1200 V SiC MOSFET single-tube design with enhanced interconnect packaging technology for high-efficiency welding machine

"introduction"

In recent years, mandatory regulations on energy efficiency of welding machines have emerged as a result of the need for more energy-efficient products to achieve a better sustainable use of natural resources. The improved silicon carbide CoolSiC™ MOSFET 1200 V is packaged in a TO-247 package based on .XT diffusion solder technology. Its unconventional packaging and thermal design approach improves energy efficiency and power density through improved design.

By: Jorge Cerezo, Senior Application Engineer, Infineon Technologies

Inverter welding machines are usually designed with power module solutions to achieve higher output power, thereby helping to reduce the cost, weight and size of energy-saving welding machines [1].

In the welding industry, trends such as increased efficiency, reduced costs, and enhanced portability (i.e., reduced size and weight) have been driving forces for continuous development. For example, several standard regulations have or will mandate specific levels of power supply efficiency for welding machines. One example is the latest European Union (EU) regulation for welding equipment that came into effect on January 1, 2023 [2]. As a result, it has now become very difficult to follow these trends for medium-power welding machines ranging from 10kW to 40kW, which use power modules as a typical solution.

Infineon's CoolSiC MOSFET 1200 V uses a TO-247 package based on .XT diffusion solder technology, which greatly improves the device's thermal performance and reliability. Combined with a specific cooling design ("to increase heat dissipation, the device single tube is directly mounted on the heat sink without any electrical isolation" [3]), it provides a better device single tube solution (Figure 1). It can achieve higher output power, improve efficiency and power density, and reduce the cost of medium-power welding machines.

Figure 1: Welding machine power supply using a single 1200 V CoolSiC MOSFET without heat sink isolation

CoolSiC MOSFET single tube using .XT diffusion welding technology

The enhancement-mode CoolSiC MOSFET 1200 V takes advantage of a modified TO-247 package based on Infineon’s .XT diffusion soldering technology. This technology uses an advanced diffusion soldering process. As discussed in detail in [4], the main benefit of this packaging technology is the significant reduction in solder layer thickness (Figure 2), where a specific combination of metal alloys significantly increases thermal conductivity. This feature reduces the junction-to-case thermal resistance (Rthj-case) and thermal impedance (Zthj-case) of the device.

This welding process avoids chip deflection and solder overflow and achieves a nearly void-free welding interface, thereby improving the reliability of the device. In addition, it improves the performance of the device under thermo-mechanical stress, which means that the device has better performance under active and passive thermal cycle test conditions. Overall, the CoolSiC MOSFET 1200 V in TO-247 package based on .XT diffusion welding technology can achieve better thermal performance and reliability in welding machine power supply design.

Figure 2: Infineon XT diffusion soldering technology compared to conventional soft soldering process

Design of 500 A welding machine power inverter using CoolSiC MOSFET device

A unique 500 A power inverter design for a welding machine from a large manufacturer shows a modified solution for medium power welding machines based on the CoolSiC MOSFET 1200 V in TO-247 package with .XT diffusion solder technology. It uses the cooling concept discussed earlier, as shown in Figure 1, with the device mounted on a heat sink without electrical isolation. In addition, to confirm its better performance, it was compared with the main competitor's SiC MOSFET under the same test conditions.

The welding machine power supply consists of a three-phase input, full-bridge topology inverter, using four 20 mΩ 1200 V CoolSiC MOSFETs in TO-247 4-pin package based on .XT interconnect technology (IMZA120R020M1H) provided by Infineon.

Table 1 lists the basic technical specifications of inverter welding:

Table 1: Basic technical specifications of welding machine power inverter

Note that the ultra-high switching speed of SiC MOSFETs enables a significant increase in the typical operating switching frequency compared to typical IGBT module solutions used in medium power welding machines that operate at 10kHz to 20kHz switching frequencies. This helps reduce the size of magnetic components and passive devices, thereby reducing the size of the inverter.

In addition, appropriate heat sinks and air flows were selected to provide appropriate thermal time constants to meet the requirements listed in Table 1. All heat sinks reached thermal steady-state conditions after approximately 5 minutes, and the cooling system design also reached thermal steady-state conditions (Figure 3). This allows the SiC MOSFET devices to reach thermal steady-state conditions within the 60% soldering duty cycle required for maximum operation.

