Category Archives: Cold Plates

ATS Power Solutions on Display at APEC 2018

Advanced Thermal Solutions, Inc. (ATS) recently attended APEC 2018 in San Antonio to showcase the company’s thermal solutions for power electronics, meet with industry representatives, and learn the latest trends in the power industry.

ATS has been a member of the PSMA (Power Sources Manufacturers Association), which is the sponsoring organization for APEC, for the past three years because ATS has a strong connection to the power industry and extensive expertise in keeping the industry cool.

ATS Power Solutions

Product Engineering Manager is interviewed about ATS cold plates by Alix Paultre, the Power Editor at Aspencore, in the ATS booth at APEC 2018 in San Antonio. (Advanced Thermal Solutions, Inc.)

Vice-President of Sales and Business Development Steve Nolan and Product Engineering Manager Greg Wong manned the ATS booth during the show and took visits from sales representatives, distributors, and engineers from across the industry, some were familiar faces and some were learning about ATS expertise in thermal management of power electronics for the first time.

The highlighted products were ATS liquid cold plates, which boast 30% better thermal performance than comparable products on the market and are easily customizable to meet a variety of applications, and the line of ATS high-performance round and flat heat pipes, which is expanding by the end of 2018 to give ATS the broadest offering of off-the-shelf heat pipes on the market.

“When people start having thermal issues, it’s because they’re dissipating a lot of power and then you start to need things like heat pipes and liquid cold plates,” said Wong. “In most of these applications, people are talking about custom designs, which is where we have a lot of strength working with the customer and designing these custom applications.”

ATS cold plates were also featured in the Texas Instruments (TI) booth as part of a “98.5% efficiency, 6.6-kW Totem-Pole PFC Reference Design for HEV/EV Onboard Charger.” The base of the design was silicon carbide (SiC) MOSFETs with a C2000 microcontroller with SiC-isolated gate drivers, according to information presented by TI.

Underneath the prototype that was on display at the TI booth was a custom ATS cold plate to meet the charger’s thermal requirements.

ATS cold plates were on display at the TI booth, as a thermal solution for a new TI design. (Advanced Thermal Solutions, Inc.)

“It’s a great example of how we can customize our cold plates to meet a particular application,” Wong added. “A lot of people were taking pictures of that piece at the TI booth and a lot of people were talking with TI about it. The booth was mobbed every time I went over there.” 

ATS participation in the TI booth at APEC 2018 is a continuation of the strong working relationship between the two companies. ATS has also been a key reference design supplier of heat sink solutions for TI’s audio evaluation module.

Wong and Nolan also learned a lot about the future of power electronics, including the prevalence of SiC and gallium nitride (GaN) components in the industry and the increasing popularity of liquid cooling, to keep ATS current on industry trends and ensure that the innovative thermal solutions that ATS provides can meet ever-rising power demands.

While there is a lot that is new in the industry, IGBT designs continue to be popular and the standard IGBT footprint matches perfectly with ATS off-the-shelf cold plates to make an easy fit for engineers designing liquid cooling solutions.

“If people are still making devices in that IGBT footprint then it will bolt directly to the cold plate, which is good news because that package is very popular, so it’s good to have those standard products,” Wong explained.

Watch the video below as Greg Wong of ATS was interviewed by Alix Paultre of Power Electronics News at APEC 2018 about ATS heat pipes and cold plates.

ATS has the expertise, products, and resources to provide off-the-shelf and customized thermal solutions for the power electronics industry. Learn more about the full line of products at https://www.qats.com or contact ATS at ats-hq@qats.com.

How to Achieve Localized Cooling with Cold Plates

Many applications in electronics cooling require a cold plate to remove heat from discrete components laid out on a board. In these circumstances, it is more efficient that the liquid does not completely fill the cold plate, but is only transferred to areas that need to be cooled.

