This is the last in a series of three articles on thermal diodes. The previous two articles of our 'thermal diode' series, How to Measure Temperature in Integrated Systems and The ‘Thermal Diode'…A Diode or a Transistor? demonstrated how 'thermal diodes' are used to measure temperature and developed an understanding of the internal variables of 'thermal diodes' used in temperature measurement systems. This article addresses an external variable -- noise coupling. It discusses the effects of noise coupling on 'thermal diode' systems and methods to prevent it.
In thermal systems relying on a linear temperature-sensing element, such as a thermistor, the coupling from a constant noise source can be averaged out. This is equivalent to the fact that over a 'long' period of time, the average voltage of an AC coupled sinusoid into a constant resistive load is 0V. Although the ability to average out noise coupled onto a thermistor is attractive, a discrete bipolar junction transistor (BJT) is a more desirable temperature sensing device in many applications, because accurate results are obtained at low cost.
The method used to detect temperature through either a thermistor or 'thermal diode' is to measure a voltage for a specific forced current. While the method is similar, the I-V response of a thermistor compared to a 'thermal diode' is completely different. At a fixed temperature, the I-V response of a thermistor is linear while the I-V response of a 'thermal diode' is logarithmic (as shown in Equation 1). This implies that the average of an AC coupled sinusoid terminated into a 'thermal diode', as compared to a truly linear device, is not 0V.
* η= Ideality Factor
* k = Boltzman’s constant
* T = Temperature in degrees Kelvin
* q = electron charge
* IF = forward diode current
* IS = reverse saturation current
If temperature is fixed, the IS term in Equation 1 is also a fixed value for a specific diode. Because the other terms in Equation 1 are all constants, Equation 2, simplifies Equation 1, showing that VF is proportional only to the natural log of the forward diode current.
Figure 1 graphically presents a typical 'thermal diode' system implementation. Let’s assume that INOISE is a sinusoidal current source coupled onto the anode (DP) line of the 'thermal diode.' Noise may be coupled by routing a printed circuit board (PCB) trace too close to the inductor in a switching power supply. This is a common situation found in both PC and embedded applications.