phase node spike

reduction, improving the converter performance.

defined as:

Equation 1

So, the input-output relationship of a buck converter is given by:

Equation 2

1.2

Low-side switching transients analysis

The low-side device works as a “synchronous rectifier” because its on-time interval is

synchronized to the high-side FET on-time; moreover, it works alternatively in an OFF state

the load current flows from source to drain.

D

t

ON

T

S

---------

=

V

OUT

DV

IN

•

=

Figure 2.

HS/LS waveforms during LS turn-off

HS waveforms

LS wavefor

LS wavef

e o

ff r

(1)

(2)

(3)

(4)

AM14939v1