MightyWatt: Lessons learned

Smart people learn from mistakes of others. And now you have an opportunity to learn from mine :-)
When I was developing MightyWatt, the Arduino Electronic Load, I had to change a lot of things from the first desing on breadboard.

Using FET in linear mode is generally a bad idea. But these devices are so much more convenient at high currents than BJTs. During development, I learned what is FBSOA and that all datasheets are not created equal. FET experience thermal instability in linear mode at high Vds and Ids. This instability is known (now even to me) as the Spirito effect:

P.Spirito (M. IEEE), G. Breglio (M. IEEE) V. d’Alessandro, N. Rinaldi (M IEEE)
Dept. of Electronics Engineering, Universita di Napoli “Federico II”, Via
Claudio 21, 80125 Napoli, Italy. 0-7803-7318-9/02 IEEE paper 2002

But not all datasheets account for it in their FBSOA curves. Mostly International Rectifier datasheets do have this kind of information.
So during testing, I fried three FETs in linear mode before finding the correct device. Luckily, IXYS semiconductors manufactures an entire series of FETs designed to operate in linear mode. The series is designated "L2" and is the kind of FET I finally settled on. The drawback is that they are more expensive at the same Rdson/Ids/Vds combinations than the standard "switching" FETs.

Measuring the temperature
Knowing the juction temperature of a FET is very important for the "load" kind of project. It directly determines the maximum safe power you can dissipate. But how to measure it? There are some articles that use the intrinsic diode as a temperature sensor. Fine, but it would probably need calibration of every single FET. The principle is basically this: You heat the FET in linear mode, then switch the load off and immediately measure the forward drop of the diode. It is proportional to junction temperature so you should be able to estimate it. I didn't go that way.
Thermal imaging is a convenient way of measuring the surface temperature. Provided you know the emissivity of the material and you actually have access to a thermal camera. I am lucky enough to have access to Flir i7 thermal camera at the institute where I study. But such technique is of course only good for calibration of other temperature sensor. The case temperature is lower than the junction temperature but not that much. Yes, there are thermal resistances from junction to case in the datasheet but they relate to the metal pad that is connected to the heatsink and hence not visible to the thermal camera. And you won't be able to accurately calculate the heatflux from the side of the FET to the surrounding air unless you do a lot of mathematical modelling. So an educated guess and a healthy safety margin is necessary. 

Here are the techniques I tried for measuring the case temperature:
1) Mounting a TO-220 digital thermometer in the same fashion as the FET, thus measuring the heatsink temperature. This turned out to be the slowest method. The TO-220 package has high thermal capacity so its thermal impedance is high.
2) Gluing a through-hole thermistor on the side of the FET using a thermally-conductive epoxy. This turned out to be faster than the TO-220 thermometer but also quite slow. The reported temperature was also a lot lower than the actual (thermal camera) case temperature. The good thing is that this method is cheap but you have to mess with epoxy.
3) Gluing a small PT100 thermistor on the side. This was faster than the regular thermistor because the PT100 was much smaller. But then again, there is the epoxy and you need at least one precision op-amp to convert the resistance to useful voltage for the ADC. Good method but expensive.
4) Using a thermocouple. Very fast response because the thermocouple is very small and very well thermally conductive. But needs even more expensive circuitry than the PT100 and epoxy again.
5) Placing a tiny (0402) thermistor beneath the FET and coupling it using thermal grease. This turned out to be faster than the PT100 but slower than the thermocouple. It lagged behind the thermocouple by just a couple of seconds. The advantage is very low cost and no need for epoxy. This method is the one I finally settled for. Easy, cheap, and adequetely fast response. The temperature is still different from the one from thermal camera but it can be easily recalculated. In fact, when the thermal camera reported 105 °C, the themistor was at 92 °C. If you know this, you only have to adjust the thermal limits. Say, if the junction temperature is 10 °C above the case temperature, then it was 115 °C. Ambient was 22 °C so it was 93 °C more. The thermistor reported 92 °C, 70 °C above ambient. 
This means that the actual difference is 1.33 times the measured difference. Recalculating to 25 °C ambient and 150 °C junction, the thermistor would report 119 °C. I set the maximum temperature to 110 °C to give me some safety margin. This would imply 94 Watts maximum power so the 70 Watts is very conservative.

Measuring the current
I was already convinced to use INA210 series current shunt monitors but my first attempt was with a 2512 power resistor acting as the shunt. It turned out that in the recommended layout, with sense traces heading to the centre of the resistor, a part of the current being measured goes through solder. And you know what? Solder has a horrible thermal coefficient for resistance. I got extremely unstable current values, differing up to 10 % from the real values. Because the resistor is directly connected to FET, you don't have to worry about power rating very much. It is thermally coupled to the heatsink so it is cooled by it. But it is also heated by the FET. I had to replace the resistor with a more special "four terminal" model from Vishay Dale. Now no solder would be between the measuring and power "terminals". Problem solved.

Preventing oscillations
FETs have large gate capacitance and that means oscillations when driving them with op-amps. I knew that but the actual solution to prevent the oscillations was the winner of a trial and error contest. What works on breadboard does not necessarily work on PCB. I tried various models of snubber circuit (RC, R-RC, ETC, I mean, etc.) but finally, the best solution was to actually increase the gate capacitance by placing a large, 10 microfarad, capacitor from the gate to ground. Without resistor, just utilizing the op-amp's output resistance. The oscillations only occur when the load capacitance is large enough than some threshold (different for different op-amps) but they fade out when the capacitance is very large.

Boosting the voltage
Gate voltage of such a power FET is more than the 5 V (more like 4.5 V) the USB can provide. I opted for swichted capacitor voltage doubler because the op-amp does not need a very large current or precise voltage. Anything between 7 and 20 Volts is OK. The first one I tried was LM2767 but it turned out to be too noisy, with its 11 kHz switching frequency. I replaced it with LM2766 with much less switching noise but unfortunately, the pinout is different so I had to change the PCB too.

Selecting thermal grease
High-performance thermal grease is a must for such an application. Luckily, thanks to the overclockers of the world, there is a big market for performance non-conductive thermal greases. I chose Arctic Silver 5 and it is perfect. But the wide selection thinned after I tried to get the service temperature range of the greases. The PC-modding industry does not produce datasheets. They produce PR leaflets. And CPUs run at much lower temperatures than I was planning to utilize. The Arctic Silver 5 was amongst the three greases I was able to get the service temperature for.

These were the major obstacles in my design efforts. I hope the knowledge will help someone, maybe you, to save some design time, components and swearwords.

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