Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!uunet!tektronix!midas!jeffw
From: jeffw@midas.STS.TEK.COM (Jeff Winslow)
Newsgroups: sci.electronics
Subject: Re: Running a Mac/SE from batteries and inverter
Keywords: Mac, batteries, inverter
Message-ID: <4243@midas.STS.TEK.COM>
Date: 11 Apr 89 21:41:09 GMT
References: <13671@jumbo.dec.com> <2446@iscuva.ISCS.COM> <4224@midas.STS.TEK.COM> <2452@iscuva.ISCS.COM>
Reply-To: jeffw@midas.STS.TEK.COM (Jeff Winslow)
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 35

Question is, will a typical off-line switcher's input rectifiers and 
capacitors have a reduced life for square-wave input compared to sine input?

In article <2452@iscuva.ISCS.COM> jimc@iscuva.ISCS.COM (Jim Cathey) writes:
>In article <4224@midas.STS.TEK.COM> jeffw@midas.STS.TEK.COM (Jeff Winslow) writes:

>>Secondly, rms running current will be lower for a square wave, since the
>>conduction angle is much greater than for a sine wave, even the chopped-off
>>version. The rectifiers will run cooler, last longer. The capacitors too...

>I disagree, the conduction angle may well be larger, but the _peak_ current
>will be very high in the early part of the cycle.  Thus higher RMS current,
>and the exact opposite results.

I agree that the peak current will be higher, but I don't think it will be
*enough* higher for long enough to counteract the reducing effect of the
wider conduction angle, especially when you consider that the inverter is
going to have some non-zero output impedance. But I honestly couldn't tell
you for sure without doing some measurement, simulation, or equation-bashing.

If the square does turn out to be worse, one could experiment with putting
an inductor bridged with a power resistor between the inverter and the
switcher to knock off that initial peak.

>Or maybe the math says otherwise.  I'm just quoting what the data sponge in
>my head says it saw several years ago.  Original source was a power supply
>design article (quoth the sponge).  Any real experts care to comment?

Well, I designed a few of the buggers. Doesn't that count? :-)
I hope the article wasn't from Power Conversion International or its
precursor, Solid-State Powerconversion. I remember seeing some rather
amusing things in there while I was subscribing to it. Unintentionally
amusing, I mean.

						Jeff Winslow