EEVblog #861 - Rigol DP832 PSU FAIL & REPAIR - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 03:08:20 am
content: Dave repairs his Rigol DP832 Lab Power Supply.Why did it need repairing? That's the FAIL part Rigol might want to look into...

author: optoisolated
author_rank: Unknown
timestamp: March 17, 2016, 03:45:24 am
content: I've had my 832 shorted many times with relays, and no issues to date. Definitely be keeping an eye out for that.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: March 17, 2016, 03:53:24 am
content: Did you actually have 3A current limiting enabled?Not saying that this is the cause, however interrupting full load DC can cause a large voltage spike (see DC circuit breakers and fuses). I've seen kV spikes when using interrupters on 48VDC/50A power supplies. If this was the case with the relay contacts then it may have been enough of a spike to damage the mosfet. A varistor on the output rail can clamp this kind of spike.

author: Brumby
author_rank: Unknown
timestamp: March 17, 2016, 03:56:40 am
content: A classic - and simple - example of troubleshooting. I would classify this as recommended viewing for anyone starting down the track of equipment repair.

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 03:58:04 am
content: Quote from: DrGeoff on March 17, 2016, 03:53:24 amIt'salwaysenabled, you can't disable it.

author: c4757p
author_rank: Super Contributor
timestamp: March 17, 2016, 04:10:46 am
content: I'm dumbfounded at your annoyance by the hand-soldering around the pass transistor. How else are they to attach it to the heatsink?

author: optoisolated
author_rank: Unknown
timestamp: March 17, 2016, 04:12:48 am
content: The hand-soldering would be fine, if they cleaned up afterwards.

author: BravoV
author_rank: Super Contributor
timestamp: March 17, 2016, 04:15:17 am
content: Not an expert, but when everytimes I saw a typical switching power mosfet used in linear mode, it worries me on it's reliability, especially its not a special manufactured "linear" mosfet.We had a discussion while back about this, and even the mosfet's rating is heavily derated, still its somehow a guessing work.Love to hear any experienced members here to comment on this topic.

author: c4757p
author_rank: Super Contributor
timestamp: March 17, 2016, 04:15:49 am
content: optoisolated, nobody ever does that for a small number of hand-soldered parts in a low impedance area. It's a waste of time. The flux doesn't do anything except annoy Dave.BravoV, yeah, I worry about that too. I disagree with Dave's general assessment of "MOSFET = bad for linear", but if the SOA curves don't explicitly allow this operation, it should not be used that way. The datasheet for this one doesn't seem tohaveSOA curves, so... naughty Rigol.Edit:I stand corrected, it does. And they look fine. So sorry Rigol, not naughty forthat, but something else isn't quite what it should beI'd be interested in how this thing behaves as the contacts are closed and opened. I wonder how well they're doing to absorb the inductive kickback, and I also wonder how the current control loop responds - quickly shorting and opening is a good way to 'trick' the current loop in many PSUs into delivering more current than it's programmed to...

author: Smokey
author_rank: Super Contributor
timestamp: March 17, 2016, 04:25:55 am
content: Dave Says:"...I would not expect a lab power supply of this price and grade to blow a pass transistor..."Really? It's a $450 power supply from Rigol. That is pretty much the supply price and grade I would expect to randomly blow a pass transistor. The Agilent/Keysight listed below is 80W and cost 3.2 time as much. Now that is a supply of price and grade!Some name brand 3 output supplies at tequipment now:Rigol DP832 = $450 USDhttp://www.tequipment.net/RigolDP832.html?b=y&v=7906BK Precision 9129B = $695.00http://www.tequipment.net/BK/9129B/DC-Power-Supplies-Lab-Power-Supplies/Rohde & Schwarz HMC8043 = $1,300.00http://www.tequipment.net/Rohde-&-Schwarz/HMC8043/DC-Power-Supplies/Lab-Power-Supplies/?v=7421Keysight/Agilent E3631A 80W= $1,442.00http://www.tequipment.net/agilente3631a/?v=7421I'm not saying the DP832 isn't a good supply for the (small) money, but it's silly to pretend it's a high class instrument.

author: c4757p
author_rank: Super Contributor
timestamp: March 17, 2016, 04:28:48 am
content: Blowing a pass transistor because of an inductive/intermittent is faulty design, though. It's a general-purpose supply, it'smeantfor driving loads like that. I think it's perfectly reasonable to expect your device to benot faulty, whatever you happened to pay for it.

author: amspire
author_rank: Super Contributor
timestamp: March 17, 2016, 04:43:04 am
content: Quote from: BravoV on March 17, 2016, 04:15:17 amThe mosfet is rated for linear use - it has a DC Safe Operating Area curve and 38V/3A is within it. However, if you could get 100A peak for 1mS while waiting for the current limit circuit to start up, that could cause a cell in the mosfet to commence thermal runaway, and from that point, you can throw out the SOA curves. The MOSFET does have low channel resistance and low gate voltage, so it is certainly vulnerably to thermal runaway. Transistors tend to have self current limiting - they cannot put out any more current then the base current drive allows, and the higher the current, the lower the gain. This MOSFET is driven by voltage, and the device is probably capable of conducting 1000A under the wrong circumstance.A more likely issue is that you have to keep the gate-source voltage under +/- 20V under all circumstances. Dave was ripping the source down from 30V to 0V in nanoseconds, possibly. If the gate voltage does not follow the source voltage down immediately, it can blow. The addition of the diodes looks like they were trying to add extra protection - probably to the gate. The trouble is that diodes still have a turn-on time, and if the gate is going over-voltage for a few nano-seconds, that may be enough to micro-puncture the gate, and then it is only a matter of time before it fails completely.Bring back the 2N3055!

author: dreaquil
author_rank: Regular Contributor
timestamp: March 17, 2016, 05:03:28 am
content: I'm thinking it was probably caused by a latent ESD fault.. maybe it had one foot in the grave already and all it needed was a good amount of current to push it over the edge. Either way, either bad design or bad quality control.

author: BravoV
author_rank: Super Contributor
timestamp: March 17, 2016, 05:03:41 am
content: Quote from: c4757p on March 17, 2016, 04:15:49 amQuote from: amspire on March 17, 2016, 04:43:04 amExactly like that, as always, when it comes to this particular topic, the discussion was and will likely to drift out to the driver and control mechanism, loop control & behaviour, etc .. etc that ended up way too complex, and from what I've observed, cmiiw, that it self made it less and less appealing than BJT.Again, cmiiw.Quote from: amspire on March 17, 2016, 04:43:04 amOr even better, this reminds me back the General Purpose Power Supply Design of yours using "TONS"of high hFE 2N2907As. I have been patiently waiting for almost 3 years for you to finish it, at least the analog section.

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 05:11:37 am
content: Quote from: c4757p on March 17, 2016, 04:15:49 amI neither said nor implied such a thing. I said they can have some potential drawbacks, but in fact they had a lot of positive aspects for such use.

