The usual, Power Supply design, critique please - Page 1
Forum section: electronics / projects

author: Asim
author_rank: Regular Contributor
timestamp: September 05, 2014, 08:05:54 am
content: Alright, let's get into it, don't be easy on me even though I am not that experienced.The design is based on LM317, the design is attached.info about the design:a1 is a differential amp, the shunt is 10(1% tolerance) 10 ohms resistors in parallel to give me 1 ohms . the 10.2k resistors all around the design are 0.1%a2 is another diff amp, but it is used to eliminate the current taken by the lm317 and lm334 current sourceb1 and b2 are comparators used for current limiting sectionc1 and c2 are for the set voltage , the output of c2 goes from -1.25V to vmax, when the current limit kicks in the transistor will pull the non inverting of c1 to ground which will give an output of -1.25v to the regulator ( around 0V output < there will be offset errors>)lm334 is used to ensure minimum load requirement .to get a zero volt output , i needed a nigative rail so i used isolated DC-DC converteraccurate panal meters will be used to display the readingsI bread boarded the design and it works with little errors with the Iout because i was using 1% 10K ohmsedit:I forgot to add a diode in the schematic for the lm317 across the input and output

author: void_error
author_rank: Frequent Contributor
timestamp: September 05, 2014, 08:42:33 am
content: You didn't mention any value for Vin.You need at least 1V across the LM334 to keep current regulation.No need for an isolated DC-DC converter, just use a charge pump - TC7660/ICL7660 or even a 555.You could place the LM317 in a an opamp's feedback loop.

author: Asim
author_rank: Regular Contributor
timestamp: September 05, 2014, 09:05:44 am
content: Quote from: void_error on September 05, 2014, 08:42:33 amvin is going to be 16V or so ,Quote from: void_error on September 05, 2014, 08:42:33 amyea, thats the plan for lm334, this supply will always be higher than 1V unless it's in current limit, I can't even think of something i will supply with 0.5vi don't have TC7660/ICL7660 on hand, i will look into the 555 timer thoughQuote from: void_error on September 05, 2014, 08:42:33 amummm, what do you mean exactly, isn't it in a feedback loop for the current ?Thanks

author: Asim
author_rank: Regular Contributor
timestamp: September 05, 2014, 09:10:33 am
content: one more thing to add:Ididn't use feedback resistor from the output to the adj pin i. that would be because the feedback resistor will sink Variable current through out the output voltage range, thus generating an error in Iout measurement which i want to be as accurate as possible

author: penfold
author_rank: Frequent Contributor
timestamp: September 05, 2014, 09:16:30 am
content: Why include so many additional op amps when something like an LM723 or UC2832 contains all the blocks you're looking for, plus there's some great Unitrode application tutorials that go with them.

author: void_error
author_rank: Frequent Contributor
timestamp: September 05, 2014, 09:28:42 am
content: Quote from: Asim on September 05, 2014, 09:05:44 amThis is one of my early designs. U3.1 controls the voltage, U3.2 controls the current. U2 is basically a current to voltage converter. I don't think I tested this one so I don't know if it works properly.Here's my Bench PSU thread, you mght find a few useful ideas there.https://www.eevblog.com/forum/projects/bench-power-supply-design/

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 05, 2014, 11:19:10 am
content: Hmm , I agree with some of the other posters.You have way too many parts in there.The LM317 is superflous, you could replace it with a darlington transistor and get all the way to 0v and have the same dropout voltage, and not need a negative supply.If you use a depletion mode MOSFET or JFET as the small transistor in the darlington, then drop out voltage is ~ 0.8v . You would obviously need to close the loop around the transistor, (but this gives the opportunity for voltage sense wiring to the load to eliminate lead resistance effects).You mention the need to measure current with high accuracy , yet neglect the bias current of the LM317 and LM334 , these will be voltage and temperature dependant.Some other points , Amplifier A1 needs to have inputs at the positive rail , most opamps won't do this, so pick your part carefully.For the current sense, I would instead use a purpose built chip (like the ZXCT1010 cheap or INA168 more accurate).Many comparators will work down to the negative rail (so don't need -3.3v) but check the output stage , many comparators (LM339) have an open collector output (this saves you the NPN transistor , but b2 output won't work as drawn)amplifier C2 doesn't do much , you could feed the DC offset into C1 just as easily.You have also connected a capacitor from the output of an opamp to ground , very naughty, can cause spontaneous oscillation.Watch the LM358 , it won't pull more than about 2v below the positive rail (like most op amps)I'm not sure why you are fixated on the 0.1% resistors , the LM317 is nowhere near that accurate, and in the end a human a gets to spin the knob, and unless you have a $20 ten turn pot with a $40 knob you will never be able to set it that accurately.Also remember the lead wires and connectors to your project will easily total 100mohm (so that's 0.1V error per amp) or 1% of 10vThe DC DC regulator will inject HF noise onto the 5V rail (and to a lesser extent the -3.3v) So your better off connecting the volt reference directly to the input (unless this can get higher than 16v)

author: void_error
author_rank: Frequent Contributor
timestamp: September 05, 2014, 11:58:40 am
content: Quote from: salbayeng on September 05, 2014, 11:19:10 amAlso check if the comparators have built-in hysteresis. If they do you can't use them to replace the opamps. It will probably cause instability/oscillations.

author: Asim
author_rank: Regular Contributor
timestamp: September 05, 2014, 12:37:36 pm
content: Quote from: penfold on September 05, 2014, 09:16:30 amLOL, true so many parts. that's an interesting chip first time hearing about itQuote from: salbayeng on September 05, 2014, 11:19:10 amI took it into consideration with ( a2) which is basically (a1 - the current taken from the lm317 + lm334)I am trying to make it as accurate as possible from easily obtainable parts as a learning experience ( I live in a part of the world who isn't into diy electronics) the parts mentioned as suggestions will take a month or two to arrive if i order them onlinethank you for your insight about the other pointsQuote from: void_error on September 05, 2014, 09:28:42 amI was following it ( silent observer)The initial design was using 3 op amps, but I kept adding one after another because I love them as building blokesmaybe i will go back to that design xD

author: void_error
author_rank: Frequent Contributor
timestamp: September 05, 2014, 02:33:28 pm
content: Whoops, looks like a linked a schematic with errors in it. The INA138/168 needs a common mode voltage of at least 2.7V to work properly so the current limiting won't work properly here... but you can replace the INA with a differential amplifier.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2014, 03:14:58 pm
content: The other posters covered most of what I would have said. I will add:The uncontrolled gain of the transistor at the output of B1 is going to totally screw up the frequency compensation of the current control loop.The capacitor at the output of C2 is probably going to cause more problems than it solves.If you use singled ended current sensing in place of instrumentation amplifier A1, then accuracy will not depend on common mode rejection from matched resistors. If the input supplies are floating, then current can be sensed in the ground return instead of the high side.Quote from: salbayeng on September 05, 2014, 11:19:10 amIntegrated regulators are uniquely useful for their built in protection features when used as power pass elements.

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 08, 2014, 05:33:14 am
content: Yep agree with all David's points, I had some doubts whether the OP's circuit had any thermal protection , and the LM317 will do that for you.