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R&S HMP4040 voltage spike at PSU turn-off
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nctnico:

--- Quote from: mahi on September 30, 2023, 10:14:32 am ---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.

--- End quote ---
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:

--- Quote from: nctnico on September 30, 2023, 10:20:16 am ---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.
--- End quote ---

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.
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