EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: McBryce on January 07, 2017, 04:51:09 pm
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Hi.
I have a test circuit connected up to my DP832. The device pulls approx. 1.1A at 5V in normal usage, but when a certain part of the device switches on, it will have a sudden spike of up to 1.8A (about 10nS long) before it drops back to 1.3A. The device runs perfectly on a old 5V 2A DC PSU (transformer, not SMPSU), but if I connect it to the DP832 the device resets when the spike occurs. The DP is set to 5V 2.1A. I've connected a scope to the DP output and the voltage is dropping to almost 2.5V when the spike occurs. I've tried all 3 channels and also turning off OCP and OVP but it still happens. The cable from the DP to the device is short (30cm) and thick enough to handle 10A. Is this normal for a DP832? I've never had any other supply do this on me. The DP works completely normal and accurate otherwise. Have I forgot some hidden setting?
McBryce.
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it is not a normal behavior. A current spike like that should be managed with no issues.
The circuit is inductive?
What happens if you try to drain with a resistive load from the DP832 say 1.8A or more continuosly?
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How do you know that the current during the spike is 1.8A, did you measure it?
Maybe it's higher than that and the power supply sees a current of more than the 2.1A limit and behaves correctly?
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Yes, it's an inductive load. Yes, I have measured the current spike and it really is just 1.8A. I tried with the OCP turned off, so it should in theory then be able to supply up to 3A even with the current set to 2.1A?
It will happily supply 3A constantly to a load.
McBryce.
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Did you try to suppress any inductive kickback coming from the load to the PSU (with a diode for example) and see if it change anything?
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The inductive load is behind a DC/DC converter, so I would assume that the schottky diode is already doing that.
McBryce.
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How are you measuring that current? current probe or resistor shunt ?
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You cannot expect a power supply to output a 10 nano second long
half amp current pulse through a cable.
You need local decoupling for that.
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I was measuring the current with a Hantek CC65 current probe connected to a scope. The device does of course have local decoupling, 2x 100µf electrolytic on the power rail and local 100nf caps on the ICs. The secondary side of the DC/DC has a 680µf on the output for the inductive load.
However, I have just solved the problem, or to be exact, it solved itself. I tested the DP832 with an electronic load and brought it slowly up to 3A to check that all was working fine. At 2.8A the 5V suddenly dropped to 0.48V and I immediately switched off. I left it powered down for 30 minutes and since powering back up, the problem has gone and it also now supplies a steady 3A at 5V. No idea what the problem was, but it's gone. It no longer dips to 2.5V when the inductive load switches in.
One side note: This particular DP832 is very new and had never been used above 500mA since it arrived. Could it have needed some sort of "burn-in" on any of the components? I have an older DP832 which regularly saw 3A, but I don't remember having had similar issues.
McBryce.
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You cannot expect a power supply to output a 10 nano second long
half amp current pulse through a cable.
You need local decoupling for that.
Thus my question about the current measuring, but you nailed it, In my opinion its a problem with local decoupling of the device itself, not a dp832 problem
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I was measuring the current with a Hantek CC65 current probe connected to a scope.
And how exactly did you measure a ns current pulse with 20khz bandwidth probe?
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Oops, that was meant to be mS, but I didn't get that from the current measurement either. The voltage drop was 10mS and I derived it from that.
Eitherway, local decoupling doesn't seem to be the problem anyway, because it now works fine since the restaart.
McBryce.
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I tested the DP832 with an electronic load and brought it slowly up to 3A to check that all was working fine. At 2.8A the 5V suddenly dropped to 0.48V and I immediately switched off.
few message above you stated
It will happily supply 3A constantly to a load.
so we assumed the PSU was working fine at 3A load, while it wasn't tested properly at that amperage.
One side note: This particular DP832 is very new and Could it have needed some sort of "burn-in" on any of the components?
I think it's more a mechanical solder/connector issue that has been thermally modified by the stress you induced with the "gradual" load.
It could be a small issue not noticed thru test/calibration procedure and came out from mechanical stress during transport.
Sorry to say this, but I think you could see that fault again in the next future.
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That was the strange bit. I ran it at 3A with a load resistor and it supplied 3A fine, it was only when I ramped it up slowly that it didn't do what it was supposed to.
Mechanical issue: That's exactly what I was thinking too, so I plan to open it up soon and give the board a serious inspection.
McBryce.