LDMOS, gate blown in under a week of (constant) use :-( - Page 1
Forum section: electronics / rf-microwave

author: Fflint
author_rank: Frequent Contributor
timestamp: April 07, 2025, 06:49:52 pm
content: I've built this kit amp with ldmos mrf186 for 70cm band (435mhz). Sadly after not even 2 weeks of use the gate shorted to the source (30 ohms). I have no idea why because the device on the input is incapable of driving over 2W, output was relatively well matched and the PSU would switch off on resonance (I assume).Starting from the beginning. After some time tweaking the matching networks I got it to work pretty well in testing achieving 75w from under 3w of drive @24V and 6.5A. At barely under 800mA quiescent current. The ldmos was new, and gate voltages were nicely matched.I calculated absolute max input at 50ohms to be 4.5W so I wrote on it "max 4W" and I paired it with a preamp that at absolute peak delivers 2W. (it would make almost 45W with this drive). Also a stabiliser lab psu current limited and set to switch off above 6A (not just limit and continue, switch off). The output was connected to a relay that flipped between dummy load and a well matched antenna. Considering the device can survive 10:1 swr at full power brief hot switching at much reduced power shouldn't break it.It was left powered 24/7. First week it worked great and I used it a lot. Then I didn't have much time to play with it so it remained powered on but it was used few min per day. The weather got cooler and I noticed my quiescent current dropping to 770mA from 800 when first on and 860mA during operation. I still used it fine.Today I noticed no power. I went to see what's going on and saw the PSU still on but consumption only 8mA. I checked voltages and I found 30ohms to ground on top gate :-(And now I wonder. What on earth could've caused it? Picture included. There was no funny smell nor any signs of overheating. That fan/radiator couldve easily dissipated 150W if it needed.I don't mind swapping a part once a year but after under two weeks is way too short. If not for a fact this preamp is incapable of driving over 2w maybe 3W at absolute peak I'd say it looks overdriven, but how?Even more importantly, how do I prevent this?Also another question , these parts are all obsolete. Are the "new" Aliexpress parts really new or do they send them for clever relating or something ? I'm trying to decide if Id better buy "new" or used. (no my "proper" supplier stopped selling them long ago)

author: Fflint
author_rank: Frequent Contributor
timestamp: April 07, 2025, 08:13:35 pm
content: Is it reasonable to assume if it had anything to do with SWR, connectors coming loose etc it would be the drain region that suffered not the gate oxide?The fact the gate to source resistance dropped to 30 ohms suggests or is the oxide breakdown. So hopefully I can discount the possibility of extremely bad swr occurring at some poin.Thinking how such damage might occur:1 - let's say temperature increase caused the quiescent current to increase which caused it to shoot way too high. That would result in overheating. There are no signs of overheating.2 - overvoltage. Maybe the little 1W amplifier got into resonance and suddenly output 4.5W? Unlikely but not impossible. Static electricity could be something to consider but the gates are powered by resistive voltage dividers. That would prevent any static buildup.So I'm thinking about making some sort of stack of pin diodes that would short rf if it got too powerful. The problem is that it would have to clip at 14V. So the stack of roughly 14 diodes. Now try making that not have too much inductance at 430mhz. If anyone has ideas let me know.

author: pienari
author_rank: Regular Contributor
timestamp: April 08, 2025, 04:52:27 am
content: I have read from several sources this is common failure in ldmos amps.When tx is switched there have to be some spikes what is too high and will damage ldmos.I have build Ampleon ART1k6 fet amp and i used -3db att in input without any issues.

author: polcad
author_rank: Contributor
timestamp: April 08, 2025, 08:12:25 am
content: Quote from: Fflint on April 07, 2025, 06:49:52 pmThat is not entirely true. They do not tell you that but this is tested with pulsed signal at the input and not CW. No device would withstand 10:1 SWR at full power.Anyway, coming abck to your problem. These transistors are normally very robust. Even ESD discharge can not brake them easily. You will have a hard time debugging it as I do not see any obvious reason for the failure. It is possible that switching the output creates some transient condition that is beyond the transitors's specs. The second thing, I would put a ferrite bead on the power supply leads as close to the pcb as possible. Maybe the output switching causes some voltage spikes that create outp of spec Vgs , Vgd condition. Also add larger cap on power supply.Also, the devices included in those chinese kits are very often questionable quality. Just buy the transistor from a trusted source.See also this topic:https://radionecks.co.uk/viewtopic.php?t=1769BTW. I am looking for an RF amplifier design. A good design. With all the supporting circuits like bias, protection, etc. designed at the top level (not just a bare transistor with a crude bias like the amp in this topic). It has to work with 0dBm drive and deliver at least a few watts of output power. Target frequency is 30-500MHz but I can make the changes to the origala design to get the desired bandwidth. Does anyone know of such a design? ()source pcb files are highly desired, though not necessary)

