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Looking for a reference design for a DC-DC power converter.

I'm looking for a reference design for a DC-DC power converter. The goals of this little project of mine is to convert the power from a

24VDC source into a single fixed output voltage that ranges from 3VDC to 48VDC just by changing the transformer, diodes, and sense resistors. Because the source on the 24V rail is from a battery, the input voltage can vary widely depending on the battery's state of charge, so the converter needs to have a minimum input voltage of 16V. Maximum output power ranges between 5W and 50W depending on components. High efficency (>80%) is a must because of the battery.

I *COULD* use a standard buck, boost, or flyback design, but I want a break in the positive rail in case of component failure...as an added margin of safety to the down-wire electronics. The input will also be fused according to the power rating, with a SCR crowbar to protect the load in case of over-voltage.

I plan on making a number of these devices with the same basic design with variations of output power and voltage.

A few interesting numbers that I came up with when deciding transformer ratios. Also are the resistor numbers for the voltage divider network at a current of about 1mA and a 1.0 volt reference.

24v-8v 3:1 ratio at 5v input is 15v; Rsense 4K/1K 24v-6v 4:1 ratio at 3v input is 12v. Rsense 2K/1K 24v-72v 1:3 ratio at 48v input is 16v. Rsense 47K/1K

I have a conceptual drawing here:

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Any ideas or suggestions?

Thanks.

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Daniel Rudy

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Daniel Rudy
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Well, the manufacturers of these switch mode controller tend to have a bunch of application notes together with the datasheets.

Rene

-- Ing.Buero R.Tschaggelar -

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Daniel Rudy wrote:

Reply to
Rene Tschaggelar

Since the negative rail is common to input and output, why not use a tapped coil as both primary and secondary and use only two diodes for the output? Both these changes should potentially improve the efficiency.

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John Popelish
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John Popelish

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Reply to
Genome

As long as the center of the auto transformer is connected to the negative rail, the rectifiers cannot put out a positive rail voltage from a blown fet. The inductor just saturates and blows the input fuse.

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John Popelish
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John Popelish

I can't picture the capacitor you mention. Are you talking about a resonant design, or some sort of snubber capacitor?

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John Popelish
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John Popelish

At about the time of 12/1/2004 9:53 AM, Genome stated the following:

What is your problem?

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Daniel Rudy

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At about the time of 12/1/2004 2:37 PM, John Popelish stated the following:

The two diode full wave rectifier is a better idea than the bridge circuit that I considered using. I'm sure that an autotransformer would work, but I need a break in the positive rail in case one of the switching transistors short out. Looks like more pSpice work needs to be done.

Thanks.

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Daniel Rudy

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At about the time of 12/1/2004 5:55 PM, John Popelish stated the following:

I like that idea, and it looks good on paper when I draw it out. I have seen designs like this up to about 100 watts or so. I just thought of something. The capacitor that sits across the coil primary, I wonder what would happen if I replaced that with a diode, or even a syncronized MOSFET...

Thanks for your help.

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Daniel Rudy

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    Boris Mohar
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Boris Mohar

Daniel Rudy

SEPIC?

Reply to
GPG

At about the time of 12/3/2004 1:52 AM, GPG stated the following:

Not quite, but it will be hard to get a 24v battery down to 12v or even

5 volts without damaging it to made a SEPIC design feasible. 
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Daniel Rudy

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Daniel Rudy

Resonant.

I'll try and draw it...

------------_/--------+-------)||(--------|>|----------- | | )||( PWM | )||( - )||( - )||( | )||( GND +-)||(-+ |---+--| | GND

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Daniel Rudy

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