Transistor working temperature

canvas · 5th May 2011, 01:17 AM

My amp's power transistors (right channel) were burned by accident. Faulty transistors are replaced with the new ones (matched pair), but I found they are cooler than left channel. I am sure both channel has same bias current, and I am wondering what could be the cause of this problem? If it is the hFE, which channel is higher? Thanks.

nightanole · 5th May 2011, 01:35 AM

If it was bad thermal transfer, then the heat sink will be cooler, but the transistors are hotter. maybe the original side is running hotter because it doesnt have fresh pads?

canvas · 5th May 2011, 02:33 AM

I think it's not the thermal transfer. Everything is the same, except the transistors. If I touch the transistors, they are different in temperature.

WuYit · 8th May 2011, 12:19 AM

Less losses at high frequencies?

wahab · 8th May 2011, 03:11 AM

The only explanation would be that the transistors are not
mounted such as the thermal dissipation is isotropic in respect
of their positionning..

That often occur when one side of the heatspreader
has better thermal contact with the case than the other side,
the transistors being mounted symetricaly on the heatspreader...

dewardh · 8th May 2011, 04:45 AM

Quote:

Originally Posted by canvas

I found they are cooler than left channel. I am sure both channel has same bias current,

How are you "sure" about the bias current? Did you measure it?

And don't ignore what nightanole is saying . . . dried up (old) heat sink compound on the left channel would cause the transistors to run hot . . . consider re-mounting them (like the new ones) with new grease, pads, or whatever was used on the right channel.

canvas · 8th May 2011, 08:30 AM

Yes, both bias current are monitored by two DMMs. The power transistors are 2SC3519/2SA1386. They are, of course, not from the same batch. Do you think different hFE would cause this?

DF96 · 8th May 2011, 11:15 AM

No, heat comes from current x voltage. You have measured the current, and presumably the voltage is the same, so the lower temperature must mean they are being cooled more efficiently. hFE is not an issue.

WuYit · 9th May 2011, 09:49 PM

Hi,
are all devices manufactured by SanKen? Could it be oscillation?

canvas · 9th May 2011, 11:23 PM

Hi,
they are all Sanken transistors. On my scope, there is no sign of oscillation. I am going to replace the left channel to see what happens. This time, all transistors are from the same source with identical batch number.

5th May 2011, 01:17 AM	#1
canvas diyAudio Member Join Date: Nov 2003 Location: Taiwan	Transistor working temperature My amp's power transistors (right channel) were burned by accident. Faulty transistors are replaced with the new ones (matched pair), but I found they are cooler than left channel. I am sure both channel has same bias current, and I am wondering what could be the cause of this problem? If it is the hFE, which channel is higher? Thanks.

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5th May 2011, 01:35 AM	#2
nightanole diyAudio Member Join Date: Feb 2009	If it was bad thermal transfer, then the heat sink will be cooler, but the transistors are hotter. maybe the original side is running hotter because it doesnt have fresh pads?

5th May 2011, 02:33 AM	#3
canvas diyAudio Member Join Date: Nov 2003 Location: Taiwan	I think it's not the thermal transfer. Everything is the same, except the transistors. If I touch the transistors, they are different in temperature.

8th May 2011, 12:19 AM	#4
WuYit Banned Join Date: Oct 2010	Less losses at high frequencies?

8th May 2011, 03:11 AM	#5
wahab diyAudio Member Join Date: Nov 2009 Location: algeria/france	The only explanation would be that the transistors are not mounted such as the thermal dissipation is isotropic in respect of their positionning.. That often occur when one side of the heatspreader has better thermal contact with the case than the other side, the transistors being mounted symetricaly on the heatspreader...

8th May 2011, 08:30 AM	#7
canvas diyAudio Member Join Date: Nov 2003 Location: Taiwan	Yes, both bias current are monitored by two DMMs. The power transistors are 2SC3519/2SA1386. They are, of course, not from the same batch. Do you think different hFE would cause this?

8th May 2011, 11:15 AM	#8
DF96 diyAudio Member Join Date: May 2007	No, heat comes from current x voltage. You have measured the current, and presumably the voltage is the same, so the lower temperature must mean they are being cooled more efficiently. hFE is not an issue.

9th May 2011, 09:49 PM	#9
WuYit Banned Join Date: Oct 2010	Hi, are all devices manufactured by SanKen? Could it be oscillation?

9th May 2011, 11:23 PM	#10
canvas diyAudio Member Join Date: Nov 2003 Location: Taiwan	Hi, they are all Sanken transistors. On my scope, there is no sign of oscillation. I am going to replace the left channel to see what happens. This time, all transistors are from the same source with identical batch number.

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