Simple circuit blowing Mosfets - Page 1
Forum section: electronics / projects

author: sparksflying
author_rank: Contributor
timestamp: May 04, 2016, 12:58:00 am
content: I'm doing a project that requires the use of a large electromagnet. I have salvaged a large microwave transformer, removed the "I" part of the core as well as the secondary windings. The magnet is powerful and draws 11A at 20V.I wanted a simple way to vary it's power so simply put a MOSFET between the coil and ground. I have a diode across the coil to catch the inductive spike when turned off. The gate of the MOSFET is powered by a potentiometer in series with a 10k resistor to provide a gate voltage between 0-10V. It worked for a while, but the MOSFET eventually shorted drain-source. The MSOFET is mounted on a large heatsink. I tried parelleling 3 MOSFETs, but they still short out.MOSFETs:https://www.fairchildsemi.com/datasheets/FQ/FQP13N06L.pdfSince it is such a simple circuit, my error should be very obvious.Thanks guys.

author: Audioguru
author_rank: Super Contributor
timestamp: May 04, 2016, 02:03:32 am
content: I think the little Mosfets get too hot when the voltage across them is 10V and their total current is 5.5A. That is why we use Pulse-Width-Modulation for things like this then the Mosfets stay cool.

author: sparksflying
author_rank: Contributor
timestamp: May 04, 2016, 02:28:52 am
content: The heatsink is the size of my hand and each MOSFET can apparently dissipate 45W. They seem to short out before becoming hot.

author: KE5FX
author_rank: Super Contributor
timestamp: May 04, 2016, 03:54:55 am
content: Quote from: sparksflying on May 04, 2016, 02:28:52 amDepending on your circuit layout, you might be seeing some parasitic oscillation that destroys them due to overvoltage. Try adding some resistors (100 ohms or so) to the gate of each FET, right at the part.

author: mmagin
author_rank: Frequent Contributor
timestamp: May 04, 2016, 04:03:41 am
content: Make sure you're not exceeding the DC area on the safe operating area graph.There are a lot of possible problems when using modern power mosfets in their linear region. AFAIK, if you're paralleling them up, you might want to add a small resistance in the source of each, say 0.1 ohms.

author: mij59
author_rank: Frequent Contributor
timestamp: May 04, 2016, 04:06:00 am
content: The mosfets are used in linear mode, paralleling will not result in equally sharing of the load current between the mosfets.Paralleling could lead to oscillations.Use a 12V zener diode between gate and source to protect against voltage spikes.

author: Zero999
author_rank: Super Contributor
timestamp: May 04, 2016, 07:53:38 am
content: Go with PWM. If the frequency is high enough, the inductance in the coil will mean the current through it doesn't stop flowing, until the duty cycle is near zero.PWM can easily be generated with a 555 timer or some logic gates.

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 04, 2016, 08:49:10 am
content: I think it is simpler than that. At 20V supply, and 1.8Ohms resistance of the electromagnet, the current flowing through it is larger than 11A.The datasheet of the mosfet says Rdson is 110mOhm at Vgs=10V. Depending on the setting of your pot, you have Vgs=10V or smaller, so I would expect you are in the Rdson of 100mOhm is a realistic value.This yields a total power dissipation on the mosfet (in case you use just one) of 12W. That will definitely kill it, in case the heatsink is not properly sized, or if the mosfet is not perfectly attached to it. Even with 3 mosfets in parallel, if these would be identical you would get 4W on each. In practice, they are not identical and one of them will dissipate more. That one will die first.Regards,Cristian

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 04, 2016, 09:14:52 am
content: Quote from: brumbarchris on May 04, 2016, 08:49:10 amThis ^.The mosfets are being overstressed by operation in the linear region. Fix by using PWM instead of the variable gate potentiometer, and beefier mosfets (TO-247 packages, higher current ratings).

author: Pjotr
author_rank: Frequent Contributor
timestamp: May 04, 2016, 09:20:59 am
content: Quote from: sparksflying on May 04, 2016, 02:28:52 amKeep in mind that mosfets not specially made for linear use cannot dissipate the rated power in linear mode due to hot spots on the chip. Usually not more than 1/3 of the rated max disspation. Also take into account derating above 25 degC of the heat sink (or actually the TO220 case).

author: danadak
author_rank: Super Contributor
timestamp: May 04, 2016, 10:13:25 am
content: Not killing the SB560 which would then lead to over stress of MOSFET.Or SB560 not a fast enough diode to prevent transient exceeding Vdssor Vgss. I noticed SB560 datasheet shows no switching times.You would know this if you turn on supply for the first time, leave it runningat full load. If no failure thermals probably OK. Use one of the cheapo IR tempreaders to check case temp, and backtrack to Tj to see if its OK. If all wellthen cycle supply a few times, maybe use a DSO to capture Vd looking fortransients.Regards, Dana.

