psu design questions - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: April 13, 2021, 01:09:47 am
content: Hi all,ive just come across this,just a couple of questions before i build it, 1,does it look viable to add bigger filter caps and transformer +another 3 or 4 pass transistors + emitter resistors and change the current sense resistor to get say 20 amps,also will this work as cc/cv or just current limiting?,cheers in advance m3vuv.

author: xavier60
author_rank: Super Contributor
timestamp: April 13, 2021, 01:58:26 am
content: Does it use an lm723?

author: coromonadalix
author_rank: Super Contributor
timestamp: April 13, 2021, 02:20:36 am
content: what design(s) are you using, you don't give much to work on

author: m3vuv
author_rank: Super Contributor
timestamp: April 13, 2021, 07:29:48 am
content: sorry i forgot to attach the pdf,try this.

author: Ian.M
author_rank: Super Contributor
timestamp: April 13, 2021, 08:50:58 am
content: That's an interesting variation on the usual two OPAMP diode ORed CC/CV PSU. The LM723 is only used to provide a stabilized 7.5V reference to the two OPAMPs, and the whole control circuit is powered by a +/-16V (approx) unregulated supply referenced to the positive output terminal. It will suffer from the usual defect of overshoot when transitioning quickly from CC to CV mode and visa versa, as the OPAMP which isn't 'in charge' will be railed due to its feedback loop being effectively open, and its recovery time after transition will be determined by the OPAMP slew rate. Its also got 47uF of output capacitance so I wouldn't rely on its CC mode for stuff like testing LEDs as the surge current when initially connecting them could be problematic.Increasing its output current to 20A would be theoretically possible by redesigning the pass transistor array to have many more output transistors, multiple driver transistors and add a pre-driver, but with approx 45V on its reservoir capacitor when unloaded, shorting the output with the current limit at 20A would require the pass transistor array to instantaneously dissipate 900W, dropping by a couple of hundred watts as the unregulated input voltage settles to its fully loaded voltage. That would make it a dammed good room heater, with a massive heatsink and nearly enough fan cooling for vertical takeoff!TLDR: High current medium voltage linear CC/CV PSUs aren't really practical without a pre-regulator or tap switching to reduce the dissipation.I attach the schematic extracted from the PDF for ease of discussion.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 13, 2021, 10:57:43 am
content: the best approach would be switched secondary tranformer taps, to limit excessive dissipation lossesThe lm723 could be subbed by a precision reference too ...For higher current(s) you need to recalculate the shunt resistor (R21) and the pseudo darlington circuit (T2 T3 ...)But as mentioned the lm723 as a max vcc supplyThere's a limit to the filtering capacitors (C10, C1,C2) check google for calculations, you need to add 0,1 uf 100v in parrallel of themAnd you could add an anti reverse protection diode at the output terminals, it will help if you drive inductive loadsAnd yes this circuit may dissipate a lot, add temperature controlled fans, or a small fan controller circuit ??You have many leaked Mastech (and the clones) psu's on the web, you can check how they implement the secondary xformer tapsAdd multi turn pots instead of a single turn

author: m3vuv
author_rank: Super Contributor
timestamp: April 13, 2021, 11:31:07 am
content: thanks but i alredy thought of all those mods,it seems dooable,also thought of sort of transfo preregulator taps too cut down the pass transistor power dissapation,its just a design im putting together,also thougt of an adjustable crowbar circuit maybe mechanicaly linked switches to controll it.

author: David Hess
author_rank: Super Contributor
timestamp: April 14, 2021, 10:23:22 pm
content: Quote from: m3vuv on April 13, 2021, 01:09:47 amThat will usually work to increase the output current capability. However doubling the number of output transistors will also increase the drive requirements which may need to be addressed.