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May 2004

AN443

APPLICATION NOTE

SERIES OPERATION OF FAST RECTIFIERS

R

EV. 2

INTRODUCTION

The use of several rectifiers connected in series is necessary to obtain voltage ratings beyond the capa-

bilities of single diodes and also when some special requirement, such as very low switching losses, re-

quires the implementation of several low voltage ultra fast diodes.

Rectifiers connected in series tend to unequally share the voltage across the string in blocking conditions

because of the variations in reverse characteristics: leakage currents and turn-off switching parameters.

To ensure that each diode operates within its voltage rating, it is generally necessary to add a voltage

sharing network.

This paper gives the rules of calculation of this auxiliary network and shows how this circuit could be op-

timized: reduction of power dissipation and cost.

STEADY STATE VOLTAGE SHARING

The difference in blocking characteristics results in unequal steady state voltage (see Figure 1).

Figure 1. Dispersion of diodes reverse characteristics. The reverse current through the string D1,

In order to equalize the voltage, a resistor is connected across each diode (see Figure 2).

Figure 2. Use of shunt resistors for steady state voltage sharing

Dn

D2

D1

Vn

V2

V1

V

IR

I

VRRM

D1

R1

IR1

V1

D2

R2

IR2

V2

D3

R3

IR3

V3

Dn

Rn

IRn

Vn

+VM