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 05:18:04 am
content: Quote from: Smokey on March 17, 2016, 04:25:55 amOk, I should have left out the word price.The simple fact is Rigol pitch it as a fairly high end lab grade precision power supply.http://beyondmeasure.rigoltech.com/acton/attachment/1579/f-01c1/1/-/-/-/-/DP800%20Datasheet.pdfQuoteIf it's possible to blow the pass transistor by shorting the output then it's no such thing. Most ebay no-name cheapies won't fail by doing this.

author: c4757p
author_rank: Super Contributor
timestamp: March 17, 2016, 05:19:31 am
content: Quote from: EEVblog on March 17, 2016, 05:18:04 amYes they will

author: Brumby
author_rank: Unknown
timestamp: March 17, 2016, 05:21:37 am
content: There wasn't any apparent inductive load that I saw to cause back EMF on the channel that failed, so I'm wondering how that is being brought into the discussion?The relay coil was energised from channel 2 - which didn't have a problem.

author: amspire
author_rank: Super Contributor
timestamp: March 17, 2016, 05:43:57 am
content: Quote from: Brumby on March 17, 2016, 05:21:37 amIt could be something similar, but capacitive instead of inductive.When the wires were being brushed together, if you could get the initial contact just long enough to drag the mosfet source down to zero, but not long enough to discharge the output capacitor via its ESR, then at the end of this initial contact, the source can jump back up to near 30V due to the output capacitor charge. If a protection circuit has just dragged the gate down to near zero, there may now be up to -30V applied to the gate after the source voltage rebound.So it could all come down to getting exactly the right initial contact time when brushing the wires together. It might be that a short of just the right amount of time blows up the supply. Shorter or longer and it is fine. Sometimes, power supplies deliberately use high ESR capacitors on the output for stability reasons.

author: bktemp
author_rank: Super Contributor
timestamp: March 17, 2016, 07:34:06 am
content: Quote from: amspire on March 17, 2016, 04:43:04 amThe datasheet looks too good to be true:My rule of thumb: Whenever you see a straight line in the DC SOA curve with no derating at higher voltages, you know it is wrong.Every transistor is more susceptible to second breakdown (or whatever it is called for MOSFETs) at higher voltages, therefore the SOA curve always has a steeper slope towards higher voltages.Here it shows a constant 300W limit right up to 150V.And the SOA curve is only valid for Tc=25°C. In any real power supply, it will be much warmer because it is dissipating power.And 300W in a TO220 package? You probably need liquid helium to make it work realiably.The datasheet says:QuoteHigh density means it is less suited for linear operation, because the die is small.It looks like the designer of the power supply simply trusted the specs in the datasheet, without thinking about if it is plausible.There is nothing wrong with MOSFETs for linear operation, but you need to understand the limits, and it looks like the Rigol engineers did not.Did Dave in one of his videos measure how fast this power supply goes into current limit? Then we could estimate the power dissipation in the pass transistor.If Dave has not done it, it would be a nice follow up video to measure the power dissipation (measures voltage across pass transistor and current using the same scope and multiply + integrate the dissipated energy and compare it to the datasheet ratings).

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 07:55:43 am
content: Quote from: c4757p on March 17, 2016, 05:19:31 amName one.I've shorted countless cheap linear supplies over the years, never killed one that I can recall.Only exception would be the Korad, but that wasn't a short with leads, it was oscillatory constant power mode on a load.I wouldn't say you can't of course, but IME it's not common.

author: Skimask
author_rank: Super Contributor
timestamp: March 17, 2016, 08:16:37 am
content: Where have I seen that damn fuse before?https://www.eevblog.com/forum/testgear/rigol-dp832-smoked-channel-1/msg580527/#msg580527Good info all around though for future reference when I blow up something else on it...

author: tggzzz
author_rank: Super Contributor
timestamp: March 17, 2016, 08:25:51 am
content: Quote from: amspire on March 17, 2016, 04:43:04 am100A through 1mS is 100kW, so I would have thought it would cause thermal suboptimalities!Or were you thinking of milliseconds?

author: hayatepilot
author_rank: Frequent Contributor
timestamp: March 17, 2016, 08:32:34 am
content: Great video!Is this now the end of the EEVblog repair curse?Let's hope for the best.

author: AndyC_772
author_rank: Super Contributor
timestamp: March 17, 2016, 08:35:56 am
content: Maybe there's a follow-up video to be had here?Hook up a scope to the MOSFET and measure Vgs and Vds.Then, apply various conditions at the output terminals, and see what the MOSFET is subjected to in each case. Maybe stick a differential probe across that current sense resistor too, so we can see the drain current and calculate the instantaneous power dissipation in the die.Then, see if there are, say, transient spikes on the gate which exceed its Vgs rating. See what the maximum Vds is, and what the power dissipation would be. Try and identify if there are load conditions which place undue stress on the MOSFET, and perhaps come up with a modification to protect it better.

author: BravoV
author_rank: Super Contributor
timestamp: March 17, 2016, 08:43:24 am
content: Quote from: AndyC_772 on March 17, 2016, 08:35:56 am+1

author: bktemp
author_rank: Super Contributor
timestamp: March 17, 2016, 08:49:07 am
content: The worst case is setting the power supply to max voltage and current, leaving the MOSFET heat up by shorting the output for some time, then disconnecting the output and shorting it again. When disconnecting the load, the power supply will change the transformer tap to the maximum voltage. When shorting the output again, all the caps are charged to the max voltage.The MOSFET has to dissipate a huge amount of power (at least >30V*3A= >90W, probably much more for a shorter time because the current regulation needs some time to respond). Because the MOSFET is already warm due to the previous load the silicon may reach a temperature where it fails.That is probably exactly what happend when Dave did the relay video.

author: McBryce
author_rank: Super Contributor
timestamp: March 17, 2016, 10:10:58 am
content: Dave,did you notice that Channel 1 was displaying "UR" as the mode instead of "CC" or "CV"at the start of the video? Could this have been a factor in the failure or was it a symptom of it having failed? I haven't seen this before and the DP832 Manual doesn't say much about it, just a one-liner.McBryce.

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 10:33:11 am
content: Quote from: McBryce on March 17, 2016, 10:10:58 amI didn't notice that.I presume that UR means Under Regulation?In that case I would expect that based on the fault. I'm surprised it would even display anything for that, that's a good thing though.

author: station240
author_rank: Unknown
timestamp: March 17, 2016, 10:35:59 am
content: Don't just turn it on, blow it apaaarrrrtt!By the way I've killed a 3055 series pass transistor in a PSU, the back emf protection was on a seperate board and the wire got disconnected.Thing it this was a stupidly high current PSU so it wasn't one 3055, it was nine!White smoke everywhere (heatsink compound), had to evacuate the shed.

author: Monittosan
author_rank: Regular Contributor
timestamp: March 17, 2016, 11:00:46 am
content: Quote from: EEVblog on March 17, 2016, 10:33:11 amUR Sounds like Agilent Unreg status in their power supplies. That mite have been why it's blown the 5amp fuse.Have you measured your incoming line voltage? Strange to be pooping 2 fuses so close to each other.

author: mikerj
author_rank: Super Contributor
timestamp: March 17, 2016, 11:35:42 am
content: Quote from: Smokey on March 17, 2016, 04:25:55 amYou likely have very different views to most people then. For $450 I would expect something engineered well enough to do it's job. If this was one of the $50 piles of junk being sold eBay then you just might have a point.Quote from: c4757p on March 17, 2016, 04:10:46 amYour dumfoundedness confuses meIt sounded (and looked) to me as though someone has previously been inside this and replaced the transistor before Dave got it, hence his suspicion.

author: Synthetase
author_rank: Regular Contributor
timestamp: March 17, 2016, 11:54:15 am
content: So what benefits do MOSFETs have over BJTs in this application?

author: nowlan
author_rank: Frequent Contributor
timestamp: March 17, 2016, 12:07:56 pm
content: Lower RdOn usually? compared to VCE.

author: matkar
author_rank: Regular Contributor
timestamp: March 17, 2016, 12:15:30 pm
content: Quote from: EEVblog on March 17, 2016, 07:55:43 amI repaired a 3055 based 20A chinese power supply last year. I had to change 6 transistors. Nothing else was wrong with it. It was destroyed when making repetitive shorts in a lab experiment.I wouldn't say MOSFETs are more prone to fail in power supplies. It all comes down to how good the protection is done.

author: mcinque
author_rank: Unknown
timestamp: March 17, 2016, 12:26:55 pm
content: Quote from: AndyC_772 on March 17, 2016, 08:35:56 am