author: Fflint
author_rank: Frequent Contributor
timestamp: April 08, 2025, 11:10:29 am
content: There are no "trusted sources" for mrf186 ... It's an obsolete part. That's why we get it so cheaply.The psu is a lab power supply by korad, a chinese company but one considered a good one. I'm not convinced it could have anything to do with the output side.There is a 5V zener in the bias circuit. I have no idea what would have to happen for there to appear 20v there. But the ferrite is a good idea. I have a large 1.5KW RF amplifier nearby. It was powered but not "enabled" (not biased) so I doubt it could be any stray rf, but I'll protect it with some ferrite anyway.Quote from: pienari on April 08, 2025, 04:52:27 amBut this was on all the time. RF was sent or not, but the power was on for these 2 weeks non stop.But let's say there were some rf spikes when the TX is on.I already had 13.5dB attenuation between the sdr and the preamp... Now I also added a limiter after the attenuators to not exceed 0dBm.My topology was is:SDR -> a 10dB attenuator and a 3dB att.-> two stage preamp (mmic 20dB + 1W mos up to 14dB) -> the mrf186 amp.Signal levels are:SDR:(as measured by me)Absolute maximum +10.5dBmThen 13.5dB of attenuation so it becomes up to -3dBHere I added my 0dBm limiter.Then we have the preamp that boosts this to 30~32dBm (1W ~ 1.5W).Then my mrf186 amp.I don't have any power to spare between the preamp and the main amp to put an attenuator there now. I've added a 250w 50 ohm resistor on the input but it's leg is lifted so I can connect it if I get a bigger preamp. But then, does this extra attenuation make any difference (other than improving input match) if I use a bigger preamp?Then there is also a mater of there not being clarity about how much rf power mrf186 can withstand. We have max gate to source voltage of +-20V which with a bias of 4.5V gives us 15V max. It one would need over 20W at low impedances these devices have at 70cm.For example they give input impedance at 920mhz as 2.5 + j6.9, let's use that for calculation. You'd need 15W to reach 15V peak. But we're at 435mhz. I haven't got this measured so let's assume half for now. 7w. That shpukd be plenty of margin. But the device died. Also I suspect counterfeit transistor (bought new from China) but I'm not sure how to check for that.

author: Fflint
author_rank: Frequent Contributor
timestamp: April 10, 2025, 08:46:54 am
content: My "counterfeit transistor" suspicions were unfounded. I cracked the ceramic and had a look at the dies. It looks exactly as it should based on other mrf186 pictures online.I wonder if a bad input match could result in some high voltages inducing... I've had swr of around 3 before (it is not trivial to measure swr as large signal response is very different than tiny signal).Anyway after adding an extra trimmer I've managed to get the swr down to pretty much 1 in the middle of the range and 2.2 on the edges.I've also replaced the driving preamp with a unit that can deliver 4x the power(4W). It seems to work fine, but for how long?I also lowered the cold bias (it is not temperature controlled) so it reaches the correct bias when at temperature.I've ordered 2 more mrf186 transistors from China.... Well see how it goes.

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2025, 09:18:49 am
content: i mean it could also be highly used or previously overloaded or something.That is something i was interested in before, to determine what kind of characterization could determine what a transistors integrity is.

author: Fflint
author_rank: Frequent Contributor
timestamp: April 10, 2025, 02:26:57 pm
content: Quote from: coppercone2 on April 10, 2025, 09:18:49 amFunny thing the one that blew was "new". No signs of soldering whatsoever. Gate thresholds within a milivolt of each other. I thought it really looked new.I'm not sure how can they make a used part appear like that.The one that I now fitted is a used one, sold as used, with solder and so on.I have another one of these "new" ones. Instead of using it I ordered two more of "used" variety.I'm not sure which option is better.The used one I have fitted now has gate thresholds differ by 0.1V but if it lasts a year instead of a week that is still better.

author: CaptDon
author_rank: Super Contributor
timestamp: April 10, 2025, 09:20:56 pm
content: I ran across an article looking at LDMOS specs that warned about uncontrolled impedances on the output like switching under load or momentary open load conditions can easily overvoltage the gate/source junction. I didn't save the link however.

