Booster diode

AN2649

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3

Booster diode

The booster diode is 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 (please refer to 3 in

Bibliography on page 18). The STPSC806D (600 V/8 A) SiC diode matches these

specifications and is especially suitable for this application. This part offers the best solution

for the continuous current mode operation due to its very fast recovery time, 15 ns typical.

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

losses.

The conduction losses can be estimated by:

Equation 5

with

Equation 6

The switching losses are:

Equation 7

where

●

●

●

●

●

●

Qrr= total inverse recovery charge of diode

At low-line input voltage the conduction losses are bigger with respect to the case of high-

line voltage while the switching losses are always negligible due to the small value of Qrr for

every value of di/dt of current imposed by the MOSFET (at turn-on). The last instance is not

true for the silicon diode, because Qrr is bigger and greatly depends on the di/dt value.

Furthermore the silicon diode performance are temperature-dependent (Vf, recovery

current, etc.), while the SiC diode has the same behavior also for high temperature (please

refer to 1 in Bibliography on page 18). In the worst case:

Equation 8

Equation 9

value. At startup the output capacitor sinks much current (it is discharged) and the boost

P

Don

V

to

I

out

R

d

I

2

Drms

⋅

+

⋅

=

I

Drms

P

out

V

lpk

-----------

16 V

lpk

⋅

3

π V

out

⋅⋅

---------------------------

=

P

sw

V

out

Qrr f

SW

⋅⋅

=

P

Don

V

to

I

out

R

d

I

2

Drms

0.9V 0.5A

0.065

Ω 1.28

2

A

2

0.55 W

=

⋅

+

⋅

=

⋅

+

⋅

=

P

SW

0W

≅