Figure 1 shows the typical RC snubber networks for voltage sharing for switches (S) connected in series in a capacitive discharge circuit. A static voltage sharing resistor R_S is required so that the switch with the lowest leakage current is not forced into avalanche and a dynamic voltage sharing capacitor C_S is needed so that the slowest switch is not forced into avalanche voltage breakdown during turn-on. A compromise must be reached between the number of switches in series, values for R_S and C_S and cost of the total switch.



The values of the resistors R_S and capacitors C_S can be computed from the following:

1. Static voltage sharing resistor R_S:

R_S ≤ (nV_S(MAX) – V_DC)(n – 1)^-1 I_S^-1

where: n = number of devices in series

V_S(MAX) = maximum allowable voltage across a switch (normally 80% of the maximum switch voltage rating)

I_S = maximum leakage current of a switch.

Power dissipation in resistor R_S:



2. Dynamic voltage sharing capacitor C_S: Assuming no reverse current flow through the switches, then the major factor to consider in sizing capacitor C_S is the voltage buildup on the last switch to turn-on. It is desirable to prevent the MOSFET from avalanching in order to limit its turn-on losses. The worst case scenario is that the switch sustaining the highest voltage is also the slowest to turn on.

Then:



where: ΔV = Avalanche voltage - V_S(MAX)

Dt_D(ON) = difference in turn-on times



Solving for C_S: