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DPRG: Re:

Subject: DPRG: Re:
From: Jim Brown brownjim at airmail.net
Date: Fri Nov 19 06:26:11 CST 1999

At 11:24 AM 11/19/99 -0000, you wrote:
>   DEAR SIR,                A THIRD YEAR STUDENT AT EAST LONDON
>UNIVERSITY,CURRENTLY RESERCHING  CLASS D AUDIO AMPS.I WOULD BE GRATEFULL IF
>YOU COULD ENLIGHTEN ME ON THE  FOLLOWING: 1.WHAT IS THE RATIONALE BEHIND
>N-CHANNEL ONLY  H-BRIDGES,AND COMPLEMENTARY H-BRIDGES/ 2.WHAT DRIVE
>CIRCUTERY WOULD YOU RECOMMEND BETWEEN  MODULATOR,(GENERATING PWM PULSE),AND
>H-BRIDGE? I LOOK FOWARD TO HEARING FROM YOU.                              
>MIKE 

N-Channel mosfets are very cheap like $0.39 cents, while P-Channel mosfets
are very expensive like a few dollars.  If you can use all N-Channel mosfets
you might save yourself some cash especially if you plan on making a bunch of
them.  The way it works is to overcome the breakdown voltage of the N-Channel
gate by doubling the voltage (ok, more like 7v or 10v higher).  If you 
already have the higher voltage supplied, it might make sense to go with all
N-Channels on your h-bridge, but if you have to add a voltage doubling
circuit, then you might as well just buy P-Channel mosfets.  The pulse
generating circuit could be done a number of ways.  Egads, you could use
a 555 timer (but don't tell anyone I recommended that), or if you're already
hooked to a microcontroller, have software spit out a PWM signal (my
prefrence), or you could use some sort of pwm timing chip, some people
even just use a EPROM programmed with about 16 different pulse widths and
send them to the h-bridge.  However, unless you just need the kind of
amperage a mosfet can supply (like 50amps) and if you really only need
at most 2-3 amps go with a canned h-bridge circuit like the L298 or
the LMD18200.  These can drive motors up to about the size of a coke can
which fits most needs.  It's much easier and cheaper than designing your
own circuit.

Here's some web pages I hope will help:  
	http://www.dprg.org/hbridge.html
	http://www.dprg.org/l298.html  
	http://www.dprg.org/hbridge_theory.html

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jim_brown at bigfoot.com jbrown at dprg.org http://www.cyberramp.net/~jbrown
Cole's Law:  Thinly Shreaded Cabbage

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