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2025, 10:36:29 pm
content: Quote from: Fflint on April 10, 2025, 02:26:57 pmyou underestimate the effort counterfitters go through.The parts are cleaned from excess solder in a solder pot, the leads are then chemically stripped down (correct mixture of certain acids) to their base material (and then possibly chemically polished using a 'bright dip" if the stripping treatment is not good enough (a different mixture of oxidizing acid), the leads are reformed in a precision ground die if bent, and then the coating (nickel, tin, possibly gold for high end operations) is reapplied with a electroless deposition mixture (or possibly electroplated on for high ends parts, I only ever saw it done with electroless.)They have some other process for re-doing the text too, you can only pick it up if they are lazy/cheap with their refinishing process...This made me seriously concerned for reusing parts because in the video they were throwing them into chemical baths (submersion) and just assuming the hermetic nature of the seal was intact, meaning that various chemicals can get into the part through cracks. Regardless of how well you wash em, I imagine these parts might be considered seriously contaminated in the most stringent applications.They literally do all the same steps you would do in the factory to prepare the stampings for die attachment prior to the plastic seal, just with the finished part instead of the unbonded and un-encapsulated stampings.One method to check for this is to verify the coating thickness of the tin on the parts. If the stampings on the real part happen to be electroplated, the coating will be much thicker then the one you can get with a electroless deposition solution (i.e. liquid tin). But that assumes they do a full strip, and it also assumes they are not doing a electroless metal coat on parts fresh out of the solder pot, and he can still get a thick coating if he dips the parts in a hot pot of pure tin and then does bright dip to try to make it look 'new' or less like it was dipped in a solder pot for the final coating.It might depend on how motivated and well equipped the counterfitter is. Some bullshit artist might get found easily but if someone is doing high volume on the same package they might really develop some masterful counterfitting techniques in a setting that basically has commercial quality with ISO procedures and shit. Issac newton got these kinds of guys hung by the crown !https://blog.sciencemuseum.org.uk/isaac-newton-and-the-royal-mint/also look at the edges of the leads, for stampings, you might see excess wear on the corners (rounded), and you can measure the thickness of the leads in a micrometer carefully to compare them, I doubt they will be that good for cheap parts. But for expensive parts, I even heard of die-swap operations to really get it perfect looking from the outside, but I assume inside you might see signs that the part was re-wire bonded.... unless they found some way to touch up the bonding pads..

author: Fflint
author_rank: Frequent Contributor
timestamp: April 11, 2025, 03:17:01 pm
content: Wow, yes, it does make you think...I'd so much prefer to buy these parts from Mouser (my currently favourite supplier) or even overpriced Farnell, but they simply do not carry these parts. It is as if these ldmos devices had a lifetime of a couple of years tops. Insane.I could understand if these older devices had horrible flaws or very bad efficiency. Nope, their only fault was that they were probably over produced and became too cheap.Still for a hobbyist they would be a very good deal if we could buy them.Ive had a look for "modern" equivalents. They are 10x the price of.As for Chinese parts. I start to think it is better to buy parts sold as used. For two reasons. One: if both are used you may want less pressing and chemical etching and do on, not more.Two, if you solder the part in and then it fails immediately you can still return it.if this was a new looking part good luck with that

author: joeqsmith
author_rank: Super Contributor
timestamp: April 12, 2025, 01:13:05 pm
content: Spreader?

author: Fflint
author_rank: Frequent Contributor
timestamp: April 15, 2025, 05:58:11 am
content: Quote from: joeqsmith on April 12, 2025, 01:13:05 pmWhat spreader?

author: cgroen
author_rank: Unknown
timestamp: April 15, 2025, 07:52:19 am
content: Whenever I do PA's, I ALWAYS use a copper heat spreader, never mount devices directly to the alu heatsink. Was the heatsink milled completely flat before screwing down the LDMOS ?Couple of pictures and links to how I have done it (23 and 70cm PA)70cm:https://www.moonbounce.dk/hamradio/70cm-500w-amplifier.html23cm (not yet fully finished):https://www.moonbounce.dk/hamradio/23cm-600w-power-amplifier.html

author: G0HZU
author_rank: Super Contributor
timestamp: April 15, 2025, 04:33:02 pm
content: To be fair, the MRF186 has a fairly huge package/footprint with the heat spread between two devices inside the package so I'm not sure a copper spreader is mandatory here. Over the years I've used copper spreaders and also direct to aluminium (sometimes via a foil gasket).One thing to watch out for with flanged LDMOS parts is instability. This can be due to poor RF grounding due to issues with the PCB ground to heatsink connection (at RF) and also with the RF grounding of the flanged device. Also it's worth it to evaluate the low frequency gain of your PA design in case it has huge gain at LF which can lead to squegging.Instability can kill an LDMOS part in a few RF cycles.The old evaluation board below is for a high power GaN part and this device has a much smaller footprint than the MRF186 and yet it connects direct to the aluminium heatsink. I would prefer a copper spreader but GaN parts are quite rugged.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 15, 2025, 05:43:17 pm
content: Quote from: Fflint on April 07, 2025, 06:49:52 pmLooked like you were using laminar vs turbulent flow and with no spreader, little effective surface area being used. Even 75 continuous seemed like a stretch, let alone claimed 150.Did you measure and calculate junction temps?

