AN443 APPLICATION NOTE

10/14

This condition can be met by using very fast rectifiers in applications where the dIF/dt is low (like in some

EQUALIZATION BY TRANSIL DIODES

TRANSIL diodes are avalanche diodes designed for operation in breakdown characteristic and they are

used as clamping devices in a wide field of applications. To limit the voltage across the rectifiers of a string

below the maximum value, TRANSIL diodes can be used according to diagram in Figure 12.

Figure 12. Voltage sharing by TRANSIL diodes

TRANSIL operates as a voltage limiter at steady state, during the switching phase, and also in case of

external voltage transients.

Steady state

In the blocking condition, the TRANSILS connected across the diode D1 (which has the lowest reverse

current) operate in the breakdown characteristic. The current through these TRANSILS is IR and the pow-

er dissipation is:

.

(

δ = duty cycle)

lower than in the case of sharing by resistors and TRANSILS in axial packages can be used.

Switching phase

When the fastest diodes of the string switches off, the TRANSILS across it operate in breakdown charac-

teristic, and the reverse recovery current of the other diodes flows through these TRANSILS. The charge

remaining in the string at this moment is:

(n-1)

and we can estimate the maximum energy in the TRANSILS with

This relation does not take into account the losses due to the capacitive current through the string.

∆Q

RR

V

–

() C

Jn

+

[] V

–

n1

–

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

<

∆Q

RR

C

V

M

–

()

n1

–

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

<

D1

D2

D3

Dn

+VM