R&S HMP4040 voltage spike at PSU turn-off

[EEVblog]

HMP4040 mains turnoff with a 3V output, no load, just into the scope.

[EEVblog]

Same test into a 4 ohm resistive load

[mawyatt]

Is that the correct image with the 4 ohm load? It has the same time code and looks the same.

If correct image that's over 1.5 amps at the peak of 6 volts 

Best,

[EEVblog]

Is that the correct image with the 4 ohm load? It has the same time code and looks the same.

There was an error in the upload. But it displays ok for me now, time in the images is different.

[electr_peter]

Images in @EEVblog posts are the same, although small thumbnail images are different. Can you reupload them? There is some forum issue if images are quickly added to several posts, happened to me as well few days ago. Reupload solved the issue.

HMP4040 mains turnoff with a 3V output, no load, just into the scope.

Ouch, maybe that is related to relay opening.
Can you check voltage spike issue when output is OFF and then PSU is turned off as well? This is what sparked original topic.

[tautech]

There was an error in the upload. But it displays ok for me now, time in the images is different.

Welcome to the attachment problems that have been plaguing us for years.

It drives members nuts that this don't work as it should and that they need do things weird to work around it.

[mawyatt]

Would expect the R&S representative to step in and comment, or at least acknowledge this issue.

This isn't about an erroneous reading or measurement, but about potential damage to customers equipment/circuits under test

Have any folks with these defective Power Supplies contacted R&S?

Best,

[nctnico]

Is that the correct image with the 4 ohm load? It has the same time code and looks the same.

There was an error in the upload. But it displays ok for me now, time in the images is different.

Not at my end... Still seeing the same timestamps.

[rf-messkopf]

My HMP4040 does not do that. See the attachment: 3V directly into the scope input, and then mains power off. It's a fairly recent unit, bought in 2019, if I recall correctly.

I vaguely remember that there was a recall or a firmware update due to this issue. Read about that in a German electronics forum. Maybe a R&S representative can provide some information about this.

[pdenisowski]

Would expect the R&S representative to step in and comment, or at least acknowledge this issue.

Sorry, didn't notice this thread until now.  I'm personally unaware of this issue and don't have one of these in my office right now, so let me check with the product line.

Has anyone here contacted R&S support about this issue?

Will post an update as soon as I have one. 

[electr_peter]

R&S support was contacted on voltage spike issue (PSU ON + channels OFF -> mains off), still waiting for a response.

My HMP4040 does not do that. See the attachment: 3V directly into the scope input, and then mains power off.

Initial observation was on voltage spike with channels turned off (PSU ON + channels OFF -> mains off). Can you check what happens in that case?
(PSU ON + channels ON -> mains off) spike could be a different issue (but not conclusive at this point).

[rf-messkopf]

My HMP4040 does not do that. See the attachment: 3V directly into the scope input, and then mains power off.

Initial observation was on voltage spike with channels turned off (PSU ON + channels OFF -> mains off). Can you check what happens in that case?
(PSU ON + channels ON -> mains off) spike could be a different issue (but not conclusive at this point).

See the attachment: PSU switched on and output directly into scope, channel off; then PSU mains power off. The output ramps up to 1V and hangs there for about 6 seconds.

Not pretty, but not much of a problem for most circuits.

[rf-messkopf]

See the attachment: PSU switched on and output directly into scope, channel off; then PSU mains power off. The output ramps up to 1V and hangs there for about 6 seconds.

Same test but with 47 ohms load. Then the voltage only ramps up to a bit over 50 mV.

[tooki]

Is that the correct image with the 4 ohm load? It has the same time code and looks the same.

There was an error in the upload. But it displays ok for me now, time in the images is different.

FYI, despite showing different file names, sizes, and thumbnails, it’s actually loading the same image for both, at least for me.

[electr_peter]

There is response from R&S, paraphrasing:

Firstly, R&S recommends to turn-off channels normally and only then switch off PSU - this way voltages spikes are not generated. In R&S HMP FAQ section this is mentioned Best way to switch off the HMP series power supply
Channels are controlled via relays and PSU even tries to be gentle on contacts Switch cycles of the HMP Series

Second, on the main issue about voltage spikes with channels turned off, there is no official fix for this behavior. As I understand, these spikes are very limited in power (2.3-2.6V, ~2-3mA max) and probably can be mitigated by capacitors and parasitic loads on PCBs anyway. From posts we see there are at least several revisions of PSU HW with different resistances on output. My PSU has ~10 kOhm across the output, but with external 2 kOhm load voltage spike is reduced to OP mentioned levels of 0.5-1.0V. I can deal with that.
But don't directly connect some sensitive laser diode to PSU and switch it on/off willy-nilly.

