DPRG mailing list archives: [DPRG] All N-channel MOSFET H Bridge a myth ?

Sun Oct 8 12:05:25 CDT 2006

Vimal,
 All N-Channel H-Bridges are not a myth, it can be done. However, they
have a few problems that must be overcome which makes them less
desirable than the alternative. 
 First off, an all N-Channel H-Bridge requires that the two upper FETs
be supplied with double the supplied voltage in order to turn them on.
In other-words, if your motor voltage needs to be 12v, then the upper
two FETs require 24v to turn on. Most designs do not offer 2 voltages,
so that becomes a problem. But don't fret, this can be accomplished by
using a voltage doubling IC like the ICL7662 which does the job just
fine. Here's a link to a working system.

http://www.1sorc.com/guide/hbridge1.html

 Another downfall to this type of N-Channel H-Bridge is that it will go
into whats known as "Runaway overheating" if continuously switched
between forward and reverse. This can be a huge problem for robot
applications, especially battlebot type robots where reversing
constantly and quickly is a must. This too can be overcome by giving
the circuit time for the EMF to collaps between dirrections. It
litterally takes just a split second for the feild to collaps.  You can
write a short pause between directions into your code and solve this
problem completely. 

 So, there you have it, N-Channel Myth busted.

-Dan Gates

--- Vimal Shankar <vimalsshankar at yahoo.com> wrote:

> I know, I know...
> 
> All the veterans thinking "Here starts the H Bridge
> topic again".
> 
> Crops up every century, now and then, I see....
> 
> But the problem is, reliable information is really
> hard to come by.
> 
> Found a lot of bipolar MOSFET designs on the web, and
> found a few all N Channel designs too, but the best
> documented one, unfortunately, has "OBSOLETE" marked
> on it by the author.
> 
> Is an all N-channel MOSFET H Bridge a myth ?
> 
> PS : Does this email warrant a few links to
> substantiate my thoughts ? Let me know.
> 
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