Power Supply Design - Current Limit working too well - Page 1
Forum section: electronics / projects

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 27, 2011, 03:28:30 am
content: I'm building the prototype of this power supply. The bottom left portion of the circuit makes a -1.2V reference to allow the output voltage adjust LM350 (IC2) to adjust down to 0V (actually going down to -0.1V). The VR1 is a header connection on here, but gets a 5k pot for voltage adjust of IC2.IC1 is where I'm having the problem. This is setup in the normal current limit setup. S1 will go to the rotary switch and back through the proper current limit connector. The current will be limited to what causes a 1.2V drop across the resistors. The higher current ones are in parallel just for power rating through 2W resistors.I'm driving this with around 55VCT transformer, giving me around 28V DC at the rectifier. I'm getting a max output of about 2.5V. I'm losing almost all of it across IC1. I had to mount a heatsink to the prototype, otherwise the chip went into thermal shutdown almost immediately. I'm not sure if I have a bad LM350 or did something wrong on this circuit. Anyone see any bonehead issues?

author: RayJones
author_rank: Frequent Contributor
timestamp: June 27, 2011, 08:07:49 am
content: Perhaps I am misinterpreting the action of IC1, but it would seem that is actually configured as a constant current source, by constantly striving to achieve 1.2V between Vout and ADJ via the selected resistor(s).

author: RayJones
author_rank: Frequent Contributor
timestamp: June 27, 2011, 08:30:19 am
content: OK, I've had a bit more thought about it and see how this may work.I can see that the output voltage from IC1 will rise with lack of current draw.I can also see that when you switch ranges with a load attached it is possible that all the current will attempt to pass down through the ADJ pin of IC1 which is not good.Your switch must be make before break, or perhaps include a default high value R permanently across Vout and ADJ for the lowest current limit option....Have you switched ranges with a load attached?

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 27, 2011, 01:33:04 pm
content: I have only hooked it up with R5 as the current limit resistor. No switching or anything. I also have no load connected. This is what has me stumped. With only quiescent current being drawn for IC2, I would expect IC1 to be setting the output to nearly 1.25V less than the input of IC1.I don't remember where I pulled this circuit from. I have had it in my notebook for over a year. I can see what you point out with range switching. I think it would be a good idea to have a low mA level resistor attached permanently to feedback.I guess I just need to double check everything for the 10th time and I think I'll swap out IC1 with another, just to eliminate the possibility that I killed it some how.

author: Zero999
author_rank: Super Contributor
timestamp: June 27, 2011, 05:12:49 pm
content: It looks like it should work.Yes, you should include a resistor permanently connected to allow something like 5mA to 10mA through.It's not a very good way of doing it, using ordinary diodes to bias the adj pin below 0V isn't very good because their foward voltages will vary as the temperature changes. The correct way to do it, is to use a 1.25V bandgap reference or better still use an op-amp to measure the voltage on the adj pin and subtract it from the output.The LM350 current source has a high dropout voltage (worst case 4.25V at 3A) taking the over all droput voltage to 7.25V.At low output voltage settings the safe area protection on the LM350 will kick-in, limiting the current to below 3; this happens when the voltage across Vin and Vout exceeds 15V to 26V.