Basic linear PSU design review, please! - Page 1
Forum section: electronics / projects

author: Boscoe
author_rank: Frequent Contributor
timestamp: July 17, 2025, 12:08:17 pm
content: Hi all,I'm looking for some feedback on the attached design for a linear PSU with current limiting. The intention is to reach 1mV accuracy and digital control with a combination of a decent voltage reference, good DACs, some trimmer pots and calibration!My main concern is with the stability of the circuit with the multiple control loops. I have simulated all the cases I can think of:- Switching voltage setting- Switching current limit setting- Current limit switching to cross CV, CC- Switching output loadI used a 1kHz square wave with 1ns rising / falling edges.Does anyone have any suggestions on likely issues when building a real circuit? The design output is 0 - 24V up to 5A maybe.Many thanks!

author: PGPG
author_rank: Frequent Contributor
timestamp: July 17, 2025, 01:37:54 pm
content: Based on what you have selected RTR030N05 for this project?

author: Boscoe
author_rank: Frequent Contributor
timestamp: July 17, 2025, 02:13:44 pm
content: No, I was just using those for simulation as the gate charge and RDSON is similar to what I intend to use.

author: MathWizard
author_rank: Super Contributor
timestamp: July 17, 2025, 02:26:04 pm
content: At a glance, the current from M1 can only go through R1=10, so that will have a huge voltage drop, for any size current, and use a huge amount of power, and probably make the op-amp go to the rails right away.I've been working on E-loads too much lately, so if this is a power supply, where's the output ??

author: Boscoe
author_rank: Frequent Contributor
timestamp: July 17, 2025, 02:29:56 pm
content: R1 is the load and U2 has a gain of 1.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 17, 2025, 02:42:44 pm
content: I would not use a MOSfet for a linear power supply. Most MOSfets are not rated for linear use and they may develop "hot spots" when you attempt to use them this way. And second, the high Ugs (often 4.5V) is a quite big loss for the maximum output voltage, unless the gate is powered from a separate power supply.With a BJT, the B-E junction voltage drop is around 600mV, and this is small enough to be insignificant. It is often also used with a power shunt resistor as an over current limit.If you want to make a four quadrant power supply, then start by looking at power audio amplifiers. There is over 60 years of development in those things, and a gazillion schematics are floating on the 'net. And they can also be easily modified for DC use.

author: JohanH
author_rank: Frequent Contributor
timestamp: July 17, 2025, 02:45:21 pm
content: Quote from: Boscoe on July 17, 2025, 02:13:44 pmIf this is a linear supply, RDSON doesn't matter. SOA under DC conditions is more important.

author: Boscoe
author_rank: Frequent Contributor
timestamp: July 17, 2025, 03:22:42 pm
content: Quote from: Doctorandus_P on July 17, 2025, 02:42:44 pmI do plan on using a higher voltage rail for the op amp as seen in the schematic.The problem with the BJT in the current sink is the base current will cause error in the constant current function, I would like better than 1mA of accuracy. Darlingtons will not fix this problem because the base current will always exit through the emitter. One could be used for the voltage regulator, though.This is only a two quadrant supply and that is all I need.

author: Boscoe
author_rank: Frequent Contributor
timestamp: July 17, 2025, 03:23:15 pm
content: QuoteNot for simulation, though. SOA is trivial to calculate for DC in this circuit.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 17, 2025, 03:55:15 pm
content: Intriguing ideas. I dont think that the INA offers any advantages as the sense voltage amplifier.I'm not sure what this low side reference (U3 etc) is for. It will be certianly be unstable unless there is a resistor from the inv input of U3 to the source of M2.Its easier, simpler and a lot cheaper to use a a precision opamp to sense the output voltage with a Kelvin, aka 4-wire, connection. This senses the output voltage at the point of load rather than inside the psu. A precsion supply should include a 4-wire option.MOSFET followers ar a pain because of the need to drive the gate to Vload+Vgs. A Darlington or Sziklai pair may simplify the design. The latter needing less than a volt of headroom.Frequency compensation of the control loop can be tricky. When you simulate this be sure to check the load step response.You're more likely to see an instrumentation amp used in a current sensing circuit. You will need that in your build to protect your psu and your kit from catching fire!I've sketched out the idea- dont take my values as in any way correct or that the schematic is complete, I am not a psu guy!Think of a power supply as an amplifier of the set point.It is often preferable to be able to source and sink current from the load in all four quadrants. A bidirectional high current follower is what's needed there.

