R&S HMP4040 voltage spike at PSU turn-off

[electr_peter]

Rohde & Schwarz HMP4040 PSU has outputs enabled/disabled by relays. Output relays should eliminate negative/positive spikes (or DC leakage in off position). That is what you would think until you discover that all outputs at PSU turn-off become charged to 2.3V!

Still investigating WTH is happening.

[nctnico]

In what time frame? Scope image showing the effect?

[electr_peter]

In what time frame? Scope image showing the effect?

~500ms rise time, see attached pic. Didn't capture a tail, it dropped off quickly after that. Voltage was visible on DMMs with 10M and 1M input impedances for few seconds.

I will upload more detailed shots in next few days, need to check across all 4 channels at once and check potential power.

[electr_peter]

There are some measurement results of switching PSU on/off via mains switch:

• Measurement setup - each channel of HMP4040 is connected to a Micsig STO1104E scope (ground on output-, probe on output+), 10x input.
• All 4 channels behave almost the same each time - there is small spike on PSU turn-on and bigger spike (charge & decay) on PSU turn-off.
• Turn-on spike is ~100ms and ~0.5V
• At turn-off, voltage ramps up to ~2.3V in 220-300ms, stays stable for 5.4s and then decays in 5.8s (with no load, only 10M scope probe).
• With 10kOhm, 1kOhm and 100Ohm loads, turn-off voltage spikes are lower in magnitude, but have the same duration and shape.
• Estimated current with different loads - 10M scope probe 0.23uA; 10kOhm load 190uA; 1kOhm load 650uA; 100Ohm load 1.2mA.

HMP4040 is capable to produce sustained (5-10s) voltage spike (up to 2.3V and up to ~1.2mA) on it's outputs when outputs are in turn-off state and PSU is switched off from mains. Likely same happens on R&S HMP2020/HMP2030 and HMP4030 as they are very similar inside (same channel module).

Potential power to damage circuit under test is limited, but not zero. Should there be a concern with 2.3V/1.2mA turn-off spike?

I do not have schematic, but observations point to PSU sense circuitry. Sense terminals are always active, there is no option to turn them ON/OFF. In off state there is 10kOhm resistance on output terminals for each channel, that should be sense circuitry. Output is disconnected via relay, while sense terminals are probably disconnected via some transistor. My guess is that during PSU turn-off transistor may be opened earlier than voltage decays inside.

[nctnico]

That doesn't look too good and a proper PSU should not behave like this. It also looks as if there is quite a bit of energy available. There are plenty of chips out there that run from 1.8V and can't take much more. It is surprising nobody else has noticed this though so you might have a defective unit.

[electr_peter]

Take in mind that sample size is one until other HMP users confirms this. This issue is sneaky and hard to notice unless looked at very carefully.

In Gough Lui road test review of HMP4040 something related was noticed, but not looked in too deeply.

The only potentially major issue was the behaviour of the rails when power is removed from the power supply unexpectedly. In the test case, an unloaded 1V rail rose to 10V as the hardware power switch was actuated – so it’s best to turn off the rails first using the output button before turning off the power to the supply to ensure the DUT is not damaged.

It was also of interest to investigate what the behaviour of the HMP4040.04 is when power is abruptly removed during operation. Some other power supplies have specific warnings not to do this as regulation can be lost on such a power-down. Looking at the scope traces above, an abrupt power-down using the red hardware power button results in a less-well-defined rail power down timing, along with significant over-voltage on the first rail configured as 1V which shoots up to 10V. As a result of this behaviour, in case of emergency, it is better to use the master output switch rather than the hardware power button to ensure no risk of damage to the DUT.

Links:
R&S 4-Output Bench Power Supply, Prog (HMP4040.04) - Review
R&S HMP4040.04 PSU RoadTest in Depth – Ch5: Instrument Performance Testing

[mawyatt]

About 40 years ago we lost some very expensive ($50K) custom chips to a power supply that produced an over voltage glitch at turn-off like OP has shown. Expensive lesson and this was a quality, expensive lab PS, think it was Power Designs.

