Mosfet HV regulator : 2 multimeters fried - Page 1
Forum section: electronics / projects

author: Rutherberg
author_rank: Regular Contributor
timestamp: June 23, 2016, 08:39:00 pm
content: Hi,I'm trying to make a pretty basic mosfet regulator but so far I failed miserably.Have a look at the schematic attached.My multimeter is between Va and ground, Vdc 600V selected, and when I turn my transformer on, I hear a pretty bad clicking noise (typical noise of a faulty semiconductor or short, you know what I mean) at 100Hz and the DMM is gone.Same result with a second multimeter.My multimeters were pretty cheap but have done the work for many years. Its rated CAT II 600V, but I saw the thread on the cheap-ass multimeters shootout...But c'mon, its at best 450Vdc so I don't understand why it fails like that.There is probably a problem with a part in the schematic, but I replaced every semiconductors and the wiring after that and the problem stays the same.Even without the DMM connected I have this bad clicking noise.Meanwhile, I ordered a descent Fluke 179, but I hope it will hold this situation...Have I done something wrong ? I don't understand what the problem is. Any advice would be appreciated !Thanks !

author: ZeTeX
author_rank: Frequent Contributor
timestamp: June 23, 2016, 09:18:55 pm
content: Quote from: Rutherberg on June 23, 2016, 08:39:00 pmHow about measuring the output with a resistor divider (:100) ?

author: ovnr
author_rank: Frequent Contributor
timestamp: June 23, 2016, 09:35:57 pm
content: Your circuit is probably switching hard and generating inductive kickback fromsomethingup the line. And of course you use a cheapo DMM that can't deal with the transients.

author: Rutherberg
author_rank: Regular Contributor
timestamp: June 23, 2016, 10:19:41 pm
content: Ok thanks for the fast replies.I should receive the Fluke tomorrow, I will try the measurement with a divider.Without the 10k connected to Va, I don't have the bad clicking noise. So I assume there is transients related to current. Is this what you mean by hard switching ?Would a 10 ohms resistor (just after the mosfet) help to tame the transients ?Or a zener between the drain and R2 to suppress the ripple of the mosfet gate ?I'm puzzled, it should work as it is. Or do I miss something ?Thanks !

author: Rutherberg
author_rank: Regular Contributor
timestamp: June 23, 2016, 11:33:37 pm
content: Quote from: blueskull on June 23, 2016, 10:39:19 pmObviously, thanks to point it out. Nevertheless I added it to the schematic for clarity, there is an internal Vgs protection diode in the 2SK2842.Quote from: blueskull on June 23, 2016, 10:39:19 pm179 precision and features are way good enough for my usage and budget.

author: Someone
author_rank: Super Contributor
timestamp: June 24, 2016, 12:00:13 am
content: Quote from: Rutherberg on June 23, 2016, 11:33:37 pmIgnore the haters, the Fluke 179 is a very respectable meter and will do you well. I've seen them as standard issue in several engineering workplaces (including for electronics).

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: June 24, 2016, 02:39:10 am
content: If C2 went to AC instead of GND, I think you'd have a voltage doubler.

author: Circlotron
author_rank: Super Contributor
timestamp: June 24, 2016, 02:45:09 am
content: Circuit could be oscillating. Put a 1k ohm in series with the mosfet gate, right next to the mosfet.

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2016, 09:03:07 am
content: Quote from: blueskull on June 23, 2016, 10:39:19 pmYes and it's not a proper regulator, just a voltage divider with a common drain amplifier.To make a regulator a voltage reference such as the TL431 is required.What's the frequency of the input voltage?

author: chris_leyson
author_rank: Super Contributor
timestamp: June 24, 2016, 10:13:53 am
content: QuoteThat was my first thought even before I had read the post, and D9 is the wrong way round.QuoteThat's also a sign of high frequency oscillation, the amplitude builds up causing the DC bias to shift until the oscillation stops and then the whole thing repeats.

