[SOLVED] PWM MOSFET excessive power dissipation losses?

[KlausST]

Hi,

Why is symbol in datasheet drawn like there is a parallel diode already integrated with MOSFET?

When designing a MOSFET then (accidentally) there is the so called body diode.
It is not specially designed in.

But this diode often is too slow with reverse recovery time. When switching inductive loads in a half bridge configuration, then this diode becomes conductive. With high switching frequencies the big reverse recovery time makes problems. It causes big but short current pulses. This generates heat as well as EMI.
To prevent the diode from becoming conductive one typically uses externah shortly diodes with their lower forward voltage.
Another approach is to put more effort in design and production of a MOSFET to
* make the body diode faster
* to prevent it to exist (like in standard FETs = non MOSFETs)
* or to additionally design a Schottky diode into the MOSFET-package.

Klaus

 

[betwixt]

The diode in the MOSFET body will clamp negative voltages on the drain to ground (assuming the source pin is ground). What was suggested was a diode wired across the load so it also clamps any reverse polarity from the LED wiring as well. It has to be a fast recovery diode such as a Schottky type and wired cathode to + supply, anode to the MOSFET drain (opposite way to the LEDs) so it does NOT conduct in normal operation.

Brian.

 

[msmiha]

Thank you every one for explanation.