AN2123

2-Level turn-off

9/15

Practical tests were made with 1200 V - 50 A IGBT modules Fuji 6MBI50S120L. The results shown in

Figure 14 point out how the 2-level turn-off feature can consistently reduce voltage stress on the IGBT in

the event of over-current.

During this test, the 50 A-rated IGBT module has to turn-off a 300 A current simulating an application

faulty condition.

The left-hand graph in Figure 14 shows a standard commutation. The driver OUT pin voltage is abruptly

pulled from 16 V to 0 V and the IGBT gate is discharged through the gate resistor. The fast turn-off of the

IGBT generates a voltage spike on Vce reaching 1kV, which is dangerously close to the IGBT absolute

maximum rating (1200 V). The calculated turn-off energy reaches 19 mJ.

The right-hand graph in Figure 14 shows how the TD351 and its 2-level turn-off feature can help deal with

this situation. During the first phase, the TD351 OUT pin is pulled from 16 V to 9 V during 2.5 µs. In the

second phase the OUT pin is pulled to 0 V. As a consequence, the IGBT turn-off is slightly longer and the

Vce voltage spike is advantageously reduced to 683 V. The calculated turn-off energy reaches 31 mJ,

but the resulting overheating can be more easily managed than the destruction of the IGBT by over-

voltage stress.

Figure 13. Waveforms of the 2-level turn-off function (COFF timing exaggerated for illustration)

Figure 14. Reduction of IGBT over-voltage stress using 2-level turn-off feature

Ic=300A

Vce=400V

overshoot 1kV

TD351 OUT

IGBT Vge

Ic=300A

Vce=400V

overshoot 683V

TD351 OUT

IGBT Vge

standard commutation

2-level turn-off w ith LVoff=9V