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AN443 APPLICATION NOTE

Figure 7. Use of shunt capacitors for transient voltage sharing

Calculation of sharing capacitors

The calculation of capacitance C is also based on the worst case situation.

We suppose also that the corresponding capacitor C1 is at the lowest limit of tolerance (a) while the others

are at the upper limit

so:

C1 = C

C2 = C3 = ... = Cn = C(1+a)

When all the stored charges of diode D1 have been evacuated, the charge remaining in the other diodes

is

At this time the voltage across D1 is V1 and the voltage across the other diodes of the string is:

So these diodes can be assimilated to a capacitor

Figure 8. Equivalent diagram when D1 switches off. Diodes D2, D3, ....Dn are equivalent to a

In these conditions the voltage across D1 is:

D1

C1

V1

D2

C2

V2

D3

C3

V3

Dn

Cn

Vn

+VM

V

2

V

3

....V

n

V

M

V

1

–

n1

–

()

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

==

=

C

D

∆Q

RR

V

n

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

∆Q

–

()

V

M

V

1

–

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

==

V1

V2

V3

Vn

+VM

D1

D2

D3

Dn

C

C

C( 1+ a)

C( 1+ a)

C( 1+ a)

D

C

D

C

D

V

1

∆Q

–

() CV

+

()

+

Cn

a

+

()

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

=