Symmetrical power supply design (+-15V 1A) - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: December 23, 2020, 11:19:35 pm
content: Hello,for a piece of instrumentation, I need a symmetrical 15 V power supply. The maximum expected current draw is up to 1 A, however for some strange reason (and a few legitimate ones) I have decided to not use integrated 7815/7915 series regulators.Reason one being the power supply will need to start up with a heavy capacitive load (up to some 3000uF), so I wanted a something with a better defined current limit. And speaking of current limiting, fold-back current limiter would be likely better option, rather the 78xx/79xx like "full current until fried enough". Maybe I'll even add some temperature protection to make it more robust.What I thought of was to use the plain simple topology in the picture below, however I have quickly found a major showstopper and that was how to power the opamp.The input side comes from a mains transformer, so the expected input voltage range is quite high, from 18 V up to some 27 V worst case. Together, that would be 54 V maximum for the opamp, which is ... a bit problem. The design goal was to use a 36 V rated opamp (TL072 for example, as I have a ton of those).Limiting the opamp's supply voltage using zeners does not work too well - as the supply voltage needs to calculate with enough headroom, for one output of the opamp to be at about 15 V +2*Ube (darlington transistors) and the other at the negative same voltage, which results in a requirement of extremely close to maximum 36V supply voltage, for this to work - one opamps output has to sit at 17.5 V, the other at -17.5 V. Bummer! So it may not even be much possible to do like this.So this is not a solution. Is there any neat trick, to get rid of the need of the full supply voltage across the opamp? Rewiring the thing as two identical power supplies in series is not possible, I have only 3 wires from the transformer. I also thought about using an NPN transistor for the negative rail and supplying current from a common-base transistor. But that would make the negative regulator with common emitter output a completely different beast to tame - which I am quite afraid of honestly.Thanks for any suggestion!Y.//EDIT: By using a common-base transistor for supplying control current to the negative side, I meant like this:

author: dmills
author_rank: Super Contributor
timestamp: December 23, 2020, 11:51:12 pm
content: Two single opamps?Cascode transistor with the base fed from a potential divider to hold its base at a few volts or so, emitter to the opamp, collector to the doings up top?

author: Yansi
author_rank: Super Contributor
timestamp: December 23, 2020, 11:58:37 pm
content: Quote from: dmills on December 23, 2020, 11:51:12 pmIsn't the cascode transistor exactly the principle I've drawn in the second picture? I do not understand exactly what you mean by "collector to the doings up top?" (you meant an NPN in the cascode, controls at the negative side?)

author: David Hess
author_rank: Super Contributor
timestamp: December 24, 2020, 12:00:30 am
content: Quote from: Yansi on December 23, 2020, 11:19:35 pmThe 78xx and 79xx do implement foldback current limiting which also takes junction temperature into account.Vbe current limiting can be added to an 317/337 adjustable regulator on the common side by pulling the adjustment pin toward common as shown below.QuoteIt is called a floating regulator, which is what the 317/337 implement internally. The 723 can also be configured as a floating regulator which should give some idea of how it is done.