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Old 24th May 2005, 09:24 PM   #1
AR1 is offline AR1  United States
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Default FET's?

For OPS design which is preferred:

MOSFET
L-MOSFET
HEXFET
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Old 24th May 2005, 10:14 PM   #2
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I personally like the 2SK1529/2SJ200 for outputs. These are Lateral types, But the negative TempoCo doesn't kick in till 6 Amps or such, so they still need a Vbe multiplier.

I've Used standard Laterals (SK1058/SJ352 and the higher voltage Equilivant) but find them to be a bit touchy. I've blow through too many of them while experimenting with circuits, so I use the above Mosfets as they are very robust and I've lost very few of them. Also the Above Fets are touted as being made specifically for audio.

Seems the other popular candidates are the I.R. devices (IRFP240/140/040 Etc.). I've used the IRF540/9540 and they seem alright. However the Vgs on is a tad higher than the above devices. I still use the IR device for power supplies.

This is just my experience/opinion, hope it helps.

-D.
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Old 25th May 2005, 03:00 PM   #3
ilimzn is offline ilimzn  Croatia
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A word of warning about vertical MOSFETs (including T-FETs, HEXFETs etc):
Types claimed by the manufacturer to be complementary, are not so electrically, but merely by the fact that N and P doping is reversed (oversimplification but close enough). This means that the P parts have higher Rdson and lower current capacity and GM than their N counterparts, and this is simply a matter of physics, born out of lower carrier mobility in P channel parts.
In audio design, complementary means the closest you can get in major electrical parameters, for amplifiers in particular, you want to see matching current and power handling, GM, treshold voltage and gate/drain and gate/source capacitances. Maximum voltage handling is secondary and needs to be higher than rail-to-rail voltage plus a security factor - higher will not make much of a difference. In order to get good audio from IRF/IRFP and similar parts, keep in mind that IRF/IRFPxxx and IRF/IRFP9xxx are not the complements you need - a bit wider search in the databooks will reveal a couple of better combinations, which will come VERY close or even be indistinguishable from LMOS, at a small fraction of the price. Hint: P parts will have lower Vdsmax than N parts...
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Old 25th May 2005, 03:37 PM   #4
AR1 is offline AR1  United States
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Default OPS devices

In a related matter I am repairing an old Carvin FET 1000 Amplifier, I am using IRFPXX and IRFP9XX parts to replace the output stage devices which are all fried. I did not have the original manufactures part number as the OPS was apparently already replaced once and the tech obviously replaced them with IRFPXX devices.

I did notice that the Ron Spec was notably different between the IRFPXX and the IRFP9XX. Is this going to be an issue in performance. Sounds like their might be some crossover distortion issues, correct? Perhaps this is why it fried in the first place?
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Old 25th May 2005, 03:42 PM   #5
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P channel devices will have higher Ron than the equivalent N channel devices (law of physics problem!).

Ron is not a very important parameter in linear class A or B amps as the output devices are operating in the linear region. Ron is normally only important if the FETs are being used in a fully-enhanced (saturated) mode, such as in Class-D amps. (and is very important in keeping losses low in high-current switch-mode regulators).
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Old 25th May 2005, 04:21 PM   #6
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Both vertical and lateral MOSFETS can be good for audio. I'm sure searching the forum will come up with some of the many discussions there have been comparing the two types. Personally I prefer lateral types, particularly those by Exicon.
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Old 25th May 2005, 05:42 PM   #7
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by Ouroboros
Quote:

P channel devices will have higher Ron than the equivalent N channel devices (law of physics problem!).
Ron is not a very important parameter in linear class A or B amps as the output devices are operating in the linear region...
True, but the same mechanism responsible for higher Ron in P parts is also responsible for lower maximum current and lower GM, and for linear operation both certainly are a problem.
Due to this issue, not only does eg. IRFP240 have 21A Idmax compared to 12A for the IRFP9240, it also has nearly 50% higher GM. I know I would not call that complementary, yet these are invariably used as complements. Something to do with Lemmings, I suppose
Lower GM on the P part not only introduces crossover distortion, but also asymetrical current limit, with resultant distortion under heavy load being the least problem - short term power dissipation issues and even a particulairly nasty form of thermal runaway (if the current limit is set close to the max for the part) can occur.
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Old 25th May 2005, 05:58 PM   #8
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In my circlotron amps I find that IRFP250N and IRFP250N is a very close match - very complimentary!!
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Old 25th May 2005, 06:27 PM   #9
AR1 is offline AR1  United States
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Quote:
Originally posted by ilimzn
[B]
Ron is not a very important parameter in linear class A or B amps as the output devices are operating in the linear region...


Sounds like I am going to have some issues if I use the IRFP240 and the IRF9240. I already bought the devices so I might as well use them and see what happens.
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Old 25th May 2005, 06:49 PM   #10
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by AR1
Sounds like I am going to have some issues if I use the IRFP240 and the IRF9240. I already bought the devices so I might as well use them and see what happens.
Try IRFP340 and IRFP9240
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