Start up spikes - looking for knoweldge - Page 1
Forum section: electronics / testgear

author: Electro Fan
author_rank: Super Contributor
timestamp: August 17, 2013, 11:10:07 am
content: Just checking to see if any of our resident EEVers who happen to be strong with power supply design skills might be willing to walk the rest of us through what it takes to get an acceptable turn-on circuit within DC power supplies?I happen to have a DC power supply and my hunch is that it doesn't incorporate a super graceful turn-on, but I can't prove it for lack of knowledge, skill and test equipment.In looking at the Rigol 832 (see recent EEV video) it appears that it might have a sufficiently acceptable turn-on capabilty, or it might have a bit more of a spike than what is preferred. I'm not sure how much of a spike in amplitude over what duration for various uses might be acceptable vs. not so acceptable.In looking at this post by IanB it appears that some reasonably priced power supplies (such as this particular TTi model) can in fact turn on gracefully:https://www.eevblog.com/forum/testgear/comparing-two-power-supplies/Given what Ian shows in his graphs (for the TTi but not for the Extech) it appears that the spike for a PS can be relatively small in amplitude and of a duration of perhaps as short as a few microseconds.Here is a spec given by TTi and another by Rigol for transient response time:Rigol: “Less than 50 (micro) sec for output to recover to within 15 mV following a change in output current from full load to half load or vice versa.”TTi: “Transient Response: <50 (micro) s to within 50mV of setting for a 5% to 95% load change.”I'm not sure if this type of transient spec is germain to the discussion of PS startup performance, but it seems like it would be; maybe someone can tell us?I can't quantify the thresholds of current and duration that are acceptable but I have sense that if a Power Supply is outside the scope of whatever is defined as acceptable that the consequence for a Device Under Test and for the happiness and success of the test engineer/technician might be less than favorable.I'm hoping this thread might shed some light on what is a good and what is a not so good Power Supply startup curve. To be clear, I see one event as happening when the AC power is applied to a PS, and another event happening when the PS user turns on the output(s) from the PS to the DUT. It's this second turn-on that I'd like to better understand. Unless I'm mistaken (which very well could be the case), it would seem that current (and voltage) should be at zero when the PS output is off and the current should rise cleanly to the targeted level with relatively little if any overshoot and to the extent overshoot occurs the current should be quicky corrected to the targeted level.Something that would be interesting to understand is what does this turn-on curve look like in a well performing Agilent power supply (perhaps for models in the roughly $1k range)? Do they perform still better than the TTi in Ian's graph or about the same? If you were working on a serious hobby or professional project, what kind of PS turn-on performance would be sufficient that you would no longer be concerned about it?As you can see, I can barely frame the question - so thanks in advance for any info.EF

author: PedroDaGr8
author_rank: Super Contributor
timestamp: August 17, 2013, 03:49:08 pm
content: I performed a test like this the other night on my rebadged Array 3645A. I turned on the output with a ~50W load attached (16V into a 5R0 100W resistor). I don't have a current trace for this but since it's a resistor, not a light bulb, the current should reasonably mirror the voltage through the resistor. As you can see turn on is a nice steady increase to the voltage, with no overshoot.Sent from my EVO using Tapatalk 4

author: tom66
author_rank: Super Contributor
timestamp: August 17, 2013, 06:28:35 pm
content: Really you just need to compensate the control loop properly.My guess is that because Rigol PSUs have the step function, where the voltage can be programmed to step in increments, that the designers were scared of limiting the slew rate, to make it closer to an ideal step function.Compare, loop 1 has no compensation, and is unstable on step function from 0V output to 5V output, it overshoots to 5.7V, undershoots and then settles. In the real world this may even oscillate under certain load conditions. Notice how Vbase, the base voltage of the driver transistor varies considerably... the only thing offering smoothing here is the output capacitor.Loop 2 has adequate compensation, a 3.3nF across the 90k feedback resistor, which slows the rise rate of the output and no overshoot occurs at all. There are a few glitches, but nothing serious. The base waveform is also smooth, which is a clue that the control loop is working.Loop 3 has huge compensation (330nF), making the rise time very slow. This is perfectly adequate for most power supplies not needing step capability, but might be too much for the Rigol (or any user-interface driven supply, because it would lag quite a bit.)Further improvements can be made by using an integrating topology, which is naturally a lot more stable than this topology.

author: IanB
author_rank: Super Contributor
timestamp: August 17, 2013, 07:26:35 pm
content: Quote from: Electro Fan on August 17, 2013, 11:10:07 amBe aware that Dave tested themains power on/off buttonwhen he saw the spike. He was not testing the electronic output on/off button that you would normally use (and that I tested on my TTi and on my Extech).In Dave's video I saw no problem at all with the normal output on/off control (the one you would normally use).The trouble with what Dave showed is what happens if you happen to plug in the supply and turn on the mains switch with some device under test still attached to the output. You would think "this power supply output is off and is safe", yet the connected device would get an unexpected zap.What you need to ask about other supplies is if they do the same thing. My hunch is that the likes of Agilent and TTi will not.

