Power supply for home lab - do I really need a R&S?

[rf-messkopf]

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.

Agreed, that should definitely not happen and is out of the setting accuracy spec. R&S should fix that.

I have not seen something similar on the HMP4040 or any Agilent supply, though.

[nctnico]

I think you need to define it differently. An ideal current limited power supply has two control loops running at the same time: one loop regulates the output so the set current limit isn't exceeded, the other loop regulates the output so the set output voltage is not exceeded. And this should be an AND function. Set current AND set voltage should not be exceeded.

From the graphs it looks like that in some power supplies the current limit control loop is disabled for a short while causing the output to be regulated to the set voltage but ignoring the requirement for the current limit.

I don't think this is a matter of specification but the very basic behaviour of a lab power supply. The set current limit and set voltage should never be exceeded under any circumstance as this will cause damage to devices connected to the power supply; hence there is no need to specify this. 

[mhsprang]

@pdenisowski: Although I am an electronics engineer, I am also a "user". So, if I purchase a power supply with CV and CC capablities, then to my circuit I design I am the engineer but to the power supply I am "merely" the user. In my opinion, I do not need to get to understand how my power supply works, based on assumptions and experiments. Instead, I rely on its specifications in the datasheet.

If I set it to be used as a current source with a certain max. voltage, I may expect, as a user, that those setpoints are honoured. I don't care about control loops and arguments about how the V and I loops interact, I only care that if I set the max current to be 10 mA, that the current will not exceed 10 mA. I will accept that there is a large capacitor at the output terminals, and that is why I use the "output enable" button to enable the power, instead of plugging in the cables to my delicate circuit, assuming the output cap is after the switch. Even this should be in the specifications, IMO.

To come back to my example in reply #37: I connected the 50 Ohm resistor to the power supply, which was set to 8V and 10 mA. I enabled the power to the resistor by pressing the Output button on the NGE100 while the resitor was connected already. So from that point onwards, it is the power supply's responsibility to deliver what I asked it to: 10 mA of current with a max. voltage of 8V. Then I hit the output button, as a user.

Seeing the current overshoot to 140 mA for 30 ms is one thing, althoug it exceeds the 200 us in the specs, but seeing the current drop from 20 mA to the setpoint of 10 mA over 12 seconds is a blatant violation of the specifications of this power supply. One could also say: I did not change the load. I merely enabled the output to a constant load.

[blackdog]

Hi pdenisowski, :-)

I have a suspicion that you don't understand what test was done....

Preparation
Set a LAB Power Supply to 8V and set the current to max 10mA.

Now connect a 50 Ohm resistor across the output terminals of the channel you set.
Connect a scope across the 50 Ohm resistor.

Set the scope to say 100ms time base time and single shot and the trigger level to say 30% of the expected value.
If you want to do it nicely, set the scope for your used channel to current measurement and then adjust the sensitivity in the probe settings to 50V/A.
Now the peak current is neatly readable on the scope screen.
Connect the scope with a BNC-Crock cable 1:1 low cost probe, as in my previous photo.

If everything is set up correctly and you turn on the LAB Power Supply via the enable switch, you will see the behavior of the Power Supply you are testing.
The maximum value of the current through the 50 Ohm resistor should not exceed 10mA.
This above is a good test for the behavior of the I-Loop control.

This test is slightly different from the test for the dynamic Voltage behavior of a LAB Power Supply.
In this, you vary the load current over a short period of time and see how well the LAB power supply responds in terms of the U-Loop.
Many manufacturers specify from 50 to 90 or 100% load, and there are some that test 10% to 100% as a test in this dynamic U-Loop test for e.g. within 50uSec within a few mV of the set voltage value.

By the way, these are just two of the tests you can do on LAB Power supplies dynamic behavior.
Like this measurement which I also think is important, i have never seen a manufacturer specify how much commonmode interference comes out of a LAB Power supply output.
Especially the ones that have switching behavior, linear post-regulation usually doesn't help with that.

