[NOT-SOLVED] B&K Precision 8500 DC Load blowing IRFP250N  - Page 1
Forum section: electronics / repair

author: Cyclotron
author_rank: Unknown
timestamp: July 08, 2025, 05:25:33 pm
content: About a year ago I picked up this B&K Precision 8500 DC Load. Using it for tests when repairing some PSUs. It developed a hard short across the terminals and my diagnosis of that short discovered one of the MOSFETs (IRFP250N). I replaced this component and added additional top side heat sinks while I had the board out just for good measure. The unit worked with the few small tests I ran after the repair so I assumed it was fixed.I don't have schematics for this device but based on the components and layout, it appears there are 8 of these IRFP250N attached to the heatsinks that are switching the loads and sinking the heat out. There are 8 corresponding wirewound RX21 0.05 Ohm 8 Watt resistors down the center and I believe two TL084CN OpAmps driving the array.Yesterday I needed a high amp load test of 20 amps which is easily within the 30A rating of the instrument. I started gettin really strange results and smelled burning.Shut down the testing and took the load apart to find the resistor in line with the previously failed MOSFET had gotten REALLY hot. (see pics). The same MOSFET had failed with a dead short.These are proper MOSFETs ordered from DigiKey. So I don't really think the replacement part is fake.I replaced the IRFP250N again and I've taken the resistor out to repair the board beneath. All of the traces and components around the resistor are showing fine though there is now a blister in the bottom of the board. I've take another resistor from the opposite side out and these two indicate they are just about precisely 0.05 Ohm so while the resister got very hot it appears to have survived the ordeal. (I haven't found replacements or I'd just swap it out)Now. I'm perplexed. Without a schematic I'm digging in the dark but I suspect that the 8 MOSFETs/Resistors are used to sink the max amount of amperes but I'm not sure how it is logically done. Do they use them all all of the time or do they switch in more as the current rises?My suspicion is that part of the array isn't coming online to assist with the loads and this is putting excessive loads on the MOSFET That keeps failing. I'm starting this thread to document my journey through analyzing the issue and hopefully a repair.

author: Kwakerman
author_rank: Regular Contributor
timestamp: July 08, 2025, 05:40:38 pm
content: You may need to replace ALL the MOSFETs at the same time from the same supplier (i.e. a matching batch). Assuming they are all in parallel then it could be that minor differences in performance means that the 'new' one is taking more of the strain than the remaining originals. The same device from different manufacturers can ofter have subtle performance differences.

author: Cyclotron
author_rank: Unknown
timestamp: July 08, 2025, 05:51:51 pm
content: Quote from: Kwakerman on July 08, 2025, 05:40:38 pmI originally ordered enough MOSFETs to replace them all for that reason and then thought I was being pedantic. I'm going to go over it for some paths but before powering it up and running live analysis I will probably go ahead and swap them all as you said. Seems reasonable and the limited bit of tests I've done does indicate the new ones are slightly different than the originals.Thanks

author: Grandchuck
author_rank: Super Contributor
timestamp: July 08, 2025, 06:57:07 pm
content: https://hackaday.io/page/399508-mastering-mosfet-paralleling-a-deep-dive-into-current-sharing-amp-thermal-management-for-high-power-circuitsA nice read.

author: MarkF
author_rank: Super Contributor
timestamp: July 08, 2025, 07:38:53 pm
content: Quote from: Kwakerman on July 08, 2025, 05:40:38 pmThat's not the way eLoads are designed.Yes, there will be variations. But the MOSFETs are not blindly just paralleled.Which would lead to the failure mode you're referring to.Normally each MOSFET has its own sense resistor and control circuit.Therefore, each MOSFET may be pulling slightly different current.But one will not run away because of overheating.Refer to this schematic for a HP6060 load that uses 8 MOSFETs.

author: Cyclotron
author_rank: Unknown
timestamp: July 08, 2025, 08:48:57 pm
content: Quote from: MarkF on July 08, 2025, 07:38:53 pmThis makes sense. When I get time to get this on my bench area for live testing I'll see what each MOSFET is contributing. Unless something is wrong with the control sense/control of the one that keeps blowing up, I theorized that some peers aren't sharing the load.Of course, if there's other failure theories. I'm open to contemplate and test those ideas too.The circuitry in this 8500 does seem to line up with this HP in a lot of ways. I need to start making a sketch of the circuitry. That brings me to another question. Anyone have a circuit sketch suggestion for doing it on computer. I normally just draw it with pencil and paper but I'd like something on computer if its fairly easy to use and can generate nicer quality output than my hands can (chickens can).