author: G0HZU
author_rank: Super Contributor
timestamp: April 15, 2025, 08:19:11 pm
content: Maybe I'm missing something here but it looks like the part failed when driven to only 45W output.The footprint for the MRF186 is huge and so it should be fine to interface it without using a copper spreader even at higher power outputs as long as the heatsink and the rest of the thermal management is good. However, it is still easy to goof things up especially if the various thermal interfaces aren't mated well or if something works loose across numerous thermal cycles.The fan and heatsink don't look great but I doubt this is the weak link if it's run at 45W output. But then I don't know how effective the fan arrangement is and I don't know what the heatsink looks like underneath.LDMOS parts can be fragile, especially if they go unstable or if they are fed into a poor load. Also the integrity of all the various grounds needs to be good (over time) to prevent instability. I know I said it already, but check for excessive gain issues right down to very low frequencies. Instability due to squegging can kill LDMOS parts.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 15, 2025, 10:19:49 pm
content: Positioned so there is basically no convection cooling. Maybe a 1/3 of the sink has some sort of laminar flow. Doesn't look like a lot of surface area (fins a not close). Who knows how much air flow... Then we have alumimum with basically a vertical path to a point source. Sure there's some mass there and it will take some time to build heat. Again, what's the junction temp calculate out to?***Not to mention looks like they are preheating the air as the fan is doing double duty.

author: G0HZU
author_rank: Super Contributor
timestamp: April 16, 2025, 12:30:59 am
content: The lack of convection cooling is irrelevant with that setup. As long as the fan can blow lots of air through the heatsink this is a fairly conventional way to cool a PA module. The fins look a bit feeble but I’d expect the heatsink to be OK with the settings that were being used. The worst thing about using that method is the noise from the fan as it blows air through the heatsink. At work there are various test fixtures like that although there’s more metal in the heatsink fins and the fan(s) blow hard. Often this technique can make enough noise to dominate a large room.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 16, 2025, 03:28:17 am
content: Quote from: G0HZU on April 16, 2025, 12:30:59 amThere is no air flow through much of the sink as shown and with no convection cooling, may as well get out the hack saw as it may absorb some heat but it sure isn't going to remove it. Normally, I am running the numbers. Do I need a spreader, how much surface area, how much air flow..... Not much to discuss as its basic EE 101 stuff.Quote from: G0HZU on April 16, 2025, 12:30:59 amI have no idea if it is OK or not, which is why I asked them to show me the data. You are certainly free to guess but most of us don't. We run the numbers, and verify the results.Quote from: G0HZU on April 16, 2025, 12:30:59 amMaybe it is even overkill. I can't say. When they start showing me the details we can go over it. Until then, it's just a guessing game.

author: Fflint
author_rank: Frequent Contributor
timestamp: April 16, 2025, 07:06:29 am
content: Quote from: cgroen on April 15, 2025, 07:52:19 amAh,that spreader :-) I didn’t think it's necessary for a 100W device that really runs up to 45W in this application. I do have it for my big HF 1500W amp (still in pieces - I've got an automatic vacuum tube amp for HF with same power, but I need something that doesn't generate so much waste heat on standby for summer).Quote from: G0HZU on April 15, 2025, 04:33:02 pmWhat is the mechanism if this failure? I mean how does it happen, which part of the device has its limitations exceeded? Do we know? Is it basically overvoltage of the gates?Quote from: joeqsmith on April 15, 2025, 05:43:17 pmI haven't. It may be an interesting exercise.The only time it ever got to 75W was in initial testing. In actual use it would not see over 45W.I have a thermal camera and the device (on the ceramic outside case) got to 50C after approximately 2s at 75W before I added the fan.At ~40-45W it would get to same temperature at 10s maybe. After I added the fan it would never exceed 49C even when I kept it at 45W for 30s.Quote from: joeqsmith on April 16, 2025, 03:28:17 amI find the idea it overheated pretty remote. There is no need to calculate if you can put your finger on the part and keep it running well below the threshold of pain (55c in most humans). Entire radiator would get warm before the part got hot in my testing at higher power without the fan. That's why I added the fan. Also there is a thermal paste and surfaces are flat as much as your typical radiator removed from a working device (I have not milled or surface ground them - I have the capability, but I thought it a waste of time. I considered the cooling an overkill as it is).Additionally I check all my hobby projects with a pretty good thermal camera (512x256 50fps - the fps is what makes all the difference as you can scan it across a surface and get a lot more apparent resolution).If it did fail due to overheating it would have to do so in.a way not to affect surrounding parts (it would have to be very quick not to discolor the board, not to leave a "funny smell" over it). So the only mechanism of such failure I can imagine is if it got into self resonance and started going at full blast. But then if it went really to the top the PSU would cut off on the overcurrent. It was set either to 6 or 8 amps I don't remember at the moment.So it woukd have to sit below the cutoff (144W thermal - likely 75W rf or if at 8A 192W thermal 96W rf). Would this setup survive 144W of thermal heating for long? I think it would last at least few minutes(consider air temperatures were around 8C in this unheated shack). At 196W it would probably overheat the junction eventually. That my "feel"for these things talking not calculations. I'd be interested what detail is needed to calculate it. The dimensions ? The surface flatness? The area of the fan open and the air speed? (I do have the air speed meter). Anything else?Then we also have to think, what breaks when the device overheats. I read somewhere the drain region not gate oxide, but who knows.I'll appreciate all informed answers and corrections!

