DC Power Supply - Mode Crossover - Page 1
Forum section: electronics / testgear

author: engrguy42
author_rank: Frequent Contributor
timestamp: March 19, 2020, 01:29:57 pm
content: Apologies if this has been discussed before here (I tried searching and didn't see a reference) but there's a youtube video discussing one of the low-end (Dr. Meter) power supplies, and the transient response when powering after the supply was left in constant current mode.In this case it was in constant current mode, and the voltage dial was set to maximum (32 VDC) prior to turning on the PS. The CC setting required only around 3 volts to push the constant current setting, but the turn-on transient jumped up to over 10 volts, then quickly dropped down to the required 3 volts.I'm assuming this is a result of "mode crossover", going from CV to CC mode upon startup, but this seems a bit extreme.I'm curious if this overshoot (of this magnitude) is typical for the higher-end devices, or if they employ some logic to minimize this. Seems like it's only an issue when the voltage dial is left on a setting above what is required to push the constant current value.Also, how do folks protect their circuits from this type of overvoltage?Thanks.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: March 19, 2020, 01:45:48 pm
content: I haven't tested it but the Rigol DP832/DP832A has a max voltage setting for each channel, maybe this is why?

author: capt bullshot
author_rank: Super Contributor
timestamp: March 19, 2020, 01:49:40 pm
content: As an experienced Engineer, I do expect the average lab power supply to behave as you describe (can't be bothered to watch the video). Only specially made (like a LED constant current PSU) or specified (like a high end SMU for testing expensive semiconductors) are expected to not overshoot under the use case you show.I do expect from every sensible / experienced lab worker / engineer to expect a common lab power supply to "mis-behave" in this way. Indeed, typically it's quite easy to kill e.g. a sensitive device by setting a lab to a CC limit and somewhat higher CV than expected, turning it on and then connecting the DUT. Most lab power supplies have a substantial amount of excess energy that gets dumped into the output when CC takes over, may it result from circuitry flaws (CC error amp must de-saturate) or simply a decoupling cap at the output.As an engineer, one is supposed to know what you do and to think about failure modes while doing so, at least after the first two or three blown expensive devicesOne cannot expect the lab supply taking care of your faults or lazyness.Edit: From my experience, it's not just the cheapish crap lab supply from today that behaves this way, such stuff exists since lab power supplies exist and even expensive brands can suffer from that. Always check and get known to your tools.

author: nctnico
author_rank: Super Contributor
timestamp: March 19, 2020, 02:13:16 pm
content: Quote from: capt bullshot on March 19, 2020, 01:49:40 pmNo. A good power supply shouldn't behave this way. The video shows the power supply overshooting with a load connected when turned on (as usual you can skip through it quickly to compress it to 5 seconds of actually interesting video).

author: engrguy42
author_rank: Frequent Contributor
timestamp: March 19, 2020, 02:14:28 pm
content: Thanks. Yeah, there's a Keysight blog describing the "mode crossover", but it just seems strange that there's no built-in protection for something like this.http://powersupply.blogs.keysight.com/2012/07/why-does-my-power-supply-overshoot-at.htmlAnd the supply in the video has no on/off switch to allow you to first configure everything and then turn on the output terminals. I guess its no big deal if you remember to turn the dials down before energizing, or your circuit doesn't care about a 3-4 times overvoltage, but geez it seems strange this is accepted as a thing

author: nctnico
author_rank: Super Contributor
timestamp: March 19, 2020, 02:29:38 pm
content: The Keysight website shows something different. It shows the case where a load suddenly is connected to a power supply. In that case you'll have two effects:1) The capacitors need to discharge2) The control circuitry needs time to adapt.These effects are normal for every power supply.Showing peaks at startup / shutdown with a load connected should not happen. A good PSU regulates the output up to voltage limit or current limit (whatever limit is hit first) when it is powered on with a load connected. A simple fix to improve switch-on behaviour is to add an output enable switch. Usually this is easy to do by connecting a switch parallel to the voltage adjust potmeter which either just shorts the potmeter or switches between ground and the wiper.

author: engrguy42
author_rank: Frequent Contributor
timestamp: March 19, 2020, 02:39:32 pm
content: Cool. Thanks.I guess the lesson learned is: If you're using a low-end power supply, and haven't verified it won't overshoot when powering on with your circuit connected, make sure your CV voltage dial is turned down real low so it doesn't zap anything. Or think about adding a zener for protection.

author: nctnico
author_rank: Super Contributor
timestamp: March 19, 2020, 02:46:07 pm
content: Quote from: engrguy42 on March 19, 2020, 02:39:32 pmTurning the CV dial down isn't enough. Many low cost power supplies will overshoot at power-on and/or power-off.

author: rstofer
author_rank: Super Contributor
timestamp: March 19, 2020, 02:47:01 pm
content: And a really good supply, like the Rigol DP832, has a separate current and voltage limit adjustment which, if exceeded, causes the PS to shut down all outputs. I haven't spent time testing the feature but it is a front-panel setting that comes into play when OVP or OCP (Over Voltage/Current Protect) is enabled. This is separate from the Voltage and Current setting.I like the idea of having a hard limit on voltage/current in addition to the control loop setpoint.I'm also fond of the idea of a front panel setting for voltage/current and the idea that you have to set the current limit by shorting the output and twiddling a knob isn't all that impressive. It just feels wrong.

author: engrguy42
author_rank: Frequent Contributor
timestamp: March 19, 2020, 03:01:14 pm
content: I think he mentioned somewhere that it only happened when the CV dial was set significantly higher than the voltage required to push the constant current setting. I think that's why he mentioned this mode crossover thing (different video?), assuming that it first started in CV mode with the dial cranked up, voltage overshot to the high CV setting, then crossed over to CC mode and then dropped quickly to the 3 volts or whatever was needed for CC. Somewhere he showed a similar trace for a low CV setting and there was zero overshoot in voltage, it just ramped up smoothly with no overshoot.Though maybe that's a peculiarity of this particular power supply.