Figure 3: Thermal steady-state conditions and heat dissipation capabilities of heat sinks

The power inverter test conditions are as follows:

• -Output power: 408 A, 47.7 V, ~19.5 kW. Target output power: 20 kW, 500 A, 40 V

• -Duration rate: 60%, 6 minutes on, 4 minutes off

• -Inverter DC bus voltage: 530 VDC

• -Switching frequency: 50 kHz

• - VGS (20 mΩ CoolSiC MOSFET): 18/-3 V

• - VGS (competitive 20 mΩ SiC MOSFET): 20/-4 V

• - Low side heat sink Rth: ~0.36 K/W

• - High side heat sink Rth: ~0.22 K/W

• -Thermal conductivity of thermal paste: 6.0 W/mK

• -Placement clamping force: 60 N (13.5 lbs)

• -Ambient temperature: room temperature

• - Forced air cooling

• - RCL load

As expected, due to proper gate driver, RC snubber and PCB layout design, there is no significant difference between Infineon CoolSiC MOSFET and competitor SiC MOSFET, both of which show similar waveform performance (Figure 4).

Figure 4: Typical SiC MOSFET waveforms during welding machine power inverter operation

However, the heat dissipation and power loss test results show that the CoolSiC MOSFET performs better. The temperature curve (Figure 5) shows that the 20 mΩ IMZA120R020M1H CoolSiC MOSFET performs significantly better than the competing devices. On average, the CoolSiC MOSFET has a heat sink temperature of about 6% lower, an estimated power loss of 17% lower, and a case temperature of 14% lower than the competing devices.

In addition, the CoolSiC MOSFET reached thermal steady-state conditions after 5 minutes of operation, which was in line with expectations based on cooling design data. On the other hand, the competing SiC MOSFET never reached thermal steady-state conditions, which means that its power loss was still increasing after 6 minutes of system operation.

Figure 5: Heat dissipation and power losses of a 20 mΩ 1200 V SiC MOSFET at 60% welding DC supply operating state - Infineon CoolSiC MOSFET IMZA120R020M1H compared to key competitor devices

Finally, even taking into account a maximum ambient temperature of 40°C, this single SiC MOSFET solution can easily meet the maximum heat sink temperature requirement of 80°C.

In summary, the test results confirm and demonstrate that the CoolSiC MOSFET single-transistor solution can help realize the inverter design of medium-power welding machines of 20 kW and above, where power module solutions are usually selected, by adopting a cooling concept of directly mounting the device on the heat sink without electrical isolation.

Conclusion

Tests have confirmed that CoolSiC MOSFET 1200 V in TO-247 package based on .XT diffusion solder technology, combined with a well-known unconventional cooling design, can achieve better welding machine power supply. This design greatly improves heat dissipation and achieves higher output power levels than power module solutions. The advantages of Infineon's .XT interconnect technology help improve heat dissipation and thus increase inverter reliability and service life. The single-tube solution proposed in the article can achieve higher efficiency and power density, helping to meet the demand for more energy-efficient welders while complying with welding industry development trends such as reducing cost, weight and size.

References

[1] This article is an updated version of the paper “Improving welding machine power efficiency using single 1200 V CoolSiC MOSFETs based on .XT interconnect technology” presented by the author (Jorge Cerezo) at PCIM Europe 2022 in Nuremberg. https://pcim.mesago.com/nuernberg/en.html

[2] Commission Directive (EU) 2019/1784 of 1 October 2019 lays down ecodesign requirements for welding equipment in accordance with Directive 2009/125/EC of the European Parliament and of the Council.

[3] “TO-247PLUS IGBT Single Tube Helps Improve the Power Density of Welding Equipment”, AN2019-10, Infineon Technologies AG.

[4] M. Holz, J. Hilsenbeck, R. Otremba, A. Heinrich, P. Türkes, R. Rupp, et al., “SiC power devices: Improving products using diffusion bonding technology”, Materials Science Forum, vol. 615-617 (2009), pp. 613-616.