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Reply-To: "fred bartoli"
From: "fred bartoli"
Subject: Re: 2.5kW-3kW-SMPS-(cheap)-suggestions?
Date: Tue, 17 Dec 2002 11:00:19 +0100
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John Jardine a écrit dans le message :
> 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
> (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
> 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?
> 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 :-)
your load is resistive. Take advantage of it... Does your output voltage
have to be regulated ? As it's heating element I guess the answer is no.
The last question is do you need isolation ?
In case yes I'll go for a simple push-pull, fixed near 50% duty cycle,
supplied by a bridge with a *low* value bypass cap (only HF decoupling) and
nothing other than the load resitor on secondary side. Doing this you'll
have the PFC for absolutely free.
Just wind the transformer to have the right stepdown ratio.
Set the duty cycle a bit under 50% so that you could use a current feed back
scheme to equilibrate peak current in each half primary, avoiding core
If you need some regulation, do it on a (mains) cycle by cycle basis and
leave the heating element thermal inertia do the job for you.
Simple, rudge, efficient and hard to beat the cost.
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