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H-bridge. Diodes in at the MOSFET gate. What are they for?

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H-bridge. Diodes in series with MOSFET gates. What are they for?

Folks,

Sometimes a resistor is added in series with MOSFET gate. Together with gate capacitance, it forms an RC, which slows down the turn on of the MOSFET, which sometimes is useful. On the web, I saw several schematics of H-bridges having a diode parallel to the resistor. See an example below.

In this picture, the diodes in question are MBR150 in series with 100Ω resistors. What are they for?
My guess is that they make the MOSFET turn on more slowly than turn off. This is a kind of a dead time generator, which prevents a vertical shoot-through. What do you think?

Any suggestion, insight or reference is really appreciated!

Cheers,
- Nick

dcmotorcontroller.jpg
 
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Hi
My guess is : this high speed diode used for making turn off quickly it's MOSFET than turn on the other.
Regards.
 
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    kender

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Yes, the diode is meant to make the H-L gate transition fast,
after having put the 100-ohm resistor in there to make the
turnon transition drain dV/dt slow. So the Schottky Rs ought
to be on the order of, or less than the 100 ohms (or else it
is just a waste of money).

The diode capacitance is trivial compared to the FET, you
can bet onthat.
 
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    kender

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Hi

The diodes kept in parell to 100 OHM resitor, wile turning on the FET minimum rise time is rquired to smooth turn ON, While turn OFF normally delay in falltime not required because the QGD has to discharge fastly. Due to this the diode is kept for discharge path

Thanks
 
I can't see a picture, but I assume you're talking about a diode connected in anti-parallel across the gate resistor, with anode connected to the gate and cathode to the input from where gate signal comes from.

This is to discharge the gate capacitance (almost) instantaneously. The gate resistor slows down turn-on and also turn-off (gate discharge). But by placing this diode across the resistor, the resistor is bypassed (as the diode has a lower resistance than the gate resistor when forward biased) when the gate capacitance is to be discharged and it is almost instantaneously discharged.

Hope this helps.
Tahmid.
 
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