System and method for temperature sensing using thermopile integrated with rigid printed circuit board
Robust estimation of temperatures inside and outside a device can be achieved using one or more absolute temperature sensors optionally in conjunction with thermopile heat flux sensors. Thermopile temperature sensing systems can measure a temperature gradient across two locations within the device, to estimate absolute temperature at locations that are impractical to measure using absolute temperature sensors. Using heat flux models associated with the device, the thermopile temperature sensing system can be used to estimate temperature associated with objects that contact an outer surface of the device, such as a user's skin temperature. Additionally, the thermopile temperature sensing system can be used to estimate ambient air temperature. Within a device, temperature measurements from the thermopile temperature sensors can be used to compensate sensor measurements, such as when the accuracy or reliability of a sensor varies with temperature.
1 . An electronic device comprising:
an absolute temperature sensor configured to estimate a first temperature;
a back crystal;
an optical sensor comprising one or more photoemitters and one or more photodetectors; and
a heat flux sensor comprising:
a printed circuit board (PCB) comprising an optical spacer between the optical sensor and the back crystal of the electronic device, the PCB comprising a thermopile, wherein the thermopile comprises a plurality of thermocouples in series;
a plurality of vias in the PCB comprising one or more first vias from a first layer of the PCB to a second layer of the PCB comprising a first conductive material with a first Seebeck coefficient and one or more second vias from the first layer of the PCB to the second layer of the PCB comprising a second conductive material with a second Seebeck coefficient, different from the first Seebeck coefficient;
a plurality of conductive traces on the first layer of the PCB and on the second layer of the PCB, the plurality of conductive traces interconnecting the plurality of vias; and
sensing circuitry coupled to the thermopile and configured to measure a voltage proportional to a temperature gradient between the first layer and the second layer of the PCB; and
processing circuitry configured to estimate a second temperature at the optical sensor and/or at the back crystal using the voltage proportional to the temperature gradient and the first temperature.
2 . The electronic device of claim 1 , wherein each of the plurality of thermocouples comprises one of the first vias and one of the second vias coupled by one of the plurality of conductive traces.
3 . The electronic device of claim 1 , wherein the plurality of vias comprise through-hole vias through the first layer and the second layer.
4 . The electronic device of claim 1 , wherein:
the PCB comprises a third layer and a fourth layer, the third layer and the fourth layer between the first layer and the second layer; and
the plurality of vias comprise through-hole vias.
5 . The electronic device of claim 1 , wherein:
the PCB comprises a third layer and a fourth layer, the first layer and the second layer between the third layer and the fourth layer; and
the plurality of vias comprise buried vias.
6 . The electronic device of claim 1 , wherein:
the PCB comprises a third layer and a fourth layer, the first layer and the second layer above the third layer and the fourth layer or the first layer and the second layer below the third layer and the fourth layer; and
the plurality of vias comprise blind vias.
7 . The electronic device of claim 1 , wherein the sensing circuitry is mounted on a surface of the PCB.
8 . The electronic device of claim 1 , wherein the first Seebeck coefficient is a positive Seebeck coefficient and the second Seebeck coefficient is a negative Seebeck coefficient.
9 . The electronic device of claim 1 , wherein the first conductive material is copper and the second conductive material is constantan.
10 . The electronic device of claim 1 , wherein the PCB has a thickness greater than 300 micron.
11 . The electronic device of claim 1 , wherein the PCB has a thickness greater than 1 millimeter.
12 . The electronic device of claim 1 , wherein the first temperature corresponds to a first location within the electronic device and the second temperature corresponds to a second location within the electronic device, the second location different than the first location.
13 . The electronic device of claim 12 , wherein the second location is separated from the first location by the PCB such that the second location is separated from the first location across a thickness of at least the PCB.
14 . The electronic device of claim 1 , further comprising:
a display comprising a second PCB.
15 . The electronic device of claim 1 , wherein the processing circuitry is further configured to estimate a physiological characteristic using the optical sensor and a wavelength of at least one of the one or more photoemitters.
16 . A method comprising:
at an electronic device including an absolute temperature sensor and an optical sensor comprising one or more photoemitters and one or more photodetectors:
measuring a first temperature at a first location using the absolute temperature sensor;
estimating a heat flux through a printed circuit board comprising, an optical spacer between the optical sensor and a back crystal of the electronic device and further comprising a plurality of vias that comprise one or more first vias from a first layer of the PCB to a second layer of the PCB comprising a first conductive material with a first Seebeck coefficient and that further comprises one or more second vias comprising a second conductive material with a second Seebeck coefficient, different from the first Seebeck coefficient, wherein the plurality of vias are interconnected to form a thermopile; and
estimating a second temperature at a second location different from the first location corresponding to the optical sensor and/or the back crystal using the first temperature and the estimated heat flux.
17 . The method of claim 16 , wherein estimating the heat flux through the printed circuit board comprises:
measuring a differential voltage across the thermopile; and
estimating the heat flux using the differential voltage, a thermal resistance of the printed circuit board, and a thermoelectric sensitivity of the thermopile.
18 . The method of claim 16 , wherein the second temperature measures a skin temperature at a point of contact between a user's skin and the electronic device comprising the printed circuit board.
19 . The method of claim 16 , wherein the first temperature corresponds to the first location within the electronic device comprising the printed circuit board and the second temperature corresponds to the second location within the electronic device, the second location separated from the first location by the printed circuit board such that the second location is separated from the first location across a thickness of the printed circuit board.
20 . The method of claim 16 , further comprising:
estimating a physiological characteristic using the optical sensor and a wavelength of at least one of the one or more photoemitters.