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Mounting a small signal diode on heatsink - need suggestion
Mounting a small signal diode on heatsink - need suggestion
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Old 3rd November 2009, 02:28 PM   #11
peranders is offline peranders  Sweden
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Mounting a small signal diode on heatsink - need suggestion
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Originally Posted by IG81 View Post
Hello,

I am currently designing a class-A power amplifier.

I was looking to mount the diodes, that I plan to use in the current source that will set the output stage bias, on the heatsink, in order to thermally compensate the BJT's.

I am looking for suggestions as to how to physically mount the diodes on the heatsink.

Thanks!

ig
The normal thing is to use a Vbe multiplier on the heatsink. Do you know what a Vbe multiplier is?
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Old 3rd November 2009, 02:46 PM   #12
LineSource is offline LineSource  United States
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Can you still change to the On-Semi ThermalTrak transistor+diode? These transistors are used in many top rated amps. There are several good threads on how to optimize tracking with the attached diodes.

For TO3P packages you can use a large washer to clamp a small diode to the top metal tab. The diode leads need to be insulated, and the washer cannot make contact to the heatsink.

To minimize the thermal lag of a large heatsink, the diodes can be clamped or glued to the plastic back of the power transistor. A Dremal tool can be used to grind a grove into the plastic package for a small diode. Some engineers use a large aluminum bar running over the back of all transistors as a uniform pressure clamp which complicates this type of diode-package attachment.
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Old 3rd November 2009, 02:50 PM   #13
IG81 is offline IG81  Canada
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Quote:
Originally Posted by peranders View Post
The normal thing is to use a Vbe multiplier on the heatsink. Do you know what a Vbe multiplier is?
Yes, a.k.a. rubber diode.

The talk about using transistor BE junctions led me to consider this too.

I know they are often used in push-pull stages. My design is single-ended, but it could be used to set a constant voltage drop for a current source. One of my requirements is that I need this voltage drop to be around 1.3V to 2V. I don't have time to look at transistor spec sheets or to run simulations right now, but would a rubber diode be at least as performant as a straight Vbe drop at the mentionned voltages?

ig
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Old 3rd November 2009, 02:59 PM   #14
IG81 is offline IG81  Canada
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Quote:
Originally Posted by LineSource View Post
Can you still change to the On-Semi ThermalTrak transistor+diode? These transistors are used in many top rated amps. There are several good threads on how to optimize tracking with the attached diodes.

For TO3P packages you can use a large washer to clamp a small diode to the top metal tab. The diode leads need to be insulated, and the washer cannot make contact to the heatsink.

To minimize the thermal lag of a large heatsink, the diodes can be clamped or glued to the plastic back of the power transistor. A Dremal tool can be used to grind a grove into the plastic package for a small diode. Some engineers use a large aluminum bar running over the back of all transistors as a uniform pressure clamp which complicates this type of diode-package attachment.
That's a cool product!

I think I'll stay with my current output devices, NJW0281G, since I already have a number of them on hand.

But that's good to know such a thing exists, thanks!

ig
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Old 3rd November 2009, 05:12 PM   #15
east electronics is offline east electronics  Greece
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here is my 2 cents

sine i ve noticed that 1N4148 worked fine for my application what i ve done is actually digg a hole bigger the size of the diode but not realy deep in the heatsink with my dremel exactly down under one driver then filled in the space with very nice white paste and used the driver transitor bolted on atop to keep things together

as about insulation only the leads of the diode ( S ) need to be insulated

regards sakis
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