4.2. Temperature Characteristics
The ECU heatsink of the 36V ECU system is made of aluminum. Various materials with higher thermal conductivity and lower heat capacity were considered to enhance the heat transfer rate of the ECU heatsink [
30].
Figure 9 shows the temperatures, thermal resistance and heat flux of the 36V heatsink ECU system with different ECU heatsink materials. In this study, copper, steel, aluminum alloy and aluminum were considered as the case materials for the ECU heatsink. Aluminum is widely used in heatsinks since it has a higher thermal conductivity and is lighter in weight than most metals. Steel and iron are heavier and less reliable than aluminum and are less preferred for use in heatsinks [
31]. Aluminum alloy is made of 85% aluminum and 15% of copper manganese, silicon, tin and zinc [
32]. The temperatures of the ECU heatsink, the MOSFET and the capacitor of the were 35.59 °C, 57.33 °C and 41.92 °C, respectively for aluminum and 33.16 °C, 54.27 °C and 41.78 °C, respectively for copper. These values show that the temperatures of the ECU heatsink, MOSFETs and the capacitors with a copper heatsink were lower by 6.82%, 5.33%, 0.33% in comparison to that with aluminum. In comparison to aluminum, cases with iron resulted in an increase in the temperatures of the ECU heatsink, MOSFET and the capacitor by 11.60%, 5.02% and 8.25%, respectively. This could be due to the lower thermal conductivity and higher thermal resistance of iron in comparison to aluminum [
33]. The temperatures of the ECU heatsink, MOSFET and capacitor with aluminum alloy were 35.49 °C, 56.39 °C and 41.15 °C, respectively, which are lower by 0.28%, 1.64%, and 1.84%, respectively than that of with aluminum. However, aluminum alloy is not a common material that is used for ECU heatsinks for electric bicycles [
34]. In general, the temperatures of the ECU heatsink, MOSFET and the capacitors were the lowest when copper was used. This could possibly be due to the low thermal resistance and low specific heat of copper [
35]. The heat flux indicates that the heat flow intensity or heat flow density per unit area per unit time [
36].
Figure 9b shows the variations of total thermal resistance and heat flux of the 36V ECU system with different 36V ECU heatsink materials. The heatsink with copper showed the lowest heat flux for the heatsink, capacitor and MOSFET, because copper has the lowest temperature difference with compared to the existing aluminum heatsink. The existing aluminum heatsink exhibited 8090.66 W/m
2 heat flux for the MOSFET and 355.56 W/m
2 for the capacitor. In comparison to the existing aluminum heatsink, the copper heatsink showed 0.081%, and 0.74% lower heat flux for the MOSFET and capacitor, respectively. Steel is composed of iron and carbon. Therefore, steel has an increased heat capacity and a lower density than iron [
37]. In comparison to the existing heatsink, the heat flux of the MOSFET and the capacitor for the 36V ECU system was increased by 0.26% and 0.05%, respectively with an iron heatsink and by 0.18% and 0.02%, respectively with a steel heatsink. This is because the thermal conductivities of iron and steel are lower than that of aluminum [
38]. Low purity in the aluminum alloy results from the manufacturing process which causes the poor thermal conductivity and increases the thermal resistance. Thus, the 36V ECU system with the aluminum alloy heatsink showed 0.16% and 0.01% higher heat flux for the MOSFET and the capacitor, respectively [
39]. Comparison of all the materials used in this study revealed that copper has the lowest thermal conductivity. Therefore, it showed the lowest total thermal resistance. The total thermal resistances of the 36V ECU system with the copper heatsink and the existing aluminum heatsink were compared. The copper heatsink of the 36V ECU system showed a decrease in the thermal resistance by 8.26% in comparison to the existing aluminum heatsink. The total thermal resistances of the 36V ECU systems with a steel heatsink, an aluminum alloy heatsink, and an iron heatsink were compared with the existing 36V ECU system with an aluminum heatsink. The total thermal resistances of these heatsinks were increased by 9.2%, 9.0%, and 10.16%, respectively, compared to those of the 36V ECU system with the existing aluminum heatsink of 8.676 °C/W. This increase in the thermal resistance is a result of the decreased thermal conductivities in MOSFETS and capacitors in steel, aluminum alloy, and iron heatsinks.
