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Posted on 01 May 2019

Optimizing Thermal Solutions

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Software cuts design costs

CFD (computational fluid dynamics) thermal analysis software is freeing engineers to create smaller, more reliable boards and systems, while simultaneously reducing development time and costs. The software solves the differential equations that model airflow and heat transfer and presents the results as 3-D color-coded simulations that accurately show thermal conditions inside an enclosure or across a board. Airflow and temperatures can be overlaid so that the engineer can detect the impact of one on the other.

By Peggy M. Chalmers, Daat Research Corporation

 

Development time and costs are dramatically reduced because the need for prototyping is eliminated. Want a different heat sink, thermal pad, additional vents, or a more powerful fan? A few keystrokes will modify the design or alter component placement. Because optimum configurations can be developed in a fraction of the time previously required, time-to-market is dramatically reduced, even on small projects.

Designing a smaller rectifier

A leading international provider of comprehensive power quality and backup power management solutions uses Coolit CFD thermal analysis software from Daat Research Corp to design its rectifier modules. In one application, a 3 kW unit dissipating 300 W, had to be mounted in multiples of three on shelves in order to achieve balanced three-phase currents. This meant the new design had to be at least 50% smaller than the company’s existing rectifier designs. In addition, the components had to be positioned in a predefined sequence which limited the possible locations for heat sinks, vents and fans.

Using Coolit, the design engineers quickly determined where hot spots would develop and explored numerous "what if" scenarios without expensive and time-consuming physical prototypes. They designed a tiny, but highly effective heat sink with optimized fin size and spacing and pinpointed preferred fan and vent locations. The resulting design exceeded the power density of competitor units and was small enough to fit six units on a 19-inch (48 cm) wide shelf.

A better power supply

When electronics designers wanted to make quick circuit improvements in very small power supply modules that provided highly precise power conversion for a military helicopter’s night vision and target acquisition system, they turned to Coolit. Each module consisted primarily of two high-density printed circuit boards mounted on opposing sides of a finned heat sink, and the engineers had to address the overall heat of each board, as well as the heat generated by hot spot components. Though they were new to CFD, it took them less than 4 hours to load the software, run the tutorials, perform test sample simulations and then begin building and running actual design scenarios. They were able to quickly characterize a fin design that would remove heat from the boards and into the customer's specified airflow.

CFD simulation of rectifier color-codes surfaces

The highest risk components for hot spots were the FET drivers, small, SO packages with moderate thermal dissipation, but with a high, 40C/W, junction to case resistance. While the engineers had pre-conceived notions as to which chips would present thermal problems, the Coolit simulations revealed other hidden risks. Once the problem chips were identified, the engineers were able to fix most problems by splitting a thermal load so that part went to a remotely located resistor instead of having the driver chip handle it all.

Airbus power supply simulation indicates maximum cooling occurs when chips are mounted directly over stator blades

Air-conditioning controls

AMS Technologies, AG of Munich, Germany used Coolit to design cooling for the power supply in Airbus’ air-conditioning controls. The IGBT module's is built without conventional base plate, so that IBGT chips and power diodes are mounted directly to the die cast housing of the turbine that circulates conditioned air throughout the cabin. Heat is conducted through the housing to stator blades that are immersed in the air drawn through the turbine.

To adequately cool the power module, the turbine's die casting had to be thick enough to spread the heat to the blades, and the number and pitch of the blades sized to dissipate the heat at 70°C ambient and inlet temperatures.

The approach velocities in the turbine are above 50 m/s and the velocity between blades approaches the speed of sound. The heat transfer coefficient to air is close to maximum and cannot be improved. Therefore, the junction-air thermal resistance is governed by the heat conductance from the module to the blade surface and by the effective cooling surface area.

The analysis showed that optimum heat transfer occurs when the components are positioned precisely above the blades and that even a small offset is detrimental over the entire design temperature range. Varying other design parameters, such as number of blades, blade pitch, etc. produced a critical variation of temperature drop of 10-17 K between junction for the hottest chip and the sensor of the IGBT module.

Honeywell used Coolit analysis to balance conflicts between cooling requirements

AMS also investigated the heat dissipated by the electrical windings in the core of the annulus. Initially, there was concern that this heat might raise the IGBT temperature significantly. However, the Coolit analysis showed that even maximum temperature conditions only slightly increased the junction temperature of the IGBT.

Using Coolit's flow visualization capabilities VT Miltope discovered that much of the air was bypassing the cooling fins

Saving time and money developing thermal solutions

As electronic designers are pushed to deliver smaller and more powerful packages, they are finding thermal issues more and more difficult to solve. To solve these tough problems in a timely fashion, engineers are finding powerful and accurate thermal design software to be essential, particularly in today’s competitive market. CFD thermal analysis software will deliver an optimum thermal solution in the least time and for the lowest cost. Predictions are typically accurate to within 5-10 percent, and there is no waiting for prototypes to be built and tested-- --they aren’t needed.

 

 

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