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Old 8th April 2009, 10:02 PM   #1
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Default Practical use of ThermalTrak diodes

My current amp prototype just uses the Thermaltrak diodes in a chain with a series resistor for adjustment but I'm thinking about more cunning ways to use them.

My next experiment will be to use just two in series with the emitter of a conventional Vbe multiplier.

What I'm thinking is whether I actually need to use all Thermaltrak devices. What if...

I adopt the topology (I don't know the name) where you have an output device as a driver which actually provides the output at low levels with the main output devices biased into pure class B (minimal bias current).

I could then just use the Thermaltrak devices for the drivers then conventional packages for the rest of the outputs transistors. That would then provide two diodes for heatsink temperature sensing and control of the bias of the drivers. The disadvantage is not having as good a sensing of the temperature as if the diodes were in the main output devices but should still be better than a conventional Vbe multiplier using a seperate device?

Any thoughts people?
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Old 9th April 2009, 01:02 PM   #2
h_a is offline h_a  Europe
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I'm only aware of large package thermal-tracks, so it seems a little strange to me that you want to use them as drivers. You are using for sure some high-beta outputs where such drivers are not necessary - at least for the standard output powers.

If you want tight thermal tracking I think that using them as they currently are is the more precise option (although the thermal track diode has a different temp. coefficient as the bjt itself).

Also if you want pure ClassB there is no bias to keep constant - but you probably mean optimum biased ClassAB as per D. Self?

Have fun, Hannes
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Old 9th April 2009, 01:29 PM   #3
Pingrs is offline Pingrs  United Kingdom
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I once wondered about the usefulness of thermal track TO220's as drivers, if they existed.

Presumably this will be an EF stage with the drivers clamped to the main heatsink. If driver H/S is seperate, then there will be very little change in temp to control the bias anyway.

I'm also looking for a way to reduce the number of TT devices. Maybe Bob Cordell's two TT arrangement, with the remaining OP devices as standard, would do it.

Brian.
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Old 9th April 2009, 01:45 PM   #4
AndrewT is offline AndrewT  Scotland
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A CFP output needs to monitor the driver stage temperature.

A nice beefy NJL driver pushing multi-paralleled MJ21193/4 could be very nice.
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regards Andrew T.
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Old 9th April 2009, 04:19 PM   #5
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I think I need to draw a diagram to explain what I'm proposing.

I'm looking at one ThermalTrak as a driver/output device, with maybe as many as 5 other output devices being driven from it in standard darlington configuration. This would be for fairly serious output power. All devices on the same heatsink.

Talking of high current gain output stages, I've been working on some Studiomaster amps which use a four-tier Darlington output stage!

By the way, has anybody looked at which standard OnSemi output devices match the ones inside the ThermalTraks?
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Old 9th April 2009, 06:00 PM   #6
h_a is offline h_a  Europe
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Quote:
has anybody looked at which standard OnSemi output devices match the ones inside the ThermalTraks?
It's just a different package with a different prefix in the name; if you replace the leading N with an M you're with the standard parts. I'm not 100% sure on the letter as it has been a little time since I played with bjts last.

Have fun, Hannes
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