author: joeqsmith
author_rank: Super Contributor
timestamp: April 16, 2025, 05:26:26 pm
content: QuoteSeems good enough.As far as books on heatsink designs, many of the old databooks from Motorola and other will go over it. Maybe look for their old linear regulator databook. The old heat sink extrusion companies also provided a good reference. Those should be free and on-line. I have a hard text book on SMPS designs that covers it.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 17, 2025, 01:08:33 pm
content: With the wooden bench, did you follow basic ESD practices when assembling and working on it?

author: G0HZU
author_rank: Super Contributor
timestamp: April 19, 2025, 12:38:48 pm
content: QuoteThanks. Visually, there's nothing obviously wrong with that setup if the aim is to cool a 45W LDMOS PA that might be 40% efficient. It's a physically huge dual LDMOS part only being run at 45W Pout on a big heatsink that is being cooled with a fan. Sure one could add a copper spreader and a bigger heatsink and add more fans but it wouldn't be necessary at 45W Pout.Sadly, it's always going to confuse some people if someone mounts a PA heatsink upside-down because they are going to start claiming stuff about 'no convection cooling' and 'vertical paths to point sources' and 'laminar vs turbulent' airflow. It's all irrelevant in this case. I think some people just want to sound experienced

author: G0HZU
author_rank: Super Contributor
timestamp: April 19, 2025, 01:09:31 pm
content: QuoteIf you only require 45W Pout then maybe consider running it at 20V. There can be an increased risk of instability when running power FETs at lower drain voltages but you would get better efficiency at the lower drain voltage. There would be slightly more distortion at the lower operating voltage but maybe you can manage OK with this.The closer you run the part to its datasheet limits the less margin you have for operation into a poor load. The Tj looks to be 200degC and the J/case is 0.8degC/W. So things will quickly become much more critical if you were to run it a lot harder into a poor load. Normally, these parts are designed to be run up towards 100W and it then becomes very important to manage both the thermal aspects of the design and whatever is connected at the input and output and how much drive level it is given. Also, if it is then fed into a really badly mismatched load or it goes unstable, especially at a low frequency then no amount of thermal management (in terms of spreader, heatsink, fans) may save it because it can fail due to an internal breakdown due to excess RF voltage.I've designed a couple of wideband LDMOS PAs a long time ago, one was bulletproof because I used a 120W push pull part in a 15W PA and I ran it at a low drain voltage. The other one tried for 80-100W from a 175W part. This was a bit more problematic in terms of reliability. I ended up using a VDMOS part from Semelab. I think I used a D1009UK but this was many years ago.

author: G0HZU
author_rank: Super Contributor
timestamp: April 19, 2025, 04:54:18 pm
content: Hi Fflint, it's hard to tell from your image but there doesn't appear to be an RC network near the input of each LDMOS gate. This is a bit unusual as this is normally included as a measure against low frequency instability.I can explain what can happen in terms of input and output loading vs instability (if it isn't there) if that helps? It's generally quite risky to not have it there with a big common source FET amplifier. Especially if the input and output loading (vs frequency) can be changed by the end user.