Figure 10 shows the temperature, total thermal resistance and heat flux variation of the seven MOSFETs and the two capacitors of the 36V ECU system with the total power dissipation rate from the MOSFETs and the capacitors. In order to reflect the various operating conditions of the 36V ECU system, the total power dissipation was varied within 2.57 MW/m
3 and 4.26 MW/m
3. Based on the manufacturer’s datasheet and Equations (10)–(14), the power dissipation was 0.77 MW/m
3 for the CD228H capacitor at a rated voltage of 63 V, capacitance of 0.22 mF, dissipation factor of 0.09, ripple current of 660 mA rms, height of 20 mm and diameter of 12.5 mm. The power dissipation was 3.2 MW/m
3 for the SIHFZ34 MOSFET with a length of 10 mm, thickness of 3 mm, width of 10 mm, drain voltage of 60 V, gate voltage of 20 V, drain current of 30 A, reverse transfer capacitance of 100 pF and drain-source on-state resistance of 0.050 Ω [
25,
26].
As shown in
Figure 10a, the temperatures of the ECU heatsink, the MOSFETs and the capacitors of the 36V ECU system at a total power dissipation of 4.26 MW/m
3 were higher by 20.95%, 30.31% and 21.54% °C than those at the total power dissipation of 2.57 MW/m
3. This could be due to an increased generation of heat with the increased flow of input current to the capacitors and the MOSFETs. This results in an increased generation of heat by the Joule effect [
40]. The heat generation of the semiconductors depends on the input power. The existing model had a total power dissipation rate of 3.2 MW/m
3 and it showed 110,094.85 W/m
2, 80,051.32 W/m
2 and 14,687.81 W/m
2 heat flux for the MOSFETs, the capacitors and the heatsink respectively.
In comparison to the power dissipation rate of the existing model, as shown in
Figure 10b, the heat flux of MOSFETs and the capacitors decreased by 19.75% and 19.92% respectively when the total power dissipation rate was decreased by 20%. The heat generation in the MOSFET and the capacitors increased when the power dissipation rate was increased by 30% in comparison to the existing model. According to Equation (14), this results in an increase in the thermal energy of all other elements. Thus, the heat flux values for the MOSFETs, capacitors and the heatsink increased by 30.10% and 31.68%, respectively, compared to the existing model. The total thermal resistance of the existing aluminum heatsink ECU system was 8.67 °C/W. When the total power dissipation rate of the existing system was decreased by 20%, the thermal resistance of the system decreased by 6.63%. When the total power of the ECU system was increased by 30%, the total thermal resistance increased by 11.18%. This is due to increased generation of heat corresponding to increased temperature of the ECU system [
41].
Figure 11 shows the variation of temperatures of the heatsink, seven MOSFETs and the two capacitors of the 36V ECU system with the ambient temperature. At an ambient temperature of −20 °C, the temperatures of the heatsink, seven MOSFETs and the two capacitors were −9.02 °C, 13.6 °C, 0.55 °C, respectively. With the increase of the ambient temperatures from 20 °C to 30 °C, the temperatures of the ECU heatsink, seven MOSFETs and the two capacitors increased by 32.93%, 24.86% and 25.14%, respectively. The temperatures of the ECU heatsink, seven MOSFETs and the two capacitors increased by 24.75%, 9.93% and 22.04%, respectively, with the increase of the ambient temperatures from 30 °C to 40 °C. The temperatures of the heatsink, seven MOSFETs and the two capacitors were 50.81 °C, 72.75 °C, 58.81 °C, respectively when the ambient temperature was 40 °C. This increase in the temperatures of the heatsink, MOSFETs and the capacitors with increasing ambient temperatures is a result of the decreased heat transfer rate. The decreased heat transfer rate is due to the decrease in the temperature difference between the ECU system and the environment. The heat generation of the capacitors did not specifically depend on the ambient temperatures because these temperatures did not directly affect the ESR value as given by Equation (13) [
42]. In this study, the maximum temperatures of the MOSFET, capacitor and heatsink were recorded, when the ambient temperature was 40 °C.
Figure 12 shows the temperature distribution of the MOSFETs, capacitors and heatsink when the ambient temperature is 40 °C. As shown in
Figure 12a, MOSFETs that are located near the middle of the PCB showed a higher temperature than other MOSFETs. As shown in
Figure 12b, the inner capacitor also showed a higher temperature than the outer capacitor due to the outer MOSFETs and outer capacitor being located near the ends of the PCB. These areas of the ECU show lower temperatures as shown in
Figure 12c. Furthermore, the part of the heatsink near the MOSFETs showed a higher temperature due to the temperature of the MOSFET.