Mounting a small signal diode on heatsink - need suggestion

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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 old solution was to use diodes in a metal can and a small metal strap clamp. The old SWTPC Tiger amps did this. You could pot the diodes in a small piece of brass tubing and do the same thing using thermally conductive epoxy. You could wrap the diode in a piece of Sil-Pad type material and use a flat spring clamp. You could also find a suitable diode in a TO-220 case and mount it in the conventional way. You could use one junction of a small metal body transistor.
 
Well now, GK, I am unclear on your suggestion now.

Are you saying to connect using the collector AND base as a diode? (as implied in post #3) Or are you suggesting they connect the base TO the collector, leaving the B-E junction as the diode? (as implied in post #5, which was the suggestion syn08 made)

In other words, do you want to say connect the collector to the base, or do you want to say connect to collector and base?
 
In addition to the above, I assume the best transistor to use as a diode would be the same model as the output transistor to bias, as they will exhibit the same temperature coefficient. Would that just be splitting hair, or would any cheaper transistor behave close enough to be almost just as good?

And when it comes to setting the current through the "diode transistors", I guess the Ic v.s. Vbe plot in the spec sheet is what I should look at to make the initial prediction to determine Vbe voltage drop right? And I would assume the former is the reason why the collector should be tied to the base in this situation.

Thanks,

ig
 
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?
 
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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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
 
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
 
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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