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AN443 APPLICATION NOTE

Example

– Given:

– Part number used: BYT12PI-1000

– Power dissipation per diode: P = 7W

– Case temperature: Tcase = 52°C

– Rectifier specification:

– Rth j-c = 4°C/W

–Problem:

Calculation of sharing resistors for 3 diodes in series.

– Solutions:

a) Simplified method:

With

n = 3

Thus

Rmin = 100 k

Ω

Power dissipation per resistor: 3.45 W! (with duty cycle

δ = .5)

b) Calculation with relation (2):

General data for fast rectifiers:

k = 0.8

Intermediate calculations:

Tj = P.Rth j-c + Tcase = 80°C

=0.85mA

Assuming we use resistors with 5% of tolerance, then a = .10

Let: Rmin = 220 k

Ω

Power dissipation per resistor = 1.58W (with

δ =.5)

Question: is it possible to remove the sharing resistors ?

With the relation (1) we can find the value of V1 when the value of R tends to be infinite. Then we calculate

the condition to have

R

nV

RRM

V

M

–

n1

–

()I

R

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

<

R

V

+

() V

+

()

–

()

∆I

+

() n1

–

() I

RM

–

1k

–

() 1a

+

() nV

RRM

V

M

–

()

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