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Author Topic: Transistor Ratings  (Read 3221 times)
KT0DD
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Posts: 277




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« on: December 21, 2012, 02:21:08 AM »

I see on RF Parts site that they rate the 2SC2879 transistor at 100W PO. Is this maximum continuous ratings like plate dissipation on tubes, or is this the maximum linear peak output power?

I see some outrageous claims of 2SC2879 based amplifiers (mostly CB'er trash) and even Ameritron's ALS 500M is rated at 500 watts with 4 100 watt devices?

Just wondering how to read the ratings.  Thank You.

73, Todd - KT0DD
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M0HCN
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Posts: 473




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« Reply #1 on: December 21, 2012, 10:05:22 AM »

From the data sheet, carefully, paying attention to operating mode, and specified cooling conditions (There is a common trick to rate sand at 25C case temperature which can never be maintained in practice....IXYS Looking at YOU!).

Most RF power semis specify various limits (Pd, Tj, current, voltage), so it is not really a one number sort of game.

For typical class AB service you can assume somewhere around 50% efficiency in a typical amateur service design (It is possibly to do much better in a more specialized architecture), so a 100W Pd part is probably good for about that much output if properly cooled and matched. 
In a purely SSB service application it may be permissible to go a bit higher on PEP by lowering the drain match impedance at the cost of reliability (You get away with this because SSB has a very high power back off ratio), I wouldn't do it, but you can if smoked power fets are not a big issue to you.

73 Dan.
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KD0REQ
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Posts: 898




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« Reply #2 on: December 21, 2012, 11:22:04 AM »

you can exceed most tube specs temporarily without catastrophic results.

not so semiconductors.  when you are looking at the sheet, if they say max temperature, max voltage between any two elements, maximum current, maximum power... they mean it.  for each rating.  for any time beyond zero.

they also mean the derating curves, absolutely.  if it says capacity goes down 50 percent each 10 degrees centigrade you step beyond the 25 C benchmark, that's no fooling either.

if you buy a part that is a derated version of a higher power part, and I don't have my old spec books on hand now for examples, you might get a "batch failed" part in which the test pieces didn't make the higher voltage spec, but work at a lower voltage.  somewhere in between, most of the parts will work over the rated limit.  but you don't know where they would fail, or which ones, unless you are equipped to do curve tracing.

where you can "push the ratings" like the amp you asked about, the average load on the part will not push the component into thermal failure, and none of the instantaneous ratings are violated in any event.  so they're probably depending on your not running continuous two-tone and crossing the dissipation limits of the device at the heat sink.  you can generally add up all the instantaneous ratings of any semiconductor and find if you ran it there, you violate something like power dissipation or thermal dissipation by a country mile.
« Last Edit: December 21, 2012, 11:25:15 AM by KD0REQ » Logged
KH2G
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Posts: 257




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« Reply #3 on: January 03, 2013, 02:12:33 PM »

Most data sheets will give ccs or icas ratings. Icas is intermittent commercial and amateur service and ccs is continuous commercial service. (Just like tubes that way). Solid state devices tend to be less forgiving when ratings are exceeded even for brief periods but that's why data sheets.
Regards
Dick KH2G
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KB1GMX
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Posts: 761




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« Reply #4 on: January 03, 2013, 02:52:24 PM »

ICAS and CCS only apply to tubes.  For solid parts its pulse and duty cycle, CW/FM or SSB and max permissible junction temperature and junction thermal resistance.  Sort form is how much heat,
for how long and how fast and it cool itself on a really really large heatsink.

That part is 100W output nominal (could be 93 or 122).  two push pull maybe nearly twice
that peak.   Any claim of significantly more is BS.

Three things you cannot exceed, collector to emitter voltage, Maximum collector current, and maximum junction temperature (typically 150-200C) above those sand is likely to reoccur.

Some CBers use the IHFM rating, one pulse at 200% of ratings in to a 25 ohm load for not
less than 1 second and only has to do it once. 


Allison
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G3RZP
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Posts: 4477




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« Reply #5 on: January 04, 2013, 12:29:07 AM »

Generally, the hotter the chip, the shorter the life - diffusion effects do occur at high temperature over time and there's also metal migration, even with today's advanced metallisation techniques. At one time we had a $125 each (for a 100 piece order) 5 transistor array, with a guaranteed life of 30 minutes at 250 deg C. Used in oil drilling bits.

Diffusion at room temperature in tunnel diodes limits their life - this is because of the very high doping density.
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N4ATS
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« Reply #6 on: January 04, 2013, 06:14:12 AM »

Interesting claims....

http://www.eham.net/ehamforum/smf/index.php/topic,66330.msg442395.html#msg442395
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WX7G
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Posts: 5977




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« Reply #7 on: January 04, 2013, 06:27:16 AM »

http://www.rfparts.com/pdf_docs/2SC/2SC2879.pdf

Lookiing at the 2SC3879 datasheet we see why it is rated for 100 watts output. The IMD is only -24 dB at 100 watts. This is without negative feedback. The Ameritron ALS-500 uses negative feedback for improved IMD performance.

http://www.ameritron.com/pdffiles/ALS-500M.pdf

Note that the collector dissipation rating is 250 watts. This is based on a 25 deg C cold plate and the transistor junction at 175 deg C. From this we calculate a theta JC of (175-25)250 = 0.6 deg C/W. Key down at 100 watts power output and given 50% DC-RF efficiency the junction will run 60 deg C above the heatsink temperature. So, 100 watts continuous output is possible given proper heatsinking.
« Last Edit: January 04, 2013, 06:34:22 AM by WX7G » Logged
ZENKI
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Posts: 916




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« Reply #8 on: January 04, 2013, 10:01:48 PM »

The IMD sweet spot is about 60 watts at -37db 3rd. Most ham transceivers use a pair of these for 100 watts of output  which is a nice conservative way of running them.
When I read the  silly CB output  power specifications  I just switch off!

