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Figure 1: Custom vacuum-brazed cold plate assemblyFigure 1: Custom vacuum-brazed cold plate assembly

With high heat loads, Original Equipment Manufacturers (OEMs) often require liquid cooling for their power electronics. In most cases, an off-the-shelf standard product will not meet an OEM’s specifications. Discovering their need for a custom solution, an industrial OEM's engineering department designed a cold plate using commercially available software. The thermal performance of the resulting cold plate didn’t meet their expectations, so they turned to Lytron to cool the six IGBTs in their frequency converter. This converter design would ultimately be used in power generating wind turbines. Lytron designed and manufactured a custom,
aluminum vacuum-brazed cold plate that exceeded the OEM’s expectations.

The IGBT Cooling Challenge

The OEM's total heat load per cold plate was approximately 15kW (51,182 BTU/hr); 2.5 kW (8,530 BTU/hr) per 120 x 160 mm IGBT module. The maximum allowable pressure drop was 1 bar (14.5 psi). The OEM’s engineers had specified a 30:70 Ethylene Glycol and Water (EGW) solution as the coolant, with a maximum temperature of 60ºC (140ºF) at a flow rate of 50 LPM (13.2 GPM). Elevated cold plate fluid inlet temperatures such as 60ºC (140ºF) are common in power generation and traction applications where the heat is removed from the fluid by ambient air. These applications can see ambient conditions of up to 50ºC (122ºF).

The goal with IGBT cooling in most applications is to achieve the lowest chip temperature as well as a minimum temperature gradient from one module to the next to extend the semiconductor’s life. This was the case for this application. The OEM also specified a custom inlet and outlet configuration with quick disconnect and hose to allow for easy integration of the cold plate. (See Figure 1.) The IGBT mounting holes, as well as the mounting holes for fixing the cold plate within the application, had tight tolerance specifications.

The IGBT Cold Plate Solution

Figure 2: Custom cold plate performance graph Figure 2: Custom cold plate performance graph

When heat loads are high and a custom cold plate is required, an aluminum vacuum-brazed cold plate with internal fin is often the best solution. The internal fin design provides significantly more heat transfer surface than other cold plate technologies, such as gun drilled, extruded, or machined-passage cold plates. The fin geometry is selected from many configurations to achieve the thermal performance needed. The fluid path in which the fin resides is also custom configured to achieve optimal results, including the desired pressure drop and temperature uniformity. Lytron's engineers designed the cold plate’s internals utilizing proprietary F and J data for thermal modeling. By using empirical data collected over decades, Lytron is able to provide much more accurate thermal modeling as compared to traditional off-the-shelf thermal software.

After the customer approved the new cold plate design, Lytron’s project engineers handed over the cold plate’s design to the manufacturing engineering department, which ensures the part is produced as designed. During the manufacturing process, the cold plate went through Lytron's vacuum-brazing process as well as custom machining. Due to the robust vacuum brazing process, the resulting cold plate was extremely strong and contained no corrosive residual flux. The cold plate's critical dimensions were measured in-house using a Coordinate Measuring Machine (CMM), and the cold plate went through additional extensive testing to ensure it met every one of the OEM's thermal and mechanical specifications.

The test results proved that the cold plate's design met and exceeded specifications. (See Figure 2.) The custom engineered cold plate had a thermal resistance of less than 0.45ºC-cm²/W (0.07ºC-inch²/W) with a maximum surface temperature of 72.5ºC (162.5ºF) and a pressure drop of precisely 1 bar (14.5 psi) at 50 l/min (13 gpm). Also, the design ensured the temperature gradient across the six power modules was minimized. When performance matters, custom, "performance-fin," aluminum vacuum-brazed cold plates can provide a solution to the IGBT cooling challenge.

Written by Christoph Bauckhage & Tracey Barber