Subject: Re: serial port control
To: Wolfgang Rupprecht <>
From: Brett Lymn <>
List: current-users
Date: 02/12/1998 22:14:55
According to Wolfgang Rupprecht:
>Its not clear to me yet that sine wave output wins.  Square wave
>should be more efficient since you don't have the big dead-time
>between, the two peaks.

Urrgh no.  Firstly, there is no dead time - the voltage continuously
changes.  Lots of things are designed with a sinusoidal voltage in
mind, feeding then a square wave can cause problems.  The problem is
that a square wave is chock full of harmonics, these are particularly
bad for any magnetics with laminated iron cores (most electrical
machinery) as the harmonics turn up as losses which implies more heat
dissipation and, possibly, overheating.

>  If the load is your typical PC switching
>power supply then it would really prefer to have the full ~170v peak
>voltage applied all the time.

No, normally these devices have input filters to try and stop hash
getting back into the power grid.  The filter will be busily trying to
chop off the harmonics that it has been fed.  It will heat up, whether
it will do damage is a matter of some conjecture.

>  The down side of square waves is a
>touch more RFI noise.

Mmmmm I would say that banging between +- 170V rails would be
conducive to more than a touch of RFI.  Some of this will get through
on the power supply rails.

>  As an aside, normal DC-DC converters are sqare
>wave internally, as are switching supplies.

Yes but there is a couple of orders of magnitude in frequency here.
Your garden variety SMPS runs at frequencies that start at around
10kHz and go up, this makes all sorts of things easier.  Also the way
things are driven is not a simple square wave/transformer/rectifier
system.  Tricks are played to perform the regulation on a pulse by
pulse basis.  Ummmm sorry - suffice to say it is not that simple.

>  It just doesn't pay to
>round the corners of the wave unless one is running a motor or
>transformer or something like it that will get very upset at the high
>dv/dt of the square waves.

or SMPS that does not have a good noise rejection.  Having sinusoidal
output is desirable to save stressing all sorts of things.

Brett"Actually, yes I have designed a 3000V, 400W SMPS in the past"Lymn

Brett Lymn, Computer Systems Administrator, British Aerospace Australia
  +++ Divide By Cucumber Error.  Please Reinstall Universe And Reboot +++
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