Why this H-Bridge Circuit Need additional Diode

Thread Starter

Sornpraram Xu

Joined Feb 24, 2019
21
Current I am studying about H-Bridge and I found a Schemetic this one


So I want to know why this circuits Have 4 additional Diode. As far as I know Diode is use to protection the back EMF right?
But isn't each Mosfet already have it's own Diode. Why it need to add more Diode into it?
Thank you.
 

crutschow

Joined Mar 14, 2008
23,334
Why it need to add more Diode into it?
It depends upon the current-carry capacity of the MOSFET diode, as to whether additional diodes are required.

Note that your schematic is missing the connections from the load to the bridge.
Also that's not a good bridge design, since there are large shoot-through currents between the top and bottom MOSFETs when the bridge changes state, due to the finite time it takes for the MOSFETs to turn from ON to OFF and vice versa.
 

Dodgydave

Joined Jun 22, 2012
8,389
Well as your circuit is drawn wrong anyway, it's better to have extra protection.. .
 

bertus

Joined Apr 5, 2008
20,049
Hello,

The diodes in the mosfets are the body diodes.
Those will not be enough for back EMF.
BTW, the mosfets are now not connected to the motor at all.

Bertus
 

crutschow

Joined Mar 14, 2008
23,334
The diodes in the mosfets are the body diodes.
Those will not be enough for back EMF.
Depends upon the MOSFET diode current rating.

For example, the IRF540 has a diode rating equal to its forward current rating (below):
That will be more than enough to handle any inductive transient.
upload_2019-4-24_13-35-40.png
 
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