AN443 datasheet(5/14 Pages) STMICROELECTRONICS

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AN443 APPLICATION NOTE

Example

– Given:

– Maximum blocking voltage: VM = 2500V

– Part number used: BYT12PI-1000

– Power dissipation per diode: P = 7W

– Case temperature: Tcase = 52°C

– Rectifier specification:

–VRRM = 1000V

–IR (Max at Tj = 100°C) = 2.5mA

– Rth j-c = 4°C/W

–Problem:

Calculation of sharing resistors for 3 diodes in series.

– Solutions:

a) Simplified method:

With

n = 3

VRRM = 1000V

VM = 2500V

IR = 2.5mA

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:

∆IR = 0.85 IRM

k = 0.8

Intermediate calculations:

Tj = P.Rth j-c + Tcase = 80°C

IRM = IRM(80°C)

= IRM(100°C) exp[-0.0054(100-80)]

=0.85mA

∆IRM = 0.72mA

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

V1 < VRRM

RRM

–

()I

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

RRM VRRM na

() V

M 1a

()

–

()

∆I

RVRRM 1a

() n1

–

() I

–

() 1a

() nV

RRM

–

()

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