Anyway, we made sure it wouldn't do this again after taking a hammer to it, then sending it to in-house cal lab!!

This is something not to be taken lightly, it can and will destroy sensitive ICs as mentioned by nctnic0

Best,

[2N3055]

About 40 years ago we lost some very expensive ($50K) custom chips to a power supply that produced an over voltage glitch at turn-off like OP has shown. Expensive lesson and this was a quality, expensive lab PS, think it was Power Designs.

Anyway, we made sure it wouldn't do this again after taking a hammer to it, then sending it to in-house cal lab!!

This is something not to be taken lightly, it can and will destroy sensitive ICs as mentioned by nctnic0

Best,

 

[nctnico]

After checking the review: having a 1V rail shoot up to 10V is a no-go. A long time ago an intern at work tested a batch of boards (20 or so) and he used a PSU that also had the output shoot up when switching it off (way worse compared to the R&S PSU thouch) and he accidentally blew up every board right after putting a 'test passed' sticker on it.  CMOS is not forgiving where it comes to overvoltage.

[mahi]

Another Rohde & Schwarz HMP4040 user here...

I can confirm that the mains power switch does affect the (turned-off) outputs. On switch-on there's a short 0.3-0.4 V pulse on all outputs, and on switch-off the outputs ramp up to 1 V and stay there for several seconds.

However, I am unable to reproduce output 1 climbing above 1 V, let alone 10 V... I've retried multiple times, with different set voltages (outputs off), but the traces always looked the same. I did not experiment with different loads - only the oscilloscope or a multimeter.

The screenshots below are switching on and off with the mains switch, no load (other than the oscilloscope) and all outputs off.

Switch-on

In about 10 ms all outputs ramp up to 0.3-0.4 V and then fade back to 0 V in 30 ms. A fluke 289 in peak detect (250µs response time) measures the peaks between 0.33 and 0.38 V.



Switch-off

The outputs ramp up to 1 V in less than a second. Output 1 ramps up noticeably faster than the other three outputs. It gets a 600 ms head start, but even without that the slew rate is much higher.

Just under 6 seconds after switching off, the outputs suddenly drop to 0.5 V in under half a second. From there it takes another 4 seconds before the outputs finally reach 0 V again.

A Fluke 289 in peak detect does not read higher than 1.04 V over the entire period.



For reference: My HMP4040 is running the latest firmware (2.72). I don't know the hardware revision, but it is a "pre-facelift" unit (light front and single fan).

The Element14 review by Gough Lui shows a later "facelift" unit. Note that this power supply was seriously damaged in transport.

Which version do you have, electr_peter?

[nctnico]

Another Rohde & Schwarz HMP4040 user here...

I can confirm that the mains power switch does affect the (turned-off) outputs. On switch-on there's a short 0.3-0.4 V pulse on all outputs, and on switch-off the outputs ramp up to 1 V and stay there for several seconds.

However, I am unable to reproduce output 1 climbing above 1 V, let alone 10 V... I've retried multiple times, with different set voltages (outputs off), but the traces always looked the same. I did not experiment with different loads - only the oscilloscope or a multimeter.

From reading the review, the output seems to shoot up with the outputs enabled. This is something that could happen during an unexpected power outage.

[mahi]

From reading the review, the output seems to shoot up with the outputs enabled. This is something that could happen during an unexpected power outage.

Good point. I missed that.

I've retested with all outputs enabled and outputting 1 V. When switching off via the mains switch a similar profile is visible like posted earlier:



However, when zooming in at the point where the voltage first falls (or should fall), things do not look very good... Output 1 climbs over 10 V, output 2 touches 5 V and the remaining two channels peak around 4 V before dropping to 0 V (and then commencing with the ramp up to 1 V):



The output artifacts do not change with the output voltage. Thus output 1 with a voltage of 10-11 V and the other outputs 5 V suppress the effect:

[electr_peter]

@mahi, thanks for the input. Your pictures of turn-off ramp matches 1kOhm load ramp (ramp, drop, ramp to full voltage). Maybe there is voltage dependancy, channels were initialy set to higher voltages (9-12-32V).