author: dannyf
author_rank: Super Contributor
timestamp: May 04, 2016, 10:19:47 am
content: Your MOSFETs are working in their linear mode. Thus tremendous power being dissipated.

author: wraper
author_rank: Unknown
timestamp: May 04, 2016, 11:28:21 am
content: According to safe operation area graph, at 10V Drain-source voltage max current is 3.5A, at 20V only 1A, given that Tcase = 25oC. Paralleling mosfets won't help as current won't be evenly shared in the linear region, more like one mosfets taking most of the current. Depends on how well matched they are.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 04, 2016, 11:41:22 am
content: The only sensible option is to upgrade to PWM. Forget about linear control.The frequency doesn't need to be extremely high, as large electromagnets have a lot of inductance.A few kHz would be easy to implement without going to great lengths with gate drive.With PWM, you do have rather high conduction losses in the diode, though, so you need to rate it for high current, too.

author: Zero999
author_rank: Super Contributor
timestamp: May 04, 2016, 12:35:13 pm
content: There are many ways to generate PWM:555 timerNote if you're using 20V, the 555 will need a voltage regulator, the LM7812 will do.CMOS Logic gatesAgain, CMOS logic ICs have maximum voltage ratings between 5V and 15V depending on the type so a regulator may be needed. The output current isn't as high as the 555 so the MOSFET will not turn on/off so quickly. Spare gates can be connected in parallel and use to drive the MOSFET slightly faster.Comparator ICWill work off 20V but the pull-up resistor will slow down the turn on time.

author: sparksflying
author_rank: Contributor
timestamp: May 04, 2016, 01:54:25 pm
content: Thanks a lot for the repliesI thought I could get away with just a simple linear power burn, but it seems like I'll have to move to PWM and do it properly.

author: CJay
author_rank: Super Contributor
timestamp: May 04, 2016, 02:29:27 pm
content: Quote from: Pjotr on May 04, 2016, 09:20:59 amI thought that too but the datasheet specifies 'audio amplifiers' which, unless they mean class D, would suggest linear operation.Those devices are good up to ~30MHz as well, they're used in CB radios and cheap 'linear' amplifiers so oscillation could be a bit of an issue (they're touchy in CB PA stages, they release the magic smoke rather easily in the face of a mismatched load)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2016, 02:49:35 pm
content: Try this instead:The clamp diode has to be associated with the transistors, not the magnet. And it only works if the supply is solid, too, hence the capacitor.Tim

author: dannyf
author_rank: Super Contributor
timestamp: May 04, 2016, 02:52:06 pm
content: You may still be able to do that, just with a lot more MOSFETs than three.The max power dissipation happens at 10v 11amp, or 100w. That means you need 10x to220 devices here, each with adequate heatsinks.That's an expensive alternative to pwm.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 04, 2016, 03:04:24 pm
content: While this may be surprising and counterintuitive, linear is actually more complex, more expensive and more difficult to realize IMO.

author: digsys
author_rank: Unknown
timestamp: May 04, 2016, 03:12:56 pm
content: I'd go with a decent NPN Transistor instead, something with flat gain 100x ... parallel 2-3 ... I don't see ANY advantage of using FETs in their linear region.Your garden variety transistor loves linear mode. And as suggested, use a TO247 package

author: dannyf
author_rank: Super Contributor
timestamp: May 04, 2016, 03:16:25 pm
content: Doesn't matter what device you use. If you drop 10v at 10amp, you have to dissipate 100w. That's true for MOSFETs as it is true for bjt, gold plated ones included.

author: digsys
author_rank: Unknown
timestamp: May 04, 2016, 03:27:49 pm
content: Quote from: dannyfYes, but operating FETs in linear mode, especially at high power, is not that easy. There are huge SOA de-rating issues, plus MANY of the curves supplied for linear mode,are actually NOT accurate by a long shot. There are a few "true" linear FETs, and even these don't have a wide "power" SOA area. BJTs LOVE linear mode, and you canget away with JUST about any part .. and YES, you still have to dissipate the heat, but that's a given.

author: CJay
author_rank: Super Contributor
timestamp: May 04, 2016, 03:47:09 pm
content: Quote from: dannyf on May 04, 2016, 03:16:25 pm*nods* Yes, I did an envelope back calculation for a linear regulator to drop ~2V at 70 amps and decided I would buy a switch mode supply instead.It was cheaper to buy two branded switchers than the heatsink assemblies to cope with that power dissipation.It was however a good way to keep my feet warm.Now, if anyone would like two 1.5KVA 8-0-8V toroids....