But let me not digress, the current test at issue here shows nicely what a manufacturer thinks is important,
my measurements of a few LAB Power Supply's shows that fast current control at the GW Instek Power Supply is well taken care of.

I hope things are more clear now.

Kind regards,
Bram

[mawyatt]

Think most users work with a Lab Supply the way Bram describes, and this is also how we generally utilize a Lab Supply, especially when sensitive circuits are involved.

The Power Supply is Mains Powered On, Voltage and Current Limits are set, the DUT is connected either before or after the Limits set, then the PS is Enabled turning ON the Output(s).

In this use case the Power Supply should not overshoot the Voltage or Current Limits set by the user IMO.

Some Lab Supplies like the GPP4323 and SPD3303X have an ALL ON/OFF Single Button to Toggle the Outputs ON and OFF which hints at the above described use case. This is a nice feature when one is utilizing multiple outputs which is common with complex DUT circuits/systems.

Best,

[KungFuJosh]

I did blackdog's test with my SPD3303X v6.2 hw.

For values set below 30mA, there's a spike to nearly 30mA before dropping to the set current. For currents 30mA and up, there's no overshoot.

Inserting the probe into the scope was seen as nearly 15mA.

[Martin72]

However, the power supply has no knowledge of the attached load impedance before it's turned on and current starts flowing.

This is the point that should be internalized.
If you set the current limit of a power supply to 30mA, for example, this is no different than setting a controller setpoint.
If you then enable the output and allow current to flow, it takes a certain amount of time for the actual value to match the setpoint.
This is the nature of the controller.
You cannot expect the current to be limited “instantaneously”.

[thm_w]

However, the power supply has no knowledge of the attached load impedance before it's turned on and current starts flowing.

This is the point that should be internalized.
If you set the current limit of a power supply to 30mA, for example, this is no different than setting a controller setpoint.
If you then enable the output and allow current to flow, it takes a certain amount of time for the actual value to match the setpoint.
This is the nature of the controller.
You cannot expect the current to be limited “instantaneously”.

Seems beside the point when we already have demonstrated supplies that do not overshoot, and that is the desired behavior.

[2N3055]

I wanted to stay out of this discussion but am getting an itch..
 
Paul, this is not moment for clever lawyer talk that "technically" specifications are not violated because someone in R&S used clever wording in datasheet...

Current limit is exactly that: a limiter that limits that current does not exceed set current limit. You can accomplish it with some kind of electronic fuse that will disconnect load, you can enable foldback limiting (that will reduce current to minimal until you reconnect load) or you can enable constant current mode that will keep constant current.

In any case, current should not exceed the set limit. It is a limit, not some general recommendation.

If one wants to be able to deal with difficult loads that have large startup current, than that has to be separate, configurable option.

Those are, generally speaking, expectations form average user.
And when they pay 3x the money (or 10x) they expect it behave better that some cheap PSU.
Which here is not the case, because I have Rigol DP831 that behaves very well in this overshoot test, and to add insult to injury, I have two noname analog PSU that are even better in that regard, behaving like decent current source when starting. They also switch between  voltage and current control loops without much saturation in error amplifiers, making transition quite smooth.

I refuse to believe R&S cannot make it better than some noname small manufacture in China copying 50 year old HP design.
But maybe design dept and product leaders should pay more attention to analog design that to "visual identity of the brand".

R&S is brand that is famous for their outstanding devices with state of the art performance.
It is not Versace. Or Apple. Or some luxury brand that sells and designs something that is fancy and status symbol but not mean to be really pushed to the limit.

R&S is not supposed to be Vertu encrusted in diamonds but basically just an ordinary phone.
R&S is supposed to be state of the art military communication device with state of the art performance.

More like LeMans Porsche LMP race car than Porsche SUV with leather seats.

[Martin72]

Seems beside the point when we already have demonstrated supplies that do not overshoot, and that is the desired behavior.

Then they have a very, very fast controller.
You have to reach the point where you have to cut back, in any case, time passes for that.

[2N3055]

I actually looked into user manual.

