In the OBC system, temperature sensors are also critical for control and safety monitoring, so this also requires careful consideration. It is usually implemented by an analog device, such as a negative temperature coefficient (NTC) resistor. TMP61-Q1 is the silicon-based thermistor with a positive temperature coefficient (PTC).

Accuracy is the most critical factor for temperature sensors. The TMP61-Q1 provides excellent linearity and consistent sensitivity across the operating range, enabling simple and accurate temperature conversion methods. The high linearity enables users to calculate temperature without using piecewise fitting or lookup tables in the software. The sensor maintains consistent sensitivity with a 6400ppm/°C Temperature Coefficient of Resistance (TCR) at 25°C and a typical TCR tolerance of just 0.2% across the entire temperature range. Figure 3-9 shows the typical resistances versus ambient temperature.

TMP61-Q1 is designed for a long lifetime of high performance. It features built-in fail-safe behavior in case of short-circuit failures at high temperatures. With exceptional resistance to environmental fluctuations, this maintains a typical long-term sensor drift of only 0.5%. The device responds rapidly to temperature changes with a quick thermal response time of just 0.6 seconds.

Available in a compact 0402 package, the TMP61-Q1 can be positioned in close proximity to heat sources and serves as a direct replacement for conventional NTC resistors. For applications requiring higher temperature tolerance, the ELPG package option extends the operational range up to 170°C.

The reliability of temperature sensing depends not only on the thermistor but also on the pull-up resistor and the power supply. The TMP23x-Q1 devices are a family of automotive grade precision CMOS integrated-circuit linear analog temperature sensors with an output voltage proportional to temperature. Figure 3-10 shows the block diagram.

The TMP235-Q1 device provides a positive slope output of 10mV/°C over the full –40°C to +150°C temperature range and a supply range from 2.3 V to 5.5 V. The higher gain TMP236-Q1 sensor provides a positive slope output of 19.5 mV/°C from –10°C to +125°C and a supply range from 3.1 V to 5.5 V. By eliminating the need for external pull-up resistors, it achieves enhanced reliability. Furthermore, it provides built-in protection for downstream components—when exposed to power supply overvoltage conditions, the device prevents these abnormally high voltages from being proportionally transmitted to the backend ADC, effectively safeguarding the MCU from potential damage.

If the thermistor cannot be placed close the hotspot (e.g. FETs / Transformer / Shunt resistor), the accuracy and response time are usually compromised. For OBC applications, since electrical clearance and creepage distance need to be considered, sometimes the placement of the thermistor is a trade-off. To solve this problem, The ISOTMP35-Q1 is the industry’s first isolated temperature sensor IC, combining an integrated isolation barrier, up to 3000VRMS withstand voltage, with an analog temperature sensor featuring a 10mV/°C slope from –40°C to 150°C. Figure 3-11 shows the block diagram.

This integration enables the sensor to be co-located with high voltage heat sources without requiring expensive isolation circuitry. Direct contact with the high-voltage heat source also provides greater accuracy and faster thermal response compared with approaches where the sensor is placed further away to meet isolation requirements.

In addition to the above-mentioned device-level designs, redundant temperature sensor and plausibility check are also common approaches to improving system functional safety.