Remote Temperature Sensor Transistor Selection Guide
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1 Introduction
Remote temperature sensors measure the temperature of an external transistor by measuring the base-emitter voltage (VBE) of the BJT, which depends on temperature. One method of determining a transistor base-emitter voltage is the two current method, using the difference of two voltage measurements to calculate temperature, as shown in Figure 1-1. However, thermal diode signals can be sensitive to noise, series resistance, and variation in transistors’ η-factors and betas can introduce additional errors if not accommodated.
Figure 1-1 ΔVBE Measurement with Two
Currents1.1 Series Resistance
Any resistance in PCB traces will cause a voltage drop between the actual VBE at the transistor and the measured VBE at the temperature sensor, which results in a temperature offset. Using three different current level measurements shown in Figure 1-2, a certain amount of series resistance can be canceled out when solving for temperature.
Figure 1-2 ΔVBE Measurement
with Three Currents