Digital active gate drives to increase power semiconductor performance
Keywords: power density
power electronics
MOSFET
gate drive
IGBT

Active Gate Drives (AGDs) tailor the signal applied to the gates of power metal-oxide-semiconductor field-effect transistors (MOSFETs) and insulated-gate bipolar transistors (IGBTs) to improve the switching behaviour of these power semiconductor devices. The AGDs can slow down the transition when switching between the off and on states (and vice-versa) to reduce peak transient voltages and electromagnetic interference (EMI). However, the longer duration of the switching transition can lead to a higher energy loss in the power devices.
In the hard-switched bridge-leg studied in this thesis, it is found that the synchronous switching device experiences high transient voltage stresses as it undergoes its reverse-recovery, due to the parasitic inductance of the circuit as explained in Chapter 2. Using the pre-existing arbitrary waveform gate driver developed in (Rogers 2016), a method of driving the control IGBT of the bridge-leg is proposed, which can adapt the switching behaviour of the IGBT to keep this peak transient voltage below a specified limit. The AGD operates in three modes of differing complexity. The potential of each mode to reduce the peak transient voltage is assessed using an experimental method where a variety of control waveforms are in turn applied to the IGBT, and the peak voltage and switching energy loss associated with each control input is measured and recorded. The results reveal that the AGD is capable of suppressing the transient peak voltage across the diode by sharing the inductive voltage spike between the IGBT and the diode. This enables the voltage stress of the diode to be reduced with a small increase in total switching energy loss.
The implications of this increased energy loss on the power-processing capacity of the IGBT are investigated. By reducing the peak transient voltage resulting from switching, AGD enables operation of the IGBT at higher dc bus voltages. However the larger switching loss means that a reduction in the conducted current is necessary to avoid exceeding the maximum allowable device junction temperature. A method is developed which experimentally quantifies the trade-off between these two factors, the results of which indicate that a small increase in IGBT power capacity can be obtained by use of an appropriate AGD.
In addition, an investigation is conducted into the role of the gate drive in obtaining the lowest possible switching loss of a silicon carbide (SiC) MOSFET, when circuit parasitic inductance does not limit the maximum speed of the switching transition. Use of an integrated circuit (LMG5200) containing two gallium nitride (GaN) FETs as a low-impedance gate drive is found to enable a high slew-rate of the MOSFET drain-source voltage, which is further increased by raising the voltage at the MOSFET gate. The results suggest that the full potential of these wide-bandgap devices is yet to be realised, that conventional gate drives currently limit the switching speed of SiC MOSFETs, and that switching performance may be enhanced by new gate drive designs.