With this kind of design, the required volumetric flow rate of the coolant will be significantly lower than if the entire cold plate was filled with liquid. The schematic for a typical example of this scenario is given in Figure 1. [1]

Localized Cooling With Cold Plates

Figure 1. Schematic of a Board with a Localized Area of Heat Dissipation . [1]

In Figure 1, areas A, B, C and D must be cooled for the components dissipating from 5 to 15 W/cm2. The other areas, designated as open, have components that interfere with the cold plate and must be avoided in the design. Two designs were considered for this case: a drilled hole and a press-fit tube. Figure 2 shows the drilled hole concept.

As can be seen, there are multiple small holes around the heat dissipating components under the cold plate surface. Large holes are machined to interconnect the smaller holes. A technique called gun drilling was used for machining the long holes. The entire cold plate was made from a copper block.

Cold Plates

Figure 2. Schematic of the Drilled Hole Cold Plate Design. [1]

Figure 3 shows the press-fit tube design. In this approach, a copper tube with high thermal conductivity is routed through the areas of heat transfer and either brazed or epoxied to the aluminum cold plate base. This design is considerably lighter and cheaper than the drilled hole design.

Cold Plates

Figure 3. Schematic of the Press-Fit Tube Cold Plate Design. [1]

To analyze the performance of this cold plate configuration, simple analytical tools can be used for a standard cold plate design. A brief summary of the equations is described here. To analyze the problem, we first have to calculate how much flow is going through the cold plate, and evaluate the pressure drop of the flowing fluid.

Pressure drop is calculated from:

Where
Um = bulk mean fluid velocity (m/s)
f = fanning friction factor
Awet = wetted surface area of the tube
Ac = cross section of the tube
K = loss coefficients related to turns, sudden expansion and contraction, etc.

The friction factor was obtained from the following equation which is in satisfactory agreements for the laminar, turbulent and transition regimes [2]

Where

Where ν is the kinematic viscosity of the fluid (m2/s) and P is the wetted perimeter of the tube.

For the heat transfer calculation, the Nusselt number can be calculated from standard correlations in the literature for fully developed flow. The Nusselt number is related to the heat transfer coefficient as:

Where
Kf = fluid conductivity

For thermally developing flow the following correlation can be used: [3]

Where
Num = mean Nusselt number
Nu = fully developed Nusselt number
L = duct length

Then the convective resistance can be calculated as:

Where
hm = mean heat transfer coefficient

For the tube fitted design the overall thermal resistance is made of four components: convection, tube conduction resistance, epoxy conduction resistance and the cold plate. It is stated as:

Where
Rh = convection resistance
Rtube = conduction resistance of tube walls
Repoxy = conduction resistance of the epoxy
Rcoldplate = conduction resistance of the cold plate

For the drilled design the overall thermal resistance can be written as:

If the heat transfer coefficient is based on the local fluid temperature, then a caloric resistance must be added based on the fluid mass flow rate. The effective heat transfer coefficient is then:

Where
ṁ = mass flow rate (kg/s)
Cp = fluid heat capacitance (kJ/kg·K)

Figure 3 shows the pressure drop of the two designs as a function of water flow rate. It can be seen that with a water flow rate up to 1.89 l/min (0.5 GPM) the pressure drop between the two designs is almost the same, but at higher flow rates the drilled design’s pressure drop exceeds the tube design. The sharp 90-degree turn of the drilled holes, which lead to a higher loss coefficient, is the major contributor to the higher pressure drop.

Figure 4. Total Pressure Drop of the Drilled Design and the Tube Design as a Function of the Volumetric Flow Rate. [1]

Figures 5 and 6 show the effective heat transfer coefficient of the two designs as a function of flow rate. The bend and sharp increase of the curves around 0.95 l/min (0.25 GPM) is due to the flow transitioning from laminar to turbulent. The drilled hole design shows effective convection heat transfer between 7,000 and 27,000 W/m2K for the range of flow between 0 and 7.56 l/m (2.6 GPM). The press-fit tube design on the other hand shows a lower effective heat transfer coefficient of between 6,000 and 17,000 W/m2K.

