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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