uSupply, few questions about the design. - Page 1
Forum section: electronics / beginners

author: hitech95
author_rank: Regular Contributor
timestamp: September 07, 2016, 08:43:48 am
content: Hi!I just finished watching videos related to uSupply / How to design a power supply, and I have a couple of questions:Could you give me an example of why Dave has chosen a regulator instead of a mosfet? He said that things would get complicated, I would like to have an example to give me an idea. (Watching power supply teardown videos looks like "professional power supplys" do not use regulators but transistor/mosfet)The second question is related to the current limiter. Why it is before the regulator? As there are "losses" due to the currents of the regulator, the minimum load current, etc; it was not (perhaps) best (if it is possible) to move it after the regulator itself (before the diode)?Thanks, Nicolò.

author: dannyf
author_rank: Super Contributor
timestamp: September 07, 2016, 01:19:46 pm
content: Cannot answer your questions.I would suggest that you look into a typical UA picoammeter design. Keighley published their service manual. I had a long thread detailing design and construction of a PA meter, using both fancy and jelly bean parts. And I'm sure you will find other equally useful links too.It will help you understand some of the design elements.One advice: if you are to go down the IV converter path, don't ever use a t-network in the feedback path. Only professors who have designed nothing in their life will do that in a textbook.

author: KuchateK
author_rank: Regular Contributor
timestamp: September 07, 2016, 02:52:12 pm
content: Take a look at series by Gerry Sweeney. He designed PSU using discrete parts. He documented everything from simple experiments to final circuit and tweaks. Nice and educational series in PSU design.This series shows how many different problems can be encountered. By choosing regulator Dave skipped pretty much all work that Gerry did.http://gerrysweeney.com/fully-programmable-modular-bench-power-supply/

author: Kleinstein
author_rank: Super Contributor
timestamp: September 07, 2016, 05:13:14 pm
content: At first view it seems attractive to use a regulator chip instead of a classical design with OPs and MOSFET / BJT. However the way with the regulator chips has several drawbacks:The maximum power loss is limited - so this usually requires some kind of pre-regulation and thus makes the circuit complicated.Stability with critical loads can be a problem, as the chips are made to be used with a more or less well behaved, constant load. There usually is not mach room for adjustment.There usually is no separate sense pin, so that there will be a little extra DC ouput resistance. So not as precise as most OP / transistor circuits. Adding a kind of sense makes stability really difficult.Combination with current limiting can be tricky and difficult to get stable against oscillation and overshoot on cross over.The µ-supply did not work out that good. There where still some problems, due to using the regulator.A classical supply with floating regulator (similar to old HP units or many new cheap Chinese ones) is not really that complicated and more to learn from. However this is without pre-regulation, so you need a relatively large heat sink and the floating regulator usually needs a second transformer winding.

author: hitech95
author_rank: Regular Contributor
timestamp: September 07, 2016, 08:56:13 pm
content: Thank you for the suggestions to look at other designs, but it is not exactly what I had asked.@KleinsteinI have not understood if you're talking about transistors or regulators.

author: JS
author_rank: Frequent Contributor
timestamp: September 07, 2016, 09:47:45 pm
content: Quote from: hitech95 on September 07, 2016, 08:56:13 pmI think he was talking about the drawbacks of using a regulator.The resistor is before the regulator to improve line regulation, using the resistor after the regulator would mess with the reference for the regulator. If you go for transistors and your own complete feedback you could add it where you want. The looses on the feedback network are really low and very close to constant current so you could rule them out. You have the CC load to be able to archive lower minimum voltage output, and the loss through the regulation pin which is rated in the DS and used to calculate the resistor for the LT3080. It's also the resistance for the sense line, again, could be ruled out, math is cheap inside the µC.The advantage for the regulator is that is solves quite a few things for you. For the 3080 you only need to give it the voltage you want it at the output, in this case calculated using a few pieces to account for the current and the digital control. Note that this design could work without the converter, as there's no need for digital feedback, which could be way too slow to limit the current in time. The µC is giving the desired outputs limits and then sensing the values to show to you, then it could add some correction if it trusts more on the ADCs than the DACs.The things you'd need to consider if going for the discrete design are quite a few a regulator solves for you. Over temperature protection, over current (while the current feedback should take care of this a hard max limit is safe), there are some other things inside which could help with stability (or make it harder). Just accounting for the temperature cut out you already have quite a few components to work that protection out. The current limit could be ignored if trusting the ADC will not give a level too high and the electronics around are fast enough. There is one more thing, which has to do with the power driver, you can't run a power transistor directly with the voltage you want out of it, from an opamp with 1k open loop output impedance and from an open collector as an and gate. You'd need a few beans to solve that and get a decent driving capability of a suitable transistor.JS

