AN443 APPLICATION NOTE

2/14

Calculation of sharing resistors

The calculation of these resistances is based on the worst case situation.

The maximum imbalance in blocking voltage when n diodes are connected in series occurs when (n-1)

diodes have the maximum leakage current and one diode D1 has the lowest possible leakage current.

In this case D1 will support the highest voltage V1, and this tendency is aggravated by the assumption

that the corresponding resistor R1 is at the upper limit of its tolerance (a), while all the others are at the

lowest limit so,

R1 = R(1+a)

R2 = R3 =....Rn = R

In order to calculate the current in the string we approximate the reverse characteristic with a straight line.

We define the slope by the coefficient k according to Figure 3.

Figure 3. Reverse characteristic model of a fast rectifier.

With k = 0.8

The leakage current IRM of diodes D2 ... Dn under the blocking voltage V2 ... Vn is:

operating junction temperature.

In these conditions the leakage current of diode D1 is:

I

IRM

IR

k.IRM

VR

VRRM

V

I

R

I

() k

V

–

()

V

RRM

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

+

⋅

+

I

R2M

I

R3M

....I

Rn

I

V

–

()

V

RRM

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

+

==

=

I

R1

I

RM

∆I

R

–

() k

V

–

()

V

RRM

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

+

=