What's the best approach to measuring an SMD component's temperature (using an SMD temperature sensor)

Question

Question

Is there a temperature sensor (or else) which allows for the measurement of a copper pour on a PCB (and therefor the approximate temperature of a component)?

I want to read the temperature via an ADC (due to firmware complexity) to provide telemetry and shut the system down when the temperature reaches a certain threshold. It needs to be consistent and quick enough to be useful, and I realize accuracy will be limited.

Background

I would like to know how hot my MOSFETs are getting. They use SMD packages, and im cooling them through the PCB with a heatsink and a fan on the other side.

My main issue is that it seems to me that most SMD temperature sensors are geared for ambient temperature measurement. I cannot connect any of their pins electrically (and thus thermally via the copper pour) to the hot part, and it would be too expensive for my application to add thermal jumpers (not that I know for sure that a strong thermal connection to the pads would yield a good response time)

It feels like my best bet could be to just place the temperature sensor (example) really close to each MOSFET's tab, and surround it in a copper pour. Perhaps doing something fancy like a comb pattern between the MOSFET's pour and the pour on the sensor's pins to increase surface area

The fact that this seems to not have a lot of questions about it (that I could find), or products that do this (that I could find) makes me think my approach may be wrong, or that im searching in the wrong place. Perhaps with a heatsink, the temperature differential would be slow enough to not need a strong thermal link?

Answer 1

An SMD temperature sensor is going to measure the PCB temperature primarily, so I think that would server your purpose with reasonable layout. Keep in mind the way the heat flows and possible resulting thermal gradients. You could use an easy semiconductor sensor or a thermistor or RTD, for example. It won't be instant, of course, there will be a lag from the die to the case and the case to the PCB where the sensor is. Temperature sensors are cheap and you can pepper the board with them if it helps.

Another consideration is to keep noise from the switching MOSFETs from affecting the temperature reading.

Answer 2

Yes, that is doable. Beware that the time constant is somewhere between the thermal diffusivity to that location, and the thermal time constant of the components and board (or part of it).

As example, I made this transient limiter,

TVS diodes are used to dissipate power during an overload condition, semi-continuously. The dissipation and time constant are such that, at maximum ratings, the surface temperature of the diodes may exceed ratings (over 150°C), but only briefly; within a few seconds at most, the thermistor reads the increase and shuts off the circuit.

Thermal diffusivity is relevant in this case. As a thermal pulse, the diodes heat the local area, and heat spreads out as distance \$\propto \sqrt{t}\$. FR-4 is in the 0.1 to 1 mm²/s range, up to 10 or so when heavy copper is present on multiple layers. The distance here is about 3mm from SMC pad to 0805 thermistor chip, so it takes on the order of a second to begin heating up significantly, and tens of seconds to "soak" in.

During that time, heat is flowing into the surrounding material, making a thermal resistance divider, but not just resistance but the effective capacitance (heat capacity) of the materials too, hence the time dependence.

Eventually (well, not in this case, but in yours perhaps), the rest of the board reaches equilibrium, and thermistor reads a temperature given by the thermal resistances. Which means you'll always have higher T_J because the transistors are generating heat in the first place, and there is some drop from there to the surroundings (and thermistor). And so on, as temperature drops across the board (due to removal by convection or conduction).

A similar argument gives a steady state thermal spreading distance of an inch or two for typical (say 2 layer 2oz copper) PCB, meaning something like a D(2)PAK is good for a watt or two in typical conditions. With thermal pad to a heat spreader/sink, more like 10W is feasible. Obviously in those conditions, with more heat sunk across board materials, the error (between T_J and T_thermistor) will increase.

In a switching application, be careful of switching noise (heavily filter the thermistor terminal(s) -- don't neglect the common mode!), and also be careful of breakdown voltage if at higher voltages.