System and method for temperature sensing using thermopile integrated with flexible circuit
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 . A heat flux sensor comprising:
a flexible printed circuit board (PCB) including a thermopile, wherein the thermopile comprises a plurality of thermocouples in series, the flexible PCB comprising:
a first conductive layer comprising a first conductive material of the thermopile with a first Seebeck coefficient, the first conductive layer being arranged on a first plane that is perpendicular to a thickness of the flexible PCB;
a second conductive layer comprising a second conductive material of the thermopile with a second Seebeck coefficient, different from the first Seebeck coefficient, the second conductive layer being arranged on a second plane that is perpendicular to the thickness of the flexible PCB; and
a plurality of vias of the thermopile between the first conductive layer and the second conductive layer; and
sensing circuitry coupled to the thermopile and configured to measure a voltage proportional to a temperature gradient between a first end of the flexible PCB and a second end of the flexible PCB, the first end and the second end being located on opposite sides with respect to a length of the flexible PCB.
2 . The heat flux sensor of claim 1 , wherein the first conductive material is copper and the second conductive material is constantan.
3 . The heat flux sensor of claim 1 , wherein the first conductive layer is patterned with a first plurality of conductive traces of the first conductive material and the second conductive layer is patterned with a second plurality of conductive traces of the second conductive material.
4 . The heat flux sensor of claim 3 , wherein each of the plurality of thermocouples comprises one of the first plurality of conductive traces and one of the second plurality of conductive traces coupled by one of the plurality of vias.
5 . The heat flux sensor of claim 1 , wherein the plurality of vias includes a plurality of first vias and a plurality of second vias, wherein the plurality of first vias are disposed within a first threshold distance of the first end of the flexible PCB and wherein the plurality of second vias are disposed within a second threshold distance of the second end of the flexible PCB.
6 . The heat flux sensor of claim 5 , wherein the first threshold distance and the second threshold distance are less than 50 micron.
7 . The heat flux sensor of claim 1 , wherein the sensing circuitry is mounted on a surface of a rigid PCB and the first end of the flexible PCB is coupled to the rigid PCB.
8 . The heat flux sensor of claim 7 , wherein the first end of the flexible PCB is bonded to the rigid PCB with a conductive adhesive.
9 . The heat flux sensor of claim 1 , wherein the flexible PCB includes one or more signal traces or one or more power traces independent of the thermopile, the one or more signal traces or the one or more power traces configured to route one or more signals or one or more power sources from the first end of the flexible PCB to the second end of the flexible PCB.
10 . The heat flux sensor of claim 9 , wherein:
the flexible PCB comprises a third conductive layer comprising the first conductive material, the first conductive layer being between the second conductive layer and the third conductive layer; and
the one or more signal traces are implemented in the first conductive layer using the first conductive material and are shielded by the second conductive material in the second conductive layer and by the first conductive material in the third conductive layer.
11 . The heat flux sensor of claim 9 , wherein the flexible PCB comprises a third conductive layer comprising the first conductive material, the second conductive layer being between the first conductive layer and the third conductive layer; and
the one or more signal traces are implemented in the second conductive layer using the second conductive material and are shielded by the first conductive material in the first conductive layer and by the first conductive material in the third conductive layer.