Anti-Overshoot Power Supply - Page 1
Forum section: electronics / testgear

author: katzohki
author_rank: Frequent Contributor
timestamp: March 04, 2020, 08:02:47 pm
content: Hi All, I've got what I think is sort of a weird use case going on. My device pulls about 30A at 4V during testing and when the load cuts out (motor load) the (SM) power supply tends to overshoot voltage. This isn't that surprising and it's very brief, but unfortunately we have a component in there that gets taken out at 6V. That this isn't protected is a "whoopsie" in the design, but it's supposed to be battery powered so it's only happening during test. Even the shortest pulse (<100us) over 6 Volts has proven to potentially cause damage and latent failure.So my question is; Is there some kind of special power supply that would guarantee not to overshoot my Voltage set point? I'm also open to suggestions of improving my assembly / test setup if you have any robust ideas.Thanks!

author: coromonadalix
author_rank: Super Contributor
timestamp: March 04, 2020, 08:06:53 pm
content: I think crow bar protection avoids that ?? or maybe an value calculated inductance ??

author: nctnico
author_rank: Super Contributor
timestamp: March 04, 2020, 08:27:28 pm
content: Big fat TVS diode across to power supply to the board. But still it is better to change the design to incorporate a TVS. It is likely that when the battery gets bad or contacts get worn the device will see the reverse EMF from the motor anyway.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 05, 2020, 12:31:58 am
content: Crow bar protection crossed my mind, I'll have to find a self-resetting version so it doesn't annoy manufacturing. I'll take a look at finding an appropriate TVS diode as well. I totally agree that the design change would be best, but it's not an option in management's eyes. Thanks!

author: JDubU
author_rank: Frequent Contributor
timestamp: March 05, 2020, 01:30:25 am
content: Is it possible to do the testing with the battery?

author: sassywren
author_rank: Contributor
timestamp: March 05, 2020, 04:55:18 am
content: It's not uncommon to see an active pulldown (i.e. resistive load) that fires on load release on some supplies. I have seen this on crappy buck regulators that are running in voltage mode and used in USB applications where this overshoot is also a major issue. There are a couple ways to achieve this, triggering it on voltage is probably not fast enough and you would need to trigger it on large negative di/dt and simply fire it for a fixed duration of time.

