Temperature simulation

protos · 25th October 2002, 02:19 PM

Hi all,

I was thinking of upping the bias on my A5 but I was worried about the temperature increase. So I thought why don't I simulate it in the Pass matching circuit and see how much the irfp240 can handle. Anyway I used a 33ohm power resistor and 30v. The bench ps showed about .780 A current but the mosfet was at about few degrees above ambient even while the resistor was into 80C. The connections were double checked and the Vgs of the transistor was around 4+ V.
Am I going the wrong way about it? Then what is the use of matching the transistors like this if you can't get them to operate at realistic temperatures as I see in my Aleph e.g. 80-100C.

protos · 26th October 2002, 01:43 PM

Is anybody out there?

rtirion · 26th October 2002, 02:08 PM

In the matching circuit the gate is connected to the drain.
This means Vds is Vgs. Dissipation in the fet will be aprox.
4*0.78 = 3.12 Watt. If you use a small sink on the FET, it will not
get hot. The resistor however will dissipate aprox.
30-4=26*0.78=20.28 Watt. This means very hot resistor.

The matching circuit is appropriate to measure the Vgs at a fixed
current through the FET.

Hope this helps a little.

protos · 26th October 2002, 06:35 PM

So how do you simulate the real thing so that you get high diisipation ?

rtirion · 26th October 2002, 10:05 PM

Best way I can think off is to build a simple ZEN, preferably one
with ALEPH style CS.
Then you can check bias current and all relevant temperatures.
You can test at no signal at the input, and fully driven.

Zapped · 28th October 2002, 09:16 PM

This is a little off the topic of temp simulation, but directly concerns the matching circuit for mosfets.

If I use the matching circuit discussed at PassDIY and on this forum, as well as elsewhere, and I get a Vgs reading on a MOSFET that appears normal (e.g. 4.25v or something) is that an indication that the device is working. Can there be any mode or portion of the device that has failed and still have this kind of measurement result?

I tried to get an answer or opinion on this in another post (on A-75 devices) but did not get one bit of feedback despite many, many, "viewings". The devices in question are IRF231 and IRF 9231 TO-3 devices, tested with the appropriate R in the setup.

Thanks

skaara · 28th October 2002, 09:28 PM

If you read vgs around 4V at some current, the fet should work..

25th October 2002, 02:19 PM	#1
protos diyAudio Member Join Date: Mar 2002 Location: Athens+Addis Ababa	Temperature simulation Hi all, I was thinking of upping the bias on my A5 but I was worried about the temperature increase. So I thought why don't I simulate it in the Pass matching circuit and see how much the irfp240 can handle. Anyway I used a 33ohm power resistor and 30v. The bench ps showed about .780 A current but the mosfet was at about few degrees above ambient even while the resistor was into 80C. The connections were double checked and the Vgs of the transistor was around 4+ V. Am I going the wrong way about it? Then what is the use of matching the transistors like this if you can't get them to operate at realistic temperatures as I see in my Aleph e.g. 80-100C.

28th October 2002, 09:16 PM	#6
Zapped diyAudio Member Join Date: Jul 2002 Location: Seattle	Slightly off thread, but This is a little off the topic of temp simulation, but directly concerns the matching circuit for mosfets. If I use the matching circuit discussed at PassDIY and on this forum, as well as elsewhere, and I get a Vgs reading on a MOSFET that appears normal (e.g. 4.25v or something) is that an indication that the device is working. Can there be any mode or portion of the device that has failed and still have this kind of measurement result? I tried to get an answer or opinion on this in another post (on A-75 devices) but did not get one bit of feedback despite many, many, "viewings". The devices in question are IRF231 and IRF 9231 TO-3 devices, tested with the appropriate R in the setup. Thanks

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26th October 2002, 01:43 PM	#2
protos diyAudio Member Join Date: Mar 2002 Location: Athens+Addis Ababa	Is anybody out there?

26th October 2002, 02:08 PM	#3
rtirion diyAudio Member Join Date: Nov 2000	In the matching circuit the gate is connected to the drain. This means Vds is Vgs. Dissipation in the fet will be aprox. 40.78 = 3.12 Watt. If you use a small sink on the FET, it will not get hot. The resistor however will dissipate aprox. 30-4=260.78=20.28 Watt. This means very hot resistor. The matching circuit is appropriate to measure the Vgs at a fixed current through the FET. Hope this helps a little.

26th October 2002, 06:35 PM	#4
protos diyAudio Member Join Date: Mar 2002 Location: Athens+Addis Ababa	So how do you simulate the real thing so that you get high diisipation ?

26th October 2002, 10:05 PM	#5
rtirion diyAudio Member Join Date: Nov 2000	Best way I can think off is to build a simple ZEN, preferably one with ALEPH style CS. Then you can check bias current and all relevant temperatures. You can test at no signal at the input, and fully driven.

28th October 2002, 09:28 PM	#7
skaara diyAudio Member Join Date: Jun 2002 Location: Europe, Slovenia	If you read vgs around 4V at some current, the fet should work..

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