It explicitly states that PSU functions in Constant Voltage (CV) or Constant Current (CC) mode.

[thm_w]

Seems beside the point when we already have demonstrated supplies that do not overshoot, and that is the desired behavior.

Then they have a very, very fast controller.
You have to reach the point where you have to cut back, in any case, time passes for that.

The other test that could be done is to compare the speed of regulation once the output is on, though at low currents like this its complicated by the size of the output capacitor.
Maybe that would tell us if it is a regulation loop speed issue or simply a software bug (timing or sequencing of the output)?

[rf-messkopf]

However, the power supply has no knowledge of the attached load impedance before it's turned on and current starts flowing.  If you set the output voltage to 10 volts and a current limit of 1 amp, and attach a 1 ohm resistor, then 10 amps (!!!) will flow for some finite amount of time before the supply's readback function is able to detect this and lower the output voltage to 1 volt in order to honor the configured current limit of 1 amp.

As has already been remarked by others, there are supplies which do not behave in this way. They seem to ramp up the voltage slow enough when activated by the output enable button for the current control loop to prevent an overshoot.

However, it is true that when a load is connected to an already activated PSU channel set to a certain voltage, and if the load will draw more current than set, then the current will necessarily overshoot. A number of supplies (among them R&S) seem to wait a couple of milliseconds before switching to CC and the current controller actually starts to ramp down the voltage in order to regulate the current. As I wrote before (see the post above), this behavior could be designed in deliberately.

Also, we speculated that it could be this delay that makes the R&S supplies (among others like the Agilent 6632B) overshoot when activated by the output enable button with a load connected that will draw more current than set. There may be pros and cons for this behavior. I'd prefer no overshoot, or user control over the delay when transitioning between CV and CC mode.

Besides this overshoot (which may be intentional), user mhsprang has seen an issue with the settling time to the set current value in CC mode. To understand this issue one has to realize that a 'normal' bench PSU with a single series pass regulator cannot sink current. Thus, when in CC mode and when the supply has to lower the output voltage to keep the current constant, the excess charge corresponding to the voltage differential in the fixed output capacitor has to be dumped into the load. Therefore, with a resistive load an exponential decrease of the output voltage is expected and has been seen a number of times in this thread. Also, the settling time 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. Therefore the settling time spec of normal bench PSUs only refers to the voltage controller. I've never seen a current settling time spec for CC mode.

Now mhsprang has noticed the following: He connects a 50 ohms load to his NGE100 (set to 8V, 10 mA). Then there is the (deliberate?) delay of about 40 ms before the current controller becomes active. Then there is the expected exponential voltage decrease due to the output capacitor (approx. 50 ms). But then it takes another whopping 12 seconds before the output current actually reaches the set 10 mA. Take a look again at the diagrams in post #37. The vertical scale is in volts, but notice that  due to the 50 ohms resistor across the output, it is proportional to output current. After the exponential decrease is over, the output sits at 1 V (i.e. 20 mA) and slowly ramps down to 0.5 V (i.e. 10 mA) within 12 s. There is a clear distinction between the exponential part of the curve and the slow ramp to the set current.

Even though there is only a setting accuracy spec for the current controller (0.1% + 5 mA, so the 20 mA are way out of spec) and no settling time specified (as explained before, there can't be because it depends on the load), I think it is unacceptable for a PSU to take so long before a set value is reached.

I hope this makes things clearer. 

[mhsprang]

At the risk of making an unnecessary observation:  the user normally configures a voltage value and current limit.  I'm not aware of any T&M benchtop power supply where the user explicitly configures the desired output current.

Well, exactly. I configure a current limit. So I don't expect the current to exceed that limit.

My understanding is that the only way to "force" a supply to output a given current is to set the desired current limit and then set the output voltage so high that the supply will transition to constant current mode and reduce the voltage to the appropriate level when the supply output is enabled. 

