CM600DU-24FA IGBT – Removal of RTC over-current protection

The real-time current control (RTC) that can found in some IGBT modules is a protection against short circuits in f.ex. motor drives where they could have been used.

The conditions on which the RTC acts is however sharing area with using IGBT bricks in Tesla coils, as the RTC will never interact while operating the IGBT within its Safe Operating Area (SOA) there is still a risk that it will be activated when driving a IGBT hard in a DRSSTC where it could be used to switch currents many times its rating.

Below is a quote from a Powerex paper on how the RTC works and behaves. I added a few outlines and extrapolated the graph to show the typical 24 VDC gate drive in a DRSSTC.

[1] 4.0 RTC Description and Behavior

F-Series IGBTs include an integrated real-time current control (RTC) circuit for protection against short circuits, which was originally developed for intelligent power modules (IPMs). The RTC is a separate chip wire-bonded directly to the IGBT die and mounted adjacent to it. During normal operation of the device, the RTC is effectively “transparent” to the gate driver. Its power supply is drawn from the main collector-emitter path of the IGBT, so it imposes no additional drain on the gate driver. The RTC is connected to a current mirror emitter on the trench IGBT chip. A simplified diagram of this is shown in Figure 3.

When the IGBT operates in a short circuit, the RTC detects the excessive current in the IGBT and reduces the gate-emitter voltage to limit the short-circuit current. The gate-emitter voltage is reduced to less than 12V, compared with the normal recommended value of 15V. The effect of gate-emitter voltage on short-circuit current is shown by Figure 4. It is important to note that the RTC acts only to limit short-circuit current; it does not switch off the IGBT. Therefore the gate driver circuit should be designed to ensure that the IGBT is turned off within 10µs of a short circuit occurring. The RTC limits the short circuit collector current to 2-4 times rated current, depending on the junction temperature of the IGBT and the short circuit di/dt.

The minimum trip threshold for the RTC is 2 times the rated current of the device and occurs at high Tj and high di/dt. Therefore operation of the IGBT within its normal switching SOA is unaffected by the presence of the RTC

In the following video I show and explain where to locate the RTC circuit and how to disable it with a simple tool like tweezers. Side cutters can also be used but it will make a bigger mess and ruin more of the protective goop that surrounds the die and bonding wires.

References

[1] Powerex, “Featured Products Technology”