MOSFET constant temperature source - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: February 10, 2014, 06:07:29 am
content: I cooked up this cct way back in 2002 and never used it for anything. Maybe someone else will find it useful.In the diagram the fet drain is fedfrom a constant current sourceof 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 a constant 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 internal gate-source threshold voltage 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!Use a fet that has a reasonably 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.

author: cellularmitosis
author_rank: Unknown
timestamp: February 10, 2014, 02:06:44 pm
content: Cool! Added to my list of weekend circuits to try out...

author: Rufus
author_rank: Super Contributor
timestamp: February 10, 2014, 04:26:13 pm
content: Quote from: Circlotron on February 10, 2014, 06:07:29 amI don't really understand what you are saying. If you apply a voltage to the gate then why does it need the zener and diode to make it start? If you don't apply a voltage to the gate then the operating point only depends on leakage in the zener, gate, capacitor, etc.Applying a voltage to the gate does stabilize the MOSFET temperature but not very well because it is just a pretty low gain proportional controller and you need a temperature stable voltage to apply the value of which is very dependant on the MOSFET characteristics.

author: Marco
author_rank: Super Contributor
timestamp: February 10, 2014, 07:39:38 pm
content: Quote from: Rufus on February 10, 2014, 04:26:13 pmPresumably at ambient the voltage is below Vth.QuoteCould you provide the physics/math for that conclusion?Circlotron : did you ever try to measure how stable the MOSFET temperature was at varying ambient temperatures?

author: Rufus
author_rank: Super Contributor
timestamp: February 10, 2014, 09:56:11 pm
content: Quote from: Marco on February 10, 2014, 07:39:38 pmIt depends on the MOSFET - look at some data sheets. Vgs threshold might change by 10mV/C. Look at drain current vs Vds plots at fixed Vgs and guess how much a -10mV Vgs changes the drain current and how much Vds has to rise to compensate for that change. One I am looking at very roughly would be 0.4V Vds for 10mV Vgs and 0.4v at 0.5A constant current would be 200mW. The gain would be 200mW/C.

author: dannyf
author_rank: Super Contributor
timestamp: February 10, 2014, 10:08:51 pm
content: "If you apply a voltage to the gate then why does it need the zener and diode to make it start?"They allow the ccs to function even not signal is applied across that capacitor." If you don't apply a voltage to the gate then the operating point only depends on leakage in the zener, gate, capacitor, etc. "Or the g-s breakdown voltage. The design is self-contracdicting in that it requires a high voltage to work. Yet unless a current path exists for the zener to work, the same high voltage (60v or os) would also destroy the mosfet."Applying a voltage to the gate does stabilize the MOSFET temperature but not very well because it is just a pretty low gain proportional controller and you need a temperature stable voltage to apply the value of which is very dependant on the MOSFET characteristics."It is not clear what the OP meant by "constant temperature". One interpretation would be that the mosfet runs so hot that small changes in ambient temperature make no meaningful impact on its current output (aka an ocxo).Another would be that the mosfet has a positive tempco so it is self-stablizing. Lateral mosfets do a beautiful job here. Vertical mosfets (the one used here) exhibit positive tempco at so high I'd that they would be long destroyed before one could reach such current levels.

author: Marco
author_rank: Super Contributor
timestamp: February 10, 2014, 10:46:45 pm
content: Quote from: Rufus on February 10, 2014, 09:56:11 pmWhich for a TO220 even in air would be more than enough to get it back where it started at ... if insulated it's probably a couple of orders of magnitude more than that, so I don't see how that's low gain.PS. the proportional control comparison doesn't seem apt though ... the circuit is self stabilizing but I don't see how it will maintain the same temperature over a range of ambient temperatures.

author: Circlotron
author_rank: Super Contributor
timestamp: February 11, 2014, 12:36:46 am
content: Quote from: Rufus on February 10, 2014, 04:26:13 pmI had the thing running at about 70 deg C. If I switched it off and let it cool down then back on again when cold it would not begin conducting current and heat up again unless the gate voltage was raised a little. The zener and diode allowed it to restart just by switching it back on.

author: Circlotron
author_rank: Super Contributor
timestamp: February 11, 2014, 12:43:22 am
content: Quote from: Rufus on February 10, 2014, 04:26:13 pmThe fact that a mosfet is more or less a constant current sink within a wide variation of drain voltage (characteristic curves are above a certain current almost horizontal) combined with the fact that it is feed from a constant current source (VERY high impedance load) means that a very small change in gate-source voltage (or gate threshold voltage) makes a VERY big difference in drain voltage. It is quite high gain indeed. I don't remember the exact figure.

author: Circlotron
author_rank: Super Contributor
timestamp: February 11, 2014, 12:51:32 am
content: Quote from: Rufus on February 10, 2014, 09:56:11 pmExcept that the drain current isn't able to change with temperature in this cct because it it fed from a constant current source. The effect will be to change the drain-source voltage and hence the dissipation.

author: Circlotron
author_rank: Super Contributor
timestamp: February 11, 2014, 01:01:13 am
content: Quote from: dannyf on February 10, 2014, 10:08:51 pmIIRC I used an IRFP460 from a SMPS because I just happened to have one sitting on the bench.Some people get mixed up between the tempco of RDSon and Vth. Tempco of RDSon only makes a difference if the device is fully switched on; here we are only concerned with Vth varying with temperature.

author: Circlotron
author_rank: Super Contributor
timestamp: February 15, 2022, 11:44:13 am
content: Hell, it's been 8 years since I posted this.Just thought I'd bring it up to the surface for a new set of eyes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2022, 01:35:22 pm
content: Hah, cute. Didn't see it back then!Issues, hmm...Source resistance is in there, I suppose. (That is, external Vgs is internal Vgs plus Rs*Id.) Should be quite low in most cases to begin with, and can be made arbitrarily low by raising the voltage (mind that your transistor can handle the peak dissipation at Id*Vds(max), check the SOA!).Calibration stinks, you need Vgs(th) vs. T or something like that; but it's normally a pretty straightforward curve, so, once measured, should have a pretty tight say 2nd or 4th order polynomial fit and you can do whatever you like from there. Maybe the datasheet curve is close enough? Mmh, they usually have transfer for two temps and fixed Vds, not fixed Id and varying temp...Don't think it's prone to aging much or anything. It will shift if you expose it to excessive Vgs (at or beyond ratings), but, well, that'll be hard to do while it's running, and not something you'd generally do, anyway!I wonder if this has application for testing SOA, speaking of which. Though I think they do that differently (direct pulses of increasing magnitude, monitoring Id(t) or something like that for runaway behavior?).The elephant of course is the CCS. A resistor doesn't quite do because of varying Id. And anything linear will also waste power, just in a different location, which isn't exactly ideal for sensitive temperature measurements. A boost or flyback converter could be made with such a characteristic, and the filtering doesn't matter as thermal time constants are all that's needed. LED power sources might even be usable directly?Tim