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 12:30:58 pm
content: Quote from: matkar on March 17, 2016, 12:15:30 pmOf course it's about the design.But all things being equal, MOSFET's do have an inherent probability of being partially damaged by ESD perhaps leading to more field failures.

author: bktemp
author_rank: Super Contributor
timestamp: March 17, 2016, 12:43:41 pm
content: Quote from: EEVblog on March 17, 2016, 12:30:58 pmDo you have any reference for that?I have never heard it. The gate is sensitive to ESD damage, that is well known. But that should be no problem with a mosfet mounted on a board with a zener/TVS diode across gate-source.From my experience a MOSFET is much more tolerant to overvoltage conditions because of its build in diode across drain-source. The brekdown of those diodes is even specified in most datasheets (avalanche rating).

author: tszaboo
author_rank: Super Contributor
timestamp: March 17, 2016, 12:44:00 pm
content: Quote from: bktemp on March 17, 2016, 07:34:06 amIt is called forward bias safe operating area. It is a local thermal runaway effect, which is the plague of high voltage FETs. Almost none of them handle high temperature, high voltage and high current at the same time. For this 30V-ish region, I think it is safe to assume that this is less likely to happen.If you see a FET datasheet, where the DC ends with a line at above 100V, assume that DC= something like 100ms.I dont think this Rigol supply is rated correctly. 30V 3A is 90W dissipated in that FET. It is very on the limit. I did not allow more than 75W in a TO220 package, when it was put on an anodised water cooled heatsink. With a 220, you have best case 0.5K/W thermal resistance junction to case, plus case to sink. With a black anodised heatsink like this, with screw, that is like another 0.5K/W. So the FET is running 90+ degrees above heatsink temperature. Not a safe margin. I bet if Dave would short the output, put it on the sun, the supply would die on its own after a while.They should just upgrade it with a TO247. That has approximately twice the surface area to the heatsink, and 0.2K/W junction to case for the best devices.And dont get me started on the lack of output relay.So this is how many thermal issue in this PSU? Already 3? I guess chinese watts are smaller.

author: McBryce
author_rank: Super Contributor
timestamp: March 17, 2016, 12:47:08 pm
content: Quote from: EEVblog on March 17, 2016, 10:33:11 amAccording to the Rigol DP832 User Manual:"DP800 series power supply provides three output modes: constant voltage output (CV), constant current output (CC) and critical mode (UR). In CV mode, the output voltage equals the voltage setting value and the output current is determined by the load; in CC mode, the output current equals the current setting value and the output voltage is determined by the load; UR is the critical mode between CV and CC."Which doesn't say much. This is the only sentence in the Manual that mentions UR at all.McBryce.

author: Swemarv
author_rank: Contributor
timestamp: March 17, 2016, 12:48:07 pm
content: I would think its another example of poor thermal design.Lets do the math..Dave measured the unloaded voltage at the filter caps to be about 53V. This would correspond to about 37.5V RMS. With a shorted output , all this voltage will be over the transistor at 3Aso the total power dissipated in the transistor would be roughly 110W:Looking at the datasheet of the transistor we can see that the thermal resistance from junction to case is 0.5 C/W. A typical thermal resitance from case to heatsink is 0.5 C/W. We dont know the figures of the heatsink but I would be surprised if its below 0.5 C/W. Assuming 25C ambient, the junction would be at 25+(0.5+0.5+0.5)*110 = 190 C. This is 15 C above reccomended and it is therefore not surprising that it couldn't handle it.The situation for channel 2 is worse since it shares its heatsink with channel 3.

author: VK5RC
author_rank: Unknown
timestamp: March 17, 2016, 12:48:28 pm
content: Dave, how often/hours have you used this supply? Is it still in its 'running in ' period?I would be leaning toward a pre-insertion static damage, (?hand soldered).

author: bktemp
author_rank: Super Contributor
timestamp: March 17, 2016, 01:08:20 pm
content: Quote from: NANDBlog on March 17, 2016, 12:44:00 pmIt looks like the power supply switches transformer taps, so the MOSFET only has to dissipate the full >90W for a short time. But even 45W are on the high side for a TO220 package if you want a reliable design. As I said before: The static power dissipation is only one part. The MOSFET must also be able to handle short circuits after it has been heated up by the static load.When I did design my 50V 6A power supply with a SMPS preregulator I had so select quite a big MOSFET to make it short circuit proof for this case because of the high peak power (330W for a couple of 100ms + >1kWs for some 10us).

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 01:24:44 pm
content: Quote from: bktemp on March 17, 2016, 12:43:41 pmIt's an infant mortality thing. You can partially damage/weaken FET's with ESD and they still work, but lead to early failures in the field.In serious products and systems they burn-in parts to weed out infant mortality due to ESD and others factors.https://books.google.com.au/books?id=Vo0PFNielQkC&pg=PA41&lpg=PA41&dq=infant+mortality+due+to+esd&source=bl&ots=BOqYOv6xUm&sig=uhi7ODOmvnODPWBmyG_9Ig0VtJA&hl=en&sa=X&ved=0ahUKEwjc7s226MfLAhWiGqYKHVfkBRIQ6AEIPjAJ#v=onepage&q=infant%20mortality%20due%20to%20esd&f=false

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 01:31:44 pm
content: Quote from: VK5RC on March 17, 2016, 12:48:28 pmCouldn't say. A few hundred hours maybe.

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 01:35:58 pm
content: Quote from: McBryce on March 17, 2016, 12:47:08 pmInteresting. Surely it would only be in UR "mode" during the very short time it's transitioning CV <> CC. So basically just the response time of the regulator loop.I'm surprised they even detect and display this. Perhaps that shows they know it's loop response is pretty poor?

author: madires
author_rank: Super Contributor
timestamp: March 17, 2016, 01:37:48 pm
content: First the overheated voltage regulator, now a poorly choosen pass transistor? Andthisalso provides insight into Rigol's R&D capability. With a little bit more effort they really could make their products better.

author: amspire
author_rank: Super Contributor
timestamp: March 17, 2016, 01:48:03 pm
content: If the load was sourcing voltage back to the supply that was greater then the set voltage, then the supply would not be in CV mode and also not in the CC mode. I would guess they would use UR mode to indicate this state.

author: McBryce
author_rank: Super Contributor
timestamp: March 17, 2016, 02:01:40 pm
content: Quote from: EEVblog on March 17, 2016, 01:35:58 pmI just looked back at some of your videos (as I'm nowhere near my DP832 at the moment), but if you take a look at video #549 from around 11 minutes, where you were experimenting with the settings, the display momentarily displays UR several times.I can't think of any other situations where it might be displayed for longer?McBryce.Edit: What if you set it to CV 30V and limit the current a few mA. Does it show UR while the capacitor is charging?

author: bktemp
author_rank: Super Contributor
timestamp: March 17, 2016, 02:18:45 pm
content: Maybe it displays UR when it detects a repeated toggling between CC and CV mode. Instead of randomly displaying CC or CV it shows a stable UR?There are several ways to detect CV/CC mode. One way is to compare the control loop outputs and use the lowest one. Another is to look at the output of the control loop and whenever it is not at its limit (one of both loops typically requests full power while other loop actually regulates the output voltage) it can be considered as active. When it is near current or voltage limit both control loops may be active at the same time.Any idea why the power supply showed 0.7A with no load? Did there really flow 0.7A thru some discharge path or was it only the current sense circuit being out of common mode range?

