AN2626

MOSFET body diode recovery

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MOSFET body diode recovery

The cross section of a MOSFET device, illustrated in Figure 4, shows an intrinsic diode

between body and drain, that is the base-collector junction of the "parasitic" a NPN bipolar

transistor source-body-drain. In no or low load conditions this transistor could be turned on

and hence brings the MOSFET to the failure, by short circuiting drain and source while high

voltages are applied among them. In order to understand this possible failure we must

investigate the freewheeling and the ZVS steps. During step 2 the current freewheels into

the body-drain diode D2 (see Figure 4); since this is directly biased carriers are injected in

the N- epi (holes) and P body (electrons) regions of the device. Once Q2 is turned on, a

portion of the total current flows through the channel, the parasitic JFET, and the epi region:

the MOSFET is conducting in the third quadrant (see Figure 5). When the transformer

current changes direction the MOSFET Q3 conducts into the first quadrant (see Figure 6).

The internal body diode D2 now is reverse biased; since it is in parallel with the low

resistance channel regions its effective reverse voltage is low. This causes a slow minority

carriers extraction, especially from the N- region, since the holes have lower mobility than

the electron one. Obviously at low load operation the current flowing through the channel

MOSFET is much lower and so is the drop voltage; in this condition the body diode needs

more time to complete the reverse recovery, but the powering period is very short and could

not be sufficient to remove the minority carriers in the P-N junction.

Figure 4.

Current flow into the parallel body diode-channel MOSFET