AN628 APPLICATION NOTE

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Power Switch

A power MOSFET is the active switch used in most application for its frequency features. It will be selected ac-

cording with the output boosted voltage and the delivered power. There are two contributions for power losses

in the mosfet: conduction losses and switching losses. The on-state power losses can be calculated using the

formula:

One extimation of the switching losses can be done considering two separated quantities:

.

where:

To reduce the crossover losses a snubber network can be used.

Booster diode

The booster diode will be selected to withstand the output voltage and current. Moreover, it has to be as fast as

possible in order to reduce the power switch losses.The STMicroelectronics Turboswitch™ diode series match

this specifications, and are expecially suitable for this application.

The diode power losses can be split in two contributions: conduction losses and switching losses. The conduc-

tion losses can be estimated by:

2

Sense Resistor

The sense resistor produces the signal for the current feedback loop and for the overcurrent protection circuit.

An easy criterion to choose the sense resistance is to minimize the power dissipated assuring a sufficient signal to

noise ratio. In much high power applications, it could be considered the magnetic sensing approach (see fig. 22).

Figure 22. Magnetic Sense

P

capacitive

10

3

------ C

oss

V

o

1.5

1

2

---C

ext

V

o

2

⋅

+

⋅⋅





f

sw

⋅

≈

Load

iD

iQ

L

Rs

Co

-Vrs