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Posted on 11 September 2019

Low loss, High Current SiC MOSFET Module

 

ROHM Semiconductor introduced 1200V 120A full SiC (Silicon Carbide) power module composed of SiC Schottky barrier diodes (SBDs) and SiC MOSFETs. When compared with conventional IGBT modules, this module can reduce switching loss by 85%. However, this module is not necessarily enough to drive higher current, which is required from many industrial applications. This paper presents methodology to raise rating current of SiC power modules and our newly developed 1200V 180A SiC module using the same package size as the 1200V 120A full SiC power module. Characteristics of SiC MOSFETs enables to eliminate SiC SBD chips working as free-wheeling diodes and to replaced them with SiC MOSFET chips in parallel because SiC MOSFETs in on-state allow reverse conduction at the same Ron as that of the forward current and its very fast body diode assures high reliability of forward conduction. In addition, SiC MOSFETs are suitable for use in parallel – due to higher gate resistance and positive temperature coefficient of on-resistance, gate current and drain current are more easily balanced than Si IGBTs. These facts lead to the realization of low-loss high current power module.

by Masashi Hayashiguchi, Mineo Miura, Kenji Hayashi, Nobuhiro Hase and Kazuhide Ino ROHM Co., Ltd.

Newly Developed SiC power module

Recently, several companies have started commercial production of SiC switching devices such as MOSFETs and JFETs. As industrial applications tend to require higher current like 100amps or more, ROHM started commercial production of full SiC modules from March of 2012 to meet such requirement. However its application is rather limited because of shortage of its rating current. Expanding the chip size is a straightforward way to achieve higher current but this may not be the best way today because lowered manufacturing yield due to crystal defects will bring cost up. By making full use of the characteristics of SiC MOSFETs ROHM has developed a high current power module. Two key technologies were employed here; 1) eliminating anti-paralleled SBDs by turning on MOSFETs to allow reverse current flow, 2) parallel use of multiple MOSFET chips without current miss-sharing.

Reverse conduction of SiC MOSFETs

The off-set voltage of SiC PN diode is relatively high because SiC is a wide-gap semiconductor material and this may cause high conduction loss during commutation under Vgs = 0V condition. In many cases of inverter/converter drive, turn-on signal is applied to the FETs at commutation-side after dead time is finished while it is impossible to operate IGBTs at third-quadrant – thus this turn-on signal does not produce any effect in IGBT-based power electronics. However, things are distinctly different in the case of MOSFETs - relatively high Vf of body-diode can be reduced by turning on MOS channel to allow reverse conduction as shown in Fig.1 and Fig.2.

Reverse current flow in MOSFET

Fig.1 Current flow in reverse direction

Body diode characteristics

Fig.2 Characteristics of body diodeand reverse conduction of 100A class SiC module

In MOSFET-only configuration utilizing reverse conduction, body diode is conducted only during dead time. Despite very short period of dead time to prevent shoot-through current, bipolar degradation of SiC devices can be still serious problem. SiC PN diode has been suffering defect expansion after forward conduction that results in increases in both on-resistance and leakage current [1]. ROHM succeeded in suppressing defect expansion as reported in PCIM 2012 [2] and confirmed the reliability of body-diode for 1000h without any characteristics change (Fig.3).

Diode reliability test results

Fig.3 Reliability test results of body diodeconduction

As for reverse recovery characteristics, body diode of SiC MOSFETs shows as fast recovery time as SBDs (Fig.4, Fig. 5), which leads to lower EMI noise and loss level not achieved by Si MOSFETs or even by Si-FRD. Recovery current of the body diode slightly increases at 125oC while that of SBDs is the same as RT, but reverse recovery energy is very small compared with Si-FRD (Fig.6).

Reverse recovery characteristics of body diode at 25°C

Fig.4 Reverse recovery characteristics of body diode at 25°C

Reverse recovery characteristics of body diode at 125°C

Fig.5 Reverse recovery characteristics of body diode at 125°C

Diode reverse recovery energy

Fig.6 Reverse recovery energy

All of these facts contributed to realization of a SiC power module without additional free wheeling diodes.

