2017.01.12 SiC Power Device
In the previous section, we compared the reverse recovery characteristics of SiC SBD and Si PND. Next, we discuss differences in the characteristics of the forward voltage VF, which is the most basic characteristic of diodes.
Difference in Forward Voltage Characteristics of SiC SBD and Si PND
Ideally, the forward voltage VF of a diode should be as close to zero as possible and stable with respect to temperature; but of course it is not zero, and fluctuates with temperature. In order to gain an understanding of VF characteristics of Sic SBDs, we compare them with Si PND fast-recovery diodes (FRDs).
These are graphs of the VF characteristic for the forward current of SiC SBD and Si FRD devices. The data was measured under eight different temperature conditions, ranging from 25℃ to 200℃.
In the case of the SiC SBD, as the temperature rises, the VF at which IF begins to flow falls somewhat, but the resistance increases, so that the slope is gentle, and the VF rises over the normal IF usage range.
In the Si FRD, however, as the temperature rises the VF simply falls. As the curves in the graph indicate, the slope is nearly the same at all temperatures, and we see that the VF decreases constantly.
These characteristics both depend on the physical properties and the structures of the devices, and in each case there are advantages and drawbacks. Previously we discussed an "ideal" diode. Likewise, for the VF of the Si FRD to decline at high temperatures means conduction losses are reduced, which is considered a good thing, but as the VF declines the IF increases, and despite the moderate decline in losses the increase in heat generation prevails, and there is the danger of the occurrence of thermal runaway in which the VF further decreases and the IF increases.
On the other hand, in the SiC SBD the VF rises with rising temperature, so there is no thermal runaway. However, there is the drawback that the higher VF means the IFSM (maximum surge forward current) is lower than that of the Si FRD.
Improved VF Characteristics of SiC SBD
In order to enhance the characteristics of the SiC SBD with its excellent basic properties, and further improve ease of use, we have been developing next-generation devices with lower VF values. This will be ROHM's second-generation series of devices. Whereas for ROHM's first-generation products and similar products of other manufacturers, the VF is 1.5 V for an IF of 10 A, the VF of second-generation devices is lowered to 1.35 V. As reference values, graphs of IF vs. VF are shown at 25℃ and at 125℃.
The red curves represent the VF characteristics of ROHM's second-generation SiC SBDs.
Considerations Relating to trr and VF Losses
In this section we have explained differences in the VF characteristics of SiC SBDs and Si FRDs. However, we now consider losses including the trr characteristics discussed in the previous section.
In order to explain SiC SBDs, we compared the trr and VF values with those of Si PNDs (FRDs), and this was done for a reason. The diagram on the lower left was used when we explained the rated voltage ranges of Si diodes and SiC diodes at the beginning of this chapter. The important point here is that Si PNDs/FRDs and SiC SBDs cover about the same ranges, and can be used in the same applications. SiC SBDs are new devices, and so put another way, one could say that the range covered by current Si PNDs/FRDs could be covered as well by essentially substituting SiC SBDs for these devices.
In particular, selecting the optimum diode for areas in which Si FRDs and SiC SBDs compete, such as applications where fast operation is important, an understanding of the characteristics of the two is vital. And of course the "reduction of losses" is, among the various issues we have studied, the most important goal.
The trr, discussed in the previous section, corresponds to switching losses. The VF we have considered in this section causes conduction losses. Both these losses are plotted for Si FRDs and SiC SBDs in the graph on the lower right.
The faster the trr and the lower the VF value, the lower are the total losses for the device. In the Si FRD, a faster trr means the VF is higher. However, in a second-generation SiC SBD, the VF is lowered from 1.5 V to 1.35 V while maintaining the fast trr value of conventional SiC SBD devices.
At present, among Si FRD and SiC SBD power devices, second-generation SiC SBDs hold the possibility of the greatest loss reductions.
・The VF of an SiC SBD rises with temperature, whereas the VF of an Si PND (FRD) falls.
・The increase in the VF of an SiC SBD at high temperatures causes the IFSM to decline, but there is no thermal runaway, such as occurs in Si PNDs (FRDs), the VF of which declines.
・In second-generation SiC SBDs, the VF falls, and at present they can be called the power diodes that are best able to help reduce losses.