Feature: Medical
20°C and 24°C and maximum air convection rates are about 5W/ m2
K. Normal forehead core body temperatures are 36-38°C;
below 36°C the condition is called hypothermia and above 38°C – hyperthermia, both of which are considered serious and, thus, necessitate monitoring during all phases of surgery.
Layout considerations for heat flow Te CBT patch device is designed to measure vertical heat flow from the human tissue surface using two MAX30208 temperature sensors (Figure 2); the other two temperature sensors (Figure 1) help track lateral heat losses. Data from the temperature sensors, combined with thermal models for the conductive plug and insulator, are used to accurately estimate CBT on the human forehead. To accomplish this task, the rigid-flex PCB with temperature
sensing circuit must: • Incorporate temperature sensors with high accuracy; • Use low power (the temperature sensors) so as to not adversely impact the thermal system;
• Have signal traces with dimensions that minimise heat flow to and from the temperature sensor, so that the thermal system is not adversely impacted; However, the electrical connections necessary for electronic
devices conduct heat as well, which is not ideal. Figure 3 highlights the main heat-flow paths of interest. Ideally, the thermal resistance of the PCB traces should be very large compared to the conductive plug, to keep heat loss (or gain) errors negligible. Whilst implementing a commercially-available metal for signal
and power traces, a thermal-electrical design trade-off is still needed for the rigid-flex interconnections. Te design formula for electrical and thermal resistance is shown in Figure 4. Te thinner and longer the rigid-flex PCB trace is, the larger the thermal resistance. Tis provides a means to set the thermal resistance to be greater than the primary conductive plug, minimising thermal leakage from the CBT thermal system. Unfortunately, the electrical resistance also increases, leading to several adverse effects such as power-line voltage drop, increased temperature of the PCB trace, and an increased RC time constant for the I2
C communication
lines. Before considering the PCB traces, the thermal behaviour of
the conductive plug should be evaluated first to establish a design baseline. Te heat path has a cylindrical form; see Figure 5. Te thermal resistance of the CBT patch conductive plug can be
calculated knowing the material conductivity and dimensions, as follows:
Figure 4: Electrical and thermal conduction of PCB traces
than that of the CBT patch conductive plug; for example, RTH (PCB trace) ≥ 100• RTH
(conductive plug). For the thermal resistance of the PCB trace, we need to consider
several issues: • Te PCB trace thermal resistance should be significantly greater
• Power requirements of the temperature sensor must be known and the PCB trace dimensions designed so as to minimise associated heat loss from them into the conductive plug. Tis can be mitigated with a device like the MAX30208.
• Te PCB traces that are in contact with the conductive core need
www.electronicsworld.co.uk March 2024 21
Figure 2: The primary temperature sensing path
Figure 3: A simplified thermal schematic highlighting the main heat -flow paths
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