This is mostly due to the interfacial resistance and tube wall conduction. In the drilled design example, these two resistances do not exist. In a real application, the pumping of fluid is constrained by the pump and its characteristic curve. Even though the drilled hole shows a higher heat transfer coefficient for the same flow rate, the extra pressure drop caused by the drilled design may have a lower flow rate hence lowering the heat transfer coefficient.

Figure 5. Effective Heat Transfer Coefficient of the Tube Design as a Function of Flow Rate for Different Regions on the Plate. [1]

Figure 6. Effective Heat Transfer Coefficient of the Drilled Hole Design as a Function of Flow Rate for Different Regions on the Plate. [1]

Figure 7 shows another cold plate design, this one by Lytron. [4] In this design, the extended-surface cold plate material and micro-channel aluminum extrusion are sandwiched between aluminum sheets. The entire assembly is welded using vacuum brazing. It is all aluminum, which makes it very light weight. The flexibility of this design allows the placement of cooling channels in different positions to enable localized cooling.

Figure 7. Lytron Vacuum Brazing of a Cold Plate for Localized Cooling. [4]

The above analytics show that the performance of a cold plate for localized cooling can be calculated using a simple analytical tool. The designer then has to consider such factors as weight, manufacturing, cost and thermal performance to decide the best option for his or her design. The characteristic of the pump has a paramount effect on the design and cannot be neglected.

References:

1. Seaho, S., Moran, K. and Rearick, D. (IBM Corporation) and Lee, S. (Aavid Engineering), Thermal Performance Modeling and Measurements of Localized Water Cooled Cold Plate, http://www.aavidthermalloy.com/technical/papers/pdfs/water.pdf
2. Churchill, S., Comprehensive Correlating Equations for Heat, Mass and Momentum Transfer in Fully Developed Flow in Smooth Tubes, Ind. Eng. Chem. Fundam., Vol.16, 1977.
3. Al-Arabi, M., Turbulent Heat Transfer in the Entrance Region of a Tube, Heat Transfer Eng., Vol. 3, 1982.
4. http://www.lytron.com

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

Thermal Performance of Macro and Microchannel Cold Plates in Electronics Cooling

In recent years, intense activity has been gone into improving the capabilities of cold plates. Specifically, the use of microchannels has provided great improvements in cold plate thermal performance. Regardless of a cold plate’s channel size, the following equations can be used for heat transfer coefficients when determining thermal performance. [1]

Where,

= Nussselt number

Dh = hydraulic diameter

= Reynolds number

 

ν = kinematic viscosity
Pr = Prandtle number

The pressure drop can also be calculated as:

Where,
P = density
f = friction factor

In recent years, microchannel cold plates have gained popularity due to their high performance. Webb shows that the best results can be achieved when the channel aspect ratio is about 7.4, and with a fin aspect ratio of 8. [2] Figure 1 shows a Fin-H copper microchannel with a channel hydraulic diameter of 0.49 mm. Due to the small size of the channels, the flow is generally considered to be laminar. The optimization resulted in a 25-mm wide and 20-mm long microchannel cold plate. [2]

Webb considered both single-pass and two-pass designs on the water side. The two-pass version was made to determine if there was any mal-distribution of the water from the single-pass case.

Microchannel Cold Plates

Figure 1. Copper Microchannel Fin-H Used in a Cold Plate. [2]

Figure 2 shows the thermal resistance of the Fin-H for the 1-pass and 2-pass designs as a function of flow rate.

Figure 2. Thermal Resistance of a Fin-H Cold Plate as a Function of the Water Flow Rate. [2]

This figure shows that the 1-pass version has a much better thermal resistance than the two-pass model for the same flow rate. It also shows that the flow has been distributed relatively uniformly. Figure 3 shows the pressure drop of the cold plate as a function of flow rate for the Fin-H and the Thermaltake Bigwater 735 cooler. [3] The figure shows the pressure drop of the 1-pass design is only 38% of the 2-pass design.

