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From: email@example.com (Gibbo)
Date: 16 Dec 2002 22:23:55 GMT
Organization: AOL, http://www.aol.co.uk
Subject: Re: 2.5kW-3kW-SMPS-(cheap)-suggestions?
"John Jardine" wrote:
>I've little idea of SMPS beyond a 200W DC step-up, so an AC-DC 2.5 to 3kW
>step-down seems an out of reach, nightmare.
>The more I look into it the more I seem either spoiled for choice, or have
>no choice at all. I wonder if someone familiar with the black art of these
>things can throw some crumbs off their table sufficient to allow me some
>kind of compass direction to point at?.
>The suggestion is for a piece of portable kit that will be powered from an
>on-site 110Vac generator (95V-140V building site type!) and will give as o/p
>(unsorted yet) 40-50V DC (say 55amps max). The power will be dumped into a
>resistive heating element (one shot) for up to maybe 30 minutes a time.
>Operating duty cycle about 60%. ('tool' rating).
>The kit needs eventually engineering to a best cost, as it would be compared
>to existing equipment using mains transformers
>It also must make best use of the power available from crappy site
>generators. So a 95% or better PF is wanted.
>Safety 'galvanic' o/p isolation is needed but other than that all other
>spec' aspects would be pretty slack.
>Is there any 'optimum' PF pre-stabiliser topology or core(s)?.
>Drive main step-down transformer via 1/2 bridge? or full bridge? something
>Can any particular main transformer core(s) be recommended ?. (is the
>amorphous metal-glass stuff regularly used nowadays?)
>Any suggestions/best guesses/guidance would be much appreciated :-)
I have quite a lot of experience of this type of thing (but not this exact one
- mainly battery chargers upto about 5KW designed to run from generators but
the AC side is gonna be similar) and here is how I would tackle it.
Firstly, you're running from a generator so pfc OR a generator 4 times bigger
than a simple power calculation would reveal is indicated (do the sums yourself
after having scoped a genny feeding a high power bridge rectifier/smoother or
just trust me on this one !)
I'll assume you'll be sensible and go for PFC (I'll post the link for the
unitrode chip tomorrow - certainly the best around at present - the data sheet
gives a schematic that is merely scaled up for more power - even gives required
As it's generator power you'll have to lower the bandwidth on the voltage
waveform fed to the PFC controller otherwise you'll get all sorts of cycling
problems and end up with the PSU drawing huge power for a few cycles then
nothing for the next few (not mentioned in the data sheet !). Generators are
horrible things to run from with active PFC.
The PFC would feed a bog standard full bridge rectifier/smoother (at around 190
volts in the case of the usual 80 to 150 volt building site genny :) ). Then
your usual txfmr isolated converter (push pull current mode at this power level
- perhaps full bridge current mode).
The rest really is no different to what you're used to.
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