Products > Test Equipment
Power supply for home lab - do I really need a R&S?
blackdog:
Hi, :-)
KungFuJosh asked for some explanation of what scope probe I was using, which can be seen below in the picture.
Cost about 1850$ *grin*
The comment above is a hint to the major measuring instrument manufacturers who regularly charge outrageous prices for all kinds of probes.
Probing setup
The tests I did with current limiting around 10mA required a 50 Ohm resistor and about 1-Watt.
I used 2x a 100 Ohm resistor in parallel with a 1M Banana Crock cable from GW Instek and to the scope I used one 1M BNC to Crock cable.
One and all can be seen in the photo below.
I have been thinking about running the measurement setup “HF”.
But on reflection this was not really necessary, this because the LAB power supplies tested have between 100uF and 680uF across the terminals.
This does not allow fast rise times in the measurements.
I use two already older scope probes on my test benches from Micsig for commonmode measurements and current measurements, which are the DP10007 and the CD2100B.
For my work these probes are satisfactory and they are very affordable.
But Micsig also has faster probes available and still affordable prices.
In my measurements, however, these Micsig probes have not been necessary, K.I.S. was a good mindset here, so i used basic cables.
First the pictures
The used probe
.
Scoop setup
In this photo, I left the settings for channel-1 on, so it is easy to see what I worked with.
Coupling
The coupling is DC
Bandwidth
Bandwidth is 20MHz, otherwise too much noise is visible.
These measurements are about waveform and level and not because I want to see everything that happens while measuring.
I do always start with the “window wide open” when making measurements.
Then I have an impression if there are things that also come into play that I need to take into account.
Pressentation
Most measurements I show on forums are “without” noise, so that the attention goes as much as possible to the measured signal of interest.
Scale factor
So the next setting is the scale of the channel used, and this is set by using the “User1” setting of the Siglent scope.
Here the setting is 50V per ampere.
Input impedance
This should be set to 1M Ohm so as not to blow up your scope. :-DD
Unit
This should be set to Ampere unit, this because we are measuring the peak current behavior of a LAB power supply.
.
And now for some comments on LAB Power Supply's.
I've tested a lot of different circuits and I've developed a fondness for the “Harrison Consept.”
That is that the reference section has its own floating power supply and the control current for the power section usually comes from a current source.
Then there are two control loops one for current and the other for voltage.
Which then control the current through two diodes to the Power section.
Nowadays it is a whole lot easier to make a fast responding LAB Power Supply.
The advantage is that it is then possible to keep the capacitor across the output terminals low and still have a fast responding loops with good dynamic behavior.
This has not yet caught on with many manufacturers,
a thick capacitor across the output terminals and slow opamp/power transistors is much cheaper.
Better loop contro en component choices
Furthermore, a current source that always draws current from the output(when the channel is turned on) helps well for excellent dynamic behavior.
There are also opamps available these days that have a large phase and gain margin, look at the ADA4625 models from Analog Devices.
This opamp series has low noise, little bias current, fast and little offset, but above all a phase margin of 88 Degrees at 100pF capacitive load, which is excellent!
Everything I have written here is mine no AI, but translated by deepl.com
And I'm a dyslexic Monkey, which can lead to pretty crooked sentences. :-DD
Kind regards,
Bram
Edit picture
rf-messkopf:
--- Quote from: ArdWar on October 04, 2024, 03:31:26 am ---The inrush duration is also well beyond "Load recovery time" spec of 400µs, so can't use that either to handwave.
--- End quote ---
That spec exclusively refers to the voltage regulator. A 'normal' bench PSU with a single series pass regulator is unable to sink current. Thus, when in CC mode and the load resistance drops so that the supply has to lower the output voltage in order to keep the current constant, the excess charge in the fixed output capacitor corresponding to the voltage difference has to be dumped into the load. The supply cannot take up this charge. Therefore, we see the exponential decrease of the output voltage shown a number of times in this thread, and this decrease will depend on the load. There is no way around that, unless the PSU is a two-quadrant one like the Agilent 6632B which can down-program the current, or a SMU.
