Industry: SiC power electronics
Figure 1: Several different SiC switch technologies have been developed. In addition to the junction transistor, a form of which is made by GeneSiC, there are planar MOSFETs, trench MOSFETs and JFETs
For example, the silicon BJT (and the IGBT) exhibit minority carriers in the drain region, which are not present in the SiC equivalent. This allows the wide bandgap device to operate at very high frequencies, like a majority carrier device, so it is not plagued by dynamic breakdown issues, such as a poor reverse-bias safe-operating- area (RBSOA). Meanwhile, contemporary SiC MOSFETs have channel mobilities that are just 5-10 percent of that of silicon MOS devices, and the high doping levels found in the drain regions of normally-off SiC JFETs have made it very challenging to manufacture this device with high yields and uniform characteristics. The SJT, in comparison, is hallmarked by: a very high current gain, which can be in excess of 100 and allows low gate currents; and a good RBSOA profile, which is indicative of the robustness of this device.
Driver considerations
These comparisons of device performance have limited worthiness, because any critical assessment of the suitability of the device for industrial deployment must not be restricted to simply its standalone performance, or even the combination of this and its cost. Instead, meaningful judgement of the merit of any class of transistor must include an analysis of how easily it can fit in with existing drive infrastructure.
Figure 3: Turn-on (top) and turn-off (bottom) switching waveforms of a 1200 V / 6 A SJT (GA06JT12-247)
Dominance of the silicon IGBT in many motor controls and power supplies has led to widespread use of voltage-controlled drivers in these applications. Modern gate drivers generally switch at +15 V levels and feature higher current sourcing/sinking capabilities than their predecessors. Current levels are now several amperes, to accommodate high operating frequencies and large gate capacitances, in both IGBTs and high-current MOSFETs.
One of the downsides of the contemporary SiC MOSFET is that it requires a higher drive voltage than that produced by many, but not all, modern gate drivers: It needs +20 V to achieve a sufficiently low on-resistance. This higher-voltage requirement results from poor transconductance, which can be traced back to the low channel mobilities of SiC. Far lower drive voltages are possible with some classes of SiC transistor that involve a junction-based approach. Junction transistors and normally-off JFETs require just a +4 V drive, but may require non-zero continuous gate currents; while normally-on JFETs may need a negative bias of up to 30 V to turn them off.
Figure 2: The SJT produced by GeneSiC, which can be driven by a gate drive IC, must be capable of supplying a continuous current of 0.5 A to the gate of this transistor. The external parallel gate resistor, RGP external parallel capacitor, CGP
, should be adjusted to meet this requirement, while the , can be chosen to ensure an optimum level of dynamic
gate current during turn-on and turn-off initial transients. This dynamic current is essential for fast charging of the internal gate-source capacitance. The presence of this paralleled resistor and capacitor on the output of the gate driver can increase the device switching speed, reduce its switching loss and also cut driver losses
42
www.compoundsemiconductor.net July 2013
This brief overview of voltage requirements for many different classes of SiC transistor appears to imply that all devices require a non-standard gate driver. Given that, it is not surprising that many SiC device manufacturers are actively working on optimum gate drivers for their switch offerings. However, it is possible to use off-the-shelf IGBT
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179