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Old 20th December 2003, 10:23 PM   #1
akb1212 is offline akb1212  Norway
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Default How to choose what power MOSFETS to use?

I have recently found this forum, and I must say Iím REALLY impressed on the level of knowledge and know-how found here!

Ok, I have gotten my inspiration on amplifier building back after reading here! And what I want to build is a few Aleph-Xís for low midds and up (6 channels), and an X amp (2 channels) for my bass horns.

As a true DIYíer I have started looking into what MOSFETS Iíll use. I found Nelson Passís articles very helpful in this regard (thanks again NP!)! Especially the article on matching transistors and in particular part 1 of The Zen Variations. GREAT to know how to choose die size and what implications your choice makes.

But when taking a closer look at catalogs I found that IRF have came out with a new generation power MOSFETS. Itís what they cal fifth generation, and is normally indicated by putting an N after the part number. The new version of IRFP044 is then IRFP044N. What made me take a closer look at it is the fact that the new types are sometimes cheaper, and even in some instances several TIMES cheaper. This is probably because they are on their way out. One example of this is the price Arrow has on IRF540/IRF540N. The prices here are $3.49 and $0.66! Quite a differenceÖ.

In the case of IRFP044 the differences in price were almost none, but as the N-types (meaning the part number ending with an N to indicate new version not as in N-channel) seem to replace the non N-types we will probably se the same here eventually.

The thing is there ARE differences on N and non N-types. The data on the N-types are almost always better! But some data are changed, and on the IRFP044N the breakdown voltage has been lowered to 55V. The wattage on most types is also changed, but the change is different on each type. On some it is higher, and on some it is lower. In the case of IRFP044 it is 180W and for IRFP044N: 120W.

There seem to be better (lower) capacitances on all the N-types. Also the capacitance to transconductance ratio seems to have been improved on all the N-types.

When it comes to different cases I have also thought of using the not so good TO220 type IRF640(N) in place of IRFP240. When you look at the data there seems to be the same chip sitting in those transistors. In this case there is an IRF640N available, but no IRFP240N (one other thing is that IRFP240 seems to be unavailable here in Norway, at least at a decent price). And by comparing the data on the two the IRF640N comes out betterÖ. Almost the same wattage and most the same data, except it uses 5th generation chips and have a lower input capacitance.

An alternative Iím seriously considering for the IRFP044 is IRFP540N. It has both higher transconductance and lower input capacitance than the IRFP044 and a higher breakdown voltage than IRFP044N. I have found a supplier here in Norway (Arrow)that is charging about $0.34 (2.80 kr) for them too..

On the downside is that these are all TO220 packages. But to me that is not as big a problem. I have a CNC mill, and can make whatever hardware necessary to clamp them properly to the heat sinks (and make my own heat sinks). My plan is to make a device taking the heat of the transistor on both sides of the metal tab on the transistor. One other thing Iím thinking about is to make the heat sinks electrically hot. I already have that kind of setup on my Pass A-40ís. This makes for a lower thermal resistance, and should make up for more than the losses introduced by using the TO220 case types. One other alternative Iím considering is water cooling. Iím easily able to make the heat exchangers myself on my mill. This all makes me not so frightened about using the lesser TO220 types. And after all, Iím only building the stuff for myself.

Ok, all of this is only my thoughts, and comments are more than welcome. After all, that is why Iím posting them here. And sorry for making such a long postÖÖ
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Old 21st December 2003, 03:25 PM   #2
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Having spent quite a few years since 1969 as a bench tech I'd have to say that the TO-220 case was the most often failed part I replaced in what ever I worked on, be it industrial, broadcast, or consumer. Many varities of failure were found but thermal stress was probably the most often found case(pun intended).

The larger TO-247P cases are a much better way to go, especially if you're going to be building a class A output device of some sort. Other wise your device might end up having a short lifespan. There is just something about TO-220 that I don't like for a P.O. output stage...seems very lacking and limiting in capabilities for any real high power usage.


Mark
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Old 21st December 2003, 04:03 PM   #3
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by Mark A. Gulbrandsen
Having spent quite a few years since 1969 as a bench tech I'd have to say that the TO-220 case was the most often failed part I replaced in what ever I worked on, be it industrial, broadcast, or consumer. Many varities of failure were found but thermal stress was probably the most often found case(pun intended).

The larger TO-247P cases are a much better way to go, especially if you're going to be building a class A output device of some sort. Other wise your device might end up having a short lifespan. There is just something about TO-220 that I don't like for a P.O. output stage...seems very lacking and limiting in capabilities for any real high power usage.

Mark
I would agree that TO-247 is usually much better for high power dissipation. However I did design and build several generations of amplifiers using TO-220 packages for output devices, either because of the price or because of availability only in this package. I can say from my experience that if you mount TO-220 package directly onto a heatsink without isolation washers and the thermal contact is good, than you can expect it to meet it's power levels from the datasheet. BUT if you use any kind of isolation material you may need to reduce maximum dissipation 3 to 5 times depending on the material. The reason is not just an additional thermal resistance, but much more dangerous is low mass of TO-220 pad, so the transients - common in audio amplifiers - could easily overheat and kill the device before the heat could be transferred to a heatsink. Worst kind of isolation is silicon rubber pads - forget these for TO-220, best in my experience is alumina and mika with a suitable thermal grease. In my designs I've used mika washers and did have good reliability with power dissipation upto 30-40 Watts per package.

x-pro
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Old 21st December 2003, 04:27 PM   #4
Magura is offline Magura  Denmark
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I too have the same experience as Mark, actually its gone as far as developing TO-220 allergy

I simply hate them with a passion, since every time ive used 220 packages theyve caused trouble. The heatsinking is as previously stated a major problem because of the little mass and a small footprint. In some cases ive gone as far as using TO-264 packages to get around thermal problems. In general nothing makes your life as easy as overkill

I usually run a safety factor of 4 or 5 if possible, In the end of the day it isnt those few euros difference there is between the TO-220 and the bigger brothers that makes the difference between making or not making an amp.

