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Posted on 03 July 2019

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650V IGBT4 the optimized device for reduced EMI and low ΔV

The trend of the last years of all power semiconductor manufacturers to increase the switching speed of the devices offers the benefit of reduced switching losses and the possibility to improve the efficiency of the system. These power devices require optimized parasitic inductances (Lσ) of the DC link circuit. With respect of the needs of high power applications with its larger currents in various setups, now a new chip, the 650V IGBT4, has been designed to provide an additional degree of freedom. The IGBT4 device features an improved softness during switch-off and a lower overshoot voltage as the result of a reduced turn-off current slope di/dt.

By Wilhelm Rusche, Dr. Andreas Härtl, Marco Bässler, Infineon Technologies AG

 

The device was designed especially for medium and high current applications. In comparison with the 600V IGBT3 the new chip offers a better softness during switch-off and a higher blocking voltage capability. As an add-on the short-circuit robustness is significantly improved. In contrast the 600V IGBT3 has been optimized for lower power applications, or higher power in very low stray inductance applications.

Design and Technology of the 650V IGBT4

The 650V IGBT4 [1] utilizes a trench MOS-top-cell, thin wafer technology and a field-stop concept as seen in Figure 1. The combination of trench cell and field-stop enables comparatively low on-state and turn-off losses. Compared to the 600V IGBT3, the chip thickness was increased by about 15% and the width of the MOS channel was decreased by about 20% as indicated in Figure 1. Thereby, the softness during switch-off is improved to reduce the EMI effort. However, of course these measures also cause additional losses. So in order to compensate for these side effects, the efficiency of the backside emitter was increased by 50%. In addition to the optimization of the dynamic behaviour the blocking voltage was grew by 50V to 650V.

Schematic cross section of the new 650V IGBT4 and the changes implemented compared to the 600V IGBT3: increased chip thickness (y), decreased channel width (z), and increased backside p-emitter.

Results of the 650V IGBT4 dynamic characterization

The stray inductance in combination with the current gradient has an influence on the voltage characteristic during turn on and turn off as ΔV=L*di/dt. Thus the over voltage increases when switching off with larger Lσ. The turn off behaviour is quite insensitive to the gate resistance. This behaviour is well known for trench field-stop IGBT [6]. A consequence of this inherent IGBT behaviour would be in such a case a special driver stage with integrated IGBT protection functionalities and/or additional components like snubber capacitors. All these functionalities and components create design effort and cost. High current levels need, due to the high di/dt level, a DC-link design with very small parasitic inductances. As an alternative specially designed IGBTs with a soft switching characteristic like the new 650V IGBT4, can be utilized.

The difference of the fast 600V IGBT3 and the soft 650V IGBT4 in the switching behaviour becomes obvious in Figure 2, where the switch-off behaviour of high current 600A EconoDUAL™3 modules are compared.

Comparison of the softness during switch-off of a 600V IGBT3 left picture and the new 650V IGBT4 right picture

For the investigations a standard DC-link design with Ls=60nH was used. This setup is not ideally suited for a high current setup with the 600V IGBT3 [5]. Consequently, the switch-off of a current of 50% Inom, IC=300A, and a DC link voltage of 300V at 25°C effects a quite high overshoot voltage VCE,max and a snap-off with oscillations. In contrast, the new 650V IGBT4, especially designed for such high current applications, shows a smooth switch-off with a much lower VCE,max, even at the typical DC link voltage of 300V in specific high current setup.

In the given test setup the 600V IGBT3 device reaches the limit of 600V. While the 650V IGBT4 shows a smaller overvoltage of 530V. In addition to the reduced overvoltage shoot the increased blocking capability VCE_max, comes as a real surplus and offers the advantage of an increased safety margin during turn-off.

Not only the turn off characteristics but also the softness of the IGBT is quite insensitive to the gate resistance. The softness during switchoff is improved to reduce the EMI effort. In Figure 3 the Fourier Transformation spectra of a soft and a not soft turn-off waveform are given. The oscillation leads to a 5 times higher level around the oscillation frequency of roughly f=20…25 MHz, a frequency which is quite typical for chip DC link oscillations at the given parasitic inductance.