author: Kevin.D
author_rank: Frequent Contributor
timestamp: March 17, 2016, 02:22:48 pm
content: As a couple of other posters already said I think the problem is simply bad thermal design.The designer must have thought 'ok I have a pass Nfetand a preregulator which limits the max voltage across my nfet to lets say 12V and I have a max I of 3A so my max pass fet power dissipation is only 12*3= 36W' . But he has not taken into account of the slow speed of any pre-regulation due to the large filter input capacitor. Which means that in cases where the output is going from max V out then shorted to 0V that the input to the Nfet will be at max 30V and only slowly ramp down to toward lower tap Voltages as the energy in the large large filter cap is used. If the output is continually shorted and opened at a high frequency then the mosfet basically has to dissipate maxtap V *3 for shorted periods (~ 120W) . So this means designer has made a very poor choice for a pass mosfet . A to220 case really? a Nfet optimized for switching ? . If I was you Dave I would upgrade that Nfet to somthing at least with a to264 or to3p case style .(choose something with a nice lowish Cgd and higer rdson (so in other words not a highly switching optimized Nfet )

author: McBryce
author_rank: Super Contributor
timestamp: March 17, 2016, 02:48:57 pm
content: Quote from: bktemp on March 17, 2016, 02:18:45 pmJudging by the temperature of the BJT when Dave viewed it through the Flir, I'd say the 0.7A were flowing through that.McBryce.

author: bktemp
author_rank: Super Contributor
timestamp: March 17, 2016, 03:05:34 pm
content: But the current shunt is at the output of the power supply, only connected to the output terminals. So it should not measure the current flowing inside the power supply.It looks like the BJT drives the MOSFET: There is probably a current source somewhere supplying the gate voltage. The BJT pulls the gate towards GND to regulate the output voltage.

author: McBryce
author_rank: Super Contributor
timestamp: March 17, 2016, 03:12:04 pm
content: Oh, so it is. Then I've no idea where they were going?McBryce.

author: wolf32d
author_rank: Contributor
timestamp: March 17, 2016, 05:01:02 pm
content: Nice! At my university they ordered a few of these for the mechanics lab (where freshmen do their first basic physics experiments). Luckily, the electronics lab technicians decided to stick to Agilent (*cough* Keysight) power supplies.

author: SeanB
author_rank: Super Contributor
timestamp: March 17, 2016, 05:18:40 pm
content: Best redesign would be to put 2 of those pass transistors on that heatsink, with 2 0R1 5W ballast resistors to share the load between them during transient conditions, along with some 18V zener diodes and 100R series resistors right by the 2 gate terminals. That at least minimises the chances of cooking the pass devices.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2016, 06:10:11 pm
content: As the unloaded voltage in the filter caps is in the 55 V range, the poor little MOSFET has to cope with 55 V at 3 A - so one rated to 3 A up to 37 V is just not good enough. So if you really want to keep this design the MOSFET should have a rating up to 60 V 3A in the SOA curve. Reducing the transformer voltage might ease things a little (the 240 V are on the high end for the 110/230 V transformers) - I don't think you need more than 50 V to get 30 V out - something like 40 V should be enough.There are two more power devices near the rectifier - so there might be a kind of preregulator that failed and thus giving the rather high voltage. I am afraid the problem is not yet reliably solved.Anyway I am surprised to see just one TO220 size power transistor and no preregulation - using 2 transformer taps and 2 power devices in series is not that difficult.A 5 A fuse at AC to get 3 A DC out with a simple rectifier is allready quite close to the edge (especially with UL rated fuses), the RMS current is expected to be in the 4.5 A to 5.5 A range (depending on the caps) for 3 A DC.

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 17, 2016, 06:46:23 pm
content: Murphy makes an appearance on the blog....LOL. Indeed Murphy does not discriminiate. When that happens Dave should know better than to continue to troubleshoot or test, unless he loves the smell of magic smoke. Dave, I hope you wore your seat belt on the way home.

author: Basp
author_rank: Newbie
timestamp: March 17, 2016, 06:57:45 pm
content: BTW the Chinese characters next to the output voltage connectors mean "black" and "red". Assembly instructions in other words.

author: armandas
author_rank: Frequent Contributor
timestamp: March 17, 2016, 08:33:32 pm
content: Did anyone else share my pain watching the video? Initial hypothesis of a failed pass transistor. About to test the pass transistor. Tests the current shunt resistor. Tests a bleeder resistor. Finds a blown fuse. The unit is still faulty. Gets out a thermal camera and finds a hot component that is NOT a pass transistor. Finally measures the pass transistor, the hypothesis was correct.

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 17, 2016, 08:58:17 pm
content: Give the guy a break! Its an adlib video, not scripted nor instructionally precise and edited for continuity and educational value. Its entertaiment for Nerds. And as far as many of us are concerned, and I have been doing electronics design for over 50 years, 40 of those years professionally....thats how it goes in reality. I loved the "what the ****? moments " I maybe new to the eevblog but no newbie to the field. This is reality TV for techies. Take it all in. Its better with popcorn and beer

author: flash2b
author_rank: Regular Contributor
timestamp: March 17, 2016, 08:59:39 pm
content: The CEP80N15 TO220 MOSFet has been replaced by the IRFP260 TO247 MOSFet in board revision 02.20 (dated 5 Nov 2013). Dave's spare board is probably 02.00 and the fixed board 02.10.Seehttps://www.eevblog.com/forum/testgear/rigol-dp832-firmware-updates-and-bug-list/msg518139/#msg518139for picture of the latest and current revision 02.20 of the topboard.The IRFP260 is more robust, so my guess is that Rigol already knew the problem and fixed that in 02.20. Since the date code of the 02.20 board is 5 Nov 2013, issue has been long solved. My (one year old) DP832A has a 02.20 board like on the pictures on the link above.

author: Smokey
author_rank: Super Contributor
timestamp: March 17, 2016, 09:19:18 pm
content: Quote from: armandas on March 17, 2016, 08:33:32 pmThat's "Info-tainment" for ya!

author: albert22
author_rank: Regular Contributor
timestamp: March 17, 2016, 09:31:36 pm
content: Quote from: Skimask on March 17, 2016, 08:16:37 amThat thread describes that the dp832 failed in the same way, when loaded with a reversed electrolytic capacitor. Evidently the sequence is the same: at the transient, somehow, the gate puts the pass transistor in full conduction, its rdsON and the shunt resistor are almost the only limits to all the energy stored in the filter caps. Destroying the mosfet and blowing the fuse in the time that it takes the reversed cap to explode.My guess is that the driver under some conditions does not switch to CC fast enough and the CV part is trying to get 30v into a short circuit.Many power supplies have an additional "hardwired" current limit protection, perhaps rigol didnt include that one.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: March 17, 2016, 09:51:56 pm
content: The DP832 might be ripe for a few quality hacks to improve it. Particularly in the area of protection.A simple reverse-biased diode across the output terminals was usually enough to protect against a reverse polarity injection taking out circuitry.I haven't checked the circuit topology, but it also sounds like a lack of Vgs protection as well on the FET, which was probably not considered relevant in a normal linear operating mode by the designers, but absolutely necessary when doing any switching into inductive loads (eg SMPS design).

author: HP-ILnerd
author_rank: Frequent Contributor
timestamp: March 17, 2016, 09:55:18 pm
content: Quote from: armandas on March 17, 2016, 08:33:32 pmI've learned far more watching Dave go down the rabbit-hole than when he gets it right first go.I also don't think he really did this time. Even had he changed out the pass transistor first, the fuse would still have been blown...I would call this one a very realistic (and successful) debugging session.

author: armandas
author_rank: Frequent Contributor
timestamp: March 17, 2016, 10:18:29 pm
content: Quote from: HP-ILnerd on March 17, 2016, 09:55:18 pmI'm not saying the other steps were not necessary or that the video was too long. I just think that if he did the transistor first, there may have been some additional "mystery" value in further debugging (i.e. "Hey, I replaced the MOSFET, but the PSU is still faulty").And, yeah, good to see EEVblog repair curse gone

author: TheSteve
author_rank: Unknown
timestamp: March 17, 2016, 10:27:44 pm
content: Quote from: armandas on March 17, 2016, 08:33:32 pmSomewhat yes. It felt more like a "Made for TV" special then a repair video. He waited to the very end to get to the part we all knew was broken all along.