My HMP4040 has grey/white front panel (single color), FW is 2.72.

[mahi]

electr_peter: I just retested, but the set voltage does not affect the voltage ramp-up after mains switch-off at all (with the outputs turned off).

The screenshots I posted were all with 10M input impedance (1M oscilloscope + 10x probe). I don't have a 100x probe but I added 9x 10M on a breadboard for a 100M input impedance. Horrible noise but the voltage and signal profile stayed the same (1 V maximum).

Then I tested with different loads. The waveform always had the same profile but the maximum voltage was lower:

10K: Maximum voltage 0.9 V
1K: Maximum voltage 0.5 V
100R: Maximum voltage 0.1 V

Under no circumstances I get over 1 V on the outputs in your scenario.

The issue that Gough Lui observed on the other hand... Nasty.

[electr_peter]

And things get even worse. If sense terminals are shorted to to output terminals (normal operation, +/+ & -/-), during turn-off DMM reads 2.64V open or 2.2mA shorted. This also points to sense circuit as a main suspect.
I do not find voltage dependency w.r.t. to output spike level as well. In addition, turn-on spike increased from 480mV to 630mV with sense terminals connected.



With all 4 channels set to 1V and turned on, abrupt power off results in up to 5.4V voltage spike and followed by the same turn-off ramp-up/down.



In all these shots violet channel (3rd) has sense terminals shorted to output terminals. Sense terminals do not mean that they are sensitive to DUT, quite the opposite in this case.

[electr_peter]

What are output impedances measured with DMM on PSU turned-off?

With sense terminal unconnected:

• between output terminals - 10kOhm
• between sense terminals - 12.9kOhm
• between output- and sense- terminals - 1.47kOhm
• between output+ and sense+ terminals - 1.47kOhm
• between output+ and sense- terminals - 11.5kOhm
• between output- and sense+ terminals - 11.5kOhm

With sense terminal shorted to output (+/+ & -/-):

• between output terminals - 10kOhm

[mahi]

electr_peter: When shorting the sense lines to the outputs, the ramp-up voltage increases to 1.6 V maximum (instead of 1 V) on my HMP4040. This does indeed suggest that the sense circuitry is to blame for this behavior. Still, the voltages on my unit are much lower than on yours.

There is no change to Gough Lui's scenario with the sense lines shorted to the outputs.

The resistances between the output terminals on my HMP4040 are quite different from yours:

With sense terminal unconnected:

    between output terminals - 1.87 kOhm
    between sense terminals - 4.82 kOhm
    between output- and sense- terminals - 1.47 kOhm (same as yours)
    between output+ and sense+ terminals - 1.47 kOhm (same as yours)
    between output+ and sense- terminals - 3.34 kOhm
    between output- and sense+ terminals - 3.34 kOhm

With sense terminal shorted to output (+/+ & -/-):

    between output terminals - 1.87 kOhm

All outputs show identical values within tens of ohms from each other.

[electr_peter]

@mahi, resistances are quite different. There could be some error due to diodes turning on in circuitry and different DMM effect, but that would not account for such big differences. Lower impedance will dampen voltage ramp effect, so there is explanation why yours HMP is more tame.

This data confirms that HMP4040 has different HW versions for channel boards. R&S probably find out this issue and did a HW revision at some point. Do you recall manufacture/sale date of PSU? Mine is from 2015-2016 or earlier.

[electr_peter]

Based on measurements and general PSU configuration I estimated rough schematic of sense circuitry. There are three main resistance elements and two voltage cut-off points (relay for main output and transistor for sense circuit). During turn-off sense transistor becomes activated and leaks voltage. It is clear that different HW versions have different resistive values.

I will try recreating impedances similar to @mahi PSU and see how it compares in terms of turn-off spike.

[mahi]

electr_peter: I can't find any date markings on the outside, but I purchased my unit in spring 2018. Thus it was probably made in 2017-2018.

By the way, your diagram can't be right. You have a direct connection between the sense circuit and the outputs. In your diagram R+ and R- serve no (useful) purpose.