author: nctnico
author_rank: Super Contributor
timestamp: March 05, 2020, 07:14:16 am
content: Quote from: katzohki on March 05, 2020, 12:31:58 amWithout a design change it is very likely you'll get lots of warranty returns in a couple of years!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 05, 2020, 02:37:13 pm
content: TVS diode likely won't do anything: there are few available with nominal voltage below 5V (which clamps at 8-10V, useless here), and they all clamp at the same level anyway because low voltage zeners simply have softer curves and by the time you get into surge currents, they're dropping the same voltage.We aren't necessarily talking surge currents here -- this isn't induced lightning or capacitive discharge -- so the clamping voltages may be more friendly. But it still seems unlikely to work out -- there's just not enough precision available, too much slop between nominal voltage (where the TVS is drawing leakage current, some uA) and breakdown voltage (where the TVS is drawing noticeable current, some mA).There are better devices, like the snapback diode which offers low dynamic resistance (= flat voltage drop) at low breakdown voltages (2-5V). But I don't think you can get snapback diodes in high power ratings (and they're hard to find by themselves, as it is), so those are out.That leaves something less TVS-ey but more closely tuned to the application; a shunt regulator for example.You don't want a crowbar, because that latches and shorts out the supply.If the problem is supply dynamics, this is a very good solution --Quote from: sassywren on March 05, 2020, 04:55:18 amIf you don't care about efficiency, a dumb load resistor (on all the time, no switch) will do. If it's dynamics, it might only need to be an ampere or a few.If you do care, and you have the motor control signal is available, then you can simply invert that signal, run it to a MOSFET with a C+R so it turns on fast then slowly times out, and use that to switch a load resistor.This could be described as a monostable timer, but beware with digital analogies: using a timer to drive the MOSFET, it would turn off suddenly, so you need to worry about the turn-off overshoot from that, which needs another timer and FET, which needs..... So an analog hack like this should be alright. Mind, the MOSFET needs to be rated for whatever power it dissipates as it comes out of saturation.If you don't have the control signal, you're just as well off using a shunt regulator; sensing supply ramp rate I think would be a bit dicey (but is possible). For this, use a TLV431 shunt regulator, boosted with a logic-level PMOS. The connection will be:- Sense voltage divider: resistor from +V to REF, resistor from REF to GND. Can put an R+C in parallel with the top resistor to give some derivative sensitivity.- Regulator: A to GND, REF to divider, K to PMOS gate. Pullup resistor from +V to K, say 1k or so.- PMOS: source to +V, gate to regulator K, drain to GND. Optional: resistor in series with drain, to limit worst-case current draw to safe levels.Divider resistor values can be typically 10-100k. Set the ratio such that 1.24V (Vref) is reached somewhere between +V(nominal max) and +V(the other stuff blows up). So uh, 4.5-5V in this case I guess?Use a generously sized PMOS. It's not clear offhand how much energy this overshoot contains (or will contain once clamped), but it can be estimated from waveforms. Energy handling roughly tracks chip (die) and package size. So shop in terms of, say, SOT-89, SOT-223 or PSON-8 if the energy is small, or PDSO-8 or DPAK for the next size up, or D2PAK, or larger still (D3PAK, TO-247, or if more is needed, consider paralleling multiple), as needed.Note: PMOS have generally poorer performance than NMOS, but that's actually somewhat helpful in this case as you need a bigger die for the same ratings, and that extra die area means more energy dissipation!Note that the PMOS greatly increases the current gain of the TLV431, and it already has a lot of gain so that oscillation can easily show up. If this is a problem, some negative feedback can be arranged. An example would be, use the series drain resistor, but a small value (some mΩ), and connect the regulator's A there instead of to GND. This way, as current rises, the (Vref - Va) voltage seen by the TLV431 falls, reducing its gain while including the PMOS in that loop.(The TLV431 could also be boosted by an NMOS but I think it would be harder to stabilize; the PMOS has the advantage that it's a source follower configuration, so its voltage gain at least is low. A PNP could also be used, but these are pretty big in the current ratings we're talking here.)Tim

author: Fgrir
author_rank: Regular Contributor
timestamp: March 05, 2020, 03:39:59 pm
content: I've run into a similar situation with a system used for testing circuit breakers at currents up to 100A. In this case the power supply being used would overshoot and trigger it's internal OVP circuits, so there was no damage to the control circuitry but the OVP shutdown caused it's own problems to the testing system. We were able to prevent the OVP from firing by adding capacitance across the power supply to slow down the dV/dT due to the load cut-off transient until it was slow enough for the power supply control loop to handle. I can't remember the exact values of capacitance and what you need would depend on your own equipment anyway, but I think it was in the 100,000uF range.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 05, 2020, 04:16:21 pm
content: Thanks for all the help, I have a lot of things I can look at now as possible solutions to this problem! I'll try to post back with my progress at some point.

author: nctnico
author_rank: Super Contributor
timestamp: March 05, 2020, 05:09:40 pm
content: Quote from: T3sl4co1l on March 05, 2020, 02:37:13 pmAn easier solution is likely to drop in an LDO (regulator) to provide power.

author: ogden
author_rank: Super Contributor
timestamp: March 05, 2020, 05:26:53 pm
content: Use powerful system supply like HP/Agilent/Keysight 6573A. Avoid switching supplies. Some local (to load) capacitors will not hurt as well.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: March 05, 2020, 05:35:52 pm
content: 1. What is the Vcc range of the part?2. What is the maximum current draw of the part?3. How many connections to other circuits that could bounce high or negative are there?

