## 2 Materials and Methods
The same procedure can be replicated with two or more TEG cells with different interconnection configurations, in order to verify the model validity even for multicell applications. The final step is to use the aforementioned multicell TEG SPICE model for a design example of a DC/DC converter with MPPT algorithm, in order to maximize the conversion efficiency. For this purpose, a conventional boost converter has been considered, with ideal components, as depicted in Figure 4. As for many low-power portable or autonomous application, its is very important to harvest the maximum available power from the source, in order to guarantee high integrability, with the usage of small batteries. The MPPT here plays this role, therefore a DC/DC with maximum powerpoint track algorithm works more as a power pump, where the output voltage is amplified, with respect to the input, according to the system load, in order to extract the maximum energy from the source [41; 42; 43]. This can be employed to directly charge a battery or to directly feed digital systems that do not require a stable power supply. If a fixed voltage regulation is also needed, combined with MPPT, usually the latter works as a pre-regulator for a second DC/DC with fixed output voltage [44]. In Figure 4, an example of an energy harvesting power management system with both MPPT and regulated voltage is depicted, as [PERSON] et al. reported in [45]. In this case, The system is composed of a first stage DC/DC converter, which is controlled by an MPPT block that has the role to modulate the PWM (pulse width modulation) of the switching frequency \(f1\), in order to guarantee the maximum transferred power at the output. The latter is delivered to a battery and to a storage capacitor \(C_{STORE}\), so as to harvest the maximum available power. The output of this first stage is then delivered to a second standard DC/DC converter, which provides a fixed regulated voltage to the system load. In this case, a bandgap voltage reference is needed for the output voltage regulation loop. Such a configuration can store the excess of energy that is not instantly requested by the output load into the battery with the maximum power conversion efficiency, while a fixed voltage \(V_{REGULATED}\) is even supplied, together with the MPPT regulated voltage \(V_{OUT}\). Another possibility is to exploit the MPPT algorithms to instantly disable the fixed output voltage when the system load requires higher power than the available energy, in order to perform a sort of PWM output modulation that helps to keep the input voltage near the maximum power point [46]. In this case, usually a supercapacitor is employed to store power to be supplied when the energy source is temporarily unavailable or when the system requires more power. Referring to Figure 4, the purpose of this work is to show how the proposed technique to identify a simple LTSPICE model for the TEG cell can improve the design procedure of the first stage MPPT converter, which affects the power conversion efficiency of the harvestseng thermal energy. Therefore, a design example of the first DC/DC (grey dashed box) stage is presented in the following pages, since the second conversion stage is considered standard and not always requested. As well known, the operation modes of the boost converter converter can change, based on the current flowing through the inductor \(L\). If the current does not go to zero, the converter operates in continuous mode, where  
\[V_{out}=V_{\text{TEG}}\frac{1}{1-D} \tag{6}\] \[I_{\text{TEG}}=\frac{I_{OUT}}{1-D}\]  
where \(D\) is the Duty cycle of the square wave signal that controls the switch \(S\).