author: Fflint
author_rank: Frequent Contributor
timestamp: April 20, 2025, 07:36:28 am
content: Quote from: joeqsmith on April 17, 2025, 01:08:33 pmDoes periodically touching the earthing lug of the nearby electric socket count? That's what I usually do. I used to have a strap for the wrist, but it broke. I probably should buy a new one.However, this amp was running continuously and it just stopped at some point. It wasn't touched etc. So I wonder, is it possible for esd damage to be hidden like that? I don't know.BTW. Since that transistor failed and I've fitted a used good one in its place the same amp has been running fine. (I also retuned the input and I added a slightly more powerful driver - I still don't expect over 45~50W but the driver can deliver that longer for now) But it hasn't been long yet.Quote from: G0HZU on April 19, 2025, 01:09:31 pmYes, that is a pretty good idea. I'll try that.Quote from: G0HZU on April 19, 2025, 04:54:18 pmHere is the schematic (another person drew it from the pcb).There is this 50 ohm resistor and a couple caps. The resistor seems to be doing double duty here. One, for dc it is a path for the bias to get to the gate. And second it appears to be an input termination resistor (not sure of the right term)I mentioned a lot earlier in this thread I know from vacuum tube amplifiers.Due to various extra parasitics this on its own would have no chance of presenting a 50 ohm resistive load on rf, but after tuning it is reasonably close in the passband (if I remember correctly I got it to 47ohms+0.3j etc). This is after the new transistor was fitted. When I had the old obe the match was substantially worse.I also learned to look for resonances with a vna by sweeping input/output if an amp and looking for big narrow spikes in return loss. Does this apply to ldmos amps? If so there used to be a spike around 330mhz but only on the input side.

author: G0HZU
author_rank: Super Contributor
timestamp: April 20, 2025, 08:20:21 am
content: First of all don’t go near it with a VNA unless you know how to do it without risk of damaging the VNA. it will be really easy to fry the VNA.Thanks for posting the schematic. The expected RC network is missing which seems very unusual. I’m using an iPad at the moment so can’t post much data etc.

author: G0HZU
author_rank: Super Contributor
timestamp: April 20, 2025, 12:15:12 pm
content: I suppose the other thing to mention is bias drift. I can see the zener diode in your circuit but how stable is the trimmer pot over time and temperature? Have you used a newish part that doesn't have a noisy/unreliable wiper? It's unlikely but maybe the bias drifted in some way whilst it was left unattended.I think the onset of low frequency RF instability is unlikely if you just left it in the same state all the time as it would have gone unstable sooner. However, your circuit breaks the conventional rules for preserving low frequency stability because the RC snubber is missing from each gate input.Common source FET amplifiers (and common emitter BJT amplifiers) can be expected to produce negative resistance at the input if the output termination is inductive. This output inductance can come from a bias choke or from the output load impedance. This happens because of the feedback capacitance inside the FET between the drain and the gate. In the case of the MRF186, there is less feedback capacitance than usual for an LDMOS part this big but it will still produce negative resistance at the input if there is a few tens of nH shunt inductance at the output.The datasheet provides the gm for the MRF186 along with the internal capacitances so it's easy to construct a low frequency small signal model that will show that it can generate negative resistance. If a typical RC snubber is added near the gate the negative resistance at low frequencies is cancelled. This is how things are done on many, many FET power amplifiers to combat negative resistance at low frequency. I don't know why it is missing from your circuit. Maybe it saves on cost?

author: joeqsmith
author_rank: Super Contributor
timestamp: April 20, 2025, 02:51:28 pm
content: Quote from: Fflint on April 20, 2025, 07:36:28 amI have been told delayed failures are possible with ESD damage. I have never researched it.

