SMPD: An Advanced Isolated Package To Keep the SiC MOSFET Chip Up to 75°C Cooler

This article is published by EE Power as part of an exclusive digital content partnership with Bodo’s Power Systems.

Littelfuse has introduced the advanced isolated package Surface Mount Power Device (SMPD) to fill the gap between modules and discrete by offering the performance of a power module with the flexibility of a discrete.

 

SMPD and its Advantages

This top-side cooled isolated package was first pioneered by IXYS (Littelfuse) in 2012. The SMPD, as displayed in Figure 1, offers several key advantages:

1. The integrated isolation provides excellent reliability under a power and temperature cycling environment.
2. Isolation voltage rating of minimum 2.5 kV AC, 1 minute.
3. Lower junction-heatsink thermal resistance and higher power handling capability compared to discrete devices using an external isolation foil.
4. Allows fully automated pick & place and standard reflow soldering for easy manufacturing.

 

Image used courtesy of Adobe Stock

 

The SMPD simplifies how engineers address the system integration, thermal and assembly challenges of their power semiconductor designs. The SMPDs are already available with standard topologies such as rectifier, buck, boost, and phase-leg in a variety of technologies such as Si/SiC MOSFET, IGBT, Diode, Thyristor, Triac, or other tailored solutions with voltage classes ranging from 40 V to 3000 V.

 

Figure 1. SMPD internal construction and size. Image used courtesy of Bodo’s Power Systems [PDF]

 

SiC-Based SMPD Compared to Standard Discrete

The electrical performance advantages of SiC MOSFET-based SMPD compared to standard discrete in October 2022 [1]. The article emphasized the applicational advantages, including power loop optimization, when using the SMPD.

 

Thermal Performance

Most standard discrete power semiconductor packages have an electrically conductive mounting tab. It is typically desired to electrically isolate the device mounting tab from the heat sink due to safety concerns and the need to mount multiple discrete devices with different tab potentials on the same heat sink frame. External thermally conductive electrical isolation foil between the package tab and heatsink has become a widely used approach in the industry for such a purpose. However, this approach comes with the major penalties of increased junction-heatsink thermal resistance, R_thJH, reduced power and current handling capability, and complex thermal management with significant assembly efforts [2]. In contrast, the DCB-isolated SMPD doesn’t require an external isolation foil, thus improving its thermal performance significantly compared to conventionally isolated discrete. Thermal measurements were carried out to demonstrate the improved thermal performance and power handling capability of the isolated SMPD compared to externally isolated standard discrete. Littelfuse has developed the SMPD with a high-performance ceramic which further reduces the junction-case thermal resistance, R_thJC, and junction-heatsink thermal resistance, R_thJH, improving power handling capability even further [3]. Thermal measurements incorporating 1200 V SiC MOSFET chips were conducted in three packages, as shown in Figure 2.

 

	TO-247	SMPD	SMPD
Isolation	External, Thermally conductive foils with thermal conductivity of 4.5 W/mK and 6.5 W/mK	Internal, Alumina ceramic	Internal, High-performance ceramic
SiC die	Same in all three packages
V(BR)DSS [V]	1200	1200	1200
RDS(on) [mQ]	25	25	25
ID25 [A]	90	55	77
RthJC [K/W]	0.27	0.7	0.45

Figure 2. Devices for thermal performance comparison featuring SiC MOSFETs. Image used courtesy of Bodo’s Power Systems [PDF]

 

Thermal measurement results for the 1200 V, 25 mΩ SiC MOSFET chip in different packages at heating current, I_H=40 A, are illustrated in Figure 3.

