bench/lab psu design - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: December 30, 2018, 05:26:49 am
content: lab type psu,Hi folks can you give me your thoughts on the attached schematic,cheers m3vuv

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 30, 2018, 07:45:44 am
content: Quote from: m3vuv on December 30, 2018, 05:26:49 amyou should first buy me a new pair of glasses ...can't you post a larger picture?

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2018, 10:04:19 am
content: There is a more legible copy of the schematic and a descriptionhere.The concept is good.The current control error amplifier clamps the output of the voltage control amplifier which should provide good performance. Unfortunately it does this through a common emitter transistor which is not good for stability but if it works, then it works. Adding a low resistance between the CS pin of U2 and common to stabilize the transistor's gain would help with this. Better would be to reverse the control of the current sense amplifier and clamp through a an emitter follower which has fixed unity gain.There is no output capacitance which would normally be required for stability except for RF filtering so I wonder about this.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2018, 11:40:52 am
content: The floating type regulator usually needs the output capacitor for stability. The compensation for the voltage loop also looks simpler than normal, which would suggest it should be slow to be stable at all - thus is would even need a rather large capacitor at the output.The current regulation could suffer from low precision: one point is that the base current also flows through the sense resistor. The other is that the 723 is not very low drift for the low low voltage at the shunt.The circuit looks like an old crude design idea (but not fully worked out) from the 1970s. I would not start from there.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2018, 01:07:51 pm
content: Quote from: Kleinstein on December 31, 2018, 11:40:52 amMost power supplies either require output capacitance for stability or perform better with it. I suspect the designer either got lucky or left a 100uF output capacitor off of the schematic.I do not suggest doing this design unless one is prepared to do some transient response testing and tuning which just takes a function generator and oscilloscope.QuoteI disagree insofar as the 723 differential input stage is a monolithic differential pair. Drift should be better than a 741 or 324 which have two Vbe junctions in series so say 10uV/C or 20uA/C with a 1 ohm sense resistor. That is several times better than the current shunt resistor just by itself so the 723 is not limiting performance.An input bias current of 1 microamp (from the 723 schematic) could yield an input offset current drift of 10nA/C or 30uA/C.I do wish that a jelly bean precision operational transconductance amplifier existed for these sorts of applications.QuoteIt looks like something copied from a much higher voltage design. With only a 20 volt maximum output, the floating configuration is not required.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 31, 2018, 02:09:26 pm
content: Quote from: m3vuv on December 30, 2018, 05:26:49 amthere is no insurance that + to gnd and - to gnd will be symetric. if you load the + side unequally to - side, + side will be reduced in magnitude compared to - side. yes its difficult to see on my 27" but it looks like the control only ensure the magnitude between + and -, so its not suitable for dual/bipolar tracking PSU. thats all right if thats not what you intended to do. and both circuit is not on common ground so they can swing differently like 2 boats on a raging sea, control maybe a bit wavy . ymmv.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2018, 02:56:25 pm
content: The ground only seems to be there for RF filtering or protection. These is not DC connection to the ground terminalQuote from: David Hess on December 31, 2018, 01:07:51 pmI would not expect high precision from the 723, since the current seen by the 2 transistors in the differential stage is not well balanced. Thus there can be quite some offset and corresponding TC. Even the cheap OPs like 358 have reasonably well balanced currents to keep the offset low.The circuit has another problem with the current part: the shunt signal is divided down first to shift the DC level up to get in the common mode range. So there would be quite some effects of these resistors. This is a suitable method to make the voltage regulation work down to 0, but not such a good idea for the small signal from the shunt.For trans-conductance amplifiers chips there is not that much choice. Mainly the LM13700. The offset specs are actually relatively good.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2018, 05:46:15 pm
content: Quote from: Mechatrommer on December 31, 2018, 02:09:26 pmIt is just a floating output and not a dual/bipolar output.Quote from: Kleinstein on December 31, 2018, 02:56:25 pmHow are they not balanced? Their collector voltages do not match but do they use different emitter areas to deliberately produce an offset voltage?QuoteI would not design it that way but common 1% precision resistor matching would still produce less error than the shunt resistor.A single supply operational amplifier could have been used. Old designs often do high side current control using an LM301A or JFET operational amplifier. Low side current sensing of course uses a much more common single supply part. Or do what professional designs do and use a bipolar supply for the floating control circuits or in this case, use a higher supply voltage for the 723s since the output is only up to 20 volts.QuoteThere are some operational amplifiers which are almost as good like the LM301A and descendants of the LM308 because they provide access to their transconductance output node for frequency compensation to abuse. (1) A monolithic matched pair could also be used.In practice however, operational amplifiers are completely adequate despite the extra complexity required to add external clamping.(1)Section X - Unusual Op Amp Applications - Perverting the Monolithic Op Amp

author: xavier60
author_rank: Super Contributor
timestamp: January 01, 2019, 07:37:55 am
content: I'd say that at least the voltage loop will be stable. At a first glance, the compensation looks wrong for that topology with its high impedance output stage.But because the voltage feedback divider connects to the non-inverting input, the op-amp's minimum gain is one, resulting in the correct type of compensation.This article calls it a Type II Compensator.http://www.ti.com/lit/an/slva662/slva662.pdfExtra: I give it a big thumbs down for having the voltage adjust VRs in the feedback path and the design is not fail safe to a VR going open.