uSupply - short circuit current deviation because of constant current load - Page 1
Forum section: electronics / projects

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: April 09, 2015, 06:01:41 pm
content: In his uSupply design Dave used a constant current load with e.g. 5mA to always fulfill the minimum output current requirement for the linear regulator (LT3080 in this case).Now this works fine for voltages > 0.7V or so but in a short circuit condition, the voltage drops below that and the constant current load doesn't sink its 5mA any more.So instead of e.g. 100mA current limitation, you get 105mA in case of short circuit.This might be nitpicking, but being off by 5mA because of something so trivial is a bit frustrating.I could imagine several ways to detect either the low voltage situation and reduce the current limit accordingly or to detect a low current situation and only activate the current source for low currents.Yet none of these approaches seems to be very elegant. I could also imagine that trying to switch or regulate the 5mA current source could introduce additional ripple/instability in cases of current limitation (voltage drops -> current source switched off -> voltage rises a bit -> current source switched on etc.). So is there an elegant solution for this?As a side note: I wonder if there's some kind of common understanding what transient cases a proper lab power supply should handle and how.So e.g. when jumping from 10% to 90% load current and back: how to define the slew rate and what times needed for regulation, over-/undershoots etc. are acceptable?

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: April 10, 2015, 06:42:38 pm
content: Ok, I came up with the idea of an N-FET current mirror to create the 5mA load. In LTSpice it works down to about 45mV (instead of 700mV for the BJT solution) and to 23mV with a 1mA drop (4mA instead of 5mA).So even with a 0.2Ohm load and a 100mA current limitation, the current will be only 1mA too high.Then again, the problem remains for low current limits. E.g. if you set the current limit to 20mA and apply a 0.2 Ohm short, the voltage drops to 4mV. Even this improved current load would only sink ~1mA in this case, so the limit would be 24mA instead of 20mA. Well, yeah, slightly esoteric case obviously.So is this a dumb idea???With a IRLML2502 the current load works even to lower voltages. With a 750Ohm (better 768Ohm) resistor, 5mA are flowing until the voltage goes down to about 30mV and 1mA drop is reached at around 12mV. At 4mV, there are still around 2mA flowing, so the error would be 3mA.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 10, 2015, 11:39:46 pm
content: Hi 0xdeadbeefQuote from: 0xdeadbeef on April 09, 2015, 06:01:41 pmYes there is, It's this 'If your product is poor at a particular spec then don't publish one'.You would expect for a good supply for a CC Load variation transient response of a similar order to the CV load transient reponse (so somthing like recover to within .1% of initial setting < 20uS would be expected), but most often CC load regulation is sacrificed for better C.V regulation and stability and so will be noticeably missing from the performance specs.On the LT3081 Have you even considered why your using a integrated V regulator ? These are great when your building a product and your needing a quick one part p.s solution since they save you having to provide all the discreet parts that would otherwise be necessary to construct a discreet Vreg (oPs,the oPs supply,voltage refs,loop compensation scheme etc) just pop in a single part and your done ! . But when you start adding those bits around one in anyway to create external control loops, like here then you have already lost most of the benefit's that using an I.Vreg bought to you in the first place. And your still left with all the disadvantages they have (like having to work around all the little extra idiosyncrasies they may come with,extra cost,poorer thermal sinking ) when compared to a single transistor solution.Here then is where a discreet transistor solution is much prefered.If your still determined to use this device and want to continue in self flaggelationthen I can only offer you this small tip, I would be getting rid of C7 altogether, it's what is responsible for the oscillation you mention going on between the prereg and post reg V cntrl in your other thread, it's also mostly responsible for the slow cc mode and oscillation .You do still need to add some compensation around op's to cure but getting rid of C7 is a first step.Good luck .

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: April 11, 2015, 12:06:03 am
content: Honestly this started as quick'n'dirty uSupply clone and I never intended to put so much effort into this. I don't intend to build the best power supply ever. I just don't want to create a complete piece of crap.Anyway, removing C7 is not really an option with this kind of current regulation (comparator, switching Vset to GND). Removing it will result in terrible noise and instability.The problem however is indeed that the larger the capacitance, the less of the current regulation noise will be on the output voltage and current but the slower the regulation gets.Dave used a 20µF capacitor in his uSupply design, so even though there or quite a bit of other differences in the mean time, I guess this will be even slower than my current design.This being said, adding a constant current load to the output is quite independent of the regulator design, isn't it? I guess most regulators will need a certain load for proper regulation...