500V CoolMOS

10

Application Note AN 2012-04

V1.0 April 2012

The MOSFET channel is turned ON and diverts a

portion of the current through the channel away

from the body diode, that is still forward conducting

(MOSFET can conduct current in both directions).

This lower current flowing through the body diode

will reduce the generation of minority carriers but

will not stop it.

The external circuitry reverses the current flow

through the device -> small amount of reverse

current flows in body diode (small due to the very

weak voltage generated by very low current flowing

in the low resistive channel, especially at light load

operation). Some minority carriers will be removed

from the p-n-junction, but not all due to a

conduction period that is short in relation to the

intrinsic carrier lifetime.

If the MOSFET completely turns ON the current will

be completely diverted to the channel, but if the

MOSFET turns OFF when there are still minority

carriers in the body diode the following happens:

The MOSFET will begin to block voltage imposing a

higher reverse voltage on the body diode, with high

dv/dt. The application of high reverse voltage on the

body diode will sweep the remaining carriers across

region are swept towards the p+ body. -> if this

reaches a magnitude sufficient to activate the

intrinsic bipolar transistor, second breakdown will

occur destroying the MOSFET.

In a conventional MOSFET the hole current, fed by reverse recovery charge, flow laterally into the p doped

region crossing the area below the n region before they reach the top side of the device below the gate

npn-junction and consequent triggering of parasitic BJT.