According to R&S, both points are related to complicated regulated PSU circuits which have limited control of outputs when turning off (there are at least 3 rails in each PSU channel).

[mawyatt]

There is response from R&S, paraphrasing:

Firstly, R&S recommends to turn-off channels normally and only then switch off PSU - this way voltages spikes are not generated. In R&S HMP FAQ section this is mentioned Best way to switch off the HMP series power supply
Channels are controlled via relays and PSU even tries to be gentle on contacts Switch cycles of the HMP Series

Second, on the main issue about voltage spikes with channels turned off, there is no official fix for this behavior. As I understand, these spikes are very limited in power (2.3-2.6V, ~2-3mA max) and probably can be mitigated by capacitors and parasitic loads on PCBs anyway. From posts we see there are at least several revisions of PSU HW with different resistances on output. My PSU has ~10 kOhm across the output, but with external 2 kOhm load voltage spike is reduced to OP mentioned levels of 0.5-1.0V. I can deal with that.
But don't directly connect some sensitive laser diode to PSU and switch it on/off willy-nilly.

According to R&S, both points are related to complicated regulated PSU circuits which have limited control of outputs when turning off (there are at least 3 rails in each PSU channel).

Haven't some folks experienced/shown significant levels of overshoot and energy at turn off? It's hard to tell with this web site's ability to totally screw up images, arrangements and such (really needs to be fixed, even messed with Dave's own images)!!

WRT to R&S response, one should Not turn the power off before Disengaging the outputs, that's fine and certainly good practice but as mentioned by believe nctnico, what about a Power Outage??? Does one expect to lose valuable CMOS circuits with a random power outage??

Also, if we owned one of these R&S supplies (we don't), would be asking R&S how come other supplies do not exhibit this level of overshoot at turn off of main power? BTW we check for similar kind of behavior (just disengaging the output) since way back we lost some very expensive custom ICs to a similar issue with a rogue Power Designs power supply. We just checked our supplies for the condition with the outputs engaged and cycling the power off, all behaved nicely without any hint of overshoot!! So we know our various Siglent's (3), GW Instek, and Korad (2) don't do this!!!

Honestly quite surprised at apparently R&S more lackadaisical response to this issue.

Best,

[rf-messkopf]

Haven't some folks experienced/shown significant levels of overshoot and energy at turn off? It's hard to tell with this web site's ability to totally screw up images, arrangements and such (really needs to be fixed, even messed with Dave's own images)!!

There must have been a circuit change down the line. My fairly recent HMP4040 does not overshoot at all (see my posting above). Also, other folks seem to be aware that units up to a certain (unknown) production date can overshoot at mains turnoff or power failure, see e.g. this posting in a German forum.

The issue with the outputs of disabled channels ramping up to about 1V open circuit (on my unit) at mains turnoff seems to be pretty benign as this voltage reduces to a few tens of millivolts with only a light load. Certainly this is not nice, but I don't think this is a big problem, at least not as far as I am concerned. Anyway, it is hard to understand why this is happening at all because the PSU seems to disconnect disabled outputs by relays. Anyone interested to do some reverse engineering?

As I'm not affected by the overshoot at mains turnoff I can say that I've been quite happy with the HMP4040 so far.

Edit: In this posting in a German electronics forum someone claims that he knows cases in which the overshoot issue at mains turnoff or power outage has been fixed under warranty on HMP2020 and HMP4040 PSUs, and that newer units already have a modified circuit (mine does, obviously).

Maybe pdenisowski can comment on this.

[pdenisowski]

Maybe pdenisowski can comment on this.

We're looking into it.  As you say, it's not an issue with current production HMPs and I'm hoping to have an update shortly.

[tautech]

(Und danke für die Links - die hatte ich auch noch nicht gesehen)

Some R&S secret code ?