author: thinkfat
author_rank: Unknown
timestamp: March 05, 2020, 05:48:59 pm
content: Isn't this why lab power supplies typically have a separate overvoltage protection? My Agilent 66309D has that, iirc. The Rigol DP832 has it, too.

author: boffin
author_rank: Unknown
timestamp: March 05, 2020, 06:00:08 pm
content: Quote from: katzohki on March 04, 2020, 08:02:47 pmDon't use a power supply for the test, use Batteries, and make a circuit that switches the batteries from a charger to load under test, back to charger.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 05, 2020, 08:20:02 pm
content: Quote from: nctnico on March 05, 2020, 05:09:40 pmYes-- instead of focusing on shunting the excess, why not ride through the one little thing that needs it?Tim

author: ogden
author_rank: Super Contributor
timestamp: March 05, 2020, 08:30:33 pm
content: Quote from: thinkfat on March 05, 2020, 05:48:59 pmNo. Crowbar is protection against failure of regulator or accidental backfeeding with another source of voltage. Quality lab (linear) supplies do not overshoot when load goes from 100% to 0.

author: Fungus
author_rank: Super Contributor
timestamp: March 05, 2020, 08:36:36 pm
content: Quote from: katzohki on March 04, 2020, 08:02:47 pmAre you sure it's the power supply and not an inductive spike from the motor?

author: nctnico
author_rank: Super Contributor
timestamp: March 05, 2020, 09:22:03 pm
content: Quote from: ogden on March 05, 2020, 08:30:33 pmThe problem is that the load is feeding back into the PSU. Some HP power supplies typically have a downprogrammer (current sink circuit) which can dissipate excessive energy at the output (up to certain limits). The older 6012A / 6024A and 603xA series come to mind (all high power switching PSUs).

author: ogden
author_rank: Super Contributor
timestamp: March 05, 2020, 09:35:54 pm
content: Quote from: nctnico on March 05, 2020, 09:22:03 pmWhere this info came from? Perhaps I missed something.QuotePurpose of downprogrammer - to improve response time of the supply, when target voltage is decreased and there is not enough external load to discharge supply output caps fast.

author: nctnico
author_rank: Super Contributor
timestamp: March 05, 2020, 10:04:06 pm
content: Well, it also works as a brake on (small) DC motors. I have tried this myself. It may be good enough to get rid of a spike.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 05, 2020, 10:19:09 pm
content: Quote from: T3sl4co1l on March 05, 2020, 08:20:02 pmI'm not sure what you mean by ride through, but at 4 Volts or adjustable output I'm not finding anything that can provide at least 15A that would still be called an LDO. At any rate, if I can just whack on a linear regulator and call it a day I'm in. Although this shunt regulator idea sounds fun.Quote from: Fungus on March 05, 2020, 08:36:36 pmI have to admit that motor control is not my strongest suit, so here's what I can tell you. It's a 3-phase Brushless DC motor, with hall sensors, we're using sine-drive control, DRV8301 controller with some decent sized FETs for switching. The 4V battery is boosted by a 2-phase synchronous boost converter to 8V which is what the FETs pull from to run the motor at 8V. There's 1500uF of caps on the 8V side. I'd be really interested to know how I can find out whether it is coming from the motor or not.Now that I think of it - I did probe the 4V input line while running the test on battery power and didn't get the same spike as with the power supply, so if that counts as proof then I guess it's not coming from the motor.

author: ogden
author_rank: Super Contributor
timestamp: March 05, 2020, 10:19:18 pm
content: Quote from: nctnico on March 05, 2020, 10:04:06 pmOh, yes indeed. When supply voltage is programmed/set from nominal to 0V - then downprogrammer indeed is DC motor brake. On the other hand OP said that supply overshoot happens after DC motor *disconnect* from the rest of the circuit. Overshoot is typical for (slow) switching supplies. As nearly everybody suggested - linear supply is solution here. For up-to 1A I would suggest DIY "linear postregulator", but30A50A supply is project on it's own, better just buy something used. There's Keysight-6573A-35V-60A-DC-Power-Supply for 500$ on *bay now for example.