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2016, 07:34:12 am
content: Integrated regulators are more limited in their voltage, current, and power handling capability than discrete transistors but they can still be used in lower power designs. Their built in protection features like current limiting, safe operating area protection, and thermal limiting can be an advantage for robust operation.Their limitations regarding output resistance without remote sense, high quiescent current making current regulation imprecise, and poor reference stability and high noise can be eliminated by using external feedback in which case they become something like an inexpensive alternative to the LM395 "Ultra Reliable Power Transistor". That leaves their largest drawbacks as low power handling and safe operating area. Safe operating area is something you have to watch out for with discrete pass elements anyway.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 08, 2016, 05:27:54 pm
content: The integrated regulator solves a few problems (actually not much more than over temperature protection, and SOA protection), but also cause new ones. And the new ones are nasty.Using a regulator chip with an external feedback is tricky: the regulator has a lot of internal feedback and so stability against oscillation can get really tricky - so external FB usually has to be limited to low frequencies. Having two loops makes the cross-over from voltage to current regulation even more tricky.I would consider it easier to get the feedback loops from ground up with discrete power stage, than to somehow add current limiting and improve stability / accuracy around a regulator. One problem here is that even with a good datasheet, there still can be surprises in the exact behavior of the chip. Transistors are much more predictable as they are much simpler devices. Even if you have a prototype around a regulator chip you never know if ones with a different date code will behave the same.At first it looks like the regulator chips does the feedback loop, but it only does it in a really poor way, if at all. At best you end up with a circuit that kind of works, but not really good and can have surprises like transient spikes or oscillation under certain conditions.So there is only one conclusion:It is a really bad idea to use a voltage regulator chip as the output stage of an lab supply.So commercial lab supplies use a discrete power stage (BJT or MOSFETs) for the output.

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2016, 08:48:14 pm
content: I disagree with it being tricky to use a monolithic regulator within a feedback loop; frequency compensation is always a concern and I have had just as many problems with device and loop oscillations in discrete pass transistor designs. In both cases the designer should make transient response measurements over the entire operating range.Check out the old LM309 (roughly equivalent to a LM340 or 7805) datasheet for an example using a 308 operational amplifier, external reference, and remote sense.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 09, 2016, 06:49:08 am
content: The circuit from the LM309 datasheet may work, but I am not that sure it is stable with any load. Also small variations in the regulator used (e.g. use the similar 7805 instead, or maybe just the TI Version of the LM309) might be enough to make is oscillate. This exactly one of those circuits where external loop an internal loop act together. The specs are just not accurate and complete enough to be sure it works.Quite some of the circuits shown in datasheets as possible applications are of marginal quality. So they somewhat work, but are not good. Especially poor examples are most circuits that us extra transistors to boost the output current from regulator chips like lm317.

author: David Hess
author_rank: Super Contributor
timestamp: September 09, 2016, 11:35:44 pm
content: Quote from: Kleinstein on September 09, 2016, 06:49:08 amThat circuit can be found with minor variations in a bunch of other datasheets but the link for the LM309 showed up first and it makes a good example. For instance I always use an emitter follower at the output of the operational amplifier which some other application notes show, pole-zero compensation around the amplifier unless it supports external compensation like the 308, and phase lead in the output divider to increase phase margin. If you use a regulator intended for variable outputs like a 317 which is a good idea for lower output noise, then you may need a level shift (or gain) between the amplifier output and the regulator. Conveniently what would be the top resistor in the voltage divider for the 317 then provides the minimum output loadandsets the operating point for the transistor buffer.The circuit as shown in the 309 datasheet is actually undercompensated via R1; usually unity gain compensation for a 308 would just require the 100pF capacitor to ground. That is easy enough to explain because the 308 is pretty slow (200 kHz?), the delay though the regulator is not significantly degrading the phase margin of the slow 308, and the R2/R3 feedback divider is increasing the gain margin anyway.I have had no problem with these circuits while using the much faster LT1007 or OP-27 for lower noise but without external frequency compensation, they absolutely will oscillate. Transient response was equal to or better than the regulator alone.QuoteI think the manufacturers make an assumption that the person implementing their application circuit will do the engineering to make sure it works correctly in their application.I never noticed any significant differences between the regulator families within the same type (Darlington outputs, Sziklai pair for low dropout, etc.) but there is aninteresting discussion hereabout the difference between the 309/340/7805 series regulators. I suspect these Darlington output integrated regulators have characteristics similar to an LM395 integrated power transistor which itself would make a fine protected pass element if a 317 were not cheaper.For my own designs, I do transient response testing with a function generator and oscilloscope and then derate the phase margin for what I expect will be the worst case variations. If a bode plot is then drawn, the effect of the regulator can then be extracted. A more sophisticated designer might use a network analyser.Low dropout regulatorsaremore difficult to use within a feedback loop because of their output characteristics but they are also more difficult to use by themselves if the input and output decoupling recommendations are not followed and I am not surprised designers regularly get into trouble with them with and without an external feedback loop.Usually I see both composite and discrete designs go wrong when uncontrolled gain elements are added within the feedback loop like common emitter/source transistors; they should at least include degeneration or local shunt feedback. Of course if you add gain within the feedback loop, then you are also asking for trouble but in some cases this cannot be helped and usually there is attenuation somewhere offsetting it anyway. Without local feedback or at least some way to control the operating point, the loop gain and sometimes frequency response will vary all over the place making things very difficult. In comparison, integrated regulators are tame.Cascaded operational amplifiers like you find in many current limit circuits are almost as bad and novice designers often go to heroic efforts to get them to work. The first constant voltage and constant current lab power supply I designed long ago was a real dog in constant current mode because of that; you couldseethe delay.