author: Boscoe
author_rank: Frequent Contributor
timestamp: July 17, 2025, 08:38:18 pm
content: Quote from: Terry Bites on July 17, 2025, 03:55:15 pmU3 is used for the constant current function. It essentially will reduce the voltage available at the output choking off current flow to maintain the voltage set on U3's non inverting input pin at the current sense resistor. This amplifier has a -5V rail attached to is Vs pin to give this room.I think the output swing for this is adequate as it will have a headroom of +5V on the positive supply as well. MOSFETs are used here for the zero DC bias current needed as apposed to BJTs.I used an instrumentation amp here for two reasons. 1. The constant current operation will be moving the virtual output ground up and down through the whole output voltage range so we need to remove this common mode voltage. 2. Using feedback resistors you have creates another current path introducing measurement and constant current operation error. For example 2k at 20V is 10mA.I can add force and sense terminals.There is a CC operation as I mention.I have simulated these circuits as I mention for stability. I have a decent phase margin and apart from the obvious ringing here and there it seems stable even with a majorly capacitive load. I would like someone more experienced than me on this subject to offer some advice, tho.I don't think a bidirectional current sink is needed. This is intended as a power supply only.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 17, 2025, 09:11:08 pm
content: Is C3 really 1f ?Quote from: Boscoe on July 17, 2025, 02:13:44 pmWhat MOSFET do you intend to use?You will need to work backwards, find power devices with large enough die and cooling and SOAR to manage the peaks of 24V/5A delivery.120W is a lot to try to get out of a single device.The current limiting section uses 0.1R, which is quite high, and needs 2.5W part at 5A - usually a lower R and lower offset opamp (or current sense amp) is chosen here.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 18, 2025, 01:28:09 am
content: Quote from: Boscoe on July 17, 2025, 03:22:42 pmThere are other ways to manage that, if you are base current paranoid.See #1 and #50 herehttps://www.eevblog.com/forum/projects/precision-current-sink-i-found-a-gem-of-a-circuit/You could also look atMOSFETS designed for linear/Audio use. These have excellent SOAR and VGS increases with temperature. Not cheap, but proven for linear use.Looks like they recently upgrade their FAB partner too, must sell enough to be a viable niche.https://us.profusion.uk/int/audio/transist/lateral-mosfets

author: Kleinstein
author_rank: Super Contributor
timestamp: July 18, 2025, 06:31:31 am
content: The circuit would be 1 quadrant only, except for maybe a minimal sink current in the µA range.Getting the whole power from a single MOSFET is tricky. Combining multiple in parallel is not that easy with MOSFETs. The gate capacitance can be limiting to the speed the OP-amp can drive things.For the stability analysis the tricky part is getting stability with different external loads. For the CV mode the difficult case would normally be a relatively large load capacity with low loss. For the CC case the difficult case is an inductive load. The 10 ohm lard resistor as shown in the simulaton already make the load less critical, but actual loads may be more tricky than this. Different load currents can also change things. In most cases one needs some lossy capacitor or RC series element across the outputs.Even of the small signal part is stable, they may be some sort of large signal instability with the regulator(s) going in an out of saturation. This part would be visible as longer than normal ringing after large load steps.As it looks the inactive regulators would wind up and the transition between CC and CV mode could be a bit slow with some possible overshoot - usually OK for the current (if not too extreme), but it could be an issue for the CC to CV transition at relatively low set voltage. The seprate output stages for the CC and CV mode not only adds costs, but also tends to slow down the transition and makes it very hard to limit wind up.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 18, 2025, 07:46:45 am
content: SorryI made a couple of errors and I've modified my repsonse.I thought the INA was single supply and didnt spot the current sink.The current sink still needs the additonal resistor as I said.A current limit would be easier to implement than a contant current sink.Sensing the high side current can be simpler to implement, eg INA293You still need a second error amp to control the current and the stabilty of that will be a factor in the over all performance.

author: Boscoe
author_rank: Frequent Contributor
timestamp: July 19, 2025, 07:02:29 am
content: Quote from: PCB.Wiz on July 17, 2025, 09:11:08 pmIt's 1 fempto.I think I will use something like the audio devices you posted below now this has been brought to my attention.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 20, 2025, 04:49:42 am
content: Quote from: Terry Bites on July 18, 2025, 07:46:45 amYes, another advantage is you do not need to cool two sources of possible high heat.A separate current limit stage is only used sometimes, but needs the same peak cooling as the regulator stage.There are parts like AP4320, designed for CC+CV regulation with dual opamps.

author: peter-h
author_rank: Super Contributor
timestamp: July 20, 2025, 02:25:31 pm
content: Those 10R resistors in series with MOSFET gates make no sense. The current there is negligible.Otherwise, MOSFETs are fine for this. Check the SOA.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 20, 2025, 04:09:14 pm
content: Quote from: peter-h on July 20, 2025, 02:25:31 pmA resistor is used to prevent oscillations.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 20, 2025, 05:00:13 pm
content: Quote from: peter-h on July 20, 2025, 02:25:31 pmTransistor current sources happen to have negative input impedance for some frequencies. Adding few positive ohms in series with it makes the most sense.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: July 24, 2025, 09:42:45 pm
content: as it merely concept and devil in details , then ...usually ... power supply as a single quadrant designed to have transformer with at least 2 independent sections as a power section , and for control circuit ±15V ( let say .. ) in this case ground of ±15V connected to high of power section , in this case current measure on high side would be better, and you measure it relative to "0" , then simple to manage mosfet as well , relative to '0' ....( if .... most practical way to have ± variable , as combination of 2 single , in this case you need to measure current on high side )for mosfet you need to look at safe area to manage it in a linear mode ...