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DPRG: Re:

Subject: DPRG: Re:
From: Ralph Tenny rten at metronet.com
Date: Mon Nov 22 15:21:08 CST 1999

Please note: The statement "MUCH lower ON resistance" relative to
P-channel MOSFETs is very true. However, compare these specs:
	Sampling from the International Rectifier summary sheet:
		N-Channel IRF7601  0.035 ohms, 4.6 Amps @ 70 deg. C
		P-Channel IRF7604  0.09  ohms, 2.9 Amps @ 70 deg. C
These two parts are "logic level" (direct drive from standard logic) and
are surface-mount devices. There may be some requirement of a minimum
amount of copper on the board for heat sinking.

		N-Channel IRFU3303 0.031 ohms, 33 Amps @ 70 deg. C
		P-Channel IRFU5305 0.065 ohms, 28 Amps @ 70 deg. C

These are through-hole parts requiring heat sinks for this level of power.

Since I'm concentrating on surface-mount parts, I didn't print out the
entire summary sheet. Go to www.irf.com to get the whole thing. To
actually buy parts, compare specs in catalogs to see which parts are
actually stocked somewhere.

Bottom line: the listed parts don't really have high ON resistance, even
in the P-Channel versions!
Ralph

 

On Tue, 23 Nov 1999, Larry Kerns wrote:

> I might also add that the N-channel MOSFETs have a MUCH lower "on" resistance
> than a comparable P-channel device thus causing the P-channel device to dissipate
> MUCH more power and thus heat.  To buy a comparable P-channel device with lower
> "on" resistance is what costs all of the money.
> 
> Larry Kerns
> ==================================================
> 
> Jim Brown wrote:
> 
> > 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
> >
> > - - ____ - - - - - - ___ - - - - - - - - - - - - - - - - - - - - - - - - - -
> >     \/\_\@  ____    /  /\ __  ___          ___    http://www.dprg.org
> >     / / /\ / / /\  /--/ //\_\/\_/\ /\/\/\ /\_/\  brownjim at airmail.net
> > /__/ / / // / / / /__/ // / /__/ //_/_/ // // /  (972)495-3821
> > \__\/  \/ \/\/\/  \__\/ \/  \__\/ \_\_\/ \/ \/ james_121 at yahoo.com
> > jim_brown at bigfoot.com jbrown at dprg.org http://www.cyberramp.net/~jbrown
> > Cole's Law:  Thinly Shreaded Cabbage
> 
> 
> 

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