author: ebclr
author_rank: Super Contributor
timestamp: June 24, 2016, 10:42:31 am
content: Buy a pair of Varistor and put in parallel with the meter probe ( the 2 in series )https://world.tmall.com/item/522030797114.htm?spm=a312a.7700714.0.0.U6sJes

author: ebclr
author_rank: Super Contributor
timestamp: June 24, 2016, 10:49:05 am
content: 1N4007 can't be used if you switch frequency is on the Khz magnitude, Must use a Fast diode

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 24, 2016, 11:31:06 am
content: All kinds of things wrong with the OP's circuit. The usual way of doing this is described nicely athttp://electronicdesign.com/power/efficiently-reduce-high-supply-voltages-accurate-low-voltage. Here is the schematic from that page:Note that while the example is for a >24V input, 3.3V output regulator, the input and output voltage are only limited by the Vds rating (and dissipation, of course). It is not mentioned in the article, but modern MOSFETs optimized for switching tend to perform poorly in linear operation (in fact, they tend to oscillate!); look for MOSFETs expressly designed for linear mode (e.g. IXYS L2 series). Also note that this circuit is not protected from overcurrent/shorts, but neither was yours. It's easy enough to add over current protection with the usual NPN shunting the MOSFET gate to its source when Vbe exceeds 0.6V.I also agree that the Fluke 179 is a perfectly fine meter for electronics use; it's my general purpose meter of choice, in fact.

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2016, 11:53:57 am
content: Quote from: ebclr on June 24, 2016, 10:49:05 amWe don't know what frequency the 300VAC is. I assumed it's 50Hz mains from a transformer because the OP is in Europe but it could be from an inverter.If it's a much higher frequency, then not only should fast diodes should be used but the 10nF bypass capacitors should be removed.Quote from: MagicSmoker on June 24, 2016, 11:31:06 amThat will work but only for output voltages under 15V - the MOSFET's threshold voltage. If a higher output voltage is required, then additional transistors are needed to reduce the voltage across U1 to 15V or below.

author: retiredcaps
author_rank: Super Contributor
timestamp: June 24, 2016, 12:01:36 pm
content: Quote from: Rutherberg on June 23, 2016, 08:39:00 pmI can't read a schematic or help with your design, but if you want to try and fix your two dead multimeters, start another thread in the repair section and I can try to help.I'm travelling this weekend so my responses will not be quick and limited to a few minutes here and there.

author: Rutherberg
author_rank: Regular Contributor
timestamp: June 24, 2016, 12:17:24 pm
content: Thanks for the replies, the schematic has been modified, D9 is correct now.Quote from: Circlotron on June 24, 2016, 02:45:09 amI'll try that, thanks.Quote from: Hero999 on June 24, 2016, 09:03:07 amThe purpose of this circuit is to vary the HV of a cathode biased push-pull guitar amplifier to vary the power output, I don't really need a regulation.The main frequency is 50Hz.Quote from: MagicSmoker on June 24, 2016, 11:31:06 amOk thanks, I will check for a more appropriate mosfet.Is it OK to use the zener (D9) as a current protection device instead of the classic NPN ? I mean connect the zener at the gate andafterthe current sensing resistor ?Attached, you will find the schematic updatedQuote from: retiredcaps on June 24, 2016, 12:01:36 pmI appreciate it but those were really cheap and I don't think it worth the repair timeThanks !