author: David Hess
author_rank: Super Contributor
timestamp: March 21, 2020, 05:28:56 am
content: Quote from: engrguy42 on March 19, 2020, 01:29:57 pmThere are two common causes:1. Integrator windup in the control loops causes overshoot when switching modes. This can be largely ameliorated with proper design which improves the response time or completely eliminated with anti-windup circuits.2. During startup or shutdown, the error amplifiers may not operate correctly when supply voltage to the control circuits is low. If this is a problem, then a good design includes a circuit to disable the error amplifiers and control the output until their supply voltage has stabilized.QuoteOvershoot is common in poor designs.QuoteNone of the power supplies that I have bought suffer from overshoot. And now that I am a careful and experienced designer who is aware of the problem, none of the power supplies that I have designed suffer from overshoot either.Quote from: capt bullshot on March 19, 2020, 01:49:40 pmI expect cheap piece of trash power supplies to suffer from overshoot. I do not expect it if the power supply comes from a reputable company.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 21, 2020, 06:57:52 am
content: Quote from: David Hess on March 21, 2020, 05:28:56 amAgree for the reputable company. One should assume they've learned their stuff, but there will be exceptions.The modern Keysight E36312A overshoots at power on, before the digital control part has fully started. Afaik it is fixed now, and units suffering from that were replaced if the customer was aware of that issue, read the notice somewhere hidden and contacted Keysight. The older model 6632B takes in the default (slow) mode noticeably long to move from CV to CC, even at rather lowish CV setpoint.So never blindly trust your lab supply, check your equipment before destroying sensitive components. One useful habit is to disconnect the DUT before turning off the main power switch of the lab supply, and don't reconnect the DUT before the supply is fully operative. One can skip that under the right conditions, as an experienced engineer one should know when it is safe to skip that.

author: engrguy42
author_rank: Frequent Contributor
timestamp: March 21, 2020, 09:05:28 am
content: Thanks David.Attached is a chart of three transient startup waveforms from a low end ($90) power supply, with the PS set to deliver CC of about 50mA, which requires about 1.5 volts. It's feeding a simple circuit of two 10 ohm resistors in series, measuring volts across the second resistor.The only difference in the waveforms is the position of the CV dial. It's a 30 volt PS, and therefore max volts across each resistor is 15 v.The highest peak is with the CV dial set to max (30v), the middle one with CV set to about mid-range, and so on.As you can see it seems to start in CV mode, then hits a peak near the CV dial setting, then decays and seems to re-start into CC mode at the required 1.5 volts.I'm trying to understand how exactly to characterize this and what exactly is at fault.It seems that many folks aren't really aware of this phenomenon, or know enough to test to see if it exists, or how to test for it. And since these low end Chinese power supplies are so common, and the overvoltages can be so huge compared to what's expected for CC mode (in this case 10x), I think it's important that folks investigate and reviewers look into stuff like this.Yeah, I know we should all "get a decent power supply", but that doesn't change the fact that people don't, and if they aren't aware of stuff like this see no reason to spend the extra money.Thanks

author: masterx81
author_rank: Frequent Contributor
timestamp: March 21, 2020, 12:41:19 pm
content: My cheap korad ka3305p if tested in same conditions (2x 10ohm, measuring on the second resistor, limited at 50ma), if powered on not have any overshot, but obvously if i suddently connect the load, it start from the cv voltage and takes around 20 ms to settle down (output capacitor discharge).I attach a power on photo with 10v setting, and 50ma limit with the 2 resistor load. It settle in around 20ms at the 500mv voltage.

author: engrguy42
author_rank: Frequent Contributor
timestamp: March 21, 2020, 01:01:34 pm
content: Quote from: masterx81 on March 21, 2020, 12:41:19 pmCHEAP?? I checked online and it's being sold via WalMart at $430 !!! And elsewhere at around $300.But thanks for checking. I'm wondering if this is an intentional design flaw or some component failure.

author: masterx81
author_rank: Frequent Contributor
timestamp: March 21, 2020, 01:26:04 pm
content: Quote from: engrguy42 on March 21, 2020, 01:01:34 pmI've not paid it that much! Near 200€. You need to wait the right offer. I've bought the rnd version from distrelec.I think that is the most low budget dual power supply available...Cheaper there is the D version that is not programmable.If you need a single supply there is the 3005.If you look at the price of "professional" psu you understand why i call it "cheap".From what i can observe and what are my needs, i think that it's a quite good psu . I was so happy that i've bought also the korad dc eletronic load kel 103.

author: electricMN
author_rank: Regular Contributor
timestamp: March 21, 2020, 05:15:41 pm
content: Quote from: engrguy42 on March 21, 2020, 01:01:34 pmIt's only $245 here:https://www.sra-solder.com/korad-ka3305p-programmable-precision-variable-adjustable-30v-5a-dc-triple-linear-power-supply-digital-regulated-lab-gradeAnd on Amazon:https://www.amazon.com/gp/product/B012LMOI84/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1I bought mine 2 years ago and it cost me $210 from Amazon.

author: David Hess
author_rank: Super Contributor
timestamp: March 22, 2020, 04:11:17 am
content: Quote from: capt bullshot on March 21, 2020, 06:57:52 amIt happens even to the best of them and I suspect when experienced engineers are replaced.Quote from: engrguy42 on March 21, 2020, 09:05:28 amI learned the hard way by blowing out early LinCMOS parts. When Texas Instruments listed their absolute maximum supply voltage as 18 volts, they were not kidding.