The diagram below (taken from the user manual) is incomplete, but it shows the link between the outputs and sense circuitry.

[electr_peter]

2017-2018 is 1-2 years newer model. HMP4000 series was made by Hameg from 2012 or earlier, this model is still made today by R&S. Predecessor of HMP4040 is Hameg HM7044.

By the way, your diagram can't be right. You have a direct connection between the sense circuit and the outputs. In your diagram R+ and R- serve no (useful) purpose.

Yes, diagram above is incomplete and does not represent functional sense circuit at all, but it is the simplest circuit representation that matches impedance measurements across output and sense terminals in PSU OFF state. Resistive network is complicated by active circuitry (diodes and ICs), thus impedances look strange.

Actual circuit would be similar (but more complicated) to linked in your post above which in off state reduces to much simpler representation. Exactly what happens in off state is difficult to say without exact schematic. Also, your linked schematic gives a hint why shorting sense and output terminals can give more voltage/current.

[SparkyBruce]

I checked my HMP2030 this afternoon and I also see the 2.3V ish output on all 3 channels after turning the unit off - even with the latest firmware.
I am collecting a second identical unit tomorrow and I will also check that.
This is all unloaded (apart from the 10M of the DVM).

[electr_peter]

@SparkyBruce, can you also check date of manufacture and resistance across output (in OFF state)? I guess there are 10 kOhm on outputs.

[mawyatt]

This is serious if across all these R&S power supplies, and not just a select rogue few.

If true then seems R&S needs to step up and issue a recall. As mentioned many times this can and will destroy modern small feature CMOS, as it produces gate oxide punctures which are non-recoverable and fatal!!!

Best,

[electr_peter]

There is clearer estimated schematic of PSU with locations of sense and load discharge (?) resistors. Sense resistors are 1.5 kOhm and load discharge resistors are either 10 kOhm or 1.87 kOhm

The resistances between the output terminals on my HMP4040 are quite different from yours:
With sense terminal unconnected:
    between output terminals - 1.87 kOhm
...

By the look of PCB, 1.875 kOhm is four 7.5 kOhm in parallel. Source of PCB pic: "EEVblog #1174 - Rohde & Schwarz PSU Teardowns" at time 09:18

I checked my HMP2030 this afternoon and I also see the 2.3V ish output on all 3 channels after turning the unit off - even with the latest firmware.

Same as in posts above, just with HMP2030 instead of HMP4040. I believe this is HW related and FW would not affect it.

[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.

[rf-messkopf]

I'll dig out the cal certificate and post the date, but this will take a few days.

Okay, here it is: calibration date is 2019-02-27. This dates the implementation of the circuit change that prevents the voltage spike at mains turnoff to early 2019.

[electr_peter]

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.

Regarding voltage spikes when switching channels off with mains button, does modification fall under warranty or "service circular"?

[pdenisowski]

Regarding voltage spikes when switching channels off with mains button, does modification fall under warranty or "service circular"?

I'd recommend contacting your local / regional service department:  I'm on the product management side and am not sure how service handles this, especially in different regions.

[mahi]

For those wondering how old their HMP power supply is and can't find the original calibration certificate, you can retrieve it via the Rohde & Schwarz website: Download calibration certificate. Just enter the device ID (case sensitive!) and it should recognize the device and offer a download link.


Anyway, to get back to this issue. I got in touch with Rohde & Schwarz customer support in Germany:

The (huge) voltage spikes when switching channels off with mains button is a known issue of the HMP series that can be fixed with Service Circular 16282 by Rohde & Schwarz Service.

Rohde & Schwarz does not provide direct service to private customers (at least not in Europe) so I had to inquire for pricing via a distributor or partner. I received a quote for my HMP4040 of just over 1100 EUR (without VAT and without shipping).

That's almost half of what a new HMP4040 costs. Ouch! That's bit more than I was hoping for (maybe I am naive)...

[EEVblog]

For those wondering how old their HMP power supply is and can't find the original calibration certificate, you can retrieve it via the Rohde & Schwarz website: Download calibration certificate. Just enter the device ID (case sensitive!) and it should recognize the device and offer a download link.

