What is a reverse-conducting IGBT (RC-IGBT)?

Back electromotive force is generated when controlling the on/off of an inductive load (such as a motor or coil) in an inverter or converter circuit. When using an IGBT for this on/off control, a freewheeling diode (FWD*1) connected in inverse parallel to the IGBT as shown in Fig. 1 is required to process the back electromotive force. A reverse conducting IGBT (RC-IGBT) is a product that combines an IGBT and FWD on a single chip.

Fig. 1 Connection of IGBT and freewheeling diode：This shows a configuration in which an external FWB connected in reverse to the IGBT is attached as a countermeasure against inductive load switching.
Fig. 1 Connection of IGBT and freewheeling diode
Fig. 2 Operation of a freewheeling diode (FWD): A diagram showing how the freewheeling diode (FWD) protects the IGBT by bypassing the current against the counter electromotive force generated when an inductive load is switched.
Fig. 2 Operation of a freewheeling diode (FWD)

An inductor, which is an inductive load, has the characteristic of generating a back electromotive force (reverse voltage) in response to changes in the load current, preventing changes in the current. In inverter and converter circuits, switching operations that turn inductive loads on and off are repeated. Normally, the on/off transition occurs very quickly, so the back electromotive force is very large. During the transition from on to off, a voltage exceeding the power supply voltage is applied to the IGBT, which may destroy the IGBT.
The load current generated by this back electromotive force is commutated in the reverse direction of the IGBT (from emitter to collector) through the FWD (freewheeling diode) connected in reverse parallel to the IGBT. Fig. 2 shows a circuit in which IGBTs with parallel-connected FWDs are arranged in series as one arm of an H-bridge circuit. When Q2 is turned off, the collector current IC that flows when Q2 is on generates a voltage that exceeds the collector voltage of Q1 due to the back electromotive force generated by the inductive load, causing the FWD to conduct and the current to be commutated.

Fig. 3 Example of RC-IGBT internal structure：In an RC-IGBT with a built-in FWB, an opening is provided in the collector layer on the back side, and the n+ layer is directly connected to the collector electrode at the opening.
Fig. 3 Example of RC-IGBT internal structure

RC-IGBT is a single chip that incorporates this FWD. Fig. 3 shows the internal structure of RC-IGBT. The FWD has an opening in the p+ layer, which is the collector of the IGBT chip, connecting the n+ and collector terminals. This forms a PIN diode (n⁺–n⁻–p⁺ structure) between the emitter and collector electrodes, connected in anti-parallel with the IGBT. RC-IGBTs are used in soft switching applications such as ZVS (Zero Voltage Switching), and more recently have begun to be used in hard switching applications such as inverter circuits. However, due to trade-offs (such as the on-voltage of the IGBT and the reverse recovery time of the FWD), it is difficult to optimize both performances, so compatibility must be confirmed for applications that place importance on high-speed performance or forward characteristics.

Please refer to the following FAQ for information on IGBT on-voltage.

Please refer to the following FAQ for information on soft switching such as ZVS.

Please refer to the following FAQ for information on FWD reverse recovery time.

*1: FWD: An abbreviation for Free Wheeling Diode. It is used as a path (commutation path) for the back electromotive force generated in the reactance load.

*2: PIN diode: A diode that has a high-resistance I layer (intrinsic layer: close to an intrinsic semiconductor, with a low impurity concentration and high resistance) sandwiched between a P layer and an N layer. This structure is used for the FWD of IGBTs, which require high withstand voltages.

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