However, the power supply has no knowledge of the attached load impedance before it's turned on and current starts flowing.  If you set the output voltage to 10 volts and a current limit of 1 amp, and attach a 1 ohm resistor, then 10 amps (!!!) will flow for some finite amount of time before the supply's readback function is able to detect this and lower the output voltage to 1 volt in order to honor the configured current limit of 1 amp.

The power supply "knows" that it is going to deliver voltage and current because I press the Output button. So, the firmware could, instead of outputting 10V at once, slowly ramp up the output voltage until the current limit is detected. And this ramp up only needs to be slow enough for the control loop to keep up.
Also, I have built many current source circuits in my life and none of them have ever fried LED's. This implies that one can certainly design a power supply in such a way that the software drives hardware voltage and current sources by means of reference voltages output by DACs, where the actual control loop exists in hardware.

If your control loop is enterely in software, it is also very well possible to behave correctly in my opinion. And if I had loads of spare time, this thread alone is enough motivation for me to design such a power supply with all this in mind.

Sorry for being lazy (because I believe you put this in an earlier post), but what is the voltage drop during these 12 seconds?

The load was a 50 Ohm resistor. So, if the current dropped from 20 mA to 10 mA over 12 seconds, the voltage (that was actually measured on the scope), dropped from 1 V to 0.5 V over the same 12 seconds. Ohm's law...

[Phil1977]

I´ve often seen this behaviour on pure SMPS. They often have an analogue control loop only for the voltage, but constant current mode is then realized in software and so it takes a relatively long time to switch over. This is okay for charging batteries, but not for anything that needs CC-mode to survive.

Also old-style linear PS often have a much slower control loop for CC than for CV to prevent oscillations with inductive loads.

But I completely understand that you expect from a premium-priced lab PS to well perform in this area. Even with SMPS it´s possible to post-regulate the output with a linear regulator that quickly reacts to load changes. It´s no magic to do this within ms, and it´s no magic to define a ramp-up that prevents overshooting.

[mawyatt]

This thread is an important message for even older more seasoned folks in that it illustrates the Power Supply is not near a source with ideal behavior, especially true wrt to Voltage and Current Limits and how they operate.

When dealing with sensitive circuits consider setting the PS Current Limit slightly above the normal operating current, which of course slows the voltage ramp up time (decoupling capacitance) and usually isn't a problem. However some circuits require a high inrush which must be dealt with as needed, here we usually set the Current Limit to above the inrush or at a level where the circuit starts up properly after turning the supply ON, then manually reducing Current Limit to slightly above normal operating current.

This is a learned behavior, as we've smoked our share of circuits and components over the past 65 years

Anyway, this thread has been enlightening, delving into some PS behavior that most folks should find useful.....and it hasn't turned into a pissing contest......yet

Best,

[Thomas]

That Agilent 6630B is something special.
A 100W power supply in a 2U 19" box weighing 15kg - one would expect it to perform damn near perfect.

Anyway, I checked my supplies too - not so exciting, except for one.
My TTi EL302RT is made with potentiometers, LM324s and 2N3055s - basic, oldschool and budget friendly.
Note the time scale - this thing is fast. Nice.

[blackdog]

Hi,

My Power Supply weekend preaching and all my opinion

I think this topic is important, that's why I put time into it to explain a few things.

1
The power-on behavior of a LAB Power Supply is just one of the important features.

2
A second is the overload behavior when the current exceeds the set value.

3
Then we also have the behavior of the power supply when the output is shorted.
I have done quite a few measurements on LAB Power Supply's and that was often shocked by the very high peak currents.
Of course this is not just because of the capacitor across the output terminals, besides this short peak current you also have the peak current due to the often much too slow regulating current loop.

4
And then again how well does the U-loop react when the current reaches its normal value again.

5
Some problems can be solved by what I indicated earlier by using a current source that always draws some current from the power section.
Yes, that takes some energy, but you get a much better U and I loop from it, the power section then always remains well in its operating range.

6
It is unfortunate that a number of manufacturers are moving more and more to software-controlled U and I loops.
This usually does not improve the quality and especially not if the U and I loop is realy done by a ADC and the processor itself.