author: IanB
author_rank: Super Contributor
timestamp: August 17, 2013, 07:37:31 pm
content: Quote from: tom66 on August 17, 2013, 06:28:35 pmI often hear about "compensation" in this context, but I have never obtained an adequate answer to the question, "Compensating for what?"I guess it would be compensating for excessive loop gain at high frequencies, but I'm not sure?If I were tuning a PID control loop, I would reduce high frequency instability by increasing the integral action while simultaneously reducing the proportional gain. The integral action will tend to filter out high frequency components and damp the response.If the opposite happens, where integrating action is producing low frequency cycling, then adding derivative action to compensate for the integral action can help. The derivative term will give a predictive action that reduces overshoot.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 17, 2013, 10:21:10 pm
content: Quote from: IanB on August 17, 2013, 07:26:35 pmYes, this is a major part of the "question" and likely the first part of the overall question - for any, all, and every DC Power supply.It seems like the first job of a Power Supply is to do no damage while in an "idle" model (ie, when the PS DC output is off).In other words (and please correct me if I am wrong) it seems that with a properly designed and operating Power Supply that a user should be able to connect a DUT to the PS whether the AC mains are on or off, as long as the PS DC output is off.If the PS AC mains are off, a connected DUT should be fully protected/insulated from any type of surge or spike as long as the DC output is off. "Off" on the DC output should mean "off".When the PS AC mains are in the process of turning on, the PS should not allow the DC output to turn on until the DC output circuit is fully ready to perform in an off condition (ie, start from 0 and proceed gracefully). Once the PS AC mains have been turned on and the PS is ready to perform, then the PS DC output can be turned on, at which point the DC ouput should be able to ramp gracefully to the designated voltage and amperage (with little to no overshoot). Likewise, when the PS DC output is turned off it should be able to gracefully turn the volts and amps back to 0. At this point the DUT should be able to remain connected to the PS (or the DUT can be safely disconnected). Regardless of whether the DUT remains connected, the PS user should be able turn the PS AC mains off with no impact to the DUT.Wash, Rinse, Dry. Repeat.It would seem that before we critique almost any other specs or attributes for a PS this is the very first capability to be tested/measured/reported/confirmed.The oscilloscope traces seem to provide compelling visual evidence of how well a PS does. It would be great if we could see more of these from users for the various steps in power supply power-on and power-off, including both AC mains operation and DC output operation, with and without loads attached.It just doesn't matter what else a PS can do much less how it looks if we don't know these basic things about a PS.Feel free to correct me if I'm wrong on this. Thanks!PS, If I was a Power Supply product manager this is the first demo I would market to potential customers. It appears that the PSs that can do this well are in the minority - and any PS that can perform well in this regard would set itself above a threshold that would/should elminate many PS's from consideration. Why buy a PS that cannot perform in this manner? Not knowing because the results and evidence aren't clearly published somewhere is just rolling the dice.

author: IanB
author_rank: Super Contributor
timestamp: August 17, 2013, 10:29:07 pm
content: Quote from: Electro Fan on August 17, 2013, 10:21:10 pmYou have it right. An important feature of a lab power supply—one reason it costs much more than just any old power supply—is that it protects and safeguards the device being tested as much as possible.

author: Stonent
author_rank: Super Contributor
timestamp: August 17, 2013, 11:07:03 pm
content: I wonder why Rigol didn't just put some relays on the outputs? Then when you power on, the relay is open and nothing at all goes through. Then when you turn your output on, there's a click and it goes live.

author: tom66
author_rank: Super Contributor
timestamp: August 17, 2013, 11:10:19 pm
content: Probably because it costs more, but IMO, that's essential for any power supply controlled by any means other than an analog regulation circuit: mistakes are made, and such a relay can easily be an emergency shut off if the control loop or OVP/OCP limits are exceeded. They can partially emulate this by using the transformer tap relays, but you'll still have a ~10,000uF capacitor to discharge. I suspect that's what they're probably using, but it's not good enough. If they didn't want to use a relay (~$1?), they could still have used a PMOS with a small negative supply and zener...

author: tom66
author_rank: Super Contributor
timestamp: August 17, 2013, 11:32:04 pm
content: Quote from: IanB on August 17, 2013, 07:37:31 pmEssentially, compensation tries to slow down the control loop, which shifts its phase and makes it more stable. Though excuse my misuse of terminology I think it makes it more integrative and creates a low frequency point (pole)?I've had compensation problems with a battery discharge jig. I ended up sticking a whopping great 1uF capacitor in the feedback loop, which gave me a slew rate of approx 0.5A/second but it was at least stable... It may not be the correct solution but it worked. I was driving some FETs with approx 5nF combined total gate capacitance, which was really messing with the control loop causing 100kHz+ oscillations.