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Old 2nd October 2009, 10:08 PM   #1
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Default Best way to implement thermal protection?

Please share your opinions and thoughts on the best way to implement thermal protection for an amplifier - techniques I have seen are:

1) cut AC power
2) turn off the current source loading the input LTP (for example Adcom)
3) turn on a fan
4) trigger a limiter on the input, reducing drive

1 and 2 are drastic but effective. 3 helps the symptom but doesn't address the underlying cause. 4 has the advantage of not stopping the party but could be ignored leading to long term damage.

I'm experimenting with shunting current away from the bases of the OPS drivers, similar to the SOA protection circuits discussed by Doug Self, but triggered by over temp rather than SOA constaints. It occurred to me that with ThermalTrak transistors it is really easy to monitor junction temp in the output stage.

What are your thoughts on the pros / cons of these approaches?
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Old 2nd October 2009, 10:12 PM   #2
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5) use the right size heatsink(s)
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Old 2nd October 2009, 10:14 PM   #3
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Monitor heatsink temperature and either mute the input or disconnect the output.

It's only needed though in cases of abuse, normal use it should not overheat, if it does the heatsinks are too small.
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Old 2nd October 2009, 10:23 PM   #4
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Give detecting element ( e.g. Airpax ) directly ( or near ) power device...
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Old 2nd October 2009, 10:55 PM   #5
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Quote:
Originally Posted by richie00boy View Post
Monitor heatsink temperature and either mute the input or disconnect the output.

It's only needed though in cases of abuse, normal use it should not overheat, if it does the heatsinks are too small.
The point is to be able to construct an amplifier that wouldn't need huge heatsinks sized for worst case, and conservative thermal limits. Monitoring heatsink temp for bias and overtemp tells you the integral of the power for the last several minutes, not what's happening right now.

Instantaneous junction temp can be far above the heatsink temp. You could have a much smaller heatsink designed for "average" power, and even let it run fairly hot, knowing that by monitoring the junction temp you can keep the output stage safe from harm by turning the power down.
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Old 2nd October 2009, 11:02 PM   #6
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I myself try to size HS appropriately

Then thermal fan control, followed by power down. I have never had an amp overheat to the point of the AC kill tripping, but the protection is there.
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