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From: "Richard Soennichsen"
Subject: Re: Driving High current with PWM
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
NNTP-Posting-Date: Tue, 08 Oct 2002 17:16:27 EDT
Organization: Prodigy Internet http://www.prodigy.com
Date: Tue, 08 Oct 2002 21:16:27 GMT
Thank you for the info. It appears that my resitor values are wrong and
that explains the behaviour I am seeing. Thanks again.
"John Popelish" wrote in message
> Richard Soennichsen wrote:
> > I need some help with the analog portion of a simple design. I need to
> > drive high current loads (~5A) with a micro driving PWM outputs. The
> > digital side is a snap for me but the analog, which seems like it should
> > simple is giving me fits.
> > The micro drives four outputs of PWM. (0-5V transitions) I need
> > to switch the HIGH side of the loads (12V lamps). I have experimented
> > PNP power transistors which functionally work but get VERY hot, hot
> > to de-solder themselves. Obviously the design is not sound. I am
> > the base of the PNP power with an NPN BJT. The base of the PNP power
> > pulled up to 12V through a 5K resistor, this keeps it off unless the NPN
> > is on.
> > Also, it sure would be nice to use NPN power BJTS as then I could
> > them to the chassis as a heatsink but unfortunately the tabs are at 12v
> > well, the chassis is not.
> > Thanks
> > Richard
> To get low on state voltage, you need to have plenty of gain in the
> PNP switch, which implies that it is rated for more than 5 amps. A 10
> to 15 amp rated transistor should do. Here are some rough ideas for
> drive values.
> The base drive should be a pretty fair fraction of the output current,
> say, 1/20 ( 5 amps /20 = 250 ma.) (If you have a high gain PNP/ you
> may get by with only 1/50 of output current for the base.) And the
> turn off current should be a fair fraction of that, say 1/5 or 50 ma.
> This means that the base to emitter shunt resistor should pass about
> 50 ma when the base is forward biased at about .7 volts, so the base
> to emitter resistor would be .7/.05=15 ohms. Using a higher value of
> base resistor to save drive current will slow the turn off and produce
> more heat in the PNP during this transition, so a tradeoff is
> possible.) So the NPN drive transistor has to deliver both the base
> drive and the turn off current or about 300 ma. This implies that
> about .3 amps * 12 volts = 3.6 watts of heat is going somewhere when
> the NPN driver is on. If the NPN regulates the 300 ma, then it will
> have ot dump this power. If the NPN saturates, then a series resistor
> of about 12/.3=39 ohms must handle that power.
> All this shows why people like mosfet power transistors as switches,
> since they require no DC to keep them on, but only need current to
> turn them on or off. If you used a P channel mosfet as the output
> switch, you will need a low power 12 volt driving signal for its
> gate. A CD4049 or CD4050 hex inverter or buffer with all stages
> paralleled can probably provide the current needed to swing the gate
> capacitance. An open collector NPN transistor with a pull up resistor
> can convert your micro output to a 12 volt swing to drive the CMOS
> John Popelish
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