With a higher power rating and broader application, Gallium nitride (GaN) is a promising next-generation power switch. The current four GaN HEMTs in paralleled phase leg that can block 400 V and conduct 200 A current is very beneficial, thus making the protection method on a GaN phase leg an urgent topic. This thesis starts with an overview of shortcircuit robustness among silicon (Si), silicon carbide (SiC) and GaN devices. An approximately safe operation area (SOA) for a GaN power switch will also be determined. The various common shortcircuit protection methods are mentioned. Additionally, current research on a GaN semiconductor is summarized. Among all of the protection methods, desaturation detection is selected and analyzed through simulation and then implemented in a parallel enhancement-mode high-electron-mobility transistor (E-HEMT) GaN phase leg. With this desaturation detection feature, the GaN E-HEMT can be turned off as quickly as 200 ns, and in the worst case, 500 ns, during a shortcircuit test. The phase leg survived a series of shortcircuit tests with shortcircuit protection. For the proposed protection scheme, the best-case reaction time (200 ns) is similar to others in the literature, while the shortcircuit peak current and peak energy are higher. The worst-case performance of this design is limited by both the gate driver and the device shortcircuit robustness.
Due to the fast switching speed of the GaN HEMT, the false turn-on phenomenon caused by the Miller effect can be a problem. A shoot through may occur with one switch false turn on. The Miller clamp is added to the phase leg to improve its reliability. After the hardware was implemented, the Miller clamp was tested and verified through a double pulse test (DPT). Compared to the phase leg without the Miller clamp, the gate is better protected from gate voltage overshoot and undershoot. The switching loss is reduced by 20 percent by using a new gate driver IC with higher current driving capability.
The degradation effect of GaN power switches in different shortcircuit pulses was also studied. The device passes through the shortcircuit tests, but any degradation effect that may change its parameters and influence its normal operation characteristic need to be addressed. Several GaN devices were selected and characterized after several shortcircuit tests to observe any degradation effect caused by the shortcircuit.
The degradation test results reveal a "recovery effect" of the GaN HEMT used in this project. The parameter variations on threshold voltage and on-resistance recover to the original state, several hours after the shortcircuit test. The test results match with the conclusion drawn in degradation test conducts by other research groups that the parameter variation during shortcircuit test is negligible. Also, repetitively fast shortcircuit tests on the GaN HEMT show that the shortcircuit protection limit for this device under 400 V bus should be limited to 300 ns. / Master of Science / A phase leg consists of two power switches: a top switch and a bottom switch. As a result of a wrong gate signal or the Miller effect, shoot through problems may occur that lead to a shortcircuit current running through the channel. The excessive heat brought by the shortcircuit current will kill the device if not turned off in time. The failure of the phase leg may also have a hazardous impact on the rest of the system. To improve the overall system stability, a shortcircuit protection feature can be added on the gate-drive level. The shortcircuit protection turns off the device when it runs into shortcircuit mode, and before device failure.
In this thesis, desaturation detection is selected to implement on a paralleled Gallium nitride (GaN) phase leg based on the device characteristic and configuration. Desaturation detection takes the device under test (DUT) as a current sensing component. By sensing the voltage across the DUT, the desaturation detection decides whether the DUT is operating under shortcircuit. If it is, a signal is sent to the gate driver to turn off the DUT when high voltage is sensed. A series of shortcircuit tests were conducted to verify the function of shortcircuit protection.
A Miller clamp is also implemented and tested on the same phase leg to prevent a false turn on problem and to protect the gate. Both the Miller clamp and desaturation v detection features are tested on the same phase leg. The GaN devices survive the shortcircuit tests, with shortcircuit protection times between 200 ns to 500 ns. The design is successfully validated. Along with the implemented protection features, device degradation and shortcircuit robustness tests are also included in this work. The test results show that 300 ns shortcircuit time under 400 V bus is a safe turn off goal for this device.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/83516 |
| Date | 11 June 2018 |
| Creators | Gui, Yingying |
| Contributors | Electrical Engineering, Burgos, Rolando, Boroyevich, Dushan, Ngo, Khai D. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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