MOSFETs in linear operation

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

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

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.

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

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.

+1 on the ZTC - I never use it.

Thankyou very much, thats solved the mystery. The maths makes it perfectly understandable why they are blowing up. Now I know what Im dealing with. 

Im afraid I dont really have specs to give you at the moment as its more about the concept - can I use one of these devices to control current. There might be different setups for different power levels but Im going to want to get up to around 20A ultimately, if I can. The idea was to reduce the Vds as much as possible to make it workable, power wise. Im wondering if mosfets in parallel will help, the issue there is the relationship between the drive voltage (Vgs) and the current. I may have to control parallel mosfets individually, as opposed to just sticking them on top of each other (circuit wise not physically of course). 

The reason why Ive gone down the P-channel route is because they make the most sense for the circuit. I use the stable input voltage on the source as a supply from which to drop the Vgs over a resistor.  Supplying and controlling the Vgs for an N-channel (in the high side) adds another complexity and after a look at it yesterday I decided I would rather stick to P-channel and make that work, I do appreciate the advice though.

Thanks again.

I'll offer you a couple of suggestions that I hope will help you.

The first is that 20A is a lot of current for a linear power supply of any kind. If you can adjust the source to allow only a very small drop, then you didn't really need an additional regulator. If you can't control well enough without a series regulator you should be thinking in terms of at least 2V and probably a lot more dropped across it. So thats at least 40W but maybe a lot more.

The second thing is that you really should have a spec, right now, before you start thinking about how to solve the problem. Good engineering starts with knowing where youwant to end up. (Google "requirements specifying". A lot of the stuff you'll find is over the top for where you are but you should have something.

My father, (now 94) told me when I was a kid that is usually a waste of time to measure something if you don't know what you expect the result to be.

You can extend from that to work out why you can't do a good design if you don't know what you expect it to do when you are finished.

As AJ said, don't be shy about posting an incomplete or vague spec, its way better than none. Often the problem on E14 or other such groups is that people ask component level questions when they should have explained the system level problem.

MK

Thanks thats really helpful. Absolutely, I wont be blowing any more components up unless I make some other mistake. I will do the calculations needed to make this work. The price isnt really prohibitive at the moment so not concerned about that.

As you say, getting the approach right and then scaling up is what I had in mind. I want to know that fundamentally it works, then I will see how far I can take it. If in the end I can only manage 10A max, thats ok, at this point anyway. 

As several very helpful folks here have asked for some details on circuitry (understandably!) I will put something up but Im afraid I wont be able to do that until monday now so please bare with me. In all honesty I have been rather shy, I dont do social media so posting anything let alone ideas or designs is quite daunting. The circuit I have is very simple but its purely my own design/idea and I half expected to be pelted with rotten tomatoes by any experienced engineers looking at it. 

You're welcome. I have learned a lot with your question, the element14 community is great, always willing to help those of us who are just starting out.

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What Javagoza says: you'll find that Element14 isn't like other sites and you won't get any grief at all for asking questions or posting things up.  You'll get as much help as you ask for, given politely and with encouragement.

Hopefully the comments here don't sound like flames - that is not a motive on this site. Many of us have similar stories to yours. Shy about publishing ideas and not into social media. I also burned my finger on my first linear power supply design. What we learn from these experiences is more valuable than the components we smoke.