An isolating DC-DC converter at these amperages is no trivial task either.
I understand how regular DC-DC converters work, by kicking an inductor with
various schemes, but how does an isolating DC-DC converter work?  Flying
capacitor?

--Lawrence

----- Original Message -----
From: "Peter L. Peres" <plp@ACTCOM.CO.IL>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, July 09, 2002 5:53 PM
Subject: Re: [EE]: Solar 240 Inverters


> On Tue, 9 Jul 2002, Lawrence Lile wrote:
>
> >Quite a logical approach, Peter.  You've always got the problem of
getting
> >12V (or other solar cell voltage) to be 120V (or 240 take your pick).
> >Either you have to solve this with a transformer, or an upconverter as
you
> >suggest.  An upconverter might be a really efficient way to do the job,
with
> >less weight and cost than a bulky high amperage transformer.
> >
> >Now, switching 240V*1.414 puts you in the realm of IGFETs, and out of the
> >realm of super-efficeint mosfets, which wimp out pretty much above 100V.
> >OTOH, you are switching much smaller currents, so the efficiency may wash
> >out.
>
> 60A is not so small.
>
> >You also have less isolation from input to output with this scheme vs a
> >transformer.  You probably want to tie your grounds together anyway.
Uh-oh
> >I just thought of something.  Your upconverter is going to make 340VDC.
> >Your output H-Bridge is going to flip this from one side to the other.
One
> >pole of your output is neutral, which is connected to earth.  So your
solar
> >cells CANNOT be referenced to ground, in fact they will have 340V AC
riding
> >on top of them.  ouch, this does not sound good.  What is a good way to
> >handle the grounds in this system?
>
> The charger dc/dc converter can be an isolating one. Also the batteries
> will flip around at 60Hz (both the - and the + rails) so you cannot ground
> the batteries. This is a small price to pay for getting rid of a large
> transformer. I also think that major major major losses will occur in the
> transformer proper, besides in the switching elements, if using low
> voltage battery storage. I guess I'd put the batteries in a large ABS
> cabinet (shed) built to IP2 or more and provide interlocks and an outer
> shield.
>
> You could also use a 1:1 isolation transfomer on the output. This could be
> a part of the sine filter. A 1:1 transformer will not have that much loss
> and it is probably easier to come by as a standard part even at 5kW+
> rating.
>
> >Would a shorted output transistor expose your solar cells to a 240VAC
main
> >voltage?  Maybe some good fusing*, crowbars, and transorbs are a good
idea
> >on the low voltage side.
>
> I guess using a isolated dc/dc converter for the charger is a very good
> idea. It would only need to handle peak charging current/power for short
> times. The average power would be low.
>
> Peter
>
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