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Determining diodes for dual H-bridge

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crazybuoy

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I want to create dual H-bridge using Mosfets for stepper motor that needs to run at 30-40V & 10-14A. I have N-P mosfets to work at more than 10-14A.

What amp rating of diodes should be to use in this dual H-bridge according to application requirement.

I have 10A diodes, is it better to use them here?
 

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spec

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I want to create dual H-bridge using Mosfets for stepper motor that needs to run at 30-40V & 10-14A. I have N-P mosfets to work at more than 10-14A.

What amp rating of diodes should be to use in this dual H-bridge according to application requirement.

I have 10A diodes, is it better to use them here?
External diodes are not required- the NMOSFETs and PMOSFETs already have an intrinsic diode.

spec
 

MikeMl

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Much more important than the diodes: how are you driving the four gates?
 

MikeMl

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The OP asked about the diodes, not a lecture.:wideyed:
My, are you a bit testy?

Folks that come to grief with H-bridges, especially with 30-40V rails, do so because of how they drive the gates; not because they include diodes around the FETs or not.
 

Triode

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As a beginner I appreciated the extra info people would add in. When you're just starting sometimes you don't even know you should ask about something. Many people don't realize that mosfets don't always switch efficiently off of a logic signal.
 

crutschow

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............... but you appeared to be trashing what I said, without reason.
I didn't view it as a trash at all, just an addition to this thread about likely pitfalls of that circuit.

Shoot through current from the top to bottom transistors during switching is also a concern with that circuit.
 

MikeMl

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All gates will be driven by L298 output signals and L298 will run with max 5V (2-3A) taken from ATX.
If the h-bridge high voltage and the L298 Vs is 35V, and Vss is 5V (logic supply), this will not work (you will instantly blow up the Fets).

First, you are over-driving the gates. The max gate voltage for both NFets and PFets is 20V, so the NFet gate voltage is constrained to be between 0V( NFet OFF) and 20V( NFet ON), while the PFet gate voltage is constrained to be between 35V( PFet OFF) and (35-20)=15V( PFet ON).

Second, you cannot connect the gate of the PFet to the gate of the NFet on each side of the h-bridge. It takes four non-overlapping gate drive signals to protect the h-bridge from destructive shoot-through caused by having the top PFet not yet fully OFF while turning-ON the NFet on the the SAME side of the bridge. The two gate signals (on each side) must be separated in time (so it takes four port pins on the microprocessor, not just two) so that the PFet completely turns OFF before you turn ON the NFet and conversely you must turn OFF the NFet before turning ON the PFet. The simplest way to generate the four non-overlapping gate drive signals is to use a microprocessor and use software-generated delays to guarantee the appropriate non-overlap between the various gate signals.

The L298 does not generate the needed non-overlapping gate drives even if you use both drivers inside the L298. Nor does it solve the problem of offsetting the gate drive to the PFets by 15V (as in the example I gave above). You should be looking for a "high-side H-Bridge driver".
 
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crazybuoy

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If the h-bridge high voltage and the L298 Vs is 35V, and Vss is 5V (logic supply)
How about if I run L298 with 12 or 15 V (Pin 4) and logic supply 5V (pin 9)?

You should be looking for a "high-side H-Bridge driver".
I tried to google "high side h bridge driver". I could not find any. Do you mean IR2104? Please suggest one or more if it is not.

You can also suggest any schematic using "high side h bridge driver" with Mosfets.
 
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