Air vs Liquid Cooling:
Advancements in Thermal Management for Power Electronics

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Over the past few decades, Power & Energy have emerged as two of the fastest growing industries in electronics. Power conversion, inversion, and rectification as well as battery and fuel cell technologies have become integral to technological growth across all industries.

As power electronic systems become more complex and perform at higher power ranges, the form factors are getting smaller, making heat one of the greatest limiting factors to what can be accomplished. To handle the amount of power being dissipated, air cooling solutions must be optimized and enlarged to adequately remove the excess heat. In some cases, size becomes a limiting factor for forced convection solutions. In these cases where the size or weight of an air cooled system makes it impractical, liquid cooling is fast becoming the most popular alternative method.

Switching from an air-cooled system to liquid is not a decision to be made quickly or lightly; there are many factors and possibilities to consider when improving your thermal management to handle higher heat loads. Although market trends indicate that full liquid cooling systems will eventually be the industry standard for cooling power electronics, there are many options and hybrid solutions that can apply the benefits of both as your system evolves or upgrades. If budget or timeline constrictions are such that a direct switch to liquid is unrealistic, optimizing your forced convection solution either through design improvements or by introducing two-phase cooling or liquid components are viable interim solutions.

Engineers have been developing liquid systems that are complimentary to existingair cooled solutions that can be expanded to fully replace the air cooled systemsover time. This is done by focusing on the electronic devices that can gainimmediate benefit with liquid cooling. Utilizing fluid couplings, reliable pumpsystems, and compact heat exchangers, the system removes heat from the air flowto the liquid where it is transferred and managed elsewhere. In other cases,engineers are opting to fully replace their air cooled systems with liquid cooled toimmediately enable higher power outputs and optimize thermal performance.

As you consider the switch to liquid cooling in order to improve the performance of your power electronics devices andfacilities, there are several key determining factors:

• What are your size, weight, and thermal performance requirements?

• Can you further optimize your air cooled system?

• How much longer will air cooled systems be a viable thermal solution for your application?

• Are there any limitations on liquid or volume availability?

• How long will it take for investment in liquid cooling to make a return on performance and efficiency?

• How can liquid cooling be implemented or designed into your application? What will be the effect on application/facility down time?

• How and when do you begin?

Fan Heat Sink Air Cooling

Air Cooling Benefits

Air cooled systems are significantly less expensive than liquid systems. They do not require regulated or specializedfluids and they are comprised of fewer components that are more economical than components for liquid systems. Asthey have no liquids to leak and less components to break, they also have less modes of failure. In addition to havinghigher reliability and lower cost, air cooled systems are also easier to modify or upgrade.

Air Cooling Limitations

In typical applications, air cooling systems are comprised of an extrudedor bonded fin heat sink and often a fan. When reliability is a significantfactor, engineers may forgo a fan and instead opt for passive solutions.

Both natural and forced convection have limitations. Natural convection islimited by the total surface area needed to dissipate heat, thisnecessitates large, heavy solutions that are often impractical.

Forced Convection solutions are limited by pressure drop. Heat sinks withlarge surface areas in feasible volumes create a high amount of airresistance that hinder the amount of flow and therefore heat transfer thata fan can produce. Larger forced convection solutions also require largeror more fans, increasing the amount of noise generated by the solution.

However, the biggest limitation of air cooled solutions is thermal performance. Air does not have the same capacity asliquid to absorb and transfer heat. At a certain threshold, air cooling becomes an insufficient solution and liquid coolingis necessary.

Air Cooling Modifications and Hybrid Solutions

There are three common methods of improving your air cooled system. The first is to optimize your heat sink design andfan selection. Generating more air flow, optimizing your fin geometry, or increasing your heat sink volume are ways toimprove upon your air cooled solution without introducing additional technologies. The second is to introduce twophase cooling into your design. Heat pipes may be integrated to spread higher power densities or move the heat to anarea where it can be more easily dissipated. The third most common method of increasing the performance of an aircooled solution is to start introducing elements of a liquid system such as a passive thermosiphon.

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The Efficacy of Liquid Cooling

Liquid has the capacity to transfer heat up to 4X higher than the capacity of air of the same mass. This enables higherthermal performance in a smaller solution. A liquid cooling system is a hydraulic circuit that typically consists of a coldplate that interfaces with the heat source and device, a pump that circulates the fluidthrough the system, and a heat exchanger that rejects the heat absorbed by the liquidfrom the device. Liquid cold plates have a much smaller working envelope than a heatsink that would be used in air cooling for the same application. Additionally, multiplecold plates can be connected to the same exchanger with minimal impact onperformance. Liquid cooling grants an additional level of control over the coolingsystem because it controls inlet temperature to the cold plate as well as flow rate.