Figure 3. Pressure Drop of a Fin-H Cold Plate and a Thermaltake Cooler as a Function of the Water Flow Rate. [2]

Figure 4 shows the thermal resistance of the Fin-H cooler in the 1-pass design compared to the Thermaltake cooler [3]. At 2.28 l/min the Thermaltake’s thermal resistance is 0.106 K/W. The balance point of the Fin-H for 1-pass is with a thermal resistance of 0.07 K/W at a flow rate of 0.361 l/min. This is only 16% of the flow rate for the Thermaltake cooler.

Referring to Figure 3, the pressure drop is almost the same for both coolers. The major implication is that the microchannel cold plate requires a smaller pump compared to macrochannel cold plates, and provides a 50% increase in thermal performance.

Figure 4. Thermal Resistance of a Fin-H Cold Plate and a Thermaltake Cooler as a Function of the Water Flow Rate. [2]

Another innovative approach is the concept of forced-fed boiling (FFB). [4] Figure 5 shows a schematic of this process. It consists of a micro-grooved, thin copper surface with alternating fins and channels. The microgrooves have a hydraulic diameter of 28 microns, an aspect ratio of 15, and a fin density of 236 fins per cm.

There are feed channels on top of the micro-grooved surface. The fluid is forced through these channels into the microgrooves, which are located on top of the heated surface. The fluid vaporizes in the microgrooves and moves upward, while the liquid flows beneath the escaping vapor. This keeps the surface wet, resulting in an increase of the critical heat flux (CHF).

Figure 5. A Force-Fed Boiling Cold Plate. [4]

Figure 6 shows the heat transfer as a function of the temperature difference between the inlet fluid and the surface for various values of the flow rate for R245fa, a non-aqueous fluid for low pressure refrigeration applications. The figure shows that for heat fluxes of about 200 W/cm2 or less, heat transfer is independent of the flow rate, but this is not the case at higher heat fluxes. It also shows that the slope of the heat flux decreases with increasing temperature difference.

Figure 6. Heat Flux as a Function of the Temperature Difference for the FFB Cold Plate. [4]

Figure 7 shows an interesting trend for the heat transfer coefficient as a function of heat flux for the same fluid. At first, the heat transfer coefficient increases with the increase in heat flux. This indicates that by increasing the heat flux, a phase change process takes place which changes the single-phase flow to two-phase heat transfer. After reaching an impressive peak at 300 KW/m2K, the heat transfer coefficient starts to decrease. This is attributed to local dryouts from bubble generation, which also blocks the microchannels.

Figure 7. Heat Transfer Coefficient as a Function of Heat Flux for the FFB Cold Plate. [4]

While advances in cold plate performance have been incremental, their technology is still evolving. Improvements in microchannel manufacturing will open more opportunities in this field. Microchannel cold plates provide tremendous heat transfer coefficient capacities, but limitations prevent their broad deployment.

Fouling, dryout, and fabrication issues have been major negating factors for microchannel deployment in the broader market. Microchannel cold plates may have particular value in such applications as military, space, and high capacity computing, where service and maintenance are part of the deployment.

However, from the design and problem-solution standpoint, microchannel cold plates can be an effective part of a closed loop liquid cooling system.

References
1. Dittus, F. and Boelter, L., Publications on Engineering, University of California at Berkley, 1930.
2. Webb, R., High-Performance, Low-Cost Liquid Micro-Channel Cooler, Thermal Challenges in Next Generation Electronic Systems II, Millpress Science Publishers, Rotterdam, The Netherlands, 2007.
3. Thermaltake Company, 2006.
4. Cetegen, E., Dessiatoun, S., and Ohadi, M., Force Fed Boiling and Condensation for High Heat Flux Applications, VII Minsk International Seminar: Heat Pipes, Heat Pumps, Refrigerators, Power Sources, Minsk, Belarus, 2008.

Learn more about Advanced Thermal Solutions, Inc. (ATS) standard and customized, high-performance liquid cold plates by visiting https://www.qats.com/Products/Liquid-Cooling/Cold-Plates.