author: Fflint
author_rank: Frequent Contributor
timestamp: April 21, 2025, 07:37:04 am
content: Quote from: G0HZU on April 20, 2025, 08:20:21 amThanks for the warning, but how am I supposed to tune it without the vna? The "old fashioned way"? :-)Coming back to being serious I've already fried my vna once long ago when working on a big vacuum tube amp (I had a dc charge in one of the caps I forgot to discharge). I got the parts from China and I've fixed it(a small mmic rf switch had to be replaced). So I do have spares still. But yes, not frying it is much preferred.Also the VNA is very useful for finding unexpected resonance which I'm told can cause instability as well.But one has to pay attention to maximum power and DC. I almost always use a dc block when using my spectrum analysers or VNAs. Then there are attenuators. I have one very nice attenuators that is 20dB and up to 1GHz at 200w. It is usually this one plus extra 20 or 30dB on the output before I connect it hot to a vna.Quote from: G0HZU on April 20, 2025, 08:20:21 amInteresting. I've searched for other similar amps published on ham radio sites like this one for HF by w6pql herehttps://w6pql.com/hf/2023mrf1k50n.htmor one by him for the 70cm (435Mhz) bandhttps://w6pql.com/1kw_70cm_ldmos_amplifier.htmI'm not seeing any additional subbing in either. As far as I know all these amplifiers use a variation if the schematic and pcb layout given in the original datasheet of one of first ldmos devices. But that circuit was meant for measuring parameters and such I guess.Still the ham radio community took it and ran with it.This is a very important point, because while mrf186 can be had for few bucks I'm also building a Mrf1k50 amp and these parts cost hundreds of $ each.I don't design these amps from scratch BTW. I usually buy a pcb and populate it myself. This is what I did for the 70cm amp as an "exercise" before the big 1kW hf amp. On this occasion I got the board already assembled. I just need to add the ldmos device and tune it (yes I have two... I'd like one to remain a spare). :-)The only low frequency snubbing I read about in context of these circuits is one guy with his 2m (142MHz) amplifier mentioned he installed an inductor between the gates to "keep low frequency gain under control".I'm not saying that snubber is not needed. I'm just wondering why no one added them...Quote from: G0HZU on April 20, 2025, 12:15:12 pmThe bias drifts but for different reasons. The pot is indeed new (only used parts are certain expensive caps and the ldmos). But ldmos devices even at same exact voltage open up more as temperature increases. This necessitated using lower bias than ideal to start "from cold". Many amps use temperature compensation in a way of measuring the heatsink right next to the transistor and lowering the voltage as temperature increases). This is a simple circuit and it doesn't have it.If one set the bias to target when cold I do think the part would runaway thermally sooner or later.QuoteI too have no idea after more research. But as mentioned it has been missing in all variations of this design including by people with some experience like w6pql. He is no professional from what I heard. But if his amps started popping ghe would post about it I think.I have to check the board I got from dx world (this is the hf amp I'm building). I know it has temperature condensation, but I haven't noticed any additional low frequency snubbing so far.

author: G0HZU
author_rank: Super Contributor
timestamp: April 21, 2025, 01:06:18 pm
content: QuoteThere is no 'one size fits all' answer to this but a lot depends on the final application and also whether the designer is aware of this issue. It isn't mandatory to fit a snubber, but if nothing is done to cancel the negative resistance at the gate (which can appear when there is an inductive output load) then there is always the risk of instability.However, many users will use the amp under the same repeatable conditions and so the input and output match requirements for instability will never be presented to the amplifier. In other words, it doesn't matter in this case if the amplifier isn't unconditionally stable.I can try and dig out an old LDMOS amp at work tomorrow and demonstrate the negative resistance at the input when an inductive load is presented at the output. I think I can find one that uses the old PolyFET LB401 part. PolyFET have released s parameter data for this part but only down to 100 MHz. They also quote the internal capacitances and the transconductance. In the plot below I've simulated the LB401 and looked for negative resistance when there is a 25nH shunt load at the output. I've also added some resistance in parallel with the 25nH.You can see that the low frequency VCCS model based on the datasheet gm and capacitances agrees with PolyFET's measured data of the real device. They both begin to fall into negative resistance below about 120 MHz. Once you have negative resistance you just need to add a resonating inductance at the input to make up a complete low frequency oscillator. It's over 20 years since I last looked at the LB401 but I can remember modelling it and using it in a wideband PA design at work.To cancel the negative resistance there are several techniques. One is to use resistive (negative) feedback from drain to gate via a blocking capacitor. This flattens the gain at lower frequencies and can cancel negative resistance at the input. It's also possible to add an RC snubber or maybe even just a small series resistance at the gate to act as an instability 'stopper'.

author: G0HZU
author_rank: Super Contributor
timestamp: April 21, 2025, 02:41:11 pm
content: QuoteThe shunt resistance at each gate looks to be about 51R, I can see this value in your first image. Although 51R seems like a low value, it is probably doing very little in terms of shunting away any negative resistance just below 100 MHz. It will be effective down at much lower frequencies though. It is possible to use a low value resistor here to combat negative resistance but I'd expect to see a value much lower than 51R here if the aim was to combat negative resistance at VHF. The aim would be to swamp out the parallel equivalent negative resistance at the gate and this might be -15R for example at 100 MHz.I'm not suggesting you change the 51R resistors because it will start to absorb your drive power and so it might get quite hot and you will also lose some gain at 435 MHz.I doubt low frequency instability is the issue, but it is also possible to get instability at very low frequencies down as low as audio frequencies if there is inadequate bulk capacitance in the supply and bias circuits. This can cause squegging and this can be highly destructive.Usually, LDMOS parts fail with a kind of avalanche breakdown. The reason low frequency instability is the most destructive is because the transistor will spend more time in the avalanche (peak voltage) condition per cycle as the period of low frequency waveforms is much longer. So it will get hotter and this can set up a runaway condition leading to breakdown from the drain or source to the gate. I'm not a guru on this stuff by any means but I have been through all this stuff many years ago and worked closely with PolyFET and Semelab (and Cree) back in those days. So I know a few basics. The best people to advise on this stuff are the device manufacturers rather than me.

