The switching performance of the IGBT is greatly affected by the charge time of the gate capacitance, making the selection of the gate resistor critical. A smaller gate resistor will charge or discharge the IGBT's gate-to-emitter capacitance faster, resulting in short switching times and small switching losses. The trade-off is the increased possibility of oscillations due to the gate-to-emitter capacitance of the IGBT and parasitic inductance of the leads.
A fast rate of voltage change at the gate can create significant switching noise if the IGBT load is inductive. To increase noise immunity, the gate drive circuitry should include substantial on and off biasing.
For best performance, IGBTs need different gate circuits for different applications. Staying in the safe operating area is the primary concern for hard switching applications like motor drives or uninterrupted power supplies because the rapid drop in current during turn-off can lead to excessive voltages in the parasitic inductances. To avoid this, the gate drive characteristics must be carefully selected.
At a high level, the requirements of the safe area may require a sacrifice in switching speed, leading to greater switching losses. In soft-switching applications where the switching waveform is well within the safe operating area, the gate drive can be designed for short switching times and lower switching loss.
Figure 2. Setup for characterizing IGBT with gate drive circuitry.
Optimization of an IGBT gate drive requires understanding of the device's switching characteristics under actual load conditions. Analysis is done by stimulating the gate of an IGBT with a series of pulses while the gate-to-emitter voltage, collector-to-emitter voltage and collector current are measured.
For stimulus, the tool of choice is an arbitrary/function generator (AFG) with a high amplitude output. A high-amplitude AFG can directly supply and shape the 15-V pulses needed by the gate, which greatly simplifies laboratory tasks, reduces calibration issues and enhances accuracy as compared to using a conventional AFG with an external amplifier.
On the measurement side, an oscilloscope needs a high-voltage differential probe to handle the high dynamic range of the IGBT's collector-to-emitter voltage, particularly when working with an inductive load such as a motor. The oscilloscope can measure the gate-to-emitter voltage with a standard passive probe and the collector current with a non-intrusive current probe.
Figure 3. Switching waveforms of an IGBT with an inductive load.
This analysis will produce the sort of graphs shown above. With these waveforms it is possible to determine switching energy, on-state losses and if the IGBT is operating within the safe operating area. This gives engineers a solid basis for determining if the selected pulse repetition frequency, amplitude and edge transitions are adequate to achieve the design objectives. If adjustments to the gate waveform, such as pulse width, rising and/or falling edge times or frequency are needed, modern high-amplitude AFGs can modify the gate pulse during the test without odd transitions that might otherwise damage the component.
Probe factors must also be considered during measurement. Probes have varying propagation delay (skew), offset and noise. It is helpful to use an oscilloscope with a software tool that takes care of the probe-related issues, automatically calculates the switching power losses and determines if the IGBT is operating in its safe operating area.