--- Quote from: mhsprang on October 04, 2024, 07:53:54 am ---But that is not the point of all of this. If we set a voltage on our PSU, we expect that PSU to output that voltage, no more. Similarly, if we set a current, we expect that PSU to output that current, no more. If a manufacturer specifies that a set current may be +/-5% + 5 mA, I do not expect the output current to be 14 times higher than that.
--- End quote ---
Well, transients exceeding set values are inevitable when the load resistor suddenly drops in CC mode, as the normal bench PSU cannot sink current, and the transition time will be load-dependent.
The delay time when transitioning between CV and CC mode (I'm not aware of a PSU that actually specifies that delay) is another story. I can see why they seem to be doing that. Otherwise you can get rapid oscillations between CV and CC mode. I've run into that with homebrew PSU designs and complex nonlinear loads long time ago. The question is which delay is appropriate. I think the approx. 150 ms of the Agilent are a bit too long. Would be nice to have control over it, and to have it specified at least.
--- Quote from: mhsprang on October 04, 2024, 07:53:54 am ---And if the design shows transients at enabling the output, I want to see that beforehand in the datasheet and not after the purchase on my bench.
--- End quote ---
That overshoot when the supply is activated with the output enable button and with a load connected which draws more current than set is yet another story. I agree that I'd prefer the supply not actually doing that. But one may also argue that it should behave just the same way at startup as when transitioning to CC mode in order to deal with complicated loads. Again, I'd prefer user control over this behavior.
So no idea whether this is in spec or not. :-//
KungFuJosh:
--- Quote from: blackdog on October 04, 2024, 01:26:34 pm ---KungFuJosh asked for some explanation of what scope probe I was using, which can be seen below in the picture.
Cost about 1850$ *grin*
--- End quote ---
Thank you for all the details!
Also... Damn it! I knew it was going to be an expensive probe. 😉
jayk:
All this is making me feel a lot better about my GPP-4323. If only it had a decent web interface and were a bit quieter under load it would be perfect. Still a great value for the money.
mhsprang:
--- Quote from: rf-messkopf on October 04, 2024, 03:07:59 pm ---
--- Quote from: mhsprang on October 04, 2024, 07:53:54 am ---But that is not the point of all of this. If we set a voltage on our PSU, we expect that PSU to output that voltage, no more. Similarly, if we set a current, we expect that PSU to output that current, no more. If a manufacturer specifies that a set current may be +/-5% + 5 mA, I do not expect the output current to be 14 times higher than that.
--- End quote ---
Well, transients exceeding set values are inevitable when the load resistor suddenly drops in CC mode, as the normal bench PSU cannot sink current, and the transition time will be load-dependent.
The delay time when transitioning between CV and CC mode (I'm not aware of a PSU that actually specifies that delay) is another story. I can see why they seem to be doing that. Otherwise you can get rapid oscillations between CV and CC mode. I've run into that with homebrew PSU designs and complex nonlinear loads long time ago. The question is which delay is appropriate. I think the approx. 150 ms of the Agilent are a bit too long. Would be nice to have control over it, and to have it specified at least.
--- Quote from: mhsprang on October 04, 2024, 07:53:54 am ---And if the design shows transients at enabling the output, I want to see that beforehand in the datasheet and not after the purchase on my bench.
--- End quote ---
That overshoot when the supply is activated with the output enable button and with a load connected which draws more current than set is yet another story. I agree that I'd prefer the supply not actually doing that. But one may also argue that it should behave just the same way at startup as when transitioning to CC mode in order to deal with complicated loads. Again, I'd prefer user control over this behavior.
So no idea whether this is in spec or not. :-//
--- End quote ---
If you go back and take a look at reply #37, second image, you see the current spike first, then drop from 20 mA to 10 mA (setpoint) in several steps over a period of 12 seconds. That has nothing to do with any capacitor discharge but everything with something that goes terrible wrong in the firmware.
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