LM350 Constant Voltage Constant Current 30V 1A PSU - Page 1
Forum section: electronics / projects

author: ZeTeX
author_rank: Frequent Contributor
timestamp: December 07, 2015, 09:20:16 pm
content: hi.I've designed this power supply based around the LM350 because it has protections and pretty cheap.I dont have stability issues or osciliation but I only simulated this circuit and not breadboarded.it has CC\CV Leds, and LT1074HV switching regulator.it must have a negative supply, I have center tapped transformer so this is not a problem at all.attached is the schematic in ltspice.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 08, 2015, 04:58:49 pm
content: The regulation is relatively slow, especially the current limiting. However the current sensing amplifier may not permit a much faster regulation here.There still is possibility for instabilities at the output of the regulator, as there is no capacitor directly at the output.Also the preregulator may become unstable, as the feedback is from behind the extra filter. Usually the Feedback should be from before the filter, at least tha faster part of it. For a preregulator it is no problem to have the feedback from before the filter (L2/C6).Many of the high side current sense amplifiers don't work if the output voltage is low (e.g. less than 3 V) - I have not checked the type used.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: December 13, 2015, 01:08:07 pm
content: Quote from: Kleinstein on December 08, 2015, 04:58:49 pmmight I ask why is the regulation slow & the current limting? it doesnt look slow at simulation at all.I changed some things here:(the 470u at the output is just for some tests you dont need it)

author: dom0
author_rank: Super Contributor
timestamp: December 13, 2015, 05:32:51 pm
content: Quote from: ZeTeX on December 13, 2015, 01:08:07 pmCare to show a few images of current limiting response to fast rising (<<1/10th of regulator time constant, or just 10 ns or so) currents at the input (with 5, 10, 100 % overdrive)?The current regulator U3 has a time constant of ~2.5 µs (560 pF * 4.7 k?), so about 60 kHz bandwidth. Current sense amplifier U4 has a bandwidth of..?

author: ZeTeX
author_rank: Frequent Contributor
timestamp: December 13, 2015, 07:25:29 pm
content: Quote from: dom0 on December 13, 2015, 05:32:51 pmI did not understood how exacly you want me to show u the current limting so I can show you this:blue waveform is the voltag at Q4 base, green is the current across R18, the voltage is set to 30V and current limting to 100mV (100mA) and with the ugly overshoot (cant manage to fix the overshoot) it takes around 350us to enter current limting. I dont know if 350us is slow or not but for my purpose it is fast enough..with R18 set to 30R it takes about 1ms, still fast for me.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 13, 2015, 08:52:45 pm
content: 350 µs is pretty slow. I would like something like less than 1/10 of that, better even shorter. The good thing is that the LM350 has an internal current limit, that will limit the worst case current to something like 5-10 A in case of a short.With additional amplification in the Feedback loop it is not 100% sure the LM317/LM350 will not oscillate. I may be OK with the load tested, but it may not with a different load. One problem is that the models for the LM317/350 may not be that accurate to decide this in simulations alone - not even all of the chips may be similar, as there are several manufacturers.A test for stability would be looking at the AC response to a current source a load. This directly gives output impedance as a function of frequency.The fuse outside the loop may actually help getting it stable, but limits DC (and LF) performance.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: December 13, 2015, 09:12:56 pm
content: Quote from: Kleinstein on December 13, 2015, 08:52:45 pmwell 350us is quite fast, I'm hobbisyst and most of the time this current limting will fuction just incase I short something or chraging a battery, and I think 1ms is good enough.also I dont have osciloscope (!) yet, I do have the money and I want to order the DS1504Z from ebay but I'm not sure about taxes and stuff yet. so thats a problem now.as far as I tested I could not get it to oscilate (only the ugly overshoot that I wish I could fix somehow)I have no idea how to do stability test or how to look at it, I watched "linear technology" tutorial about op amp stability in ltspice and I could not understand 70% of the things they said..well I guess now I need an osciloscope and breadboard it. also learn how to do those stability tests