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MOSFETs in linear operation

ben0127

Hi,

Im new here, first post, and am a fairly junior engineer so could do with some help regarding a charging circuit. I am designing a mosfet based current control circuit with a P-channel (needs to be high-side) and am having problems with the mosfet destroying itself. I have done some research into the use of mosfets in the linear region so I believe I am biasing it correctly and not putting much strain on it, keeping Vds low etc,  but when trying the circuit out it works for about 10 seconds before a wisp of smoke and charming aroma fills the lab. I have tried several different mosfets now and had the same problem with each of them, all setup to work comfortably within specs. 

After some more research I came across "Power Mosfets" which sounded like the answer to my problems, several articles I read talked about how standard mosfets cannot dissipate the heat being generated but power mosfets can. I started looking at the datasheets of some of these to select some samples but each one I looked at had the same issue which, with the knowledge I have so far, has ruled them out. The ZTC point on the transfer characteristics graph was very high.

My understanding of the operation of mosfets is that they need to be biased on (Vgs) above the ZTC point so that they are thermally stable. This is one of the key properties I have looked for in datasheets, along with the SOA, but what Im seeing with power mosfets is, for example, a ZTC point at 20A with a realistically useful maximum current of around 20A. This is of no use to me as there is no range. The standard mosfets have a much lower ZTC point so can be used to regulate current over a significant range which is what Im looking for. 

Im starting to doubt some of the things Ive learned about these devices and so definitely need some help with some questions:

Ive got used to thinking that I can only use mosfets above the ZTC point but these power mosfets look like using them below it would be necessary. Im wondering if it is safe to do so, as long as you have a feedback system to back off the current as it warms up?? If this is the case, is it safe to use at Vgs extending all the way down to the Vgs threshold? 

During research another confusing piece of info I came across, which completely contradicts what I had come to understand about power mosfets, was that they can fail in operation above the ZTC point even when within the SOA. I thought the whole point of a "power mosfet" was that it could handle operating in linear mode? Isnt this what differentiates it from a standard mosfet?

Im starting to wonder what it is Im missing with this as it seemed relatively simple to begin with so any help or advise on any of these points would be greatly appreciated.

  • 0

    Have you calculated the maximum power that needs to be dissipated in your transistor? Are you familiar with how to calculate the amount of heat sinking you need based on the power levels? If you have amps going through any transistor in its linear region, it will need serious heatsinking.

  • 0

    What fet are you currently having trouble with and at what operating conditions?

    Loosely speaking power FETs are generally vdmos devices, they operate vertically through the semiconductor substrate. A tradition fet operates planar to the substrate. Power FETs can suffer from hot spotting when running in saturation at high power levels but the issue isn't very well documented.

  • 0

    To make helpful comments we need more information.

    A schematic (with component types and values) and some pictures would be a good start. A specification always helps.

    Mostly power MOSFETs are not operated with any regard at all to ZTC (and I even had to look it up).

    You would normally have some kind of feedback arrangment to control the gate voltage.

    BTW, you don't need P channel parts to make high side controllers.

    MK

  • 0 in reply to dougw

    Thanks for your reply.

    Im just at the trial stages so not putting any serious power into the device. All the mosfets I tested were at pretty much the lowest current (4-5A) they could work at in the linear region, taking into account the ZTC point. The drain source voltage was around 4V which was done deliberately to avoid overheating. I was so far from the maximum power rating that the last thing I expected was overheating so didnt think about "serious" heatsinking. 

    What stood out to me was the fact that it was the front side of the package that got the hottest, and so quickly that there wasnt time for the heat to get to the tab and be sinked away. Is this the hotspotting phenomenon I have read about?

  • 0 in reply to ben0127

    If you want help you'll need get a bit more specific:

    "serious power" = how much , 1mW, 50W, 10kW - all reasonable amounts, able to be taken seriously in differnt applications.

    "so quickly" is that a few seconds or a minute or two

    You still haven't told us what the part is or what your test rig is like.

    MK

  • 0 in reply to scottiebabe

    Thanks for the reply.

    Ive tried 3 mosfets so far (SPD15P10PLG, IRF9120 and IRF6218) and all had the same problem. I was running each with a gate voltage to allow around the lowest current they could safely handle (considering ZTC point) and with around 4V across the drain-source.  I deliberately gave them an easy ride just to see them in operation. They did their jobs but after 10 seconds gave up after getting extremely hot.