author: Howardlong
author_rank: Super Contributor
timestamp: March 17, 2016, 10:37:10 pm
content: Oooo wait, I know this...For all those armchair quarterbacks out there, hello, McFly.... 100% identical fault here, how it was caused, and how I fixed it.https://www.eevblog.com/forum/testgear/rigol-dp832-smoked-channel-1/msg619848/#msg619848

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 11:16:31 pm
content: Quote from: Smokey on March 17, 2016, 09:19:18 pmAnd would have taken all of 5 minutes real time if I wasn't shooting video.Many people complain about this, but they (understandably) fail to understand the mindset of a video blogger when they are shooting a video. I'm always thinking of stuff to say and do, little hints and tibbits etc, and if I think of it them I shoot it. Sometimes that doesn't always make sense in a methodical troubleshooting scheme of things.You probably can't appreciate this unless you make videos like this.

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 11:18:36 pm
content: Some made a comment in another thread that I have an old revision board that hasn't been made since 5 Nov 2013. Revision 02.20 of the TopBoard uses a IRFP260N in TO247.So seems like Rigol knew about this problem and have potentially fixed it.In that case I think I might scrub the video I planned to do on measuring the transistor parameters.Or maybe not... I can always make a meal out of scraps

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 11:20:50 pm
content: Yep, comfirmed, I have an old board:https://www.eevblog.com/forum/testgear/rigol-dp832-firmware-updates-and-bug-list/msg518139/#msg518139New one has a much beefier FET

author: TheSteve
author_rank: Unknown
timestamp: March 17, 2016, 11:22:00 pm
content: Quote from: EEVblog on March 17, 2016, 11:18:36 pmMay as well make the video and see if the new FET they selected is really up to the task. Then find a nice way to shoe-horn the new FET into the older rev PCB.

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 11:40:59 pm
content: Quote from: TheSteve on March 17, 2016, 11:22:00 pmI notice those diodes seems to be magically gone again in the new version!

author: ion
author_rank: Regular Contributor
timestamp: March 17, 2016, 11:53:26 pm
content: Quote from: EEVblog on March 17, 2016, 11:40:59 pmMaybe they knew the FET was a weak point on the old board? If that's the case I wonder why they added the diodes rather than just go for the beefier FET in the first place.

author: EEVblog
author_rank: Unknown
timestamp: March 17, 2016, 11:59:01 pm
content: Quote from: ion on March 17, 2016, 11:53:26 pmThey knewsomethingwas up, hence the additional diodes. Then they probably discovered later that the reality is the 80N15 was too whimpy.

author: Synthetase
author_rank: Regular Contributor
timestamp: March 18, 2016, 12:17:45 am
content: Quote from: EEVblog on March 17, 2016, 11:59:01 pmAnyone looking at a TO-220 stuck to a heatsink of that size could have figured that out.

author: Co6aka
author_rank: Unknown
timestamp: March 18, 2016, 01:29:11 am
content: Any RF bypassing right at the gate of the MOSFET? (I haven't looked at the schematic.) The high impedance makes them great antennas, even in spite of the high capacitance. A few 100V nF-range SMT ceramics might be in order. The PCB traces to and from the MOSFET are essentially inductors, and essentially transmitting antennas when narrow pulses and/or sharp edges pass through them.If anyone wants to have a little fun, try probing around in powered-up linear circuits with the antenna of a handheld DMR radio while the radio is transmitting, even with low power. Haven't investigated it, but I've thought about putting a small power inductor on the end of a probe and driving it with a pulse generator through some coax, as a diagnostic tool to find all the cool stuff I can couple into and wreak havoc with.

author: Agent24
author_rank: Regular Contributor
timestamp: March 18, 2016, 03:07:14 am
content: Quote from: Synthetase on March 18, 2016, 12:17:45 amIt did seem a little odd to me as well.

author: crazyjim30
author_rank: Contributor
timestamp: March 18, 2016, 04:15:41 am
content: Dave,I showed the section of this video with the pass-transistor diagram in class today. It got rave reviews and cleared up some confusion during the Voltage Regulators/Power Supplies unit.Sent from my SM-G900V using Tapatalk

author: Skimask
author_rank: Super Contributor
timestamp: March 18, 2016, 04:16:11 am
content: Quote from: EEVblog on March 17, 2016, 11:59:01 pmYou think the addition of those diodes protected the 80N15 in my case?.....where I hooked up a battery backwards to Ch 1...at the expense of the current sense resistor.(Diodes present at 12:00)

author: amspire
author_rank: Super Contributor
timestamp: March 18, 2016, 04:29:07 am
content: Quote from: Agent24 on March 18, 2016, 03:07:14 amI disagree. This TO220 package is rated at 300W, and if you are dissipating 100W, there is only a 50 degC junction to case temperature - that is very manageable. The maximum operation junction temperature is 150 degC and to meet that at 100W dissipation, all you have to do is to keep the heatsink below 100 deg C. I am sure they manage that.There are two obvious concerns.First, even though it has a proper DC Safe Operating Area spec, it makes me very nervous. With a low gate voltage and a low channel resistance, the trenchFET cells will be very susceptible to thermal runaway in the linear mode. Vishay make the same device (SUP80N15), and their data sheet says it is optimized for PWM use. It is still designed mainly for switching. Here is the problem - even if the device can meet the SOA spec 100%, if there is a combination of events that can cause some cells to start conducting more then other cells, then thermal runaway can start anyway. It is possible to design MOSFETS that are immune to thermal runaway but the 80N15 is not anywhere near that class.When you look at the data sheet more closely, the device is rated for DC at up to about 6.5A at 40V - but at a 25 degC case temperature. There is no spec for SOA at a 90 degC case temperature but given the fact the cells are susceptible to thermal runaway, the SOA has to be degraded. How much can we expect it to degrade by? If we look at a MOSFET that is properly specified for linear use like the IXYS IXTK46N50L, it can do 7A at 100V (25degC). At 90degC case temperature, it drops down to 3A at 100V. The IXYS is designed with a much lower risk of thermal runaway - well over 20x the resistance in relation to the die size and a 50% higher gate threshold, so my guess is that realistically, the 80N15 drops to no more then 1A DC SOA at 90 degC case temperature. I cannot imaging the 80N15 actually being capable of 3A in linear mode at 90 deg C case temperature at about 38-40V.Secondly, they clearly added two extra protection diodes, and that indicates that they have been having problems. It is a big clue to issues they have been having. Sometimes when you start adding diodes to fix one problem, you can make new problems.Can you just change the MOSFET? The problem there is that it is very hard stabilizing the feedback loop of power supplies, and the power MOSFET characteristics are a major element in the feedback loop. You might fluke it, but in all probability, you will end up with a sometimes-unstable power supply.

author: EEVblog
author_rank: Unknown
timestamp: March 18, 2016, 04:46:27 am
content: Quote from: crazyjim30 on March 18, 2016, 04:15:41 am

author: BravoV
author_rank: Super Contributor
timestamp: March 18, 2016, 06:32:51 am
content: Quote from: amspire on March 18, 2016, 04:29:07 amRegarding this "neatly bodged" protection diodes that may create new problems, examples ?