[mawyatt]

Edit: In this posting in a German electronics forum someone claims that he knows cases in which the overshoot issue at mains turnoff or power outage has been fixed under warranty on HMP2020 and HMP4040 PSUs, and that newer units already have a modified circuit (mine does, obviously).

Maybe pdenisowski can comment on this.

That's good to know for folks with these that do exhibit this behavior, R&S apparently is not leaving them hanging!!

Would be interesting to know what the culprit is and the cure/fix?

Best,

[mahi]

To summarize the thread so far...

There are currently three different issues:

1) 0.5 V spike at mains switch-on on all outputs



At mains switch-on there is a short 0.3-0.5 V peak on the (unloaded) outputs. Not an issue.

2) 2.6 V (old hardware) / 1.0 V (newer hardware) for ~6 seconds at mains switch-off on all outputs



At mains-switch off, all outputs go to zero an then rapidly ramp up to 2.3 V (old hardware) / 1.0 V (newer hardware) and stay there for about 6 seconds. When shorting the sense terminals to the outputs the voltage increases to 2.6 V and 1.6 V respectively.

This happens even when the outputs were disabled before switching off.

The current is small (1-3 mA), so when the outputs are loaded the voltage is much lower.

This behavior is regrettable, but the voltage on newer hardware is low enough to be not a cause for alarm for most circuits. The 2.3 V on the older hardware is debatable. This could potentially damage low voltage devices, but the voltage rapidly drops with load.

The difference in voltage between old (before 2017-2018) and newer hardware (from 2017-2018) is due to the different output resistance: (1x) 10 kOhm for the old hardware and 1.87 kOhm (4x 7.5 kOhm parallel) for the newer. In theory it might be possible to replace the output resistors of the old hardware to get the same behavior as the newer hardware, but I don't think you should have to make such mods to an expensive high-end power supply of a renowned brand.

3) Massive >10 V spike on output 1 and lesser spikes on the other outputs at switch-off



This happens only when the outputs were enabled at the moment of mains switch-off. This is the scenario that Rohde & Schwarz warn against in their FAQ: Turn off the outputs before switching off the power supply. Under normal circumstances you should not face it, but it is a realistic scenario in the case of a power failure or absent-mindedness.

At mains switch-off, right before the voltage drops to zero (and then ramps up again - previous issue) the outputs show a 40-50 ms peak of over 10 V on output 1 and up to 5 V on the other outputs (unloaded).

This is bad, very bad, and could easily damage circuits hooked up to the power supply. This should never occur in a quality power supply.

Some sources quote spikes of 'only' 6-8 V. That might be due to load or perhaps other hardware changes we are not yet aware of.

I don't know why the voltage spike is lower the further away from output 1 (see my earlier post), but as a precaution it might be a good idea to connect sensitive electronics to the last outputs.

According to rf-messkopf, pdenisowski and messages on mikrocontroller.net, this issue is fixed in the newest hardware. It is unclear when exactly the change happened. Dave's unit is presumably from 2018 and shows the issue, and so does an early 2019 unit on mikrocontroller.net.

[rf-messkopf]

It is unclear when exactly the change happened. Dave's unit is presumably from 2018 and shows the issue, and so does an early 2019 unit on mikrocontroller.net.

It's hard to tell from the date of purchase as you don't know how long they have been sitting in a warehouse. I bought my PSU (unaffected by the turnoff-spike issue) in 2019, I think, and from a large German test equipment dealer with certainly a lot of stock turnover. I'll dig out the cal certificate and post the date, but this will take a few days.

[mahi]

rf-messkopf: In the thread at mikrocontroller.net you linked to earlier (link) the massive voltage spike is seen on a HMP4040 with a January 2019 calibration report. If yours does not have the problem, it must have been fixed somewhere in 2019 (I assume calibration is right after manufacturing).

[EEVblog]

Dave's unit is presumably from 2018 and shows the issue, and so does an early 2019 unit on mikrocontroller.net.

Mine was calibrated 6/6/2018

[pdenisowski]

Sorry again for the delay - wanted to be sure I talked to everyone on the team and had the whole story directly from them.

Short version:  there are some older HMP’s that show this behavior if channels are not disabled before switching off the device (the recommended shutdown procedure).  We implemented a hardware change several years ago that prevents this from happening, and this modification can be made by our service department if needed.