For more information about ATS thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

What Fluids Can Be Used With Liquid Cold Plates in Electronics Cooling Systems

By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc. (ATS)

Liquid cooling systems transfer heat up to four times better than an equal mass of air. This allows higher performance cooling to be provided with a smaller system. A liquid cooled cold plate can replace space-consuming heat sinks and fans and, while a liquid cold plate requires a pump, heat exchanger, tubing and plates, there are more placement choices for cold plates because they can be outside the airflow. [1]

One-time concerns over costs and leaking cold plates have greatly subsided with improved manufacturing capabilities. Today’s question isn’t “Should we use liquid cooling?” The question is “What kind of liquid should we use to help optimize performance?”

Liquid Cold Plates

Figure 1. A Liquid Cooling System for a Desktop PC with Two Cold Plates. [2]

For liquid cold plates, the choice of working fluid is as important as choosing the hardware pieces. The wrong liquid can lead to poor heat transfer, clogging, and even system failure. A proper heat transfer fluid should provide compatibility with system’s metals, high thermal conductivity and specific heat, low viscosity, low freezing point, high flash point, low corrosivity, low toxicity, and thermal stability. [3]

Today, despite many refinements in liquid cold plate designs, coolant options have stayed relatively limited. In many cases, regular water will do, but water-with-additives and other types of fluids are available and more appropriate for certain applications. Here is a look at these coolant choices and where they are best suited.

Basic Cooling Choices

While water provides superior cooling performance in a cold plate, it is not always practical to use because of its low freezing temperature. Additives such as glycol are often needed to change a coolant’s characteristics to better suit a cold plate’s operating environment.

In fact, temperature range requirements are the main consideration for a cold plate fluid. Some fluids freeze at lower temperatures than water, but have lower heat transfer capability. The selected fluid also must be compatible with the cold plate’s internal metals to limit any potential for corrosion.

Table 1 below shows how the most common cold plate fluids match up to the metals in different cold plate designs.

Table 1. Compatibility Match-ups of Common Cold Plate Metals and Cooling Fluids [1]

The choices of cold plate coolants will obviously have varied properties. Some of the differences between fluids are less relevant to optimizing cold plate performance, but many properties should be compared. Tables 2 and 3 show the properties of some common coolants.

Tables 2 and 3. Comparisons of Properties of Typical Electronic Coolants. [4]

An excellent review of common cold plate fluids is provided by Lytron, an OEM of cold plates and other cooling devices. The following condenses fluid descriptions taken from Lytron’s literature. [5]

The most commonly used coolants for liquid cooling applications today are:

  • Water
  • Deionized Water
  • Inhibited Glycol and Water Solutions
  • Dielectric Fluids

Water – Water has high heat capacity and thermal conductivity. It is compatible with copper, which is one of the best heat transfer materials to use for your fluid path. Facility water or tap water is likely to contain impurities that can cause corrosion in the liquid cooling loop and/or clog fluid channels. Therefore, using good quality water is recommended in order to minimize corrosion and optimize thermal performance.

If you determine that your facility water or tap water contains a large percent of minerals, salts, or other impurities, you can either filter the water or can opt to purchase filtered or deionized water. [5, 6]

Deionized Water – The deionization process removes harmful minerals, salts, and other impurities that can cause corrosion or scale formation. Compared to tap water and most fluids, deionized water has a high resistivity. Deionized water is an excellent insulator, and is used in the manufacturing of electrical components where parts must be electrically isolated. However, as water’s resistivity increases, its corrosivity increases as well. When using deionized water in cold plates or heat exchangers, stainless steel tubing is recommended. [5, 7]

Inhibited Glycol and Water Solutions – The two types of glycol most commonly used for liquid cooling applications are ethylene glycol and water (EGW) and propylene glycol and water (PGW) solutions. Ethylene glycol has desirable thermal properties, including a high boiling point, low freezing point, stability over a wide range of temperatures, and high specific heat and thermal conductivity. It also has a low viscosity and, therefore, reduced pumping requirements. Although EGW has more desirable physical properties than PGW, PGW is used in applications where toxicity might be a concern. PGW is generally recognized as safe for use in food or food processing applications, and can also be used in enclosed spaces. [5, 8]

