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

Still investigating WTH is happening.
nctnico:
In what time frame? Scope image showing the effect?
electr_peter:

--- Quote from: nctnico on September 28, 2023, 04:30:59 pm ---In what time frame? Scope image showing the effect?

--- End quote ---
~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.
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