    I could understand the problem if I was really working them hard but the power dissipation is low, a fraction of the SOA claims. 

  • 0 in reply to michaelkellett

    Thankyou, its interesting to hear that ZTC is largely ignored. That answers my question about the useful range I was looking for in the datasheets.

    Is there a disadvantage in using P-channel as opposed to N-channel?

    Apologies for the lack of specifics regarding circuitry and component values etc but Im just at the beginning stage of this and want to get a feel for how these operate. I dont have a complete finalised circuit or anything, just supporting components to operate the mosfet.

    Basically Im using a small mosfet to drop voltage across a resistor which is between gate and source of the main mosfet, producing the Vgs. This voltage is set (depending on the device used) at around 5.5V to produce 4 - 5A drain current and there is feedback to maintain the current - which I monitored. The devices I have tried are rated well beyond this and have power ratings of well over 100W. The IRF6218 for example has 250W max power rating and with Vds of 4V I am dissipating around 20W. 

    This is why I feel that there must be something fundamental I am missing because 20W in a 250W device surely shouldnt cause thermal failure in 10 seconds.

  • 0 in reply to ben0127

    Still would be good to have a schematic of your test set up that is blowing the devices.

    A scan or picture from your notebook would be fine, doesn' need to be CAD and all that!

    On to the substance.

    The IRF6218 in To220 package (all comments from IR data sheet), has a junction to ambient thernal resistance of 62 C/W (with no heatsink).

    Put 20W in and its aiming for 25 + (62 * 20) = 1265C. They don't quote thermal mass but a TO220 I had lying around weighs 2.6g. Let's assume that its solid copper so its thermal capacity is about 2.5 x 0.4 = 1W/C. So with 20W in, its temperature will rise at 20C per second. 10 seconds to death seems about right.

    On a good heatsink (1C/W)  this part can cope with 75W. That 250W rating isn't a use rating, but a bit of marketing puff - the only way you can actually achieve it is by holding the metal case at 25C by squirting cooling liquid at it.

    I still can't understand how you are connecting these bits up, so I'll need that diagram to help anymore.

    Can you tell us the spec of the end thing, it would be nice to know:

    input voltage range

    output voltage range

    max output current

    and then all sorts of stuff like environment , reliability, accuracy etc but we aren't close to needing to worry about them yet.

    P channel parts tend to work worse, cost more and are harder to get than N channel. So you go for N channel when you can.

    MK

  • 0 in reply to michaelkellett

    I had to look up ZTC as well.  Michael is making some useful points here: before blowing any more MOSFETs or spending money on buying more to blow in a similar manner, you should do a bit of research on power in MOSFETs and how to calculate the thermal resistance - i.e. how much it is going to heat up with the amount of power you are putting through it.  If you watch this video, (YouTube) from 10:05 onwards, the chap explains how to select MOSFET based on a datasheet and how to calculate the thermal rise from the power consumed.  It's very understandable and you'll get a feel on what to do in order to experiment further, without destroying your MOSFETs.  Hint: at the sort of power you are talking about it won't be cheap as you'll need a big heatsink!  However, you may be able to tailor your experiment down to manageable levels to at least work out that your approach is fundamentally sound and could scale up.  

    The other thing: don't get disheartened, we've all been there!  Don't worry about putting your ideas down here, a picture of your circuit as you've envisaged it so far, what your experimenting with etc.  If you want to do your own research to discover stuff/learn stuff etc, just say and we can give you a direction to look in rather than specific answers.

    EDIT: by "not cheap" I mean you'll have to lay out a few pounds for heatsinking and maybe even a fan to keep experimenting at that power level.  We're not talking hundreds of pounds (or equivalent) but maybe £20 pounds or so.

  • 0 in reply to ben0127

    I congratulate you on observing the magic smoke hidden inside electronic components. If you are running your fets without a heatsink eventually they will get too hot and give up the ghost.

    If you are applying a constant Vgs to your FETs and expecting them to provide a constant current, your FETs will succumb to thermal runaway. The threshold voltage of a FET trends negative for increasing junction temperatures. So if you apply a fixed VGS the fet heats up, resulting in a higher overdrive voltage (VGs-Vt) and sources more current, which results in more power dissipation, and the positive feedback cycle continues....