author: Cyclotron
author_rank: Unknown
timestamp: July 08, 2025, 09:37:57 pm
content: Ok, I'm starting to sketch the circuits back from the MOSFET Gates to their control circuitry.Each of the 8 MOSFETs have a diode that sits between the gate and a common rail attaching to "Q6" (BR10 or BR01) which I assume is some sort of FET itself.The "10" on the chip is on its side so I'm not sure what that means. I'm going to try to identify it before I move on.Photo attached of the diode array. This appears to match the HP diagram so far. I need to see if that circuit splits before it reaches the diode too..Thanks

author: bastl_r
author_rank: Regular Contributor
timestamp: July 08, 2025, 10:57:22 pm
content: Quote from: MarkF on July 08, 2025, 07:38:53 pmThat's right.Each MOSFET has its own OP in the TL084, which is regulated using its own source resistance.

author: MarkF
author_rank: Super Contributor
timestamp: July 08, 2025, 11:40:50 pm
content: Quote from: Cyclotron on July 08, 2025, 08:48:57 pmLet me push the design discussion a little further.It's not about other pairs picking up the load of the pair that has failed. It doesn't happen.Each of the 8 MOSFETs provides 1/8 of the load.If you remove half of the MOSFETs, the remaining 4 do not pickup the extra current.You are solely left with only half of the current capability of a working unit.Here's each pair reduced to its minimum:The voltage applied to the (+) op-amp input will be maintained across the sense resistor (0.2 ohm in this case). The op-amp will apply a voltage to the MOSFET gate in order to match the sense resistor voltage to its (+) input. Thus setting the current the MOSFET will draw. What the other 7 MOSFETs are doing is totally independent. I would start by checking the op-amp that drives the MOSFET that keeps failing. Typically when the MOSFET fails, it ends up frying the op-amp too. If the MOSFET shorts, more than likely the current from the load will be funneled into the op-amps output.As for CAD software, I like DipTrace and found it very intuitive and quick to learn.Others on the forum will suggest KiCad. It's been years since I looked at it and I was never able to pick it up. It always seemed like a foreign language to me.

author: Cyclotron
author_rank: Unknown
timestamp: July 09, 2025, 12:17:44 am
content: I see some very interesting post here that I need to soak in.I've sketched 1/8 of the MOSFETs out and it is really looking to match that HP 6060 diagram provided by MarkF!I've attached a sketch I did inhttps://www.circuit-diagram.org/editor/@MarkF, I get what you are saying about the load capacity shrinking with loss of a MOSFET's sink.I'm trying to wrap my head around this though. If the instrument is set to say 20Amperes max and it measures less than that but there's a loss of capacity due to a MOSFET or two not participating, doesn't that mean the rest of the MOSFETs are sinking more than would be expected normally?Or is it more likely that the control over the failed MOSFET is losing control and sinking too much current causing it to fail?I'm trying to work out what the possible failure modes are to test. Maybe I've just been unlucky and lost two MOSFETs in the same place?I will check out DipTrace. I've messed with KiCAD and it may be something I need at some point but its too much for me for simply trying to document and test some repairs.Thank you very much for the suggestion and everything.

author: Cyclotron
author_rank: Unknown
timestamp: July 09, 2025, 12:52:08 am
content: Quote from: MarkF on July 08, 2025, 11:40:50 pm^^^ I didn't see this paragraph earlier as I took a phone call between reading and posting so some of my post above is answered here.This is exactly what I was wondering. I need to ponder things a bit more as I imagine the network of MOSFET sinks.The idea that the OP amp is suspect is perfectly reasonable. It may have be the original culprit. This is the reason I do this as a hobby as the mystery and problem solving is so dynamic. I have a bunch of work to finish and much to prepare before heading out on vacation later this week but I'm hoping to get this unit in an operational mode laid out in my lab so I can run a few tests. It would be awesome to find the root cause as I really won't trust it till I do.Thanks!!!

author: MarkF
author_rank: Super Contributor
timestamp: July 09, 2025, 11:27:37 am
content: Rearranging your sketch a little will make it a little more obivious.VSENSE= VSETILOAD= VSENSE/ 0.050The voltage across the sense resistor (0.050 ohm) is the sole control determining the MOSFET current.Also, the diode is most likely used to turn off the load current.Pulling down the MOSFET gate will turn it off and disable any control by the op-amp.Does the load have an 'output disable' switch? This is how it's implemented.My guess would be that the failure started with the MOSFET shorting and then taking out its opamp.It might have been from over heating or just a voltage surge from the power supply you were testing.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: July 09, 2025, 02:00:15 pm
content: Check the resistor's resistance under load. It measures 0.05 ohm at room temperature but what about at operating temperature?

