The third quadrant (3rd-quad) conduction (or reverse conduction) of power transistors is critical for synchronous power converters. For power metal-oxide-semiconductor field-effect-transistors (MOSFETs), there are two current paths in the 3rd-quad conduction, namely the MOS channel path and the body diode path. It is well known that, for 1.2 kV silicon carbide (SiC) planar MOSFETs, the conduction loss in the 3rd-quad is reduced by turning on the MOS channel with a positive gate bias (VGS) and keeping the dead time as small as possible. Under this scenario, the current is conducted through both paths, allowing the device to take advantage of the zero 3rd-quad forward voltage drop (VF3rd) of the MOS channel path and the small differential resistance of the body diode path.
However, in this thesis work, this popular belief is found to be invalid for power MOSFETs with higher voltage ratings (e.g., 3.3 kV and 10 kV), particularly at high temperatures and current levels. The aforementioned MOS channel and body diode paths compete in the device’s 3rd-quad conduction, and their competition is affected by VGS and device structure.
This thesis work presents a comparative study on the 3rd-quad behavior of 1.2 kV to 10 kV SiC planar MOSFET through a combination of device characterization, TCAD simulation and analytical modeling. It is revealed that, once the MOS channel turns on, it changes the potential distribution within the device, which further makes the body diode turn on at a source-to-drain voltage (VSD) much higher than the built-in potential of the pn junction. In 10 kV SiC MOSFETs, with the MOS channel on, the body diode does not turn on over the entire practical VSD range. As a result, the positive VGS leads to a completely unipolar conduction via the MOS channel, which could induce a higher VF3rd than the bipolar body diode at high temperatures. Circuit test is performed, which validates that a negative VGS control provides the smallest 3rd-quad voltage drop and conduction loss at high temperatures in 10 kV SiC planar MOSFET. The study is also extended to the trench MOSFET, another major structure of commercial SiC MOSFETs. Based on the revealed physics for planar MOSFETs, the optimal VGS control for the 3rd-quad conduction in different types of commercial trench MOSFETs is discussed, which provides insights for the design of high-voltage trench MOSFETs. These results provide key guidelines for the circuit applications of medium-voltage SiC power MOSFETs. / M.S. / Recent years, the prosperity of power electronics applications such as electric vehicle and smart grid has led to a rapid increase in the adoption of wide bandgap (WBG) power devices. Silicon Carbide (SiC) metal-oxide-semiconductor field-effect transistor (MOSFET) is one of the most attractive candidates in WBG devices, owing to its good tradeoff between breakdown voltage and on resistance, capability of operation at high temperatures, and superior device robustness over other WBG power devices.
In most power converters, power device is required to conduct current in its third quadrant (3rd-quad) (i.e., conduct reverse current) either for handling current during the dead time or acting as a commutation switch. In a SiC MOSFET, there are two current paths in the 3rd-quad conduction, namely the MOS channel path and the body diode path. It is widely accepted that by turning on the MOS channel with a positive gate-to-source bias (VGS), both paths are turned on in parallel such that the 3rd-quad conduction loss can be reduced.
In this thesis work, it is shown that this long-held opinion does not hold for SiC MOSFETs with high voltage ratings (e.g., 3.3 kV and 10 kV). Through a combination of device characterization, TCAD simulation, and analytical modeling, this thesis work unveils the competing current sharing between the MOS channel and the body diode. Once the MOS channel turns on, it delays the turn-on of the body diode and suppresses the diode current. This effect is more pronounced in MOSFETs with higher voltage ratings. In 10 kV SiC MOSFETs, with the MOS channel on, the body diode does not turn on in the practical operation conditions. At high temperatures, as the bipolar diode path possesses the conductivity modulation, which can significantly lower the voltage drop and is absent in the MOS channel, it would be optimal to turn off the MOS channel. Circuit test is also performed to validate these device findings and evaluate their impact on device applications. Finally, the study is also extended to the commercial SiC trench MOSFET, the other mainstream type of SiC power MOSFETs. These results provide key guidelines for the circuit applications of medium-voltage SiC power MOSFETs.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110061 |
| Date | 22 April 2022 |
| Creators | Zhang, Ruizhe |
| Contributors | Electrical and Computer Engineering, Zhang, Yuhao, Li, Qiang, Orlowski, K. Marius |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf, application/pdf |
| Rights | Attribution-NonCommercial-NoDerivatives 4.0 International, http://creativecommons.org/licenses/by-nc-nd/4.0/ |
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