Influence of a current snap off to the EMI; FFT of the voltage curves of a FF600R07ME4

Even though such a procedure is not able to predict passing or failing of an EMI qualification, it obviously demonstrates the sensitivity of EMI to snap-off phenomena.

The most important aspect in all designs is the improvement of the DC-link design in order to be able to prevent additionally any kind of oscillations.

For the inductance the lower the better is a simple rule for high efficiency designs.

On the other hand, the softer switching behaviour has to be paid for with higher losses during switch-off, Eoff, and with a slightly increased saturation voltage ΔVCEsat.≈100mV@T=25°C. Taking into account common switching frequencies, this increase does not play a major role. This fact is visualized in Fig. 4 showing a simulation with IPOSIM. This tool, the Infineon Power Simulation program, can be found on the Infineon homepage (www.infineon.com). It performs a calculation of switching and conduction losses for all components, taking into account conduction and switching losses as well as thermal ratings. As can be seen in Fig. 4, the reduction of the RMS module current due to increased losses of the 650V IGBT4 is only moderate between 4 and 9% at switching frequencies of 2kHz up to 10kHz, a typical range for common applications.

Calculation of the RMS current as function of the switching frequency of the 600V IGBT3 and the new 650V IGBT4 calculated in 600A EconoDUAL™ 3 modules

Besides this standard operating the design must be robust and has also to withstand a case of failure. The established value in power semiconductor datasheets is the specification of a hard short circuit current (ISC).

Short circuit robustness

Despite the considerably reduced silicon thickness of field-stop devices as compared to non-punch-through (NPT) designs, field-stop IGBTs are known to feature a good short-circuit robustness [3, 4]. With the new 650V IGBT4, the short-circuit robustness is significantly enhanced compared to the 600V IGBT3. The increased thickness of the chip offers a larger thermal budget due to the thermal capacity of the silicon volume. In addition, the decreased channel width reduces the level of the short-circuit current, this effect is shown vice versa in [5]. In sum, the 650V IGBT4 can resist a higher short-circuit energy, and therefore the device is able to withstand a longer short-circuit pulse time without getting destroyed. In Fig. 5, a typically hard shortcircuit pulse measurement of the 650V IGBT4 is displayed. As the graph shows, the pulse time short-circuit event was 10 µs, and the short-circuit current typically is about 4 times the nominal current of the FF600R07ME4 device.

Conclusion

Infineon’s new 650V IGBT4 permits the development of inverter design especially for large current applications, to be employed in the corresponding modules. The device features reduced EMI effort as the result of an improved softness during turn-off, a lower overshoot voltage as the result of a reduced turn-off current slope di/dt, a higher blocking capability of VCE_max=650V, an operation range of increased DC-link voltages and/or higher stray inductances, an enhanced short circuit robustness with 10µs pulse time @Tvjop=150°C, and an ideal flexibility between highest output power at elevated junction temperature of up to Tvjop=150°C or highest power cycling capabilities at lower junction temperatures.

Measurement of a short-circuit pulse event of the 650V IGBT4

In sum the 650V IGBT4 provides design engineers effective degrees of freedom in their applications.
 

References:

1) A.Härtl, M.Bässler, M.Knecht, P.Kanschat: “650V IGBT4: The optimized device for large current modules with 10μs short-circuit withstand time”, Proc. PCIM Europe, (2010).
2) H. Rüthing et al.: "600V-IGBT3: Trench Field Stop Technology in 70μm Ultra Thin Wafer Technology", Proc. 15 th ISPSD, 66 (2003).
3) M. Otsuki et al.: “Investigation on the Short-Circuit Capability of 1200V Trench Gate Field-Stop IGBTs“, Proc. 14th ISPSD, 281 (2002).
4) T. Laska et. al.: “Short Circuit Properties of Trench-/Field-Stop-IGBTs – Design Aspects for a Superior Robustness”, Proc. 15th ISPSD, 152 (2003).
5) P. Kanschat, H. Rüthing, F. Umbach, F. Hille: “600V-IGBT3: A detailed Analysis of Outstanding Static and Dynamic Properties”, Proc. PCIM Europe, 436 (2004).
6) W.Rusche: Infineon Application Note “AN2003-03, Switching behaviour and optimal driving of IGBT3 modules” (2003).

 

 

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