## 3 Results and Discussion
### DC/DC with MPPT Design
The first system simulation, depicted in Figure 12, represents a load variation for 12 TEG cell connected in series with a 16 \({}^{\circ}\)C temperature difference. In the first region of the graph, the input is not connected to the DC/DC and the fractional open-circuit voltage is stored in the capacitor \(C_{3}\). The sensing time period is fixed to 30 ms, which is enough for the capacitor to be charged at 0.3 V, that is half of the open-circuit output voltage of the source. At the end of this time window, the converter switches from sensing mode to operative mode, thus the MPPT circuit is disconnected from the TEG output, which is attached at the DC/DC input by means of the switch \(S_{1}\). In order to guarantee the maximum power operative point for the source, considering a DC/DC output load of 2 k\(\Omega\), the output voltage is boosted up to 2.4 V. At the end of the region 2, the resistive load changes from 2 k\(\Omega\) to 1 k\(\Omega\). As can be observed, the output voltage is suddenly decreased, in order to keep constant the input voltage and to guarantee the source to operate at the maximum power point (MPP). At the beginning of region 4, the resistive load is reset to 2 k\(\Omega\) and the output voltage is boosted accordingly. Figure 12: DC/DC converter \(V_{OUT}\) (green trace), \(V_{IN}\) (blue trace), and \(V_{A}\) (red trace) simulation with load step variation. Region 1: input voltage sensing; Region 2: output voltage regulation; Region 3: load changing from 2 k\(\Omega\) to 1 k\(\Omega\) and output lowering for MPPT; Region 4: load changing from 1 k\(\Omega\) to 2 k\(\Omega\) and output boosting for MPPT. For the second simulation, the resistive load has is kept constant, while a source temperature difference changing has been emulated, from 16 \({}^{\circ}\)C to 10 \({}^{\circ}\)C. As shown in Figure 13, in the first two regions the fractional voltage is sensed and the output is regulated, keeping the voltage provided by the TEG source close to the desired MPP. At the end of region 2, the temperature change occurs, thus the output voltage is lowered but the TEG voltage is still close to the previous maximum power point condition. At the beginning of region 4, a new voltage sensing is performed and the FPCV value is updated to the correct value. The output of the DC/DC converter falls towards zero in this time window, since the input source is disconnected. After this sensing stage, the output is restored and boosted in accordance with the new MPP reference. In fact, the TEG source voltage is coincident to the reference value. Finally, Figure 14 shows a time window reporting a transition between MPPT input voltage sensing and output voltage regulation modes regarding a load variation simulation. Once the FOCV has been updated, the signal produced by Op-Amp \(A_{2}\), which is the result of MPPT hardware processing, begins to rise and therefore, the PWM switching signal controlling the transistor \(M_{1}\) gradually modifies the duty cycle in order to perform the MPPT regulation of the output. Figure 8:0 DC/DC converter \(V_{OUT}\) (green trace), \(V_{IN}\) (blue trace), and \(V_{A}\) (red trace) simulation with source temperature difference variation. Region 1: input voltage sensing; Region 2: output voltage regulation; Region 3: temperature difference changing from 16 \({}^{\circ}\)C to 10 \({}^{\circ}\)C and output lowering; Region 4: new source voltage sensing window and FOCV update; Region 5: output voltage regulation in accordance with the new detected maximum powerpoint.