author: RoV
author_rank: Regular Contributor
timestamp: April 21, 2025, 04:34:21 pm
content: Did you consider overvoltage from the power supply and from the inductance of the connecting cables?I see only relatively small bypass capacitors on the schematic. If the driving power is abrupbtly removed, the current absorbed on the DC line will decrease in a few ns: some power supplies respond to this with a large overshoot that can damage the amplifier.I remember this was a cause of fault in radar power amplifiers: tha PA was normally in standby in class AB, then received a short chirp waveform (a few us) followed by a long pause (10s/100s of ms), causing a transient on the DC bus that required large electrolytic capacitors placed very close to the PA.

author: Fflint
author_rank: Frequent Contributor
timestamp: April 22, 2025, 03:00:37 pm
content: Quote from: RoV on April 21, 2025, 04:34:21 pmThanks. No, I haven't. I'll put it on an oscilloscope and see.The Psu has overvoltage protection just for that, but who knows if is capable of noticing a very fast spike.

author: G0HZU
author_rank: Super Contributor
timestamp: April 22, 2025, 04:30:16 pm
content: You definitely do have to make sure there is enough bulk capacitance in the PSU and bias feeds as it's possible to set up a squegging oscillation at low frequency as well as there being possibilities for transients as described by RoV. I found an old paper from Polyfet about instability (and how to cure it) for LDMOS amplifiers. The stuff about the snubber should now make sense if you read it through.https://www.polyfet.com/mtt97.pdfI also put together a simulation using a VCCS model based on the gm and capacitances in the MRF186 datasheet. See the attached simulation.The brown trace (Rp1) is for the top schematic and it shows what happens with no resistances added at the input. There is negative resistance produced below about 120 MHz and this extends down to low frequencies.The red trace (Rp2) is for the middle schematic and it shows what happens with a 51R shunt resistor added (as in your circuit). This cancels the negative resistance at really low frequencies but not at VHF.The blue trace (Rp3) is for the bottom schematic and it shows what happens when the snubber is added. This cancels the remaining negative resistance at VHF. Often at least two methods are used to ensure there can be little chance of there being a frequency band with negative resistance.This is because the FET can also produce negative resistance in various parts of the RF cycle in the large signal condition. So it's best to adopt a belt and braces approach.Also, if you look at the datasheet for the MRF186 the application circuit that is included in the datasheet has an RC snubber fitted at the input. However, this doesn't mean that your failure was caused by RF instability. It could have been caused by any of the things mentioned so far. it could have been due to bias drift leading to overheating or it could have been a delayed ESD event or it could have been PSU/bulk capacitance related. Or it could be that the part isn't a genuine MRF186 and it had a defect that made it less reliable.

author: G0HZU
author_rank: Super Contributor
timestamp: April 23, 2025, 06:32:54 pm
content: There's some more stuff about FET amplifier stability here although it is for GaN rather than LDMOS. However, the techniques and reasons for it are the same. This is a technical paper from Qorvo.https://www.qorvo.com/design-hub/blog/model-based-gan-pa-design-basics-s-parameters-linear-and-resistive-stabilizationI managed to find an old LDMOS PA board at work and isolated just one side of the LB401 device. It was really easy to demonstrate the negative resistance below 100 MHz by simply adding an inductive load at the output (I used a 33nH Coilcraft air coil in parallel with a resistor) and then I put an inductive load at the input This was another 33nH Coilcraft coil. The LDMOS part oscillated at about 70 MHz as predicted by a simulation as a lot of negative resistance is generated at 70 MHz with this setup. I also replaced the input inductor with about 13cm of Sucu 86 coax cable with the far end shorted. This is similar to a 33nH inductor but I can ground the outer shield along its length to prevent any risk of direct inductive coupling between the input and output 33nH inductances. It still oscillated at the same frequency as expected.I could probably make it oscillate at very low frequency by playing with the bulk capacitance in the supply and bias circuits and this might cause it to self destruct. It survived the 70 MHz oscillation but I only ran it at 15V drain voltage. It's usually the very low frequency oscillations that kill these devices.