Industry: SiC power electronics
Figure 6: The external gate capacitance, CGP (c) and device energy losses
, impacts: (a) the peak gate current, IG,pk
; (b) device turn-on tr
and turn-off tf
times;
parasitic inductances in the gate drive circuit. One good remedy is to place a low-inductance resistor in series with the gate capacitance.
Another variable is the gate driver output voltage, which impacts SJT performance. This voltage must be high enough to bias the SJT gate-source junction on – it has a built-in voltage of about 2.8V – and supply the steady state gate current that follows the gate current peak. There are no trade- offs associated with increasing the voltage, which leads to a nearly linear decrease in total energy and rise and fall times (see Figure 7). However, using an excessively high gate driver output voltage is not to be recommended, because this can be a large contributor to gate driver loss and steady state driver losses. A superior solution, called a 2-level driver, is provided by Rabkowski et. al. (see further reading). This uses two current levels to drive SJTs – one during the transients, and another during the steady state operation of these devices.
Figure 8: Gate driver dependent system power loss as a function of frequency for a fixed duty cycle of D = 0.7. A 1-level gate driver is as shown in Figure 2. A 2-level driver circuit uses two gate-driver ICs, supplying different currents during transients and steady-state operation. Note that the SJT conduction loss component is not considered here
When we drive our 1200 V/6 A SJT with a judicious choice of gate driver output voltage and gate capacitance and resistance values, we obtain very low power losses. Using a duty cycle of 0.7 and a frequency of 500 kHz, the steady state loss for the driver is 3.85 W, while the switching losses for the driver and SJT are 0.54 W and 45.6 W, respectively. These switching losses are frequency dependent, and dominate below 70 kHz (see Figure 8).
Figure 7: A higher gate voltage reduces transition times and energy losses
Further reading D. Veereddy et. al. Bodo´s Power Systems, pp. 36–38 Oct-2011. S. Sundaresan et. al. Power Electronics Technology, pp. 21–24 Nov- 2011. “IXD_614 Low-Side Driver Datasheet.” IXYS Inc.
http://www.ixysic.com/ home/pdfs.nsf/www/IXD_614.pdf/$file/IXD_614.pdf J. Rabkowski et. al. Power Electronics, IEEE Transactions on 27 2633 (2012) “GA06JT12-247 Datasheet.” GeneSiC Semiconductor Inc.
http://www.genesicsemi.com /
index.php/sic-products/SJT
44
www.compoundsemiconductor.net July 2013
The discussion provided here illustrates that replacing silicon IGBTs with SiC switches may not always give engineers of power circuits the ease of use that they may have anticipated, due to the drive requirements of particular classes of this wide bandgap device. However, our measurements show that one type of device, our SJT, can work very well with a conventional gate drive. Its strengths include fast switching speeds and ultra-low losses, and it is not plagued with many of the drawbacks of other SiC transistors and bipolar silicon devices.
© 2013 Angel Business Communications. Permission required.
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