**1. Introduction**

Thermal managemen<sup>t</sup> is a crucial issue in high speed data processing today. The problem has been attacked by many authors, e.g., they propose three techniques to create sensor infrastructures for monitoring the maximum temperature of a multicore system [1]. In [2], the systematic techniques for determining the optimal locations for thermal sensors to provide high-fidelity thermal monitoring of a complex microprocessor system are presented. Another paper presents a compact thermal model that can be integrated with modern Computer Aided Design tools to achieve a temperature-aware design methodology [3].

Data processing with the use of electron devices involves heat losses, which hamper the speed of the processing. Some kinds of cooling systems are used; however, some of them consume additional energy, make noise and enlarge the dimensions of mobile devices. In previously published papers, the authors presented a new idea: one additional temperature sensor, placed on the heat sink. Measuring the temperature di fference between the processor and heat sink yields valuable information, which is able to improve the microprocessor's throughput without any changes in its design [4,5]. The theoretical research of these articles was supplemented with experiments using a portable computer: MSI U270 (Micro-Star International Co., Ltd).

It must also be stressed that in this paper, we are dealing with transient or time-dependent thermal problems. Some time ago, there was only interest in steady-state temperatures. Data books only provided thermal resistance. For the power consumption, only a Direct Current (DC) value was given. Recently, more and more attention is paid to transient analyses [6–12]. From these measurements, one can set up equivalent Foster and Cauer Resistor-Capacitor (RC) networks, also known as structure functions [13]. The time constant distribution has also been proved to give a lot of useful information about the thermal path between the chip and the ambient temperature [14]. Just like for linear electric circuit analysis, the time-dependent analysis was mainly done in the AC domain. It turned out that for thermal problems, the Alternating Current (AC) approach was very useful. These studies have been applied to electronic packages [15–18], cooling fins [19,20], underground and overhead high voltage cables [21–24], integrated inductors [25], heat pipes [26] and photovoltaic panels [27].

The title of this paper clearly mentions "additional temperature sensor". Usually, an integrated circuit is fully designed and, at the last minute, the idea is put forward to include some temperature monitoring. In order to avoid a completely new design, the decision is then made to put the temperature sensor "aside" or in a free place, if available. This creates a particular problem that will be discussed in this paper.