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 24, 2016, 01:58:53 pm
content: Quote from: Hero999 on June 24, 2016, 11:53:57 amGood catch. The LM4041 can be directly replaced with an TL431 to extend the input/output differential to ~35V, or it can be replaced with a zener for a fixed output voltage and no real limit on the i/o differential (aside from aformentioned Vds and Pd limits).Also not included in the EDN circuit is the obligatory 12V-15V zener from gate to source to protect against overvoltage there, but the OP has recognized the need for such so I didn't bother pointing it out before.

author: Rutherberg
author_rank: Regular Contributor
timestamp: June 24, 2016, 02:40:20 pm
content: So I tried the attached schematic and it works like a charm ! My Fluke too BTWI didn't implemented the current limitation circuit for the test, I will add it later.I changed the mosfet for a beefier one. This is still a fast switching type, I compose with what I have in stock.Is there any way to further reduce the risk of oscillation in this case ? Except a proper layout ?About current limitation, is it OK to use the zener (D9) as a current protection device instead of the classic NPN ? I mean connect the zener at the gate and after the current sensing resistor ?Thanks for your help !

author: bktemp
author_rank: Super Contributor
timestamp: June 24, 2016, 02:56:03 pm
content: IRFPExx should be fine up to a few 10W of power dissipation. They are rather old, so have a much larger die compared to more modern MOSFETs.A source resistor + gate zener diode helps limiting the short circuit current, but unless the current is set to a rather low value (<50mA) the power dissipation will be too high for the MOSFET to survive a short circuit longer than a couple of seconds.

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2016, 03:22:42 pm
content: Quote from: MagicSmoker on June 24, 2016, 01:58:53 pmThe TL431 does have a higher voltage rating but it also has a higher minimum drive current.For higher voltages, another transistor can be added to power the regulator IC, off a potential divider connected to the output.Attached is an example using a Darlington transistor and the LM431. It will work with a MOSFET and LM4041, with a few modifications.

author: Rutherberg
author_rank: Regular Contributor
timestamp: June 24, 2016, 03:52:48 pm
content: Quote from: bktemp on June 24, 2016, 02:56:03 pmI don't understand, the datasheet announce Pd max = 190Whttp://www.vishay.com/docs/91248/91248.pdfIn the worst case, Vgs is about 330V with a quiescent current of 30mA and 120mA peak.So, Pd quiescent is about 9.9W and Pd peak 39.6W, well below Pd max.Or am I missing something ?

author: bktemp
author_rank: Super Contributor
timestamp: June 24, 2016, 04:28:35 pm
content: Quote from: Rutherberg on June 24, 2016, 03:52:48 pmThe specified power dissipation has almost no practical meaning, especially for linear operation at higher voltages:http://www.digikey.com/en/pdf/m/microsemi-power-products-group/microsemi-make-linear-mode-workQuote39.6W should be ok if the junction temperature stays <100°C.Because there is no DC SOA curve in the datasheet, you can only guess the operating area unless you do some destructive measurements.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 24, 2016, 05:20:16 pm
content: Quote from: Rutherberg on June 24, 2016, 03:52:48 pmUt-oh, you're a young player or a new one, aren't you? To paraphrase an old saying, "there are lies, damn lies and then there are transistor datasheets".The tell is in the thermal resistance specs: the junction to case resistance is 0.65C/W which results in a 123.5C rise above ambient at 190W all by itself, but then you have to add the rise from the case to heatsink (min. of 0.24C/W) which gets you another 45.6C rise, and then there is the thermal resistance of heatsink to ambient which for a typical convection cooled finned aluminum job is going to be somewhere between 0.5C/W and 2C/W. Meanwhile, the maximum allowed junction temperature is usually 125C or 150C - occasionally 175C - but don't expect very long operational life above 100C.All in all, that means a more realistic dissipation limit for a TO-247 package is 40-60W, and even that is assuming a very good heatsink (think of forced convection or liquid chill plate).

author: Rutherberg
author_rank: Regular Contributor
timestamp: June 24, 2016, 05:37:32 pm
content: Quote from: bktemp on June 24, 2016, 04:28:35 pmVery interesting, I'll check that.Quote from: MagicSmoker on June 24, 2016, 05:20:16 pmExactly, well, kind of...I have a bachelor degree in electronics but until now I was field service engineer for 7 years. Now I try to evolve in electronics R&D so I certainly have knowledge gaps.Thanks for the informations !Any alternative in this case ?

