Products > Test Equipment
Power supply for home lab - do I really need a R&S?
2N3055:
I actually looked into user manual.
It explicitly states that PSU functions in Constant Voltage (CV) or Constant Current (CC) mode.
thm_w:
--- Quote from: Martin72 on October 05, 2024, 12:02:29 am ---
--- Quote from: thm_w on October 04, 2024, 11:41:51 pm ---Seems beside the point when we already have demonstrated supplies that do not overshoot, and that is the desired behavior.
--- End quote ---
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.
--- End quote ---
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:
--- Quote from: pdenisowski on October 04, 2024, 10:37:07 pm ---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.
--- End quote ---
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:
--- Quote from: pdenisowski on October 04, 2024, 10:37:07 pm ---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.
--- End quote ---
Well, exactly. I configure a current limit. So I don't expect the current to exceed that limit.
--- Quote from: pdenisowski on October 04, 2024, 10:37:07 pm ---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.
--- End quote ---
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.
--- Quote from: pdenisowski on October 04, 2024, 10:37:07 pm ---Sorry for being lazy (because I believe you put this in an earlier post), but what is the voltage drop during these 12 seconds?
--- End quote ---
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.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version