12V DC-DC converter

[evandude]

I am looking for a DC-DC converter design that takes a nominal 12v input, and outputs 12v also. The input voltage should be 8-15v or so... the lower the minimum, the better (6 or 7v minimum input would be great).

I need at least 2 to 3 amps output, the higher the better... 5 amps would be excellent.

It's for my car computer, which runs off 12v. This would allow it to continue running when the power dropped well below 12v during engine starting without requiring a battery backup system, as well as providing a regulated 12v output. There are similar things commercially available but most of them provide outputs for all computer voltages (+/-12, +5v, 3.3v, etc) designed to replace a computer power supply... however I already have a power supply that generates all those voltages from a single +12v so I'd rather build a simpler DC-DC to add on to the system.

I really don't know much of anything about DC-DC converter design, so I'm hoping people can offer some insight. I found a design for one of the multiple output ones, if nobody has another design handy I will probably try to reverse-engineer that one a little to get just the 12v stage from it.

thanks in advance...

 

[Sebi]

Just purchase a 12VDC to 120(230)VAC (camping)converter. Simple, cheap and reliable solution.

 

[evandude]

and ridiculously inefficient. running 12vdc up to 120vac, and then stepping it back down to 12vdc with a power supply (which I'd also have to buy)

and it also wouldn't work when the input voltage dropped!

 

[williB]

how about trying a battery isolator.
$20.00 to $30 , but it would require another battery..
http://www.surepower.com/Products/batiso.asp
but on the up side you just hook it up to the alternater and it isolates the computer battery from the car battery ..

 

[evandude]

Those look to be little more than big schottky diodes in heatsinks. Since my computer only draws 2-3 amps at most, I can do the same thing with just a $1-2 shottky diode... I am just trying to avoid that solution since the DC-DC would make things much easier for me all around. For one, it wouldn't require a large battery (even small UPS lead-acids are large as compared to the computer I am using), and it would solve the power problems i have both in the car, and on my workbench when I bring it in... When I power it off my bench power supply (a hacked AT power supply) the 12v line droops to 11.8 volts or so, which causes random read errors and restarts when used with a CD-ROM. (therefore, things like installing windows are impossible)

 

[Circuit Seeker]

You need to investigate switch mode power supplies, there have been several posts on this board about them and you should be able to find a ton of information about them easily on the internet.

There are a few links to some examples at

**broken link removed**

 

[Schneibster]

It is worth noting that there is a considerable lack of coverage of the Cuk converter, which has several distinct advantages over other converter topologies.

First, it is the only topology that does not have either input or output currents that pulsate (spend some time at zero). This means that it is quieter than any other topology. The reason is because the pulsating currents that every switching power supply must have are concentrated in the center of the converter, rather than at the input as in the buck converter, or at the output as in the boost converter.

Second, from the point of view of switch stress, it is the easiest converter to calculate the switch stress for; the stress is equal for both the Schottkey diode and the MOSFET, and straightforwardly calculated using the nominal and maximum input, and nominal output.

Third, it is the only topology to use capacitive energy transfer. This has the obvious advantage of concentrating the pulsating currents inside the supply, where they belong and can be dealt with, rather than at the input or output, where they can only cause problems, as noted above, but also presents the more subtle advantage that the least expensive transfer component is the one that requires the largest value relative to standard component values.

Finally, the Cuk converter is theoretically the most efficient converter design; and the less power you dissipate in the converter, the less stress you place on its components. Not to mention the obvious advantages when relying on battery power. And the final point about efficiency when dealing with high-amperage power supplies is that high amperage and low efficiency is going to result in pretty serious dissipation in the converter; do you really want to have to put huge heatsinks on everything?