author: JohnG
author_rank: Frequent Contributor
timestamp: July 09, 2025, 03:05:42 pm
content: Quote from: Grandchuck on July 08, 2025, 06:57:07 pmI disagree. Looks like another AI summary with a mix of good, bad advice, and inconsistent recommendations because the proper solution depends on the application. It is definitely not a deep dive.In the case of an electronic load, it's a low speed application using power FETs as linear devices. RDSon is unimportant to sharing in this mode because you are operating in transconductance mode. This is tricky because each device operates not far above threshold, and threshold voltage for MOSFETs usually has a negative tempco. This means that a small imbalance will grow and left to itself, the hottest device will run away thermally. In an electronic load, each FET will need its own negative feedback circuit, provided by the source resistor and op-amp as others have pointed out.For troubleshooting, all 50 mohm source resistors should have the same voltage withing a few mV if the current sharing is good and the resistors are the correct value. Also, in addition to the op-amp, check the solder joints and near the resistor to make sure they didn't get overheated, and if you can, check the temp on each resistor. They should be within a few degrees of each other, definitely within 10C and closer would be better in a good design.JohnJohn

author: Cyclotron
author_rank: Unknown
timestamp: July 09, 2025, 05:52:06 pm
content: I have the unit setup on my bench for testing but disassembled for access. The first test was just some basics but I may have found something already.I tested at 5v .5, 1, 2, 5, and 10 amperes so far.At 5v 10 Amps I measured the voltage drop across the 50 mOhm resistors and while they are a bit here and there, it is pretty obvious that each showing a voltage drop that puts them near each other. See image attached.What I noticed was that neither fan came on at any point. So, I need to check the theory and operation of the cooling fans.EDIT: I setup an 8amp load and let it run for a bit and both fans do come on so that's not a problem.Maybe the voltage drops indicate more than I thought? I need to do the math on them to see what they are dissipating.

author: Cyclotron
author_rank: Unknown
timestamp: July 09, 2025, 06:39:34 pm
content: Running continuous 10v 10amp loadThe front TL084 Outputs:Pin 1: 16.41 VDC (this is Gate for the MOSFET that has gone short twice)Pin 7: 16.68 VDCPin 8: 16.81 VDCPin 14: 16.80 VDCRear TL084:Pin 1: 16.75 VDCPin 7: 16.82 VDCPin 8: 16:74 VDCPin 14: 16.77 VDCI'm guessing the output on Pin 1 for the MOSFET that has failed is due to variances in the MOSFET that I used as a replacement in comparison to the originals. Eight years and operational time would account for such.Unless this TL084 goes bananas after a while, I'm not sure what might be the root cause. Heat isn't going to build up around the TL084 out on the bench like it would in the case. Maybe I'll run some heat on it to see if I can get it to act up.I've looked over the system with a thermal camera and it looks fairly symetric on all 8 MOSFET circuits.

author: Cyclotron
author_rank: Unknown
timestamp: July 09, 2025, 07:35:37 pm
content: Testing all MOSFET temps at the screw head I get between 98 and 108 degrees F with the majority of them being around 104.The hottest is the one that has failed at 108 but the one right behind it is very close to the same.All 50mOhm resistors are within 1 degree of each other in temps.I've been running at 10v 10amps for a about an hour or more. I'm going to bump it up to 20 amps for a bit.

author: Cyclotron
author_rank: Unknown
timestamp: July 09, 2025, 09:02:59 pm
content: I'm starting to suspect one of my PSUs. The previous time it blew this MOSFET I was repairing that very PSU; A TTI CPX400D.The test I was running when it blew this time required near 30amps and the TTI CPX400D I have in parallel mode is all that I have that can get that high in amps.So, the test I ran a couple days ago I used that CPX400D and blew another MOSFET. I decided I was going to run this DC load up to its max rating of 30amps so I pull that PSU down off the shelf and hooked it up in parallel mode. When I turn it it on it intermittently blinks "FAIL 4" but after sitting for a couple minutes (Warming up?) it quits the error and output can be turned on. In parallel mode I can only get 15amps out of this PSU. In fact that's all I can get out of the either left or right side of the CPX400D. Even in parallel it goes no higher than that.I am not seeing any strange AC on the output of that PSU but given it was involved in both events of a blown MOSFET I have to include it in my theories given I can't find a smoking gun as it were anywhere else so far.I may need to either find another PSU that I can use to full drive the DC Load to limits or troubleshoot/repair this CPX400D again and get it properly working.Thanks

