Análisis funcional y pragmático de las UFS

Figure 3.34 shows the SiC Schottky diode switching energy as a function of tempera- ture and MOSFET dIDS/dt during turn-off. Similar to the case of the diode turn-on,

the switching energy exhibits a U-shaped characteristic as a function of the MOSFET’s dIDS/dt. This happens since at high switching rates, the ringing of the Schottky diode

dominates the switching losses while at lower switching rates, the duration of the transient is the dominant source of increase in switching energies.

Figure 3.34: Switching energy of SiC SBD at turn-off.

Figure 3.35 shows the diode turn-off voltage transient as a function of temperature for RG = 15 Ω while Figure 3.36 shows the same plot for RG = 150 Ω. It can be seen from

Figures 3.35 and Figure 3.36 that the damping of the diode ringing during turn-off reduces with increasing temperature and the peak voltage overshoot increases with temperature, i.e. ringing becomes more sustained at high temperatures during diode turn-off. This is due to the positive temperature coefficient of the MOSFET dIDS/dt at turn-on, hence

faster switching at high temperatures contributes to the ringing. It can also be noticed in Figures 3.35 and 3.36 that the temperature dependence of ringing and its damping reduces at higher MOSFET dIDS/dt (lower RG) and increases as the dIDS/dt is reduced, i.e. at

high dIDS/dt, the diode characteristics become more temperature invariant compared to

low dIDS/dt (higher RG). It can be seen from Figure 3.36 that there is a significant

Figure 3.35: SiC SBD turn-off voltage and currents with RG of 15 Ω.

Figure 3.36: SiC SBD turn-off voltage and currents with RG of 150 Ω.

This ringing in the SiC Schottky diodes is a major reliability concern in the application of unipolar SiC devices which switch with high rates, as the ringing will be transferred to the DC link voltage and can therefore unstable the entire circuit. This will be shown with more details in the following chapters and mitigation techniques will also be employed to remove this ringing from the DC link. Additionally the ringing causes significant thermal losses on the system, and hence requires to be accurately predicted. Analytical models in the next chapter will also be developed to provide this diagnostic tool.

Figure 3.37 shows the switching energy of the silicon PiN diode at different temper- atures and IGBT dICE/dt. It can be seen from this figure that the switching energy

increases as the temperature increases for a given gate resistance. This is due to the in- creased minority carrier lifetime with temperature thereby increasing the reverse recovery charge of the PiN diode during turn-off. Hence, the switching performance of the PiN diode deteriorates as the temperature increases.

Figure 3.37: Switching energy of silicon PiN diode at turn-off.

Figure 3.38 and Figure 3.39 show the PiN diode voltage and current transients for RG = 15 Ω and RG = 150 Ω. It can be seen from these figures that the peak reverse

recovery current and the transient duration increases as RG is decreased (dICE/dt is in-

creased). It can also be observed from Figure 3.38 and Figure 3.39 that the peak diode voltage overshoot increases with reducingRG, hence fast switching using PiN diodes (sim-

Figure 3.38: Silicon PiN diode turn-off voltages and currents with RG of 15 Ω.

Figure 3.39: Silicon PiN diode turn-off voltages and currents with RG of 150 Ω.

As the turn-on dICE/dt of the IGBT increases with decrease of temperature (refer to

Figure 4.12), the peak voltage overshoot of PiN diode reduces with increasing temperature. The result of all this is that the switching energy as a function ofRG, especially in diodes,

exhibits a U-shaped characteristics at all temperatures. At low RG, the switching energy

is dominated by high peak reverse recovery currents and large diode voltage overshoots while at higher RG, the switching energy is dominated by the longer switching duration

Figure 3.40 shows a comparison of the SiC MOSFET and Si-IGBT switching energies as a function of temperature with RG = 15 Ω for both turn-on and turn-off. The SiC

MOSFET shows less switching energy compared with the Si-IGBT at high switching rates. The switching energy also shows less temperature dependency in the SiC MOSFET compared with the Si-IGBT. Figure 3.41 also shows that the switching energy of the SiC Schottky diode is significantly less than that of the silicon PiN diode.

Figure 3.40: Transistors switching energy as function of temperature, RG = 15 Ω.