SMPS boosted and regulated

[davelectronic]

Recently after testing a linear regulator circuit   ( 7812 + TIP36C X 4 ) with a 19 Volt HP power adapter, I started thinking about another power adapter. I've got a few dell power adapters 12 Volt 18 Amps. I have already converted these some time ago now, but haven't used them for anything yet. Is it possible to use a suitable high current boost converter, to take that 12 Volts up to say 20 Volts or so, then put that through a 7812 voltage regulator with multiple pass transistors ? The idea is to convert the boosted SMPS to a linear power supply output. It might sound an odd question, but I just wandered if it's possible to do this. Thoughts appreciated, and thanks for reading.

[Buriedcode]

Sure its possible, but what kind of current are we talking here?  If you wanted say 4A out @20V your boost converter would have >7A going into it with a peak current over 10A.  Thats a hefty current, and then if you were to use a linear regulator to drop it back down to 12 (for your adjustable output) thats dissipating (20-12)*4 = 32W. 

If you're talking less than a couple of amps it would be done probably with off-the-shelf modules.

A better options might be to modify one of the power supplies back to 19 or 20V negating the need for the boost converter.  Then you could use a buck converter for the adjustable output.

[davelectronic]

There's not much I can do to increase the Dell power adapters, there fixed at 12 Volts at a rating of 18 Amps. I have a few of the HP adapters, there just over 11 Amps. I've tried both makes of PSU adapters in series and parallel. The HP 19 Volt adapter PSU's are happy in series or parallel, but the Dell will only play ball in parallel. The boost converter I was looking at was a 30 Amp unit off Amazon, I've got one configured to run a couple of my universal motor tools, drill etc up to 80 Volts from this converter for drill stand use, it works with that very well. The boost converter is in the picture below, it's only the Dell PSU adapters I was thinking of boosting first before then running that voltage through a linear regulator circuit. I might be able to close the 12 volt to 20 volt gap, so long as the linear regulator has enough overhead voltage to function.

[mariush]

I don't know why you need to boost it so high, as the 7812 only needs around 1.5-2v of voltage about the output to regulate properly. Even if you use transistors, you shouldn't need more than 3-4v.

You could probably alter the 12v power supplies to output 13-14v by adjusting the resistors of the voltage reference and (if any) the over voltage protection.

Also, there's other much better linear regulators with lower dropout voltage, for example LM1084 which has a maximum of 1.5v dropout voltage at 5A and they can be used in parallel for higher current (See page 15 of  LT1083 datasheet - https://datasheet.octopart.com/LT1083CP%23PBF-Linear-Technology-datasheet-7629341.pdf -  they use a piece of awg18 gauge wire as a balancing resistor, but you can use an actual resistor to balance current between resistors).

[davelectronic]

Yes, I don't think I need to boost the SMPS 12 Volts as high as 20 Volts. Probably 15 Volts or I little higher will be ok. I've got some low dropout voltage regulators in a box somewhere, just have to find them. I prefer series pass transistors to carry the current. Just think it's a better way than parallel voltage regulators.