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Modurn lab power supply ideas and advice

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I plan to build a lab power supply into a Tektronix TM500 system plug-in enclosure because such plugins are hard to come by in the UK and tend to be unreasonably pricey if you can find them at all.  Most of the available PSU modules are also looking a little dated now.  If this works out i'll publish the project with PCBs.

* capable of operating as a split power supply, with + and - rails.  this could either be in fixed dual tracking mode or as 2 separate supplies that can be seriesed together.  the latter is slightly more flexible with slightly more potential issues around current control.
* variable voltage and current adjustable down to '0V' and '0A'... ish.  A few mV,mA is acceptable
* im considering switching pre-regulation to improve efficiency as the supplies in the TM500 mainframes are a little anaemic.
* current and voltage readouts, to be provided by a digital display.  this should be the easy bit.
* +- about 20V max.  TM500 presents each (high power)bay 2 separate dedicated 1 amp 25VAC windings and one dedicated 4 amp 17VAC winding.

* LT3081 based designs.  Linear even did a demo board https://www.analog.com/media/en/technical-documentation/user-guides/DC2132AF.PDF single channel but i think two could be seriesed up.   it's an expensive design though,  I don't mind expense if it buys me something, particularly if it buys simplicity, but here it seems over the top, £50 of semiconductors just for the two supplies alone.  the need for a minimum load on the LT3081 is annoying, though interestingly the datasheet just glosses over that with a 249Ohm resistor to ground.  at the max 20v that is only a couple of watts so maybe not a huge issue, but i'd have to calibrate the readout for that extra current draw.
* there is a power op-amp, https://www.ti.com/product/OPA549  which even has a current limit feature.  They are also expensive but very simple, potentiality.  Unfortunately hard to find in the UK.  getting a current reading could be annoying though as it doesn't use a shunt resistor, so i would have to add one, then fight with high side current sensing anyway... so..
* What about alternative power op-amps intended for audio?  for example the LM3876 in conjunction with a high-side current scene amplifier such as the INA296?  it should make for a simple topology, as an added bonus one could even feed in an arbitrary signal and use the psu as a power buffer.  I have an HP psu/amp combo like this and it is surprisingly useful.  On the down side i'm not sure how well a power op-amp intended for audio would like having it's input voltages changed on the fly, by a pre-regulator or tap switching to improve efficiency.  I am also not sure if there are hidden pitfalls in this strategy, particularly around the current limiting which can be hard to make stable.
* yes i suppose there are also legacy designs.  I don't like them.  they are hard.  they have an ungodly number of zenners, a mix of ancient integrated and discrete techniques.  yes i'm not ashamed to say i may have reached the limits of my expertise working with these designs and have failed with them brffore.  Go on, tell me i should just use an LM723, i dare you  :-DD

opinions and advice welcome.

For a +- supply one would likely build to idetentical parts that could be stacked.

With the limited space a preregulator may be a good idea, though it adds complexity. The alternative would be several transformer taps and a fan to get rid off the heat. It is kind of a balance between heat (power rating for the given size) and noise.

For the regulator the standard way is to use the floating regulator scheme and OP-amps for the control and power transistors. An integrated solution is tricky, as the heat would effect the accuracy. So it is a good thing to have the heat  and precision parts separated. It looks a bit ancient, but it is still the best way to build a lab supply. The frequency response part (compensation) needs a bit care to get good stabilty also with a difficult load. Here a simulation can really help, if one does not copy a tested design. The supply for the regulator part can also power the read-out and control part.

You might wish to specify what your voltage and current requirements are, as any advice would need to take that into account.

Yes, good point i did omit that.  edited the original post, max about 20v, + and -

You mention a 17V ‘AC’ winding. Is that 50 Hz (or 60 Hz)? Of so, a classic approach is to deploy a phase controlled rectifier using Silicon Controlled Rectifiers (SCRs) aka thyristors as a pre-regulator.

(But a 17 V AC winding won’t reliably get you a 20 V DC output, I guess. Maybe it was sized for 15 V?)


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