Wow, does any other manufacturer have this?

[mahi]

I took one for the team and had my HMP4040 serviced by Rohde & Schwarz (Service Circular 16282). Price... 1111 EUR without VAT or shipping. Ouch. It might be a bit cheaper if you can deal with R&S Service directly, but I had to go through a partner (Esenwein GmbH - very friendly people). The turnaround time was two weeks.

The power supply came back with a new calibration certificate and a elaborate 12 page test report (it was not re-adjusted, but still well within specs).


I've tested the power supply and can confirm the issue with the >10 V spike is fixed.

Measurements:

All outputs enabled and outputting 1 V before switching off with the mains power switch. The profile is identical to that from before the fix apart from the faster ramp-up to 1 V after switching off the power supply (compare against my screenshot in post #11). Thus the issue with the outputs staying at 1 V for 6 seconds after switch-off remains (but we know that is present in the latest revisions as well and it's not a big deal). The faster ramp-up seems to suggest some capacitance was changed:



Let's zoom in at the point of switch off. That's what we are most interested in. Gone are the massive >10 V spike on channel 1 and the 4-5 V spikes on the other channels. There's still a small spike of almost 2 V on channel 1 and 1.5 V on the other channels, but that is acceptable. Let's not forget that under normal conditions you should not run into this issue at all. It's only during a power failure or absent-mindedness that the power supply may be switched off without turning off the outputs first. This will not damage any devices under test:



Now with the voltage at 2 V before pressing the mains power switch:



The profile of channels 2-4 is exactly the same to that of the screenshot posted by rf-messkopf in post #33. This suggests my HMP4040 now behaves the same as the newer revisions. In fact, I'm quite sure the hardware modification applied to my power supply is exactly the same as what the newer revisions come with from the factory.

[mahi]

So, what was changed by Rohde & Schwarz Service?

I did make pictures of the power supply internals before sending it off. When I got it back I had a hard time finding what was changed. All the PCB numbers and stickers were the same, so no boards were replaced. In fact, it didn't even look as if the power supply was fully disassembled: None of the wires to the circuit boards seemed to have been (re)soldered.

After closer inspection I found 2 locations (per channel) with new solder flux: A 1000µF electrolytic capacitor was replaced by a smaller 470µF one and a previously unpopulated pad now has an 22µF 0603 chip capacitor. That's all I can find. If there's more, it is well hidden.

• Original: Panasonic 1000µF / 25V / 105°C (EEUFC1E102L)
• New: Panasonic 470µF / 63V / 105°C (EEUEB1J471)

• Original: unpopulated
• New: Unknown 22µF 0603 ceramic capacitor (measured in-circuit with an LCR meter at 120 Hz)

I can't tell what part of the circuit the components belong to. We do not have diagrams, it's a multi-layer PCB and the solder mask hides the traces very well.

Top:



Bottom:



The PCB version is 5800.5847.10. Earlier units (with higher output impedance) may have a slightly different PCB. I was unable to find PCB pictures of the earlier units.

The fact that the power supply did not need re-adjustment implies that the changes do not affect calibration. It is possible to perform these modifications with just taking off the cover. Channel 4 is a bit more challenging because of the cables passing over it. You should undo the connector to the interface board to move the ribbon cable out of the way, and perhaps take off the metal cable guide on the duct (2 screws) to get the cables to the rear terminals out of the way. That's probably how R&S Service did it. They definitely didn't take it apart completely. The biggest challenge is soldering the 0603 capacitor as it is a quite deep reach and close to a tall electrolytic capacitor and the heat sink.

Total cost of the fix? Less than 10 EUR in parts...

[electr_peter]

Thanks for an update and repair details, @mahi. Glad that R&S has responded and at least some issues were fixed in repair.
However, small voltage spike at PSU mains turn-off still remains. Users of HMP2000/4000 series (old and new models) should be aware this - in some case it can pose some risk to DUT, but likely not. Otherwise HMP are quite good PSUs.