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Just bought a GW-Instek GPP-4323 power supply-First impressions etc.

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nctnico:

--- Quote from: mawyatt on October 17, 2021, 09:57:11 pm ---
--- Quote from: Martin.M on October 17, 2021, 09:08:32 pm ---set it to:

12V  5mA to understand why the question for this small offsets is interesting  :)
We have to add it to the setting. I have payed this 800€ to have a trustful current control in small areas.
Otherwise any chinese PSU for 120,- can do the job also

--- End quote ---

Exactly!! Lots of folks that haven't worked in research labs with advanced SOTA semiconductors that are highly susceptible to damage by just slight over voltage or over current, don't appreciate having a precise voltage and current limit. Almost 50 years ago we lost well over $20K of chips because of a lab supply slight overshoot on start up, that's why I won't consider Konrad, I'll let others play Russian Roulette :o

--- End quote ---
But main thing is that wanting a few mA from a PSU which has a 3A range is too much to ask. You are working next to the noise level of the current sensing circuitry of a regular power supply in such a case. For these kind of jobs you need a PSU with dual current range (like the older Agilent 663xx series which still has a >1mA setpoint uncertainty) or an SMU.

mawyatt:

--- Quote from: nctnico on October 17, 2021, 10:27:11 pm ---
But main thing is that wanting a few mA from a PSU which has a 3A range is too much to ask. You are working next to the noise level of the current sensing circuitry of a regular power supply in such a case. For these kind of jobs you need a PSU with dual current range (like the Keysight E3631x series or the older Agilent 663xx series) or an SMU.

--- End quote ---

Agree, the best case is using a supply with the proper current limit range, and Keysight and others higher end types have these. However, some of the mid price range supplies mentioned are quite good down in the ma range, know the SDP3303X-E when augmented and calibrated (please follow the Siglent cal routine) is OK. Did quite a bit of testing before I would consider using the SDP3303X-E for the development work we are currently involved with. Haven't used any of the GPP stuff you and others have, but from what you've posted and others this seems like superb equipment  :-+

One thing about these newer medium cost range supplies, they seem to be utilizing the new Sigma-Delta ADCs and DACs chips which provide large dynamic ranges and likely why they can dive down into the ma range and still support 3 amps. These chips are remarkable considering the cost, and many are finding their way into mid range DMMs as well.

Best,

2N3055:

--- Quote from: Martin.M on October 17, 2021, 09:08:32 pm ---set it to:

12V  5mA to understand why the question for this small offsets is interesting  :)
We have to add it to the setting. I have payed this 800€ to have a trustful current control in small areas.
Otherwise any chinese PSU for 120,- can do the job also

--- End quote ---

If you want really good control of 12V in region of 5mA NONE of the general purpose PSU are to be trusted. Even expensive Keysight or R&S.
They all have output capacitors that will discharge significant energy in DUT if it suddenly starts to pull large current.
Simply put, you set PSU for 12V and 10mA and connect small LED with PSU output enabled and you will kill the LED, unless it is one of those high power types..

For instance, my Rigol DP831, outputs 4,9mA when set for 5mA and 12 V. Quite better than specs. But that holds only if you set current with outputs OFF, connect the load and then enable PSU outputs. This way output capacitors inside PSU (those soldered directly to output terminals) are discharged, and will actually contribute that output voltage will slowly ramp up until output current reaches 5 mA. For a red LED about 2 V. It will ramp up slowly because it will need to charge capacitors in addition to supplying current to DUT. But if you disconnect the load, capacitors will charge to full voltage and if you connect DUT again, it will discharge full voltage into DUT, without current limit. Current limit circuit is before output capacitors.

On top of that,  5 mA on a 3 A is well in the error band on that GW Instek PSU that in datasheet has specified:

Line Regulation ≦0.2%+3mA
Load Regulation ≦0.2%+3mA
Ripple & Noise ≦ 2mA RMS(let's say P-P on that would be 10mA)

It is not clear whether 0,2% is from full scale or set value...

Anyhow, you can have up to 6 mA offset and 2mA RMS fluctuations and your PSU would be in spec.

IN addition to all mentioned, most of the general purpose PSU you can buy, despite having both voltage and current controls, are not very good at switching from CV to CC mode and back. They can have delays in reaction, in which output is not very controlled. These are milliseconds (or microseconds) but you can  get spikes of overvoltage/overcurrent. You wouldn't see them on multimeter, but will see them on a scope.

If you have to have control of 5mA at 12 volts, you have to have something called SMU (source measure unit) that will have fine control, or at least a precision power supply that is made for small currents ( smaller than 100 mA maybe) and have small output capacitor ( so it cannot discharge significant energy into DUT).
Odds are it would be quite easier for you to make one that would be good for your purpose than find something to buy that won't cost an arm and a leg.

I'm afraid that you bought device that is not what you expected. Not that it is not a good device, it is quite good general purpose PSU, but not what you though it would be.

nctnico:
A side note: it is a good custom to set a PSU to a voltage which can't harm the DUT even when you are only interested in sending current through the DUT. The amount of energy in the output capacitors goes up exponentially with the voltage so the lower the output voltage, the less damage a discharge can do. Setting a PSU to 12V / 5mA to test an LED is not a good way of doing such a test. Look up the worst-case forward voltage first and set the PSU to a little bit above the worst case forward voltage.

mawyatt:
This sorta gets into the grey area of what is safe regarding energy discharge. Eons ago when working with Rad-Hard designs I recall a spec that was 10mJ per supply line as the maximum available energy source for safe recovery from an event. The assumption was that all the energy would discharge into the forward biased chip to substrate junction caused by the nuclear event, if the chip was junction isolated.

A little math shows the maximum capacitance as:

C = 139uF for 12 volts
C = 800uF for 5 volts
C= 1837uF for 3.3 volts

Edit: Corrected above.

Of course the 10mJ is from long ago, and one would expect modern chips to be more sensitive, how much more sensitive I don't know.

The message 2N3055 coveys is important, you want the lowest possible output capacitance from your supply. The popular concept of loading the supply lines with lots of capacitance to reduce ripple effects may not be a wise choice!!

Using a supply that misbehaves when entering or leaving CC is also a recipe for disaster, as is voltage overshoot on startup or shut down. The only supply I'll comment on is the ones we selected for the critical use in the on-going development project, which is the Siglent SDP3303X-E. These have been tested, augmented to 3303X and calibrated and behave well.

All this reminds me that we are entering an area with the ongoing project where over $10,000 of chips could be instantly ruined by a mistake. We've taken measures in the power supply design to provide protection from reverse voltage, over voltage, excessive current and so on, but need to revisit the main bench power supplies and check them out very carefully. We've already put a new ODA HV Lab Type supply thru a loaded multi-day burn in outside. It had a horrid smell from the lacquer used to seal the two power transformers, so another good reason to burn in outside :P 

Best,
 

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