Considerations for series connection of IGBT and MOSFET switches
28 May 2008
Power Electronics / Power Management
Information from Ixys
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 RS is required so that the switch with the lowest leakage current is not forced into avalanche and a dynamic voltage sharing capacitor CS 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 RS and CS and cost of the total switch.
The values of the resistors RS and capacitors CS can be computed from the following:
1. Static voltage sharing resistor RS:
RS ≤ (nVS(MAX) – VDC)(n – 1)-1 IS-1
where: n = number of devices in series
VS(MAX) = maximum allowable voltage across a switch (normally 80% of the maximum switch voltage rating)
IS = maximum leakage current of a switch.
Power dissipation in resistor RS:
2. Dynamic voltage sharing capacitor CS: Assuming no reverse current flow through the switches, then the major factor to consider in sizing capacitor CS 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 - VS(MAX)
DtD(ON) = difference in turn-on times
Solving for CS:
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