Potential Risks & Trade Offs of Liquid Cooling

Some have been reticent to adopt liquid cooling because of the additional complexity and the fear of leakage.Complexity often increases the cost of the solution and the amount of maintenance required to keep the systemrunning. However the additional costs are mitigated in that the improved cooling performance will increase the lifetimeand reliability of your device.

Because of its complexity, liquid cooling requires better planning and design to incorporate into your power electronics.Although the cold plate is much smaller than an extrusion or heat sink, the overall solutions tends to occupy morevolume once the heat exchangers, tubes, reservoir, and pumps are all taken into account. Engineers must take all of thisinto account during the initial design phase in order to avoid complications later on. With proper foresight, thecomplexity of the systems can be beneficial as there is more flexibility in system design.

Liquid Cooling Solutions

The AAVID Hydrosink

The Aavid HydroSink system is a configurable method of combining astandard set of optimized heat exchangers, fans, pumps, valves, reservoirs,fittings, sensors, and control boards with custom cold plates to design thebest possible liquid cooling solution for given requirements.

HydroSinks offer more flexibility in design and installation than standardliquid cooling systems because they are configurable and more easilyadapted to design requirements. Sealing and connecting of the liquid coldplate, control board, and customer machine controls within the enclosureare also customizable.

As Aavid HydroSinks are largely comprised of a set of standard optimizedcomponents, they are more cost effective than traditional custom liquidcooling and their air cooled counterparts.

Currently the Aavid HydroSink is available in two basic compact system sizes, Small and Medium. Actual size of thefinal customer HydroSink system varies by configuration. Sizes are based around fan size and cooling performance.Small operates at a temperature rise 7-20 °C per kW, while the Medium operates at a rise of 3-9°C per kW.

AAVID Liquid Cold Plates

Customized Aavid Liquid Cold Plates are an integral part of the HydroSink system. Aavid offers four distinct, innovativecold plate designs developed to optimize the overall system based on application and requirements. All Aavid cold platesare constructed for worry-free liquid cooling utilizing specialized certification procedures to ensure leak-free, reliablesolutions.

Hi-Contact Tube Cold Plates

Hi-Contact™ Tube Cold Plates
Aavid Hi-Contact tube liquid cold plates feature a high performanceassembly utilizing a continuous tube press fit into an extruded aluminumplate. The patented geometry used in the Aavid Hi-Contact process movesthe fluid closer to the device generating heat, achieving the best thermalperformance from a tube cold plate. To further increase the performance ofAavid's Hi-Contact liquid cold plates, a thermal epoxy is applied to the jointto provide a gap free thermal interface between the tube and the plate. Hi-Contactplates are easy to customize and are available in standard sizes.

Blister Cold Plates

Blister technology stamps channels into the base plate, eliminating channelmachining and greatly lowering manufacturing costs. A leak free joint is createdbetween the base and cover plate and the blister channels to allow greaterflexibility to drill mounting holes in the topside of the cold plate without regardfor the location of the liquid channels.

Vortex Liquid Cold Plates

Aavid Vortex Liquid Cold Plates are designed to cool extremely high powerapplications. These cold plates were initially developed for applications where ahigh compressive load may be applied such as when cooling SCR type devices.Using patented flow path geometry, both sides of Vortex Liquid Cold Plates areevenly cooled; therefore they can provide equal and consistent performanceacross both surfaces and lend themselves to creating more predictableenvironments.

Extended Surface Liquid Cold Plates

Aavid Extended Surface Liquid Cold Plates have increased internal surface areawhich allows for better overall heat transfer. Innovative technologies andmanufacturing processes are used to increase the liquid to plate contact areawithin the liquid cold plate. Their vacuum brazed construction ensures leak freejoints while maintaining high thermal conductivity. Aavid Extended SurfaceLiquid Cold Plates are specially fabricated to improve design flexibility and canbe easily customized for optimized flow paths for application designs.

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Final Thoughts

The key to effective design for liquid cooling and optimized air cooled systems is to consider your thermal managementas early in the design phase as possible. Aavid offers design, engineering, and testing services that can come in at anyphase and develop the best possible solution based on requirements, constraints, timeline, budget, and any othercritical factors.

With design centers around the world, Aavid can provide any customer the necessary engineering services to design andmanufacture a fully optimized system. Engineers are available at every phase from analyzing if there is a need for liquidcooling or air cooling, to developing an optimized, integrated system, to reliability and validation testing of the entiredevice.

To request a free consultation about your current cooling solutions or for help finding the right thermalsolution for your Power Electronic Applications click here.

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