No hair on my head thinks about such a modern LAB Power Supply here on my workbench.
My experience with modern measuring equipment and software bugs is not really positive....

7
Many people who start with electronics often make a bench power supply, often because they think it is easy, many simple schemas are available through search engines.

9
I am too old to have done that via search engines, with me they were magazines of course. :-)
Many power transistors I have blown and so I learned that a linear power supply is anything but simple.
Almost no construction project mentions the phase margin for the U and I loop.
Nowadays this is a lot better, but especially for the SMPS types and their configuration.

9
On many forums when one is asked for help to decide regarding power supplies, often the first reaction of forum members is that an SMPS type should be used and this is Wrong, Wrong, Wrong!
That usually indicates that the ones proposing that have little measured equipment with SMPS power supplies.
For those who are going to build something themselves with SMPS, they are then stuck with a very big EMC/common mode problem.

Then when I counter, the comments often come that in my scope, phone are also SMPS power Supplies and that is correct.
These are often very well designed and tested for EMC and I don't see the average homebuilder replicating this....

10
But the manufacturers of all kinds of equipment can also do something about EMC interference and common mode problems.

I have several function generators here that inject EMC into my D.U.T. with their SMPS Power Supply.
For this I bought a Scoop/Function generator model from OWON that runs on battery.
And gone are my commonmode problems with certain measurements.

More and more I have to turn off equipment on my main workbench to do measurements at low levels.

I now have a second workbench with less equipment and where almost everything is off (real power switch), so no equipment with Standby Power Supply's etc.

And then consider how I think about a Power Supply that has a switching pre-regulation and then a linear post-regulation.
Then you're still stuck with the commonmode injection of the switching part, yea... but not on my measuring bench...
This is a personal opinion. 

11
Above all, don't get me wrong, I am very happy with the SMPS technique but it is certainly not the holy grail.
That technique also has major negative characteristics.
What you are going to apply must be looked at holistically in my opinion.
What is needed, what is stable, how much power loss may occur, what are the error situations that may occur and how much EMC is permissible and of course the cost.

12
There is no “One Size Fits All” LAB Power Supply.
But for an electronic lab, you do expect it to be “clean” with both a fast U and a fast I loop and otherwise no on or off abberations of the Power switch and or the enable key.

13
The last one!
A LAB power Supply must be kind to itself and to the D.U.T that is connected to, this is perhaps the most important premise...

Kind regards,
Bram

[KungFuJosh]

My TTi EL302RT is made with potentiometers, LM324s and 2N3055

How the heck did they fit a person into a PSU?!?!?! 🤯🤯🤯

[2N3055]

My TTi EL302RT is made with potentiometers, LM324s and 2N3055

How the heck did they fit a person into a PSU?!?!?! 🤯🤯🤯

It is tight. But hey man has to put food on the table..

[KungFuJosh]

My TTi EL302RT is made with potentiometers, LM324s and 2N3055

How the heck did they fit a person into a PSU?!?!?! 🤯🤯🤯

It is tight. But hey man has to put food on the table..

🤣🤣🤣

[Furna]

Some tests regarding Siglent SPD4000X series starting at https://www.eevblog.com/forum/testgear/new-siglent-spd4000x-series-power-supply/msg5666851/#msg5666851

[mawyatt]

See new related thread.

https://www.eevblog.com/forum/testgear/lab-power-supply-turn-on-and-off-characteristics/

Best

[flo.b]

I would argue a general purpose PSU should prioritize safety and stability over speed.

It looks like the industry disagrees on that
From R&S document "Fundamentals of DC Power supplies – output characteristics":

[salomonander]

This is a bit disappointing. Im still set on buying an NGA102 as the problem wont bother me much in my applications. But after i saw Daves video where he showed this issue and studying the firmware notes i was convinced that they have tackled the issue. They are well aware of it. See the firmware notes:

https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/pdm/firmware/5601_9009_00/NGA100_ReleaseNotes~1.pdf

Does anyone here have a NGA100 with up to date firmware who can comment?