As evident in Figure 3, the SMPD with high-performance ceramic can improve the thermal resistance, R_thJH, by up to 56% compared to the TO-247 with the same chip. This directly translates into increased power handling potential and lower chip temperature at the given current. It was observed that the SiC chips in the SMPD package with improved ceramic stay up to 75°C cooler when compared to the TO-247 device with external isolation at I_H=40 A. The SMPD with high-performance ceramic enables nearly a 58% reduction in temperature swing ∆T_JH compared to the standard discrete. However, it is worth mentioning that the practical zone for T_vj within an application is usually only up to 130°C to ensure safe operation. By comparing the SMPD’s performance to the TO-247 with 6.5 W/mK foil at this junction temperature, the SMPD with high-performance ceramic improves thermal resistance, RthJH, by 48% and offers 45% lower temperature swing, ∆_TJH. Compared to conventional alumina ceramic-based SMPD, the high-performance ceramic-based SMPD offers 30% lower R_thJH and 40% lower temperature swing, ∆_TJH with the same SiC chip. The SMPD package thus can contribute to significantly improving the device’s lifetime and, in turn, the reliability of the application under realistic application conditions.

 

Figure 3. Thermal measurement comparison results on devices featuring SiC MOSFETs. Image used courtesy of Bodo’s Power Systems [PDF]

 

Application Power Output Increases

The reduction in overall thermal resistance and chip junction temperature using the SMPD package may significantly increase the potential in application power output. To demonstrate this, thermal measurements were performed such that the SiC MOSFET chip will reach the junction temperature of 130°C for the packages listed in Figure 4. The SMPD with the high-performance ceramic could handle 28% higher heating current and 130% higher power dissipation than a conventionally isolated discrete solution. The thermal measurement results can be interpreted in terms of the percentage power output increase potential of an application by using the SMPD. Consider an 18 kW power converter operating at 800 V DC-link voltage, designed with TO-247 discrete and external isolation foil. Replacing the standard discrete with the SMPD will not only reduce the number of necessary power components but also, for the given junction temperature limit of 130°C, the SMPD can push the theoretical DC power output of an 18 kW application by 48% to 26.6 kW.

 

Figure 4. Potential increase in application power output using SMPD. Image used courtesy of Bodo’s Power Systems [PDF]

 

System-Level Cost Saving

The SMPD offers significant indirect cost-saving opportunities at the system level in the application thanks to its pick & place compatibility, reduction in potential warranty claims by eliminating isolation foil, and reduced space and size on the PCB. Figure 5 illustrates the system-level direct cost-saving opportunity in the application using the SMPD versus standard discrete using the example of a 22 kW active front-end converter for a DC charger.

 

Figure 5. System-level cost-saving opportunity by using the SMPD in the application. Image used courtesy of Bodo’s Power Systems [PDF]

 

Comparison Summary

By comparing the thermal performance of the Littelfuse SMPD with standard TO-247 discrete having the same SiC MOSFET chip, it was observed that the SMPD package offered thermal resistance R_thJH reduction by up to 56% and temperature swing ∆_TJH reduction by up to 58%. The SiC MOSFET chip in the SMPD package could stay up to 75°C cooler at given DC power. The usage of SMPD in application reduces mounting efforts, enables space-saving, provides integrated isolation, and increases power density and efficiency along with simplified thermal design compared to the standard discrete packages. The Littelfuse SMPD product portfolio can be checked and inquired at the Littelfuse web page [4].

 

References

[1] A.Bhatt, F.Perraud, J.Padilla, M.Schulz, SMPD: An Advanced Isolated Packaging to Fully Exploit the Advantages of SiC MOSFETs, Bodo’s article, Oct 2022, https://www.bodospower.com/

[2] Application Note: ‘Mounting and Cooling Solutions for SMPD Packages’; www.littelfuse.com.

[3] A.Bhatt, U.Kulsoom, F.Perraud, M.Schulz, L.Gant, ‘ISOPLUS - SMPD: An Advanced Isolated Packaging to Fully Exploit the Advantages of SiC MOSFETs’; PCIM-2023, Nuremberg, Germany

[4] Littelfuse SMPD product offering; https://www.littelfuse.com/products/power-semiconductors.aspx

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine and is co-authored by Aalok Bhatt, Umme Kulsoom, Francois Perraud, Martin Schulz, and Levi Gant of Littelfuse.