author: amspire
author_rank: Super Contributor
timestamp: March 18, 2016, 07:52:52 am
content: An example...... Not diodes, but I hope it gives you the idea.I used to work in a company making inverters - I wasn't the one doing the inverter designed at the time. We were getting very regular IRFP460's blowing. There were numerous meetings and numerous fixes. One fix was "Only buy from one manufacturer - the equivalents from all the other manufacturers are crap!"Years later, I was involved in a project to redesign the product so that about 5 boards went onto a single board. I wanted to use a smaller heatsink, and I couldn't work out why the mosfets were running so hot. I was reviewing the design of the transformer coupled driver circuit and I discovered that it was faulty. A mosfet would initally turn off cleanly, but as the voltage on the drain started to increase from 0V to 400V, the drain-gate capacitance was turning the mosfet on again for about 30 nsecs. This was the problem that was wasting 10% of inverter efficiency and was blowing mosfets. The power engineers had been working on this inverter for years, and had blamed everything except their transformer coupled driver circuit. None of the fixes they had pushed through had ever actually fixed the real problem.The consequences were not just the stunning realization that we were blaming the lousy quality mosfets for years and that it was our fault all along, but it ruined my redesign. I had the new prototype board in my hands ready for testing and suddenly I had no mosfet driver circuit. There were no simple high side driver IC's at the time. It was really hard then designing a circuit that could tolerate 8 billion volts/sec slew rate and still work perfectly. I left pretty soon after that and I don't think the product redesign was ever completed.In general, fixes are often implemented without as much testing as the original device had. In the original design, the circuit often has hundreds or thousand of hours of testing. Sometimes fixes can get as little few hours of testing before approval, especially if it is an urgent fix. Sometimes a circuit is flawed and has no easy fix, but companies will try and add an easy fix anyway. A boss will stand up and say "Redesigning the board is totally out of the question. Just fix it!"Basically if a part fails, and I see a mod has been added to stop it failing, I would look into the original weaknesses, how good the fix was, and whether the fix is as bad as the original problem. The initial problem was caused because someone didn't fully understand the circuit, and there is no reason to assume anything had changed when they did the "fix". If the "fixed" part fails again, then the "fix" is probably not a proper fix.Richard

author: tszaboo
author_rank: Super Contributor
timestamp: March 18, 2016, 08:58:54 am
content: Quote from: EEVblog on March 17, 2016, 11:20:50 pmWell that pretty much summs that the original design was too small. I wonder, if with just a "back of the envelope" calculation of 3-4 forum member can come to this conclusion, what was Rigol doing? Its not like they replaced it with a different TO220 and become the issue, one of the main component was designed too small from the start.Seems like they did not have an analog designer, doing the power section.Quote from: amspire on March 18, 2016, 07:52:52 amI've seen so many cases, where there was a hardware issue, which they tried solving in firmware. Because that is "free" in management's mind. Like making complicated PID designs instead of changing a capacitor.And then the hardware patches. The original design was tested for numerous hours. Production already begin. Product is shipped, promising the new features in the future. Firmware development reached a point, where a new feature was used, which is not working well (failure of planning, it is called). And the patch is released, after testing one board for a few hours. Lets hope it doesn't brake anything else!

author: amspire
author_rank: Super Contributor
timestamp: March 18, 2016, 09:36:39 am
content: Quote from: NANDBlog on March 18, 2016, 08:58:54 amIf the chip inside the TO220 package was the right chip, then the TO220 package would have been fine.Now they are using the IRFP260 - they have gone from a device that was underspecified for DC use to one that is not specified at all for DC use. Brilliant!Who cares about data sheets anyway?

author: bktemp
author_rank: Super Contributor
timestamp: March 18, 2016, 10:13:33 am
content: Quote from: amspire on March 18, 2016, 09:36:39 amIRFP260 is an old design, somewhere from ther early 90th. They have a huge die compared to modern MOSFETs and are much better suited for linear operation. IRFP150 and IRFP250/260 are widely used in electronic loads.

author: madires
author_rank: Super Contributor
timestamp: March 18, 2016, 01:58:04 pm
content: Quote from: ion on March 17, 2016, 11:53:26 pmThey have tried the least expensive solution first. It's the same like for consumer electronics, the customer does the beta testing. It's sad that Rigol tries to save some cents/pennies at the wrong places. Everyone would be happy to pay a few bucks more for better components that matter, like pass transistors in PSUs.

author: ECEdesign
author_rank: Regular Contributor
timestamp: March 18, 2016, 06:50:42 pm
content: I was planning on purchasing a Rigol DP832 for my lab supply. Is it still a worthy purchase? I suppose I could get one from Agilent used on ebay but the functionality of the Rigol is pretty hard to beat for the price and size. The old HP supplies that provide the same power output specs are quite big devices and usually no front facing plugs.I could get like 3 of the Mobile DC source units or similar power supplies from HP that might work well. Still the programmable interfaces on the Rigol is hard to beat.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 18, 2016, 07:00:14 pm
content: Form the Vishay datasheet the SOA on this little TO220 MOSFET looks very good, not to say to good to believe it. If you just take the datasheets the IRFP260 may not be better.For the IRFP260 there are also different datasheets some specify a DC SOA curve.I hat a look at the old teardown video, that showed some triac drivers near the rectifier. So this would indicate that there is some kind of preregulation. One might want to check that if this is still present with the output turned on - the "only" 37 V with the failed MOSFETs somewhat seems to indicate there is some function.

author: jitter
author_rank: Frequent Contributor
timestamp: March 18, 2016, 08:43:45 pm
content: Quote from: bktemp on March 17, 2016, 02:18:45 pmPerhaps the DP832 compares the setting with the actual values for voltage (CV mode) or current (CC mode) and if neither are agreeing, then it assumes being out of regulation (which is also why it will display it during the transition between the two). My guess is that "UR" just means "unregulated" as Dave's DP832 shows UR all the time while the voltage is slowly ramping up on the defective CH1.Maybe a bit off-topic and maybe not, but my DC electronic load also shows "Unreg" if I take the load up too far and the DUT plummets below the setting for (e.g.) current. It does the same if I switch it on with nothing connected and I or V set to any value other than 0 as in the attached image. I guess it compares the setting to the actual value and displays "Unreg" when they don't agree (it was set to 5.000 A in the photo, but the flash reflection drowns the digit).

author: saturation
author_rank: Super Contributor
timestamp: March 18, 2016, 08:59:34 pm
content: From the home eevblog page search for "DP832 problem" as well as youtube videos not mirrored on eevblog to help inform your decision.Quote from: ECEdesign on March 18, 2016, 06:50:42 pm

author: McBryce
author_rank: Super Contributor
timestamp: March 18, 2016, 09:17:44 pm
content: I've used mine extensively and never had a problem, but I use it well within "sensible limits", powering PCBs between 2 and 12V and rarely above 2.5A. No short circuiting or other extreme situations. If you intend using it for stuff like this then you can't go wrong.McBryce.Gesendet von meinem Motorola DynaTEC 8000X mit Tapatalk 2.

author: jitter
author_rank: Frequent Contributor
timestamp: March 18, 2016, 09:51:28 pm
content: Quote from: c4757p on March 17, 2016, 04:10:46 amAgreed, finding hand soldered components in the DP832 is not strange at all.At work we often have to hand solder parts on the boards because it's impossble or impractical to solder them in the machines (reflow and/or wave soldering).Yes, that will leave flux residue on the boards, but we use the so called "no clean" solder. After soldering the flux residue becomes inert and can stay on the board without any adverse chemical or electrical effects.I can say this with quite some certainty as we do this even with metrology grade instruments that need very low leakage across pcbs. Residue of flux used in the wave soldering process usually can stay on except in critical applications. That thin liquid rework flux can definitely not stay on the boards, though. When that dries, there's a white, salt-like deposit that must be cleaned off.Quote from: optoisolated on March 17, 2016, 04:12:48 amMany fluxes from cored solder don't clean up all that well without the use of strong chemicals. We use no clean solders and very few customers ever complain that we leave the residue on, just like on the DP832. My guess is if Dave had taken a look at the other board as well, those pass transistors would have shown signs of hand soldering too.Having said that, we do have customers who require cleaning of boards as a matter of course, even though there's no technical reason! These customers typically sell their products all over the world and some of their customers tend to take a close look at the boards (esp. in Asia) and start asking questions when they see a repair or an abundance of flux residue, and given Dave's reaction at the sight of a hand soldered FET, I can understand that.