If you want good and reliable mosfets in europe, take a look at fairchild or on-semiconductor , they offer mosfets superior to IR that are a lot easier to get your hands on through farnell or RS components. IR have a funny policy on delivering the goods in europe.

You could replace irfp240/ irfp250 with fairchild HUF75945G3, the specs are fine and the price is reasonable.

Magura
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Old 21st December 2003, 04:47 PM   #5
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Quote:
Originally posted by Magura

If you want good and reliable mosfets in europe, take a look at fairchild or on-semiconductor , they offer mosfets superior to IR that are a lot easier to get your hands on through farnell or RS components. IR have a funny policy on delivering the goods in europe.

You could replace irfp240/ irfp250 with fairchild HUF75945G3, the specs are fine and the price is reasonable.

Magura
I didn't know about delivery/availability issue for IR products
in Europe.

If you're a klutz like me, soldering is definitely much easier with
TO247 devices.

x-pro's point considering TO220 and transient is very interesting.
I'm wondering if that may be less of a problem with
class A designs.

Dennis
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Old 21st December 2003, 05:34 PM   #6
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I've hardly developed an allergy for TO-220 packages yet. If they are utilized correctly as in the case of the Aleph amplifier or the Pass preamps they can be nice and handy. As an output stage though I'd agree with X-Pro about the need to mount them dorectly to the sink, or possibly use water or thermoelectric cooling.
Mark
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Old 21st December 2003, 05:36 PM   #7
Magura is offline Magura  Denmark
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OOHHH, youd be surprised how hard it is to get things here that are just a bit out of the ordinary (that is less ordinary than butter). The european market is too small for many companies to be really interested in it.

As for me, i live in denmark, thats even worse, a small country in europe (i.e. a small market in a small market)

Pretty much everything have to be ordered abroad or through the No.1 thieves Farnell or RS components. Any of those 2 companies would happily supply pretty much anything, but slow delivery (hence they have to import the goods from abroad) and high price (like twice the price of digikey or even more isnt out of the ordinary).

I usually try to limit my purchases to suppliers that have represetetives in one of the neghbour countries, else the whole thing turns into one big sponsor deal for the postal service.

Magura
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Old 21st December 2003, 05:43 PM   #8
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Why not just order directly from Digi-Key and bypass those theives?
Mark
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Old 21st December 2003, 05:49 PM   #9
Magura is offline Magura  Denmark
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Cause youd have to get it through customs, pay a fortune for the postage and end up with an even longer process.

Its unfortunately a general problem here. I race 1/5th scale rc cars, thats only possible because i have acces to a machine workshop, else i wouldnt get to race much....id just wait for spareparts(or have to stock an entire car in parts).




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Old 21st December 2003, 07:09 PM   #10
akb1212 is offline akb1212  Norway
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One thing is for sure, you guys over in the States have WAY better access to parts.... especially the IRF ones. Not to mention ebay..... I have bought LOADS of used end mills for my mill there, and have ended out with what I think is a nice collection. But I have spent almost half the price on these on shipping and other costs...... Electronics is not the only thing that is much harder to get parts for here.

One way that really seems to work for me though is to go directly to the big industry suppliers. In many cases they are willing to help out a DIY guy, and in other cases I just order them through my work. I'm lucky to have that possibility.

One other way for me to get parts is through Elfa og RS or those other thieves, but the prices there are.... well.... high!

And one more thing, as I brought up in my first post. There seems to be a new series of MOSFETS coming out that is replacing the old one. Farnell is one of my possible parts supplier, and they have already replaced the IRFP044 with IRFP044N, meaning I'll probably only be able to get the n-types.

Digikey is an option..... but as others have pointed out this has it's drawbacks. What I was looking for was to find a place to buy ALL the transistors I would need at this time. I would also have to buy enough to make the needed matching.

Oh, and the TO220 allergy thing..... Yes, I know what you mean! But I hadn't any real experience with it myself, and therefore wanted some of your thoughts and experiences here. Thanks, and as I feared it is not that good idea....

Although I'm still considering it for my midd and treble amps. I have 110dB/1W horns, so I would only need some 10-20 watts (my TAD2001 compression drivers are rated max 10W..). And as NP states, the transistors wants to run with lots of current and heat. And since these will be smallish amps I was thinking of using smallish power devices to, and run them as close to their limits as the TO3P devices is on the bigger amps.

I also want to use a regulated supply and lead-acid batteries as capacitors (as in Hiaraga's Monster) . I have access to as many 24Ah 12V UPS batteries I want (used, but they are always taken out of service long before they die), and was planning to be able to run the system off grid for a few hours without any interference from 50Hz.

Anders
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