AN443 Datasheet, PDF (1/14 Pages) STMicroelectronics – SERIES OPERATION OF FAST RECTIFIERS

AN443
APPLICATION NOTE
SERIES OPERATION OF FAST RECTIFIERS
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,
D2, ....Dn is I_R and the voltages across the diodes are respectively V1,V2, ....Vn
I
D2
Dn
D1
IR
Vn
V2
V1
VRRM
V
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
D1
D2
D3
IR1
IR2
IR3
R1
R2
R3
V1
V2
V3
Dn
+ VM
IRn
Rn
Vn
May 2004
REV. 2
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