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Old 14th September 2005, 09:39 PM   #1
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Question selecting a transistor

Hi all:

I just joined the forum and I have a question. I am planning on building a class ab car amplifier, using n-channel mosfets in the converter and bipolars for the outputs. I have a fundamental problem. How do you accurately size mosfets, bipolars etc...? say for example you were using a pair of irfp250 for the outputs (yes I know they're mosfets) and a rail voltage of +-90v with an 8 ohm load. assuming adequate heat sink of course, how would you know:
1: the RMS output power?
2: amount of heat to be dissipated?
3: the maximum output power a mosfet will tolerate before it will blow up? (if you changed the load to say 4 ohms, upped rail V etc...)

thanks
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Old 15th September 2005, 01:25 AM   #2
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Every transistor has a data sheet and an SOA (safe operating area) graph with voltage across the device against the current through it, that shows what it can handle. Best to use the DC curve. Also you have to take into consideration the inductance or reactance of a speaker coil or else....
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Old 15th September 2005, 06:25 AM   #3
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To say the least, this would be an ambitious project. For an 8ohm stereo amplifier, the amp would likely draw well in excess of 150 amps of current. The supply alone would be difficult to build.

OK, now to answer at least part of your question...
If you have 90 volts of rail, you will be able to get approximately 64 volts (90*.707) RMS. Of course, there will be losses to prevent getting quite that much voltage but that's a good value to work with.

64 volts RMS into 8 ohms is ~500 watts. A class AB amp is approximately 50% efficient. If you are producing 500 watts into the speakers, you'll be dissipating that much in the transistors. For a bare minimum, you need to use enough transistors to dissipate 500 watts. The transistors you selected can dissipate 180 watts at 25C. You will NOT be able to keep them that cool. You need to derate the transistors based on the worst case operating temperature. If we take 80C as the worst case, the transistors are good for only 100 watts (180 watts minus 1.44W/C * 55 -- the difference between the 25C power rating and the worst case temperature). At a bare minimum, you will need 6 FETs per channel (3 for each half of the waveform).

For 4 ohm operation, you need to double the number of outputs (assuming that you have a regulated power supply). 4 ohms with 90 volt rails would not likely be feasible for a class AB amp operating from a 12 volt supply.

The heatsink would have to be huge to dissipate that much heat without fan cooling. With fan cooling, you would still need a large heatsink. From experience, I can tell you that an 800 watt amp running at full power (sine wave) will cause the temperature of a 500+in^2 heatsink to reach 80C within a few minutes (even with significant forced air cooling). With music, the amp wouldn't thermal. You'll have to decide whether you want the amp to be able to operate continuously at full power or not.

You may want to look at the MJL3281A and the MJL1302A for outputs.

I would suggest that you start with a less ambitious project. If you can not get this one to work, you may get discouraged and give up on audio. If you succeed with a smaller project, you're more likely to continue with more projects.
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Old 20th September 2005, 05:11 PM   #4
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So you are not stuck using only an 8 ohm speaker, use lower rails so you can drive 4 ohm. 90V rails is crazy

Amp I'm building will have peak +/- 70V rails that should hopefully not fall below +/- 60V under load running 4 ohm. Still finishing it, and waiting to get a better transformer, but it ran no problem with +/- 50V testing so far.

Used 5 pairs of the MJL4281/4302 already mentioned in this thread. Reason I used these is because they are 230W transistors and are large plastic ones so that they contact the heatsink good. When I heat them up, they don't feel any hotter than the heatsink.

If you wanna build a powerful amp, pick a good transistor that will handle your power needs, and you can parallel them for even more power handling. Also going with better transistors gives you better SOA so you don't blow up the amp later.

If you really want good performance, you could go TO3 case style which is best thermally, but I wouldn't recommend it because it's old case style, and hard to get heatsinks for, and also requires 2 screws, and washers, and pad for each one! Flat plastic TO247 just need a screw, sometimes a pad, and that's it.
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