Parallel use of SiC MOSFETs

If devices with negative temperature-coefficient of on-resistance are connected in parallel, current might concentrate on a chip with the lowest Ron and cause thermal runaway in the worst case. However such risk is lower in the case of SiC MOSFETs as its temperature coefficient of on-resistance is positive (Fig.7) when its recommended on-state Vgs is applied (i.e. Vgs= 18V) – the characteristics make parallel connection of the switching devices much easier. Fig.8 shows temperature change at chip surfaces observed by pyrometer along with time. 4 chips of MOSFET are mounted and connected in parallel in one module. 1 chip out of the 4 chips has lower Ron and the rest has higher Ron. These chips were intentionally prepared to analyze the current crowding effect due to such Ron variation. The normal variation of Ron, in the mass-produced module is less than half of that for this experiment.

Ron vs Tj characteristic

Fig.7 Ron vs Tj characteristics

As shown in Fig.8, temperature difference among chips is larger at the very beginning of conduction but it becomes smaller with time (12°C difference at 1sec) because of self-balancing effect due to positive temperature coefficient of Ron.

Temperature change during conduction

Fig.8 Temperature change during current conduction of the intentionally prepared module which consist of 3 MOSFET chips with higher Ron and 1 MOSFET chip with lower Ron in parallel.

The other concern regarding parallel use is miss-sharing of gate current at turn-on. As for paralleled Si IGBT chips, due to its low internal gate-resistance, the balance between each gate current is easily influenced by the difference in stray inductance. Then the devices may be broken by rush current and resonant oscillation triggered by this. Contrary to Si IGBTs, thanks to relatively high internal gate-resistance of SiC MOSFETs (several Ohm), distribution of gate current among paralleled chips is well balanced without additional gate-resistance put individually.

These characteristics make multiple connections of SiC MOSFET chips in parallel much easier without miss-sharing of both drain and gate current, which helps to prevent thermal runaway or resonant oscillation.

Characteristics of 1200V 180A SiC MOSFET module

A new 1200V power module which is composed of only SiC-DMOSFETs was developed. By replacing SBDs with MOSFETs, rating current was increased from 120A to 180A with the same package size. Typical ID- VDS characteristics at room temperature are shown in Fig.9. Drain-source on-state voltage at drain current of 180A is 2.3V (Ron = 12.8m: about 40% less than 120A module). Fig. 9 and Fig. 10 shows switching loss comparison between Si-IGBT module and SiC-MOSFET module. Due to no-tail current and fast recovery characteristics of SiC devices, total switching loss is reduced by 75% against IGBT module if small external gate-resistance is used. The module is reliable for body-diode conduction and will not show bipolar degradation because high reliability is confirmed in our SiC-MOSFETs.

Typical output characteristics of 180A SiC-MOSFET module

Fig.9 Typical output characteristics of 180A SiC-MOSFET module

Gate resistance vs switching loss

Fig.10 Comparison of Eon , Eoff and Err between Si-IGBT module and 180A SiC-DMOS module

Total switching loss comparison

Fig.11 Comparison of total switching loss between Si-IGBT module and 180A SiC-DMOS module

Conclusion

We examined the methodologies to achieve SiC power module with higher current rating with the existing SiC devices. Based on this, a new half-bridge power module containing only SiC-MOSFETs was developed. Rating voltage is 1200V, maximum drain current is increased from 120A to 180A (Ron is reduced by 40%) with the same case size by replacing SBDs with MOSFETs. This module can be implemented to many industrial applications which requires high current.

Literature

[1] S.I.Maximeko, P.Pirouz and T.S.Sudarshan, Mater. Sci. Forum (2006) , Vols.527-529 pp.367-370
[2] K.Okumura, N.Hase, K.Ino, T.nakamura and M.Tanimura, Procedings PCIM (2012)

 

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