Analysis of Parasitic Turn-on Phenomenon in Si-IGBT Unipolar Supply Gate Drivers for Low-power Motor Drives

Abstract

Motor drives play a crucial role in performing numerous daily tasks of human life. The gate driver circuit, an essential part of motor drives, serves as the interface between the switching devices, such as the insulated gate bipolar transistor (IGBT), and the motor-drive controller. The general approach to operating the IGBT involves using a bipolar power supply, where a sufficient negative voltage is applied to the gate-to-emitter terminals of the IGBT during turn-off. Nevertheless, the recent trend leans towards applying zero volts between the gate and emitter terminals when the device is turned off. This method is known as "unipolar supply" for gate drivers, typically applicable for low-power motor drives with a nominal current of less than 100 A. This absence of a negative voltage supply reduces the cost of power supplies and printed circuit board (PCB) footprint. Despite these advantages, it increases the risk of parasitic turn-on, which raises the gate-emitter voltage of the complementary IGBT in a half-bridge configuration. If it exceeds the threshold voltage of the IGBT, it may cause a shoot-through across the DC-link of a motor drive. This thesis discusses several techniques to prevent such parasitic turn-on events, and these are tested on a double pulse test setup.

Specific characteristics of IGBT modules that favour unipolar supply gate drivers are identified, relating to their construction. A simulation model is developed in LTSpice, enabling the observation of interferences caused by switching transients. As developing accurate simulation models that comply with all natural phenomena in semiconductors is challenging, a real-world test setup is necessary to obtain accurate results. Due to the rapid transient nature of switching events, noises arise from the circuit. Therefore, the necessary considerations for building such a test setup are stated to correctly identify and capture the quantities of interest (currents and voltages).

The analysis primarily focuses on the influence of varying external gate resistance and gate capacitance on the gate-emitter voltage of an IGBT in a half-bridge configuration. A recent technique to suppress this increasing gate voltage involves utilising an active Miller clamp gate driver, which provides a low-impedance path when the IGBT is turned off. Its operation is tested along with other techniques to reduce the gate voltage rise by varying the gate driver component parameters appropriately. Their strengths and weaknesses are discussed and analysed qualitatively. Moreover, this analysis includes a comparison of the quantities of interest for two different IGBT modules due to the differences in their development concepts.

The test results demonstrate that the use of active Miller clamp gate driver ICs and fine-tuning passive components effectively suppress the gate-emitter voltage rise during switching events. Key insights are proposed into selecting specific solutions for unipolar supply gate drivers tailored to the cause of parasitic turn-on and circuit characteristics.