A pair of 2SC2879S are good for 100 watts of clean output. The new Mitusbishi FETS produce even better IMD performance.

http://www.rfparts.com/pdf_docs/2SC/2SC2879.pdf

Lookiing at the 2SC3879 datasheet we see why it is rated for 100 watts output. The IMD is only -24 dB at 100 watts. This is without negative feedback. The Ameritron ALS-500 uses negative feedback for improved IMD performance.

http://www.ameritron.com/pdffiles/ALS-500M.pdf

Note that the collector dissipation rating is 250 watts. This is based on a 25 deg C cold plate and the transistor junction at 175 deg C. From this we calculate a theta JC of (175-25)250 = 0.6 deg C/W. Key down at 100 watts power output and given 50% DC-RF efficiency the junction will run 60 deg C above the heatsink temperature. So, 100 watts continuous output is possible given proper heatsinking.
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ZENKI
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Posts: 916




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« Reply #9 on: January 04, 2013, 10:14:14 PM »

I got  500 watts PEP out of a pair, they lasted 1 second!  With the lack of specific information we can only  imagine what the failure rate  of these devices in the CB amps are like.

In my book the ALS500m is only really  a  200 watt output amp, that is if you are running the devices in a  conservative manner. SO in my opinion the ALS500M is really being driven to the limit at 500 watts of output power.
The IMD performance at 500 watts of output power would be very poor. 125 watts output per device and  the IMD would be really bad if the IMD figures on the datasheet is PEP IMD figures.

Even though you operating this amp from a mobile  where the antenna efficiency is poor, this is  still no excuse for driving any amplifier close to its saturation point.
I have a HF MANPACK radio that runs on batteries that has a 2 tone IMD performance of -36db. Its really easy designing clean 12 volt amplifiers. Unfortunately  most ham  solid amplifiers are just glorified
versions of  CB amp designs.

If you want to see what the  how clean  a pair of 2SC2879s can be in a properly designed amplifier. Look at the QST review of the Yaesu FT990. You wont find PA's in radios designed so conservatively  these days.



I see on RF Parts site that they rate the 2SC2879 transistor at 100W PO. Is this maximum continuous ratings like plate dissipation on tubes, or is this the maximum linear peak output power?

I see some outrageous claims of 2SC2879 based amplifiers (mostly CB'er trash) and even Ameritron's ALS 500M is rated at 500 watts with 4 100 watt devices?

Just wondering how to read the ratings.  Thank You.

73, Todd - KT0DD
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WX7G
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Posts: 5977




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« Reply #10 on: January 05, 2013, 08:52:51 AM »

The ALS-500 uses negative feedback and the transistor datasheet IMD specs cannot be taken and applied to the ALS-500.

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ZENKI
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Posts: 916




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« Reply #11 on: January 06, 2013, 01:55:28 AM »

That might be true however it does very little for the IMD performance. If negative feedback is such a panacea why is the IMD performance so poor on the ALS500?
I still maintain that anything over 100 watts of PEP output per pair is pushing  the devices  to the point where  something like negative feedback will achieve very little for IMD.

Anyway its a mobile amplifier and thank our lucky stars very few hams use them for fix station use.

The ALS-500 uses negative feedback and the transistor datasheet IMD specs cannot be taken and applied to the ALS-500.


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G3RZP
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Posts: 4477




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« Reply #12 on: January 06, 2013, 03:11:44 AM »

Once you are pushing them into gain compression, NFB won't help matters at all.
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M0HCN
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Posts: 473




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« Reply #13 on: January 06, 2013, 05:31:54 AM »

Actually NFB does still help, for all that one usually cannot apply as much of it as would really be desirable.

The pain is that most RF stages can only run a few dB of NFB because the open loop gain just is not there and feedback around multiple stages raises stability issues.
That modern LDMOS has in the region of 20dB open loop gain at HF makes NFB a far more useful technique then it was with the old 10dB bipolars, but far too often this is seen as an excuse to reduce the number of gain stages rather than increase the linearity of the whole chain. 

73 Dan.
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G3RZP
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« Reply #14 on: January 07, 2013, 02:56:15 AM »

If the stage is in gain compression, NFB cannot help. It will reduce distortion at levels below gain compression, however, and higher open loop gain helps, although stability needs looking at - Mr. Nyquist figured all that out, although I find Bode plots easier to follow. If NFB did help when in gain compression, then you could run Class C in the PA.
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