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AN827 APPLICATION NOTE

Where:

The estimated crossover switching losses (turnon and turn-off) are calculated as follows:

Where:

To reduce the turn-off losses in the mosfet, an RCD turn-off snubber has been employed. The capacitor value

is calculated as follows:

Therefore, use C11 = 820pF, 1000VDC rating

The resistors, R23-24, must dissipate the energy stored in the snubber capacitor upon turn-on of the power

mosfet. The capacitor must fully discharge during the switching cycle.

The time constant of the RC combination is determined as follows:

The power dissipated in the resistors, R23-24, is calculated as follows:

Therefore, use R23 = R24 = 510

Ω, 3W rating.

The power mosfet chosen is the STMicoelectonics Part Number STW20NA50.

perature at a safe level, the mosfet is attached to an AAVID Heatsink Part Number 61085 with a thermal resis-

tance of 3.0°C/W. This will keep the mosfet junction temperature at a safe level at worst case conditions, low-

line input voltage (88V) and full load (500W).

The thermal resistance of the heatsink may need to decrease depending upon the ambient temperature, type

of enclosure (vented or non-vented) and the method of cooling (natural or forced convection).

Boost Diode

The main criteria for the selection of the boost diode, D2, include the repetitive peak reverse breakdown voltage

Voltage Rating:

The voltage rating of the boost diode is determined by the same equation as for the power mosfet. The value

C11

I

Q1pk

t

rise

⋅

∆V

out

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

8.92 40ns

⋅

400

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

892pF

==

=

R

1

10

------

1

f

sw

C11

⋅

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

1524

=

⋅

≤

P

dis s

1

2

---C11 V

out

2

f

sw

1

2

--- 820pF 400

2

80k

5.25W

=

⋅⋅

⋅

=

⋅⋅

=