12v DC to 20v DC

How critical does the output need to be? What kind of device is it going to drive?

P.S. With 8 amps out, you'll be expecting at least 15 amps on the input side..

Jamie

Might be best to direct the OP in the general direction of a blocking oscillator style converter - with the stated load current I think forward conversion mode is more likely to do the job than flyback, so some kind of secondary winding is on the cards.

The 12V input isn't a vast headroom - a low VCEsat transistor would be well worth having.

Output voltage not critical. Say something between 19 and 22 volts. The output should feed UC3906-controlled SLA battery chargers. Right now, it is done with a 12v DC -> 220v AC converter ... aso. The purpose is to load sailplane batteries where no mains is available.

Willy

The OP never stated it had to be isolated.

A simple boost converter will do.

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Thanks,
Fred.

Is this it ?

+-------------+ | | | | | +----+-------+ | | | | |/ | +----|----+-| | .-. | |> | | | | | | 20VDC | | | === | +-+-+--------+ '-' | GND +. ,+---+ A A | + | +---- )|( +---++ | --- | | |VDC )|( +---+(-+ --- | | | +' '++--+ A A | +----|-------------+ | | ++-+-+--------+ | | | | | | +----+--|----+ | | |/ | | +-----|-----+| | |12VDC .-. | |> | | | | | | +----------------+ | | | | '-' | === +-----+ GND

CT Transformer, Caps coupled with the R's

The above is just an idea, you need to plug in the values, calculate the current needed, in this case 16 amps, to know the values of the R's that drive the power transistors..

This a cheap mulitvibrator circuit that can generate a small amount of boost which you can add in series with your existing system..

P.S. Keep in mind that while one side is building current, the other side of this push pull will be generating double the voltage at the collectors, so select transistors that can handle at least 30V on the collector and ones that can handle 15 amps continuos with good heat sink.

A more advance circuit using MosFets and a driver circuit from a fixed source would most likely work better but this will give you the idea.

Excuse my bad drawing.

Jamie

That one is not isolated. This scheme simply adds to the already available source, as noted (boost). :)

Jamie

Not to mention the ultimate saturation of the transformer in such a simple-minded inverter. ...Jim Thompson

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If you put a couple of diodes across the transistors or use MOSFETs with internal diodes AND allow for some dead time then there is enough room for the reset current to flow and prevent saturation.

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nico@nctdevpuntnl (punt=.)
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Yes, I know it's a crappie circuit, but I did ask as to how critical it had to be! :)

Actually, instead of using R's as the feed back, you use Caps and then bias the transistors as in a AB class type amp, just some minimum current to get it started. But, I did state, to couple the R's with caps. The R is to be designated only to get a small amount of current and the caps do the feed back. So, either way, it'll work.

I suppose putting diodes on the (B) of each can help prevent the transistors from over saturating, but they could also keep them not saturating enough and heat up.

It's obvious there are better solutions but not with so few of parts count :)

You should see the charge pump boost I made! I needed to keep the voltage at 13 volts minimum for 2 radios I have that will not operate below 12 volts. I was able to use a 555 timer with complemary MosFets to charge pump a large capacitor with proper regulation and safety crow bar in it, just in case ! That unit allows me to operate my FT-100 even when transmitting, down to about 10 Volts at the plug. We don't allow the charge pump to operate under that. we don't need to have a dead battery.

Jamie

Not this one, but very similar.

Willy