author: dannyf
author_rank: Super Contributor
timestamp: May 04, 2016, 03:47:17 pm
content: "Yes, but operating FETs in linear mode, especially at high power, is not that easy. There are huge SOA de-rating issues, plus MANY of the curves supplied for linear mode,are actually NOT accurate by a long shot. There are a few "true" linear FETs, and even these don't have a wide "power" SOA area. BJTs LOVE linear mode, and "Operating anything in linear mode with high power dissipation is not easy. The apparent derating of MOSFET is due to the spec from which they are defeated from: those are specs for switching applications.If anything, MOSFETs, particularly lateral types, have a significant advantage over bjts due to 1. Their resistance to thermal breakdown, and 2 a tempco that helps contain a thermal runaway.Again, you pay dearly dearly for linear solutions simplicity.

author: rstofer
author_rank: Super Contributor
timestamp: May 04, 2016, 04:02:37 pm
content: When you go to PWM, watch the rising and falling edges of the gate. What you really want is the ability to dump AMPS of current into the gate in zero time. That's the ideal situation, not necessarily achievable. But you want to get the MOSFET through the switching transition as quickly as possible. It is in this transition region (linear) that the device heats up. Higher PWM frequency, coupled with poor rise and fall time will yield yet more heat. That's why PWM control of MOSFETs is usually in the very low kHz range (like 1 or 2 kHz). Switch it on and leave it that way for a while.You absolutely can't drive the MOSFETs with 10k+ Ohms of source impedance. You can try the logic level MOSFETs but, still, the problem is to get through the linear region as quickly as possible.

author: macboy
author_rank: Super Contributor
timestamp: May 04, 2016, 04:26:55 pm
content: Quote from: dannyf on May 04, 2016, 03:16:25 pmIf you have 10 V across the transistor, then you have 10 V across the magnet coil which is 1.8 Ohm. Please do explain how 10 A of current will flow!The transistor will dissipate little power when it either drops most of the voltage (hence low current) or very little voltage. It will dissipate the maximum when it drops half the voltage. At 20 V supply, this is 10 V, and coil current will by 10/1.8 = 5.55 A. Transistor and coil will each dissipate ~ 55.6 W. This exceeds the maximum safe operating area in Figure 9 of the datasheet, which shows ~ 4.5 A max DC current at 10 V Vds. And those numbers are with an overly optimistic Tc (case temp) of 25 C, impossible to obtain with any size heatsink in a normal temperature room, plus a Tj of 175 C which will lead to early death in any case. I would not depend on dissipating more than half the power shown in that curve under normal conditions (hot heatsink, even hotter case, lower than maximum Tj).You could try a degeneration resistor for each MOSFET from source to ground. This provides some negative feedback to the control signal; as the current rises, the Vds drops (due to IR drop in the resistor). This is extremely effective for BJTs which turn on a little at 0.6 V and a lot at 1 V, but less so for MOSFETs unless you carefully match the Vds threshold voltage of the paralleled FETs. Even then, since Vdsth and transconductance vary with temperature, you can easily get thermal runaway. Discrete active control of each FET is probably necessary, or simply switch to BJTs with a small value emitter degeneration resistor for each (e.g. 0.1 Ohm).

author: danadak
author_rank: Super Contributor
timestamp: May 04, 2016, 04:54:59 pm
content: You can operate in linear mode, attached example sim shows peak Pdiss.Problem is its a narrow range of Vgs and very T and device to devicedependent.Regards, Dana.

author: sparksflying
author_rank: Contributor
timestamp: May 10, 2016, 12:01:17 pm
content: An update on how it's going...I implemented the PWM circuit using the 555 timer and a high current MOSFET. The control works well, but makes the coil buzz/ring at a high frequency (a few KHz), which is obviously very annoying.I guess a solution would be to increase the switching frequency above human hearing range? (>20KHz)I've got a 25KHz speed controller in the mail from China (about 3 weeks away) that will hopefully solve this problem.

author: CJay
author_rank: Super Contributor
timestamp: May 10, 2016, 12:57:51 pm
content: Quote from: sparksflying on May 10, 2016, 12:01:17 pmYou may find it still 'sings' but it's defintely worth a try, bear in mind the transformer you used wasn't meant to operate much above 50 or 60Hz so I suspect you'll need to play around to make it work well and with minimum/no noise.

author: sparksflying
author_rank: Contributor
timestamp: May 10, 2016, 01:31:20 pm
content: Hopefully not, I might play around with linear control again. Perhaps a couple of TO-3 BJTs in parallel might be ok, I have a few 2N3055's in stock.http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDFPower dissipation at 100deg is 60W, so two should be fine. I'll put low value protection resistors between the emitter and ground. I think i have a few 0.1ohm wire wound resistors lying around.