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From: "Spehro Pefhany"
Subject: Re: Mosfet and rectifiers
User-Agent: tin/pre-1.4-19990216 ("Styrofoam") (UNIX) (Linux/2.2.14 (i586))
Date: Tue, 10 Sep 2002 16:34:49 GMT
NNTP-Posting-Date: Tue, 10 Sep 2002 12:34:49 EDT
The renowned V8TR4 wrote:
> Hi everyone,
> I'm building a 0-90vdc motor speed controller. I have a seperate power supply
> for my motors and for my logic circuit. I have a pair of PIC processors
> outputting PWM into a pair of MOSFets rated at 5amp 400volt. The signal is
> going thru a 7407 with a 10k resistor pulling hi to 12vdc.
> My question is regarding the DC going thru the MOSfets to power the motors. I
> only have a bridge rectifier with no regulator or caps. Will this be a problem
> with the MOSfets? Will this cause noise in my logic circuit?
> As I was assemblingthe board this morning I suddenly noticed that this could be
> a problem.
You have to worry about transients on the high voltage bus. You can't
just replace SCRs with MOSFETs in the conventional phase control
bridge setup because of the parasitic body diodes. You can keep your 7407,
but add push-pull drivers to the outputs using complimentary emitter
followers (add one NPN and one PNP BJT per output).
You might have problems with beat frequencies between your PWM and line
frequency unless your PWM frequency is quite high, or unless it is locked
to line frequency (essentiall phase control).
What does your output circuit look like exactly?
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