Electrical-mechanical controllers depend on power semiconductors. Motor controls of all sorts - robotics, switching power supplies, welding and induction heating systems - all need a ready way for their power to be controlled. For the most efficient design, the semiconductors and amplifiers that drive these devices must be accurately characterized by the type of load they will be expected to handle.

IGBTs (insulated-gate bipolar transistors) have a particularly strong characterization need. The dynamic performance of IGBTs varies with their type of load. Their switching response is not linear, so characterizing their switching performance is an essential part of selecting the gate drive parameters as well as an essential part of the design optimization process.

Many MOSFET applications require characterization of their switching times to discover how they will really perform in the production circuit. In both cases, characterization will check the safe area, circuit stability, switching speed and switching losses. Characterization is often critical to the design process.

Often, this characterization is done with whatever equipment is handy in the lab. Difficulties with the measurements or instrumentation can reduce the accuracy of the results, increasing risk or even delay the project. On the input side, IGBTs and MOSFETS require a relatively high voltage at their gate, a voltage that most signal generators have problems providing without the complexity, delay and potential errors involved in using an external amplifier. On the measurements side, MOSFETS and IGBT characterization requires proper differential probes and current probes for best accuracy.

Characterizing IGBT switching performance
The use of IGBTs has been increasing lately. For many industrial control applications, IGBTs have advantages over MOSFETs. They have lower conduction losses and a lower on-state voltage drop. In addition, they also have high switching speed, high current capabilities, large blocking voltages and good gate drive characteristics. This makes the popularity of IGBTs understandable.

IGBTs are now used as part of the control circuit for variable speed motor drives, traction control, solar inverters, uninterrupted power supplies, induction heating, welding and high-frequency switching power supplies. Many high-power switching and control applications benefit from IGBTs.

IGBTs have some of the characteristics of both bipolar transistors and MOSFETs. The output switching and conduction characteristics of IGBTs resemble those of a bipolar transistor. But IGBTs are controlled by voltage, like a MOSFET. To assure full saturation and limit short circuit current, a gate drive voltage of +15 V is recommended.

Figure 1. IGBT circuit symbol and equivalent circuit.

To continue the comparison, an IGBT equivalent circuit has capacitances between the gate, emitter and collector of its equivalent circuit. When voltage is applied between the gate and emitter, the input capacitance is charged up through the gate resistor until the IGBT's threshold voltage is reached and the IGBT turns on. In the same way, the gate-to-emitter capacitance must be discharged to a specific plateau voltage before the IGBT can turn off.