Power supply design. Need 48V, -24V, -12V, +12V - Page 1
Forum section: electronics / projects

author: mkiijam
author_rank: Regular Contributor
timestamp: November 02, 2023, 04:42:33 am
content: Hi Guys,I need to build a power supply for and audio device that needs 48 (phantom power), +/-12 and -24 Volts.Any fancier solution than 24 Vac CT transformer - 7812 / 7912 for the +/-12. 7924 for the -24 and voltage doubling into a 48V regulator for the 48? Seems like a lot of wasted heat on the +/-12V supplies.

author: magic
author_rank: Super Contributor
timestamp: November 02, 2023, 08:11:42 am
content: Phantom power is not a lot so you could use a few stages of Cockcroft-Walton for 48V from 12VAC. Or a small switching boost converter.Then you are left with solving -24V somehow.And the fanciest solution is a custom transformer

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 02, 2023, 08:48:34 pm
content: You could potentially also use a negative doubler for the -24 depending on the current demand. How much power is required for the 12 and 24 V rails?

author: oz2cpu
author_rank: Frequent Contributor
timestamp: November 02, 2023, 09:32:47 pm
content: very hard to help you when the only design demand hint we could pick up : dont like wasted heat..what do you get wasted heat ?what is you usage on the voltages you mention ?and why do you need the 4 voltages.the 48V is quite obvious, but the rest we just have to guesstell us more ?size ? cost ? mains oppeates ?you got a normal transformator ?or you like small ? compact and to a special price ?how about noise ? in the audio spectrum ? you like switchmode or is that banned ?is it highend ? hobby ? studio ?stand alone ? connected to what ?

author: mkiijam
author_rank: Regular Contributor
timestamp: November 03, 2023, 04:52:09 pm
content: The -24 needs to be around 500mA. A 7924 will be fine I think. But I don't think a doubler would be okay on that(?)

author: oz2cpu
author_rank: Frequent Contributor
timestamp: November 03, 2023, 05:09:43 pm
content: making a doubler for 500mA is possiblebut you need large caps if running on mains frequencyI prefer conventional linear solutions for stuff that is analog and needs to be as clean and silent as possible,if size and loss is a thing, it is of course possible to make switchmodes, but you end spending quite a lot of timeon filtering and implementation, to get it right

author: Ian.M
author_rank: Super Contributor
timestamp: November 03, 2023, 07:17:23 pm
content: You wont have enough headroom for the regulators (i.e. they'll drop out in the ripple troughs) if you use a 24V CT transformer unless you vastly over-spec its VA rating so the regulation curve is in your favour. 26V CT gives you a little more headroom, probably enough to manage during 5% low line conditions, *IF* you use Schottky rectifiers for the main rails. Use a capacitively coupled bridge(i.e. a balanced charge pump), output relative to the main negative unreg rail, and size the flying capacitors to limit the current and thus its output to about -27V in the troughs at full -24V load current. You'll find the values are quite reasonable, though to get a high enough ripple current rating for the flying capacitors, you'll need to look at leading brand, low ESR, 105° C caps rated 50V or above.Similarly, stack the voltage doubler for the +48V phantom power on top of the positive unreg rail, and size its flying capacitors to give you enough headroom for the 48V regulator at the max phantom power current you want to support. The diodes wont be anything special, 1N4148 should have a high enough surge rating if you aren't stupid about the capacitor values. N.B. you'll need a high voltage regulator as it could see close to 100V input voltage no load during high line conditions.