author: bktemp
author_rank: Super Contributor
timestamp: June 24, 2016, 06:17:30 pm
content: Another useful application note showing why a mosfet can dissipate much more power at low voltages and why older mosfets are suited better for linear operation:http://www.infineon.com/dgdl/Infineon+-+Application+Note+-+PowerMOSFETs+-+OptiMOS%E2%84%A2+-+Linear+Mode+Operation+and+SOA+Power+MOSFETs.pdf?fileId=db3a30433e30e4bf013e3646e9381200

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 24, 2016, 06:40:22 pm
content: Quote from: Rutherberg on June 24, 2016, 05:37:32 pmAn alternative for the MOSFET you selected? I would go with an IXYS L2 series part. IXTP15N50L2 looks like a good choice and isn't too expensive:http://www.digikey.com/product-detail/en/ixys/IXTP15N50L2/IXTP15N50L2-ND/2117346It is rated for linear operation and has a thermal resistance from junction to heatsink (shorthand: Rth[j-s]) of 0.67C/W, which is about the same as just the junction to case resistance of the part you mentioned earlier. Max. recommended junction temperature is 150C but safe operating area is guaranteed up to 75C, so you have about 50C to work with. Referring to the FBSOA graph at the end of the datasheet, you can theoretically* handle 0.5A of current with 300V of drop across the drain-source, so it should be plenty robust for your application.* - you should now know why I say "theoretically"

author: ZeTeX
author_rank: Frequent Contributor
timestamp: June 24, 2016, 08:27:52 pm
content: Quote from: MagicSmoker on June 24, 2016, 06:40:22 pm

author: Rutherberg
author_rank: Regular Contributor
timestamp: June 27, 2016, 11:02:39 am
content: Quote from: ZeTeX on June 24, 2016, 08:27:52 pmThanks but could you be more specific please ?I thought back about the worst case in my application and it should be less restrictive, about Vds = 210V and Id peak = 50mABut assuming a more demanding application, would paralleling 2 mosfets be a good solution to reduce stress ? I assume there is precautions to dispatch current evenly between the two (or more ?) ?Anyway, thanks for the help so far

author: ZeTeX
author_rank: Frequent Contributor
timestamp: June 27, 2016, 04:36:19 pm
content: Quote from: Rutherberg on June 27, 2016, 11:02:39 amParalleling 2 mosfets with balancing resistors to share the current across them might help with the power dissipation but I don't think that it will do any more then helping with the power dissipation.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 27, 2016, 06:26:16 pm
content: Quote from: blueskull on June 24, 2016, 05:54:22 pmYes, some semiconductors are rated for a continuous Tj of 125C or higher - and I even said as much - but even those devices will last longer if Tj is kept below 100C. Not sure what your point is here, as your argument does not refute or invalidate the general rule to keep Tj below 100C for long life.And yes, there are *some* TO-247 parts which have a sufficiently low total Rth[j-s] so that along with a suitable heatsink (think heat-pipes, vapor phase chamber or liquid chill plate) they are able to dissipate 150W or more, but *most* TO-247 parts and *most* heatsinks are of more humble capability, and thus one finds - usually with experience - that ~50W is a reasonable maximum Pd, and even that assumes forced convection cooling of the heatsink; if relying on natural convection of even a large PCB-mounted extruded aluminum heatsink you might not be able to manage more than 20W.Quote from: ZeTeX on June 27, 2016, 04:36:19 pmDropping 210V at 50mA is well within the DC SOA of the MOSFET I suggested; perhaps you were predicting thermonuclear catastrophe with the MOSFET the OP suggested (IRFPE50) as its SOA does not go down to DC (10ms is the longest pulse spec'ed)?

author: ZeTeX
author_rank: Frequent Contributor
timestamp: June 27, 2016, 07:39:50 pm
content: Quote from: MagicSmoker on June 27, 2016, 06:26:16 pmyep, well not thermonuclear explosion but something a little bit smaller.

