You have a FWB on the LINE??? (No isolation transformer.)
And an external refernce to frame ground tied to the "common"!
Think about it!

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Chris Loiacono
> Sent: Wednesday, December 18, 2002 10:54 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]: unregulated supply help
>
>
> I have put together a setup to PWM a 90V PM motor that uses an unregulated
> 170V supply from the AC line in a low-side switching arrangement.
> The supply
> consists of a 6A 400V FWB & a 1500uF cap. I wiped out a bridge
> when testing
> this and reasoned that perhaps back EMF, or perhaps a program glitch may
> have allowed a hard pulse to the MOSFET which would have made high inrush
> current...
>
> I made sure the program gave a nice smooth  10+ s soft-start with no
> surprises and checked to be sure that the PWM would not exceed approx 50%V
> at 1khz ... and started again, but first with a 40V bench supply. All was
> well and the motor ran according to the program. Then, I connected the
> unregulated supply and tried again...
>
> Needless to say, even before applying power to the uC (FET Gate
> Low) as soon
> as I switched the Line on, the bridge failed and the line breaker tripped.
>
> So, I believe my power supply has some inherent flaw. The common
> is tied to
> the logic common, which is tied to the FET Source. There is also a bleed
> resistor (1k 1/4W) between the gate and the source, but nothing
> else in the
> path of the trouble. Basically, at this point, there is nothing on the
> supply output, except passive connections and a 1000V FWDiode across the
> motor terminals....
>
> Are there any clues in this that may lead to a cure????
>
> Tks,
> Chris
>
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