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High Amplitude Arbitrary/Function Generator Simplifies Measurement in Automotive, Semiconductor, Scientific and Industrial Applications Application Note

Figure 7. IGBT circuit symbol and equivalent circuit.

Figure 8. IGBT gate drive circuitry and switching test circuit.

Analyzing the Switching Waveforms of an IGBT

In recent years, insulated gate bipolar transistors (IGBTs) have been finding increasing use in industrial and automotive applications as replacement of MOSFETs thanks to their high switching speed, high current capabilities, large blocking voltages, and simple gate drive characteristics, but lower conduction losses and lower voltage drop in the on-state.

Industrial applications for IGBTs include traction, variable speed motor drives, uninterrupted power supplies (UPS), induction heating, welding, and high-frequency switch mode power supplies in telecom and server systems. In the auto- motive industry, IGBTs are in huge demand for ignition coil driver circuits, motor controllers, and safety related systems.

IGBTs are a cross between bipolar transistors and MOSFETs. In terms of output switching and conduction characteristics, the IGBT resembles the bipolar transistor. However, while bipolar transistors are current controlled, IGBTs are voltage controlled like a MOSFET. To assure full saturation and limit short circuit current, a gate drive voltage of +15V is recommended.

Like a MOSFET, an IGBT has capacitances between gate, emitter, and collector. When voltage is applied between the gate and emitter terminals, the input capacitance is charged up through the gate resistor RG in an exponential fashion until

the IGBT's characteristic threshold voltage is reached where collector-to-emitter conduction is established. Likewise, the input gate-to-emitter capacitance must be discharged to a specific plateau voltage, before collector-to-emitter conduction is interrupted, and the IGBT turns off.

The size of the gate resistor significantly impacts the dynamic turn on and turn off characteristics of the IGBT. A smaller gate resistor charges and discharges the IGBT's gate-to- emitter capacitance faster, resulting in short switching times and small switching losses. However, a small gate resistor value can also cause oscillations due to the gate-to-emitter capacitance of the IGBT and parasitic inductance of the leads. To reduce turn-off losses and to improve the IGBT's immunity to noise injected through the rate of change of the collector-to-emitter voltage which can be substantial for inductive loads, it is recommended that the gate drive circuitry includes substantial on and off biasing.

The IGBT's best performance varies by application, and the gate drive circuit must be designed accordingly. In hard- switching applications such as motor drives or uninterrupted power supplies, the gate drive parameters must be selected so that the switching waveform does not exceed the IGBT's safe operating area. This can imply a sacrifice in switching speed at the expense of switching loss. 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.

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