author: nctnico
author_rank: Super Contributor
timestamp: March 05, 2020, 10:30:09 pm
content: A linear power supply won't help since these have a pass transistor which only passed current in one way (out). You need a parallel sink circuit either way.I just tested with an HP6012A set to 4.5V and try to push 6V @1A in. The output voltage is capped to 4.5V while sinking 1A. Although it must be kept in mind that the current limit of the down programmer is limited to 1.5A on the HP 6012A.BTW: the 6573A is a switching PSU too. I have the 20V/100A version. And yes these series have a down programmer too although it seems to be unspecified.

author: Someone
author_rank: Super Contributor
timestamp: March 05, 2020, 11:23:42 pm
content: Quote from: katzohki on March 05, 2020, 10:19:09 pmUnless something is very wrong or its running as a bidirectional converter the boost converter shouldn't let any power flow back from the motor to the 4V power input. Probe the 8V supply under the fault condition just to be sure but the obvious way to find out if its the motor would be a current probe somewhere along that 4V -> 8V path.Assuming its just a boost converter you're probably left with it being a characteristic of the test power supply system, including the cabling etc. Remote sense and some capacitance right at the load could be all thats required, fancy 2 quadrant power supplies are when you need to push power back into the source (battery for example).

author: nctnico
author_rank: Super Contributor
timestamp: March 05, 2020, 11:34:34 pm
content: Quote from: Someone on March 05, 2020, 11:23:42 pmIt can still be induced somehow. At high currents even a small inductance can cause a nasty spike. I one of my designs I had trouble with the self inductance of planar (not wirewound!) resistors.I think this circuit is a good occasion to get a differential probe and probe around a bit to see where the spike is actually coming from.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 06, 2020, 12:18:49 am
content: Quote from: Someone on March 05, 2020, 11:23:42 pmOkay thanks, this was my thinking as well. Can't hurt to look at the 8V side of things certainly so I'll add it to the checklist as I get on with this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 06, 2020, 12:35:20 am
content: Quote from: katzohki on March 05, 2020, 10:19:09 pmYou said this --QuoteDesign oversight or not, perhaps it is time to add some protection to that component? A regulator can be added in front of it, to drop essentially no voltage under normal circumstances, but provide the same output voltage even when the input swells too high. In other words, a regulator, to ride through it.Tim

author: David Hess
author_rank: Super Contributor
timestamp: March 06, 2020, 01:12:07 am
content: Some regulated power supplies have better load transient response than others. If this is important then it should be part of the specifications during design. I have seen commercial supplies which gave it no thought and had obvious design mistakes which made overshoot worse like leaving out base-emitter shunt resistors.T3sl4co1l mentioned using a shunt regulator and I have done that before. I essentially made a precision power zener diode using a big power transistor and feedback circuit.Another common solution is to add enough bulk output capacitance to absorb the transient.

author: ogden
author_rank: Super Contributor
timestamp: March 06, 2020, 10:09:08 pm
content: Quote from: nctnico on March 05, 2020, 10:30:09 pmOuch, my bad. Thank you for clearing this out. For example 6654A I used long ago is linear, thou it was "only" 10A or so. Seems like 6x7x supplies are SMPS and 6x4x, 6x5x - linear, right?

author: Keysight DanielBogdanoff
author_rank: Unknown
timestamp: March 06, 2020, 11:23:03 pm
content: I talked to the power team about this, they mentioned that the motor loads are tricky (for a resistive loads the E362xxA supplies have <1V overshoot, but motors can vary).You should be able to get a demo of one if you want to try it out. You could parallel an E36233A's outputs to get to those power levels.

author: nctnico
author_rank: Super Contributor
timestamp: March 06, 2020, 11:54:17 pm
content: Quote from: ogden on March 06, 2020, 10:09:08 pmBased on the service manual this appears to be the case. The 6573A / 6673A still weighs 28kg though!