AN827 APPLICATION NOTE

4/20

The capacitor chosen has a voltage rating of 450VDC. The overvoltage trip level of Pin 3 (OVP) must be set

below 450VDC.

Power Mosfet

The power mosfet, Q1, is used as the active switch due to its high frequency capability, ability to be driven di-

rectly from the controller and availability. The main criteria for its selection include the drain to source breakdown

voltage (BVdss), delivered power and temperature considerations.

Voltage Rating:

The power mosfet has to sustain the maximum boosted output dc voltage according to the following equation:

Power Rating:

The main parameters to consider are Rdson and the thermal characteristics of the package and heatsink. The

main losses in the power mosfet are the conduction and switching losses. The switching losses can be sepa-

rated into two quantities, capacitive and crossover losses. The switching losses are dependent on the mosfet

current di/dt. The maximum conduction (on-state) power losses can be calculated according to the following

equations:

Where:

The capacitive switching losses at turn-on are calculated as follows:

I

Qm sm ax

P

out

η

2 V

irms min

⋅

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

2

16

2 V

irms min

⋅⋅

3

π V

out

⋅

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

–

500

0.9

----------

288

⋅

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

2

16

2 88

⋅⋅

3

π 400

⋅

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

–

⋅

=

⋅

=

P

capac i

c e

tan

3.3

C

oss

V

out

1.5

1

2

---C

ex t

V

out

2

⋅

+

⋅



 f

sw

2W

=

⋅⋅

=