Dielectric Fluid – A dielectric fluid is non-conductive and therefore preferred over water when working with sensitive electronics. Perfluorinated carbons, such as 3M’s dielectric fluid Fluorinert™, are non-flammable, non-explosive, and thermally stable over a wide range of operating temperatures. Although deionized water is also non-conductive, Fluorinert™ is less corrosive than deionized water. However, it has a much lower thermal conductivity and much higher price. PAO is a synthetic hydrocarbon used for its dielectric properties and wide range of operating temperatures. For example, the fire control radars on today’s jet fighters are liquid-cooled using PAO. For testing cold plates and heat exchangers that will use PAO as the heat transfer fluid, PAO-compatible recirculating chillers are available. Like perfluorinated carbons, PAO has much lower thermal conductivity than water. [5, 9]

Conclusion

Water, deionized water, glycol/water solutions, and dielectric fluids such as fluorocarbons and PAO are the heat transfer fluids most commonly used in high performance liquid cooling applications.

It is important to select a heat transfer fluid that is compatible with your fluid path, offers corrosion protection or minimal risk of corrosion, and meets your application’s specific requirements. With the right chemistry, your heat transfer fluid can provide very effective cooling for your liquid cooling loop.

References
1. https://www.aavid.com/product-group/liquidcoldplates/fluid
2. http://semi-therm.org/wp-content/uploads/2017/04/How-to-design-liquid-cooled-system.pdf
3. Mohapatra, Satish C., “An Overview of Liquid Coolants for Electronics Cooling,” ElectronicsCooling, May 2006.
4. http://www.calce.umd.edu/whats_new/2012/Presentations/David
%20Saums%20PPt.pdf

5. http://www.lytron.com/Tools-and-Technical-Reference/Application-Notes/The-Best-Heat-Transfer-Fluids-for-Liquid-Cooling
6. https://www.thereadystore.com/5-gallon-collapsible-water-container
7. https://www.amazon.co.uk/IONISED-WATER-Mineralised-Deionised-Distilled/dp/B00X30JKGY/ref=pd_lpo_vtph_263_tr_t_2?_encoding=UTF8&psc=1&refRID=QNAM8H7J8R1AEDP8W5FF
8. http://www.rhomarwater.com/products/catalog/envirogard-heat-transfer-fluid-antifreeze
9. http://www.skygeek.com/anderol-royco-602-cooling-fluid.html

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

Cold Plates for IGBT and Power Electronics from Advanced Thermal Solutions   For information on ATS Cold Plates, visit our Cold Plate page at https://www.qats.com/Products/Liquid-Cooling/Cold-Plates

Industry Developments: Thermal Management Solutions for IGBT Modules

By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc.

Power electronics devices are vital for the efficient generation, conversion, transmission and distribution of electric power. Power technologies are being used to improve energy efficiency, reliability, and control. Some experts expect that one day all electrical power will flow through a power semiconductor device at least once. [1]

Cooling IGBT

Fig. 1. An IGBT module with rated current of 1,200 A and max voltage of 3,300 V. [2]

Among the more widely adapted high-voltage, high-power devices are IGBT. An IGBT (insulated gate bipolar transistor) is a solid-state switch that allows power to flow in the ‘On’ state and stops power flow when it is in the ‘Off’ state.

More specifically, an IGBT works by applying voltage to a semiconductor component, changing its properties to block or create an electrical path. An IGBT combines an insulated gate input and bipolar output to provide a reliable power switch for medium frequency (5-50 kHz) and high voltage (200-2,000 V) applications. [3]

Large IGBT modules typically consist of many devices in parallel and can have very high current-handling capabilities in the order of hundreds of amperes with blocking voltages of 6,500 V. These IGBT can control loads of hundreds of kilowatts. [4]

Among the many areas where IGBT are used in high power applications are:Electric and hybrid-electric vehicles; Battery chargers and charging stations; Electric buses, trams, and trolleys; Appliance motor drives; Switch and uninterruptible power supplies; Power factor correction converters; Traction motor controls; Solar and wind power inverters; Induction heating; and Medical diagnostic devices.