author: apis
author_rank: Super Contributor
timestamp: March 18, 2016, 10:36:54 pm
content: Quote from: hayatepilot on March 17, 2016, 08:32:34 am+1

author: tech5940
author_rank: Unknown
timestamp: March 19, 2016, 04:30:06 am
content: I took mine apart and I can confirm top board ver 2.20 has a IRFP260NPBF pass transistor. However the bottom board Version 2.20 appears to have much thinner gauge wire for Ch2 wiring compared to Ch1 and a IRF5210PBF pass transistor. I thought it was interesting the top and bottom board use a different transistor for the 30v 3a Channel. I couldn't quite see the others transistors attached to the bottom board without complete disassembly so I could stand to be corrected but I believe the output of the IRF5210 was connected directly to the ch2 output. I'm not sure how the ch3 5v is connected yet as it was hard to see the other transistor part numbers. Anyway aside from the first DP832 that I had delivered that completely lost CH1 out of the box (it would deliver 30+ volts no matter set voltage) I've had no problems with the replacement they sent, and have been quite happy with it.Sent from my iPad using Tapatalk

author: liviuo
author_rank: Newbie
timestamp: March 20, 2016, 02:43:49 pm
content: Hi Dave,Can you do a video with the channel 2 loading the relay contacts and channel 1 powering the coil , of course hook a scope on the channel 2 output for the rise time when the contact open and another probe on a current sens device for the spike.This could show a design problem of supply or a weak/damaged FET in channel 1.Thank You.

author: exe
author_rank: Unknown
timestamp: March 20, 2016, 03:02:37 pm
content: @c4757p, @amspire, guys, I think the problem with all that mosfet SOA ratings is that they assume the mosfet is _fully open_. AFAIK, a big mosfet transistor is just many small mosfets sharing the same bulk. They may well have different Vgs(th) and in linear region it happens that some of the conduct while others don't. For this reason it is recommended to keep switching mosfets either fully open or fully closed and minimize transient time (this is mentioned in some datasheets).Anyway, would like to hear your opinions. This bothers me a lot because I don't like hard-switching because it is noisy. But this is a big gray area most of the time 99% not even discussed in datasheets.

author: Smokey
author_rank: Super Contributor
timestamp: March 20, 2016, 05:31:25 pm
content: Quote from: exe on March 20, 2016, 03:02:37 pmI think you're missing the point of a LINEAR regulation power supply.

author: exe
author_rank: Unknown
timestamp: March 20, 2016, 06:56:33 pm
content: Quote from: Smokey on March 20, 2016, 05:31:25 pmWhy do you think so? I was talking about mosfets in linear region, that is still valid even for switching applications:http://www.infineon.com/dgdl/Infineon+-+Application+Note+-+PowerMOSFETs+-+OptiMOS%E2%84%A2+-+Linear+Mode+Operation+and+SOA+Power+MOSFETs.pdf?fileId=db3a30433e30e4bf013e3646e9381200

author: Kleinstein
author_rank: Super Contributor
timestamp: March 20, 2016, 08:27:01 pm
content: A full SOA curve has curves for different pulse times and also DC operation. The pulse times assume a low duty cycle and fully open MOSFET in between. However for a linear power supply the DC SOA curve is the relevant - so the MOSFET is for a long time at the specific conditions.With SOA curves for MOSFETs there are a few problems:- many MOSFETs don't specify a DC curve, though there will be curve somewhere way down.- the SOA curve is not production tested for normal types - so they are more like expected by designSOA testing is expensive and even if done only checking a small part, e.g. a few points for pulsed operation- there is chance that not all samples meet there SOA specs, reliable part would need individual SOA testing.One parameter that enters is the degree of local inhomogeneity - a parameter prone to scattering- the SOA measurements may be limited to a few points. So even if there is a curve it may not be reliable. Especially if the thermal instability range is not shown in the curves, chances are this effect is not checked for and curves are just extrapolated.- The SOA curves shown are usually for something like 20 C case, so derating for higher temperature is needed.- for TO220 and similar chips non homogeneous contact to the heat sink might degrade the SOA curve.- many mosfets are made by several sources, the SOA curve may no be considered an important parameter and may vary.

author: fish
author_rank: Newbie
timestamp: March 20, 2016, 09:30:17 pm
content: I have followed Dave's trials and tribulations with this supply with some interest and concern as I only bought mine on his recommendation!That said I am extremely pleased with it. It offers me SO much more than the cheaper power supplys I have had in the past and the current limit has save some of my components form untimely deaths. And all the bells and whistles are great for understanding what's going on, even just noting the effect different bits of software have on the current draw from a pi etc.However like Dave I have had some issues with mine, (https://www.eevblog.com/forum/repair/dp832-lose-screws/msg786388/#msg786388) thank fully not to the extent of his, as I would not have managed to work out what was up. But all the same there is clearly some QC issues on the DP832 line.As for, is it cheap? Well it is very reasonably price for the performance but if you are going for hobisist, self employed, start ups and small companies then they really cant afford for things to just go pop when they cost that much! If Rigol get a bad name for this then people might just make do with less equipment which would be a big shame for them and for Rigol.Once again a great video, Thanks Dave.

author: exe
author_rank: Unknown
timestamp: March 20, 2016, 09:53:26 pm
content: Quote from: fish on March 20, 2016, 09:30:17 pmActually, has anyone measured its load response? A few years ago I was quite surprised to see 1000uF output caps (and even with them it has spikes on turn on, sometimes negative spikes, are they good for caps?). Now I realised out they used a MOSFET pass element with gate capacitance of almost 10nF... Never seen anything like that before. So I'm curious how good it actually is (I would expect it to be rather slow in response). And what about stability with capacitive load...

author: fish
author_rank: Newbie
timestamp: March 20, 2016, 10:59:17 pm
content: Quote from: exe on March 20, 2016, 09:53:26 pmI think the point I was trying to make is that for the price I don't expect supper duper performance BUT I do worry about the failure rate.Also I do find the feature set very use full.I was thinking of writing this up as a more formal question but I will note it hear first.NewFile1 shows the start up (pressing the channel button not the main switch) with no load, I have tryed it with a tiny bit of load and it was similar, (the transience of my circuit is similar so its annoying.)NewFile2 shows flicking a in line switch. (I have plots of some nasty bouncing but they are over well before the few milliseconds transience of what I was interested in)Sooo, is this pretty standard for a power supply? Is this standard for this power supply (is mine broken) ?I had a bit of a look when I got it and really struggled to get anything nearly this good anywhere else, even some poor one channel thing was well over a third of the price but maybe I didn't look hard enough. So to be honest just using a switch every now and again is really not a issue for me when I get so much more than I would other wise. That said would like to take Daves advice and build one my self, but it still will not come close to the DP832.Is there a better strategy than a switch? I was thinking about using a Micro controller and a transistor or something as that wouldn't bounce but if it did anything funky I wouldn't have the experience to tell so I think I will stick with my switch and a cap.*** I also tested the supply with the probes on the terminals and got the same answer.

