My Power Supply is F'd Up! - Page 1
Forum section: electronics / projects

author: colorado.rob
author_rank: Frequent Contributor
timestamp: September 02, 2017, 01:10:56 am
content: I was banging my head against the desk yesterday trying to figure out what I had done wrong with the design of the power system on a new PCB. This is what the scope showed on my power rails.CH1 is VIN, CH3 is +3V3, CH3 is VDD_EN signal, and CH4 is VDD (switched). As soon as VDD_EN goes high to enable the switched LDO which provides VDD and VDDA to the peripherals, VIN and +3V3 drop precipitously by ~1.5V! It stays low for (in this case) 250ms. This caused problems during start-up, as one might expect.VIN is powered by my bench PSU. Even when powering on, my board draws less than 20mA The PSU isn't going to go belly up when drawing 20mA. The power is fed through my bench DMM to measure current consumption. Nothing unusual in the current being drawn during power on.is going on?I spent hours trying to track down what was causing this to happen. The odd thing is, the amount of time it was in this zone wasn't fixed, but the amount it dropped was. It varied quite a lot, from tens to hundreds of milliseconds.It hit me this evening after switching the DMM to voltage mode to see if I could capture the voltage drop on the DMM. The problem went away.The alternate subject for this post should be:Adventures in Auto-ranging DMM Burden VoltageWith the DMM in auto-ranging mode, when the circuit is powered off, it goes to its 200uA range. When the circuit powers on, it is still at this range, with the higher burden voltage, dropping the voltage until it can adjust its range up to the 200mA range. At this point it recovers to where it should be. And this is also why the time varied. It was independent from the DUT.

author: rstofer
author_rank: Super Contributor
timestamp: September 02, 2017, 12:38:12 pm
content: Nice troubleshooting!

author: nctnico
author_rank: Super Contributor
timestamp: September 02, 2017, 12:43:04 pm
content: A good example why PSUs with a very accuracte current readout are so nice to have

author: colorado.rob
author_rank: Frequent Contributor
timestamp: September 02, 2017, 03:35:53 pm
content: Quote from: nctnico on September 02, 2017, 12:43:04 pmI do not need a Keithley 2280S.Idonot need a Keithley 2280S.I donotneed a Keithley 2280S.I do notneeda Keithley 2280S.Ido not needa Keithley 2280S.

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2017, 06:57:49 pm
content: Quote from: nctnico on September 02, 2017, 12:43:04 pmOr it is a good example why manual ranging DMMs have their virtues. My DMMs all provide 1 milliamp or better resolution on their highest current range so I cannot say that I have run across this problem and power supplies do not normally read out their current with that kind of accuracy. If you want something better, then you need a source meter.

author: nctnico
author_rank: Super Contributor
timestamp: September 02, 2017, 08:35:19 pm
content: Some HP/Agilent power supplies have sub-uA resolution at low currents and get close to source meter territory. Recently Keysight introduced a bench power supply which offers uA resolution in a low current range.

author: colorado.rob
author_rank: Frequent Contributor
timestamp: September 02, 2017, 10:49:44 pm
content: Using the sense inputs on the power supply or using a fixed range on the DMM work too. The sense inputs result in a little overshoot on VIN. Using DMM in the 20mA range gives me 10nA resolution, which is all I need. The DUT alternates between 2.9uA in standby to 17mA in run mode, which is just within the DMM's 20mA range. There's about 200nA difference in the reading between the 20mA and 200uA range.The problem there is my design goal is 3uA standby current and in the 200uA range it reads 2.88uA and in the 20mA range it reads 3.1uA.

author: GreyWoolfe
author_rank: Unknown
timestamp: September 02, 2017, 11:04:11 pm
content: Quote from: colorado.rob on September 02, 2017, 03:35:53 pmYes you doYes you doYesyou doYes youdoiYes you do!You know you want one