author: JohnG
author_rank: Frequent Contributor
timestamp: July 09, 2025, 09:50:22 pm
content: Did the fans in the load ever turn on? At 100 W dissipation, they should definitely be on if it's hit thermal steady state. If it's not, then your measurements are probably changing with the temperature (especially the op amp output. Also, the fan is used to cool the main measurement shunts (the long thick wire with the hairpin turns), as this is probably the input to the main current control loop that commands the individual loops, and it needs to be precise and repeatable. Keeping it cooler makes temp compensation on this shunt less critical.Also, the screws are not a good measurement of junction temperature. They will be cooler and much closer to the temperature of the heatsink than the FET junction, and the discrepancy will grow with temperature. The FET with the thermal pad probably has about 4C/W to the heat sink, so at 100/8=12W, that's 48C junction to sink, with the sink at about 40C, for a total of about 90C, which is hot, but ok. At 20A, you have about 98C junction to case, and the sinks will also have double the power. pushing you pretty close to 130-150C on the junction, which is bordaerline and will hit the reliability. If there is any unbalance, an individual transistor could be running near the edge.Finally, if you are looking at shiny screw heads with an IR camera, you will not get a good temp reading because you will be picking up the cooler surroundings in the reflection off the screw. They need to be painted with a high emissivity paint to get a decent reading.John

author: Cyclotron
author_rank: Unknown
timestamp: July 10, 2025, 12:55:21 am
content: Quote from: JohnG on July 09, 2025, 09:50:22 pmGreat points all!The fans do turn on.. I updated/edited my post earlier. Probably should have made a new post for clarity.My temp measurements have been several and more to find out if there was balance in the system than anything. I did measure touched a couple of places along the package and I used the IR camera to look at the top of the black package itself for readings.I am going to find some better thermal pads. Suggestions are welcome. The cooling on my bench is not as engineered given the lack of the case to direct flow but I think this instrument is better than some I've worked on for flow in this state.Before I run a full load test on it, I will want to maybe install a sensor or coupling in a good place to monitor this MOSFET. Obviously the best place would also impact the cooling but since I have an outer sink on them I may loosen that and slip something underneath to sit on the package.I may play with the feedback resistors to lower this MOSFET's contributions slightly. And I need to either purchase a high enough output PSU or fix the one I have so it will let me run a full load test.

author: JohnG
author_rank: Frequent Contributor
timestamp: July 10, 2025, 02:03:03 pm
content: FWIW, a couple jobs ago I did some experiments on trying to estimate junction temp in the same TO-247 package without modification of the surrounding circuit, PCB, or heat sinke. The winning approach was to put the temp sensor (RTD in our case) on the face of the package above the die, opposite the leadframe. We could get within 5-10C of the junction temperature as long as the sensor was well attached to the case AND there was minimal airflow on this side. This works because if there is little cooling on the top side, there is little heat flow through the package and thus the temp drop is minimal.This would not work with the mini-heat sinks because they will cool the temp sensor faster than the heat can flow through the epoxy over the top of the die.Here are some other things you can look for. They are not common, but they do happen (I can personally vouch for it happening to me):-Leads of FET are pulling FET away from heat sink due to location of PCB holes being slightly off relative to heatsink.-Small burr or raised lip at screw hole in heat sink means FET is not flat against the sink.-Small particle or bump in thermal pad so that FET is not flat against the sink.No thermal pad will work well if the FET is not flat against it and the heat sink. You can get thicker pads that are more forgiving of things not being flat, but they are thicker and will thus have higher thermal resistance.I know I am harping on thermal management here, but at full power you are dissipating ~37 W per FET. That's a lot of heat to get out of the package and if things are unbalanced there is little margin for one FET to get hotter.All this assumes nothing is wrong with the circuit. You might want to pull out a scope and look for signs of misbehavior as load current increases.Hope this helps,John