author: Rutherberg
author_rank: Regular Contributor
timestamp: June 29, 2016, 10:07:50 am
content: Quote from: ZeTeX on June 27, 2016, 07:39:50 pmYou are probably right, at Vds = 300V and Ic quiescent = 10mA thecaseof the IRFE50 is already at 50°C. The mosfet is fixed on a big aluminium chassis with a milar film in between.I will order an IXYS L2 and see how it reacts.About paralleling mosfets, the balancing resistors in this case should be connected at the source of each mosfets, right ?Is there a rule of thumb about the value of these resistors ?Thanks

author: bktemp
author_rank: Super Contributor
timestamp: June 29, 2016, 10:28:12 am
content: Quote from: Rutherberg on June 29, 2016, 10:07:50 amThat sounds wrong: 300V*10mA=3W. If the case reaches 50°C by dissipating only 3W, then there must be a good thermal insulator between the mosfet and the heatsink. Using a different mosfet will not change much. A better heatsink is more important than going to a larger mosfet.QuoteThe voltage drop on the source resistor should be much larger (10x) than the gate threshold voltage change by temperature. This translates to a couple of volts voltage drop on the source resistor.

author: Rutherberg
author_rank: Regular Contributor
timestamp: June 29, 2016, 11:29:21 am
content: Quote from: bktemp on June 29, 2016, 10:28:12 amThe junction between the mosfet and the chassis is not optimal, the milar film is pretty thick and the screw is nylon type so not really tight.Meanwhile I found an alternative candidate to the IXYS L2 :http://www.farnell.com/datasheets/1863418.pdfDC is plotted on the FBSOA curves, the thermal resistances are pretty similar and it's almost 10 times cheaper. Thermonuclear explosion or not ?

author: bktemp
author_rank: Super Contributor
timestamp: June 29, 2016, 11:49:16 am
content: Quote from: Rutherberg on June 29, 2016, 11:29:21 amIt is probably as good as IRFPE50.My rule of thumb: If the power dissipation does not decrease at high voltages, the SOA curve is bullshit. And that is the case in this datasheet: No way this mosfet can reliably dissipate more than 200mA at 900V. The junction to case thermal resistance is 0.63°C/W. So if you dissipate 198W @ 25°C Tcase, the junction will be at 150°C. That's where the 198W specification comes from. They probably didn't do any SOA test, they simply calculated the values.30W power dissipation at 900V is a more realistic value. At 300V probably 50W, but you can only guess. Same as for IRFPE50.It most likely works (if you improve your heat sink), but you can't know for sure.In contrast IXYS probably did a lot of destructive SOA tests to give you a reliable SOA curve. That's why IXYS parts are more expensive than other manufacturers.

author: tautech
author_rank: Super Contributor
timestamp: June 29, 2016, 11:58:58 am
content: Quote from: Rutherberg on June 29, 2016, 11:29:21 amMight I suggest that's a large part of your problem?Get a mounting kit with metal nuts and bolts and quality silicone heatsink pads.

author: Rutherberg
author_rank: Regular Contributor
timestamp: July 26, 2016, 04:06:48 pm
content: Hi,After few tests of the implemented system (attached schematic), the FQA6N90C seems to be OK, even in the worst case it never exceed 70°C.Now I'm facing an other weird problem. When the amplifier is pushed, so when the current increases, the system tend to oscillate.The result : when the signal peaks, I can hear a loud hum/hard clipping (at 100Hz..?) in the sound. It's getting worse when the switch is on the low position (so when Vds is the bigger).The peak current is well below the current limitation of R67/D5 so it is not the problem. I also tried without C40, no change.The weirdest : when I connect my DMM between GND and the positive terminal of C24, it works like a charm, no oscillations and weird noises.I suspect a layout problem, the oscillation created by the radiance of another component but... Am I missing something else ?Thanks