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Generating a constant temp with a hexfet.
Generating a constant temp with a hexfet.
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Old 26th June 2002, 07:46 AM   #1
Circlotron is offline Circlotron  Australia
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Lightbulb Generating a constant temp with a hexfet.

While investigating the possibilities of running my amp driver transistors at a constant temperature as an alternative way to keep the temperature sensitive Vbe and therefore the bias current stable I had been thinking about various ways of making a mini oven sort of like are used with crystal oscillators sometimes. I figured that a large fet with a variable current put through it and bolted onto the same block as the drivers would be the go, just pick a suitable temp above ambient e.g 70 deg. Of course you have to measure the temp to regulate it and that is when I had this great idea! In the diagram the fet drain is fed from a constant current source of at least 60 volts. You set the gate voltage according to the temperature you want the fet to run at. Lower temp = higher voltage. The zener and diode are for startup from cold. When the drain-source had say 20 volts across it and the current is always 200mA then it's dissipation is 4 watts and it runs at a certain steady temp. If the ambient cools for instance then the fet starts to cool a little and this raises the gate threshold causing the fet to conduct less (same as lowering the gate voltage). This causes the drain voltage to rise and seeing it has always the same drain current but now a higher drain voltage, the dissipation rises and this pulls the die up to very nearly the temp it was before. The loop gain is very high because of the current source.

There are two very good things about this setup. 1/ the temperature is sensed by the gate junction which is only microns away from the drain-source so it knows what the die temp is *now*, not in several seconds time, so it is very stable with sudden variations in thermal load. 2/ As the set temp is approached the rate of change of drain voltage slows down the closer it gets. At one stage I was measuring 25 deg to 75 deg in about 10 seconds with way under 0.1 deg overshoot. Try that with a conventional feedback loop!

Maybe someone has done all this before but it's original as far as I am concerned. I hope someone finds this to be of use.

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Old 26th June 2002, 03:08 PM   #2
mrfeedback is offline mrfeedback  Australia
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Default Pratt Constant Temperature Source

PCTS - Good work !.
I think Peter Stein (ME Technologies) mentioned 50*C as sonically optimum temperatue.

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Old 26th June 2002, 04:00 PM   #3
tiroth is offline tiroth  United States
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Very neat Circlotron. Simple, and sounds like it works really well. Coming full circle, what do you think of creating a DIY ovenized oscillator on this principle? Of course, target dissipation would be much lower.

Offhand, this seems like it would work very well...thermally bond a TO-220 FET to a metal oscillator can, raise the temperature 15C above ambient, maybe add some insulation to prevent stray air currents from causing mischeif. The canned clocks I have are specd to 70C, and industrial versions are available specd to 110C+. Ftorres, is this anything like what you are considering?
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Old 26th June 2002, 06:16 PM   #4
MRehorst is offline MRehorst
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A nice idea, but I don't think it would work. The heating in a semiconductor is generated at a very small, microscopic point (a transistor junction). The heat spreads from there through the die and then is conducted to the case and then to the air.

The thermal gradient across even a single transistor causes distortion. This thermal gradient is a function of the audio signal. This is not as big a problem with a single transistor as it is in an IC, but it is still a problem. The junction heats up very quickly due to the small mass and relatively poor thermal conductivity of silicon. So even if you raise the temperature of the whole die, you'll still have the temperature varying at the junction in response to the audio, except that now instead of heating to 50 or 60 C above your ambient room temp, you'll be heating to 50 or 60 C above the heater temp. Very high junction temperatures lead to reduced lifetime.

Most semiconductors generate more noise as temperature is raised.

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Old 27th June 2002, 05:55 AM   #5
Circlotron is offline Circlotron  Australia
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Tiroth, I think it would work just fine. Actually one thing I forgot to mention is use a fet that has a nice low Rds on because when it is turned fully on it will still generate a little bit of heat and if the constant current source is too high then the I2R loss will put a lower limit on the heat output.
A covering to prevent air currents interfering works really well. Otherwise you could measure the drain voltage and use it like a hot wire anenomometer! It is great fun to watch the drain voltage at about 0.5cm/sec because even the very slightest air currents make several hundred millivolts difference.
The diode and zener are necessary for startup because when the fet is cold it is fully off and conducts no current so there is no heat. The zener makes sure the fet is always conducting when it is cold.

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