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nctnico:
--- Quote from: Berni on March 05, 2021, 08:27:21 am ---Well the HP 6032A does have a dedicated foldback button.
It works like a normal bench PSU normally, but if you push the foldback button the LED next to it lights up and this locks the PSU into the current mode of operation. If you are currently in CV mode and the supply hits CC mode then it will instead shut down and wait for the output to be disabled and enabled again. If you are in CC mode (say powering a big LED with a set current), push the foldback button and then disconnect the load, causing the PSU to go back into CV mode it will shut down too.
Such a feature is probably even more desirable on such a massive PSU that can supply 1kW 60V 50A.
--- End quote ---
That is not fold-back but 'electronic fuse' operation. AFAIK this is pretty much standard on all digitally controlled HPAK PSUs. On the E3631xA series it is called OVP and OCP.
tautech:
--- Quote from: nctnico on March 05, 2021, 08:24:46 am ---
--- Quote from: tautech on March 05, 2021, 07:41:46 am ---Why is it that foldback current limiting isn't incorporated in a 'quality' PSU FFS, even a $30 SMPS has such a feature as it's hardly a new leading edge technology when it's been around for decades.
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How would that be compatible with constant current mode? How is a outputing a random voltage at a random current going to help? AFAIK foldback current limiting is used to limit the dissipation inside the PSU in case the heatsink is not big enough.
--- End quote ---
Experiment with your MCH K305D.
Sure it's not foldback yet even in max CC mode it limits the voltage so that the max current is maintained and it will do that indefinitely and not smoke, pop fuses or traces.
--- Quote from: bd139 on March 05, 2021, 08:17:50 am ---Also it's difficult to rationalise foldbacks when you have a variable current limit. I mean you can't go over-current. What if it's a CC source you want (charging SLAs). Foldback would be a problem.
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Indeed if you exceed the foldback limit however with the PSU's I use for SLA's which are mostly 7AH or less you don't want to squirt too much current into them anyways and as they're near the bench they're easy to monitor draw and increase the voltage to the cyclic limit and watch the current draw drop away. Couple of weeks back I forgot and left a nearly charged 24AH SLA charging overnight set to 14.5V which was drawing just 10mA next morning. :phew:
exe:
--- Quote from: bd139 on March 05, 2021, 07:55:32 am ---
Usually, taking radio power supplies as an example, you set the voltage to 13.8V and the crowbar to 15V. The specification for the load sets the voltage tolerance. Typically this is max input voltage of LDOs, capacitors, ICs etc directly connected to the supply considering power dissipation as well. Some things designed for say 9v input will quite happily run at 12v depending on how the internal power is arranged.
--- End quote ---
In you example with blown zener and output voltage skyrocketing. Do you propose to have a second reference that would only be used for crowbar?
--- Quote from: bd139 on March 05, 2021, 07:55:32 am ---There are two cases where the crowbar comes in handy as well which are almost never mentioned as well. Firstly, and this does happen with RF stuff, when there is RF noise on the sense lines to the supply. Secondly, when the load is too inductive and the supply starts oscillating. Even the best of power supplies can be coaxed into oscillating (try running something like an SMPS on the end of a few metres of untwisted wire.
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In both example, when crowbar fires the output drops to zero, is that correct? I'm not sure how useful is that, the load remains with no power. Probably, very special cases. If PSU is oscillating, the only way to "fix" it is to switch-off and do something about it. Like, adding more output capacitance, or reduce loop bandwidth, or use shorter wires, add filtering on sense wires, or something.
As of oscillating, I was thinking if power supply can detect that. I considered using window comparator, or ideal diode bridge and AC-coupling to rectify and sense peak-to-peak voltage, but it was too much added complexity to my liking. Probably, digitally sampling output voltage and processing in software is easier, but might be not as fast as a hardware solution.
I was also considering not a crowbar, but a down-programmer (essentially a load resistor triggered by comparator when output voltage, say 5%, above set voltage). Also quite tricky to make everything nice, fast and stable.
PS I'm currently making a power supply, and I made CV feedback with adjustable bandwidth: fast and slow. Slow is to drive troublesome loads. Also helps with stability at low output currents (below 1mA) when output stage has too much gain. I think a few commercial power supplies too offer switchable speed.
Berni:
--- Quote from: nctnico on March 05, 2021, 08:41:07 am ---
--- Quote from: Berni on March 05, 2021, 08:27:21 am ---Well the HP 6032A does have a dedicated foldback button.
It works like a normal bench PSU normally, but if you push the foldback button the LED next to it lights up and this locks the PSU into the current mode of operation. If you are currently in CV mode and the supply hits CC mode then it will instead shut down and wait for the output to be disabled and enabled again. If you are in CC mode (say powering a big LED with a set current), push the foldback button and then disconnect the load, causing the PSU to go back into CV mode it will shut down too.
Such a feature is probably even more desirable on such a massive PSU that can supply 1kW 60V 50A.
--- End quote ---
That is not fold-back but 'electronic fuse' operation. AFAIK this is pretty much standard on all digitally controlled HPAK PSUs. On the E3631xA series it is called OVP and OCP.
--- End quote ---
Yeah i don't like the name they gave that feature since it is misleading, i guess its to not confuse it with the actual OVP feature of this PSU that is separate feature on a separate control. But the use case is similar to limit damage when something goes wrong.
And yes lots of bench power supplies have OVP and OCP settings, but on all of the ones i used was it a UI pain in the ass to set up. Most of the time you have to manually set a OCP limit somewhere and then make sure your current limit is set higher so that it actually reaches OCP and trips out. Then once you don't need it anymore you forget to turn it off, and be confused what is going on later on.
On the other hand on this PSU there is nothing that needs setting. You just use the PSU as normal and if you want this fuse like trip out protection you simply press that foldback button and see the LED light up to indicate its active. If you don't need it anymore simply press the button again and it turns off. I want this "magical act like a fuse button" on all of my PSUs but none seam to actually implement one.
Kleinstein:
The classical crowbar circuit uses a beefy SCR. So one fired it will essentially short the output to some 1.5 V residual voltage of the SCR. One would even need to turn off the ouput before the SCR would go open again.
Detection of oscillation would be a nice feature, but it is far from standard. A good lab supply with simple outputs should not oscillate with any reasonable passive load. A few not so good ones get unstable with large low ESR caps, or maybe high Q resonant loads. With seprate sense wires one can hardly avoid oscillation if the sense wiring is poor.
The critical case for the CV mode is a capacitive load and even if not oscillating there can be quite some ringing with a tricky load. Naturally a regulated supply has a inductive ouput impedance over a larger frequency range.
A SMPS may very well show negative differential input resistance - worst case it may even oscillate when powered from a battery with long enough cables.
So the combination of poor SMPS and lab supply may show oscillation and ripple.
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