Capítulo 2: Aplicación parcial del procedimiento para el estudio del servicio al cliente
2.3 Aplicación parcial del procedimiento
a. Temperature Imbalance
The parallel-connected silicon PiN diodes are heated to different initial junction temperatures using the electric heaters and the double pulse measurement is performed in order to assess the impact of the different junction temperatures on the diode’s switching and conduction characteristics. Figure 3.10 shows the turn-on and turn-off current waveforms of two unbalanced diodes operating with DUT1 at 25 °C and DUT2 at 100 °C with a VDC of 200V. Such an extreme mismatch is not a common issue in real applications
but it is nonetheless studied as a worst case. As can be seen, the device operating at lower junction temperature conducts less current than the device operating at higher junction temperature. Moreover, this device switches off significantly faster than the device with higher junction temperature. This is due to the fact that the device at the lower junction temperature exhibits smaller minority carrier lifetime in the drift region and as a result, higher resistance in the drift region i.e. the PiN diodes are operating below the ZTC. In contrast the hotter device has a larger carrier lifetime and therefore more conductivity modulation. According to the current divider rule, it therefore conducts a higher current. It can also be seen from Figure 3.10 that the hotter Si PiN diode exhibits significantly larger reverse recovery charge. This larger reverse recovery will contribute to additional switching energy which increases the temperature of the hotter device and causes it to
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take more current since it operates below ZTC. What is also important to note in the silicon PiN diode waveforms shown in Figure 3.10 is the fact that the diode currents diverge over time with the hotter PiN diode continually taking more current than the cooler PiN diode. This indicates thermal instability and possible thermal runaway in the absence of adequate cooling.
Figure 3.10: The measured (a) turn-ON and (b) turn-OFF current waveforms of parallel connected PiN diodes with junction temperatures of 25 °C and 100 °C.
Figure 3.11: The measured (a) turn on and (b) turn off current waveform of the parallel connected SiC Schottky diodes with junction temperatures of 25 °C and 100 °C respectively.
The turn-ON and turn-OFF current switching transient of the parallel connected SiC Schottky diodes operating at different initial junction temperature is shown in Figure 3.11. As can be seen, the hotter device conducts less current thereby indicating that the diodes are operating above the ZTC point. Furthermore, the current through the parallel SiC
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diodes is constant over time thereby indicating thermal stability. It should be noted that the diodes under investigation are conducting currents within the rated specification.
Figure 3.12(a) shows the turn-on and Figure 3.12(b) shows the measured turn-OFF switching energy of the unbalanced parallel-connected silicon PiN diodes. In Figure 3.12(a) and 3.12(b), one diode is held constant at 25 °C while the junction temperature of the other diode is varied from 25 °C to 150 °C. Hence, the plots show the switching energies as a function of the difference between the temperatures of the parallel diodes. As can be observed from the Figure 3.12, the turn-ON energy is higher for the cooler device while the turn-OFF energy is higher for the hotter device as a result of greater reverse recovery charge at high temperature. The difference between the switching energies of the parallel devices increases with the difference in the junction/case temperature.
Figure 3.13(a) and Figure 3.13(b) show the similar measurements for the unbalanced parallel-connected SiC Schottky diodes at different junction temperatures. By comparing Figure 3.12 and Figure 3.13, it can be seen that difference in switching energy between the parallel diodes are smaller for the SiC Schottky diodes than for the PiN diodes. In other words, the variation in the switching energy as a function of temperature is lower for the SiC Schottky diodes than for the silicon PiN diodes. As the temperature difference between the parallel connected diodes is increased from 0 to 125 °C, the switching energy difference between the parallel pair increases by 44.3% for the silicon PiN diodes and 13.5% for the SiC Schottky diodes. This is due to the temperature dependence of reverse recovery charge in silicon PiN diodes.
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Figure 3.12: The measured (a) turn-ON and (b) turn-OFF switching energies of the parallel connected PiN diodes as a function of the difference in junction temperature.
Figure 3.13: The measured (a) turn-ON and (b) turn-OFF switching energies of the parallel- connected SiC Schottky diodes as a function of the difference in junction temperature.
b. Dynamic behaviour evaluation
The thermal transient measurements are performed by repetitive switching over several minutes until steady state case temperature is reached. Hence, the case-temperature rise of continuously switched parallel-connected silicon PiN diodes has been measured and shown in Figure 3.14(a) and for the SiC Schottky diodes shown in Figure 3.14(b). Electrothermal imbalance between the parallel-connected diodes was introduced by setting different initial case temperatures. The difference in the initial case/junction temperature between the parallel-connected diodes was set to 3 °C. The parallel-
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connected diodes have identical heatsinks and are switched at a frequency of 2 kHz with a duty ratio of 90% so that the conduction losses are dominant.
As can be seen from Figure 3.14(a), the measured case temperature difference between the two silicon PiN diodes diverge with time and is 13.5% after 600 seconds. This correlates with the double pulse measurements presented for the PiN diodes in Figure 3.10. Figure 3.14(b) shows that the case temperature difference for the SiC Schottky diodes converges and is only 2.3% after 600 seconds. However, the steady-state average case temperature rise for both PiN diode pair is 27.9% smaller than that of the Schottky diode pair after 600 seconds. Also shown in Figure 3.14, is the case temperature of the low side conducting SiC MOSFETs used to commutate current in both diodes. As can be observed from the comparison of Figure 3.14(a) and Figure 3.14(b), the temperature rise of the bottom SiC MOSFET is higher for the parallel PiN diode pair compared to the Schottky diode pair. This higher temperature rise in the MOSFET is due to the reverse recovery of PiN diodes inducing higher switching losses in the low side SiC MOSFET.
Figure 3.15 shows the parallel diode pair switched with different sizes of heatsinks, which in this case, simulates different thermal resistances and capacitances. Figure 3.15(a) shows the case temperature transient for the PiN diode pair, while Figure 3.15(b) shows that of the SiC Schottky diode pair. As can be seen from Figure 3.15(a) and 3.15(b), the diode with the smaller heatsink (higher thermal resistance) operates at a higher case temperature compared to that with the larger heat-sink (smaller thermal resistance). However, in the case of the unbalanced SiC diode pair, the difference in case temperature is smaller and appears to be converging after 600 seconds. For the unbalanced silicon PiN diode pair, the case temperature difference is larger and appears to be diverging after 600 seconds. Again, the low side SiC MOSFET used for switching current into the PiN diodes exhibits a higher case temperature compared to when the same device is used for the SiC
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Schottky diodes. Hence, although the overall average case temperature is 27.9% lower for the PiN diode pair compared to the SiC Schottky diode pair, the case temperature rise of the low side MOSFET is 53.5% higher for the PiN diode pair.
Figure 3.14: The measured case temperature rise for the (a) parallel-connected PiN diodes switched with same heatsink but with 3 °C difference in initial temperature. (b) Similar
measurement for the parallel connected SiC Schottky diodes.
Figure 3.15: The measured case temperature rise for the (a) parallel-connected PiN diodes switched with different size heatsinks. (b) Similar measurement for the parallel connected
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