author: Cyclotron
author_rank: Unknown
timestamp: July 12, 2025, 01:33:37 am
content: Quote from: JohnG on July 10, 2025, 02:03:03 pmYou are a wealth of knowledge! I didn't think about there being a "thread pull" where the torque of the screw lifts the material on the top thread. This would certainly create a bit of uneven thermal interface pressure.I ordered some alumina ceramic pads that are 1mm thick. I've been looking at thermal interface material and this material seems to be very good. I was surprised to learn that Kapton tape is also pretty good some testing I've read.When I get back from vacation, I'm going to pull the heat sinks out of the unit entirely and check that the interface surfaces are flat. This is obviously more important with alumina ceramic pads. Thinking as I type, I may do this with one side and then run tests to see how they differ left to right. The temps on the left and right front outer MOSFETs run near the same with the problem one being slightly warmer.As for oscilloscope, I have had that monitoring some but need to do some side by side from front to rear OpAmp for comparison. This again is a great idea; thanks!I have thought that I will likey replace the OpAmp anyway while its open given they are very inexpensive. This has also led me down the road of building an OpAmp tester but I see some interesting one already built out of China on Amazon that I'd like to play with too.And here we are at my primary reason for doing this as my hobby. I love the problem solving and hunt. The dynamics of thought experiments to consider tests that may review some undying truth. My earlier career years in IT as an SRE/Operations engineer had similar problem solving which I enjoyed. Now I spend most of my time trying to explain concepts to organizations and what might be considered consulting. The food and drinks are great but the interesting problems aren't there.Thank you again JohnG!

author: Cyclotron
author_rank: Unknown
timestamp: August 02, 2025, 02:37:11 am
content: Today I was able to get back to this project and replaced the OpAmps and replaced all the thermal interfaces with ceramic alumina pads and thermal paste.I am calling it successful as the thermals appear to be nearly even across the MOSFETs now. I kinda wish I had done one of these things at a time and tested.But, I'm going to lean towards issues with the OpAmps due to the prior blown MOSFETs.I'm going to mark this as solved for now. I've ran it at a 20amp load for an hour without issue and generally consistent thermal distribution. I have a 50 amp power supply inbound in a week or so and I'll us it to push this at its max load to see how it does.Thank you to all that participated in this little adventure.

author: Cyclotron
author_rank: Unknown
timestamp: September 11, 2025, 03:03:00 am
content: Ok, I did a load test on a supply today, trying to debug an error, and the 8500 popped the same MOSFET again.The Op-amps were replaced, and now, I'll replace all 8 of the MOSFETs with one new set to see if this fixes it. It's only at the top of its capacity that things go wrong.The MOSFETS in there are 30amp, I'm thinking of bumping that up one level and replacing them all to see if it makes a difference.Parts cannon, I know, but I'm not sure what else to do at this point.

author: gamalot
author_rank: Super Contributor
timestamp: September 11, 2025, 04:33:39 am
content: Q6 is an NPN bipolar transistor 2SC2412K:https://fscdn.rohm.com/en/products/databook/datasheet/discrete/transistor/bipolar/2sc4617e3-e.pdf

author: MarkF
author_rank: Super Contributor
timestamp: September 11, 2025, 05:37:16 am
content: Admittedly, I have not been following along very close.I built an Electronic Load with a single IRFP250 and a TLC272 op-amp with the MOSFET mounted directly (no insulator) onto the heatsink with a fan blowing directly on them. Mine is limited to 50W (20V @ 2.5A).The limiting factor is 'heat dissipation'. The MOSFET will exceed its max temperaturelongbefore reaching the max voltage or current. If the MOSFET that blows is close to the edge of the heatsink, it may not be getting the same cooling as the others. You might want to check that each MOSFET is pulling approx. the same current at say a power level in the middle of its max. (The easiest would be to measure the voltage across each sense resistor).The other thing to check is that none of the MOSFET are oscillating.A YouTuber built a load with a single IRFP250 and it blew-up at 5A (don't remember the voltage) because of a spike on the power supply he was testing. It took out ALL the electronics.Stating that they are 30A MOSFETs and that you ran the load at 20A is meaningless.You need to be looking a power drop across each MOSFET and calculate how much heat is being dissipated.

author: Cyclotron
author_rank: Unknown
timestamp: September 11, 2025, 07:45:17 pm
content: Quote from: MarkF on September 11, 2025, 05:37:16 amYou're absolutely right! I did the first couple of MOSFET swaps simply due to them being obviously dead. I believe I bought the replacement MOSFETs at Mouser or DigiKey so I don't think they'd be fake parts.My own ignorance/intuition suggest that a single MOSFET rated at 30a should be able to handle the loads I've used if it were properly cooled. I believe, what you are saying is that my intuition is near right.I have a Chroma 63004-150-60 now so I'm going to take the BK8500 down and try to learn something more than swapping the parts.

author: MarkF
author_rank: Super Contributor
timestamp: September 11, 2025, 08:27:31 pm
content: The IRFP250 ratings are more geared toward the MOSFET in its saturated region. They are not really stating limits when in their linear region (partially ON). MOSFETs are designed to be fully ON or fully OFF. So don't expect to achieve the maximums stated in the datasheet.When testing my simple load, I quickly would reach the maximum heat dissipation at 18V @ 2.5A.