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IGBTs Incorporating Fast Recovery Diodes (FRDs)

2023.08.30

Points of this article

・IGBTs with internal FRDs have been made available for applications in which IGBTs are paired with FRDs, such as in inverters and motor driving.

・The internal FRDs must have fast trr characteristics to reduce losses, and at the same time, with respect to EMC performance, soft recovery characteristics to minimize ringing and noise are important.

table of contents

IGBT lineups include devices with internal fast recovery diodes (hereafter “FRDs”).

IGBTs Incorporating Fast Recovery Diodes (FRDs)

In inverter and motor driving applications using IGBTs, diodes also serve as a path for backward currents that occur during switching. These diodes are called freewheel diodes, and in general FRDs are used. IGBTs with internal FRDs have been made available for applications that use IGBTs together with FRDs in this way.

In the case of an IGB with an internal FRD, the data sheet states as much, and ordinarily the internal FRD is shown in the pinout diagram, as in the example below. And, in addition to the IGBT specifications, the specs for the internal FRD are also given.

An advantage of an internal FRD is elimination of the need to add one externally, so that the number of components and the mounting space can be reduced, and reliability is improved.

Ringing and the Recovery Characteristics of FRDs Integrated with IGBTs

In inverter and motor driving applications, one important characteristic demanded of a freewheel diode is fast operation, with a short reverse recovery time trr. As explained in “Selective Use of Power Devices in Motor Application“, turn-on losses during switching are heavily affected by recovery currents, and so to reduce losses, FRDs with fast trr values are used. Hence FRDs with fast trr characteristics are also demanded for use within IGBTs.

Another important point is the relationship of internal FRDs to ringing. Because FRDs with fast trr values are used, the recovery current converges rapidly, and so ringing (noise) occurs, posing a problem from the standpoint of EMC. Hence FRD recovery characteristics with a fast trr value and “soft” convergence are sought. FRDs designed accordingly are said to be soft recovery-type FRDs.

As examples, the following graph compares the recovery characteristics of the RGS series and RGT series with internal soft recovery type FRDs versus the FRDs in ordinary IGBTs with internal FRDs.

The internal FRDs in the RGS series and RGT series exhibit “soft” convergence and recovery even while the recovery current converges rapidly, even during fast switching at di/dt = 1000 A/µs, and ringing does not occur even under high-temperature conditions with Tj = 125°C. In contrast, we see that significant ringing occurs in the case of general-use devices.

IGBTs with internal FRDs are highly convenient, but the recovery characteristics of the internal FRD must be examined closely. In systems that handle high voltages and large currents, substantial ringing and surges occur, and the effect on EMC is considerable. These are extremely important matters that require careful consideration.

【Download Documents】 Basics of IGBTs

IGBTs are one of the typical power devices and are used in a wide range of applications including motor drives. This handbook provides a basic understanding of IGBTs, including their application scope and application picture based on their features, their structure and principle of operation, and how they compare with and use other power devices.

IGBTs Incorporating Fast Recovery Diodes (FRDs)
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