author: DuPe
author_rank: Regular Contributor
timestamp: March 22, 2016, 12:17:01 pm
content: Quote from: exe on March 20, 2016, 09:53:26 pmIf you let me know a little in detail what exactly you are interested in, I may post some measurement results withthis PSU DP832 using electronic load (Maynuo M9811)CheersPeter

author: exe
author_rank: Unknown
timestamp: March 22, 2016, 01:05:02 pm
content: Quote from: DuPe on March 22, 2016, 12:17:01 pmHi Peter,I'm no specialist in benchmarking PSUs, but I think it would be great to see:1) set 12V and 3mA current limit and connect LED. We'll see if it survives. If so, set to 30V and repeat the experiment2) 12V 200mA -- 1A change in load3) 12V 200mA -- 3A change in load4) 3.3V 10mA-1A change in load5) 12V No load => 1A load.For every measure it would be great to see it in both directions. E.g., in case 4, first step load from 10mA to 1A and then back to 10mA again. I.e., 10mA => 1A, then 1A => 10mA. I think your load is very smart and can do this with ease.If your ever decide to make such test I'd like to thank you with a small paypal donation.

author: DuPe
author_rank: Regular Contributor
timestamp: March 22, 2016, 01:22:17 pm
content: No problem. I am getting so much information from this blog, so I am happy to contribute a little.Only condition: An answer will take some days till I am back in may lab. I am currently travelling abroad.CheersPeter

author: DuPe
author_rank: Regular Contributor
timestamp: March 23, 2016, 08:12:16 pm
content: Quote from: exe on March 22, 2016, 01:05:02 pmHi,here are the first results. Edit: Values are correctPower supply settings were 29V/3A, channel 1. I see nothing to be worried about.Current change was 100mA to 2A and vice versa (see file name)More to follow as soon as there is more time.CheersPeterPS: I hope the attachements do work

author: exe
author_rank: Unknown
timestamp: March 24, 2016, 03:11:55 pm
content: Quote from: DuPe on March 23, 2016, 08:12:16 pmThank you very much! Just one question, should the values be divided by 10, or multiplied by 10? If divided then it's a very-very good result. If multiplied that would be... uhm, not great at all. So, 1/10 probe means attenuation by the factor of ten, or 10x amplification?PS I wrote you a personal message about the stuff I promised.

author: DuPe
author_rank: Regular Contributor
timestamp: March 24, 2016, 03:21:31 pm
content: Quote from: exe on March 24, 2016, 03:11:55 pmEdit: Voltage readings are correct (probe ratio was set on scope)What I am interested also is what will happen if you set to 30V @ some mA and then short the output.In other words: Ho much energy is stored in the output capacitor and would a sensible device like a LED survive that.I'll post the result as soon as available. I am also planning to compare to my Keithley PSU.

author: exe
author_rank: Unknown
timestamp: March 25, 2016, 10:46:56 am
content: Quote from: DuPe on March 24, 2016, 03:21:31 pmHo-ho-ho, Keithley should "destroy" that rigol hands down. Actually, the results you obtained is already enough to conclude DP832 is a "snail" PSU.Here is, just for comparision, step response of TI's lm1085.pdf 0-3A with 100mA preload: (see first attachment).And, one of the fastest MOSFET drivers on the market, lt1575, step response 0.2-5A: (see another attachment). It was designed for Pentium 2 CPUs, that's why it is fast like hell. I can't imagine what modern CPU power supplies can do... They should respond to change of tens of A in some hundreds of nanosecond. But they all are very low-voltage, while lt1575 works up to 18V or so.PS all pictures from datasheets, so they should be taken with some grain of salt.

author: DuPe
author_rank: Regular Contributor
timestamp: March 25, 2016, 10:50:52 am
content: Quote from: DuPe on March 24, 2016, 03:21:31 pm

author: DuPe
author_rank: Regular Contributor
timestamp: March 25, 2016, 11:09:27 am
content: Quote from: exe on March 25, 2016, 10:46:56 amHi exeThe load response especially of a universal/Lab PSU depends by far more on the design of the voltage control loop than on particular chips or transistor.The design has to take care on regulator loop stability and static load regulation as well (the latter value is well suited for outnumbering in data sheets).I assume the Rigol performs bette due to a bigger output capacitor. To verfiy takes a little bit more effort.I think the weaknesses of the Rigol are somewhere else than load regulation: E.g. they do not recommend to use active loads. I.e: You should not try tocharge an accumulator with the DP832 (which is a mess)PS: Just took a quick and dirty (I know, I know) test mesuring the capacitance between outputs: Rigol 500..800µF, Keithley 130..150µF (... depending on polarity of meter)

author: exe
author_rank: Unknown
timestamp: March 25, 2016, 11:47:26 am
content: Quote from: DuPe on March 25, 2016, 11:09:27 amYeah, but those chips are complete regulators... Only that lt1575 requires an external pass element and a simple compensation, other than that they are ready to go. So I think it's a fair comparison (though they do not provide CC mode out of the box).May be I'm missing something, just I expected much better load regulation.... I'll try to benchmark my DIY PSU based on lt3080 with 4.7u output capacitor as soon as I get access to a decent scope... May be bench PSUs are intentionally slowed down to be more robust for variety of loads. But there are models on the market with ~1MHz loop bandwidth.

author: DuPe
author_rank: Regular Contributor
timestamp: March 25, 2016, 12:38:53 pm
content: Quote from: DuPe on March 25, 2016, 11:09:27 am

author: DuPe
author_rank: Regular Contributor
timestamp: March 25, 2016, 12:57:47 pm
content: I made a bad beginner mistake: I have not noticed, that on the scope ratio was correctly set and values are corrected for probe divider: All voltage vallues in the oszillogamms are 1:1 correct.I am going to edit my post for correctness.So sorryPS: Done. Information now should be ok

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2016, 02:47:50 pm
content: The step response is not that bad. It's about what you expect with such a large capacitor at the output. Even other wise fast supplies will get slow with 500-1000µF at the output. Many supplies are rather slow, as to make the layout / cables less critical. For a fast regulator you have a hard time having the power devices at the back and output at the front.Fast regulators like the LT1575 also need a suitable layout to get fast. Also the compensation for highest speed will not be that tolerant to higher capacitive or other problematic loads. So it's ok for a CPU, but not a lab supply. Its also a question if you really need a supply with lower output impedance than a foot of 20mm² cable.An active load circuit could also be a a very difficult load and might make the supply / load combination oscillate. Ideally it should not happen, but both sides could be guilty - there can be unstable active loads as well. Especially a constant power more realized in software can very easy be unstable - it's more like what you expect. So if a supply with a active load in constant power mode is oscillating, I would blame the load first.Supply circuits that use tap switching or preregulation with SCR might not like very fast changing or oscillating loads. That a way to get magic smoke out, as not properly working preregulation might overload the main output stage.

author: exe
author_rank: Unknown
timestamp: October 01, 2017, 09:19:50 pm
content: Quote from: Kleinstein on March 25, 2016, 02:47:50 pmI just conducted a few simple experiments with a step load of my LT3080 PSU. Basically, the observation is thatthe distance from output/smoothing capacitor dominates the response. So, placing a good cap (large electrolytic with short leads or a 10uF SMD mlcc cap) just right at the load changes results dramatically. Placing any capacitance on long leads does not make any sense. I can post some pics from my oscilloscope if anyone cares.The load itself is just two metal-oxide resistors in parallel (of 33 and ~3.9Ohm) and a mosfet (PSMN9R5-100XS) switching the smaller resistor. It was driven from a signal generator. At 3.3V this gives approximately 0.1A => 1.0 step response.Interestingly, putting polyester/MLCC caps at the point of load creates dumping/decaying/fading (which word is better?) oscillation. But this is explainable, mixing different caps and long wires leads to resonance problems (the output cap in PSU is 10uF MLCC).Conclusion: measuring step response makes only sense if wires are as short as possible.