Thermal Management Needs and Solutions

IGBT generate significant heat and can be affected by excess thermal energy. Using air cooling techniques, e.g. heat sinks, for high-power dissipating IGBT can be impractical because of the large sizes the sinks require to manage the high volumes of heat.

Liquid cooling provides heat transfer coefficients several orders of magnitude higher than convection cooling, thus enabling much higher power densities and more compact module and inverter solutions.

While there is sometimes a reluctance to use liquid cooling in the power electronics industry, it is essential to meet many of today’s IGBT thermal management needs. The automotive industry has been using liquid cooling for internal combustion engines for more than a century and the idea of using liquid cooling for power electronics in an automotive application is now considered a non-issue. [5]

Liquid cooling methods for IGBT include cold plates, heat pipes, turbulators and vapor cooling loops.

Cold Plates

Cold plates provide localized cooling of power electronics by transferring heat from the device to a liquid that flows to a remote heat exchanger and dissipates into either the ambient or to another liquid in a secondary cooling system. Compared to air cooling, liquid cold plates provide more efficient performance and enable major reductions in the volume and weight of power electronics systems.

Cooling IGBT

Fig. 2. Cold Plates are used to keep chip temperatures lower inside modular IGBT component packages. (Advanced Thermal Solutions, Inc.) [6]

High switching frequencies and voltages result in IGBT dissipating higher power at the die level. Thus, the goal for cooling IGBT with cold plates is typically to get the lowest semiconductor temperature possible, as well as a minimum temperature gradient from one module to the next. They provide efficient heat transfer between the cold plate contact area and the IGBT base plate.

Uniquely manufactured IGBT cold plates from Advanced Thermal Solutions, Inc. (ATS) feature a higher performance mini-channel design. For example, the CP-1000 model cold plate, at a flow rate of 4 L/min, can transfer 1 kW of heat at 5°C temperature difference between the cold plate base and the inlet fluid temperature.

Fig. 3. A superior quality, vacuum-brazed cold plate from Mersen. [7]

Mersen S.A. provides vacuum-brazed cold plates specially dedicated to the needs of industrial drives. The vacuum–brazing technology insures metal-to-metal, flux-free joints ensuring leak-free, high-performance results. [7]

Fig. 4. IGBT-cooling base plates are available with multiple metal substrates and with low cost fin and pin features. [8]

Base plates (without liquids) are also available for IGBT cooling. One supplier is Wolverine MicroCool. Wolverine’s base plates provide efficient heat transfer in part because of their patented Micro Deformation Technology (MDT), which enables a wide variety of fin, pin and micro-channel geometries in a low-cost process. Because of this technology, the base plates have a very low pressure drop without compromising thermal conductivity. [8]

Turbulators

A turbulator is a cooler assembly designed to ensure all chips in a series of IGBT modules are cooled equally and efficiently. The concept enables tailored cooling, if hot spots need extra attention, and is accomplished by designing the liquid cooling channels individually.

The Mentor ShowerPower plastic part (pictured below in blue) has several cooling cells in the ‘X’ and ‘Y’ directions and needs a manifold structure on the backside of the plastic part. This ensures that each cooling cell receives water at the same temperature. [9]

Fig. 5. The turbulator concept ensures that all chips a series of IGBT modules are cooled equally. [9]

Turbulator designs like the ShowerPower provide many benefits. By homogeneously cooling flat IGBT baseplates and modules, they eliminate temperature gradients to allow the paralleling of many power chips.

Direct Liquid Cooling

Unlike cold plates, whose metal enclosures contact the base of an IGBT with a TIM (thermal interface material) in between, the concept of direct liquid cooling puts the liquid in contact with integral fins on IGBT base.

By arranging the fins in a high-density configuration directly beneath the power chip, which is a heat-generating body, the capacity for heat dissipation between the fins and the cooling liquid is increased. The result is that the thermal resistance between the power chip and the cooling liquid is reduced by approximately 30% compared to that of the conventional structure. [10]

Fig. 6. Cross-sectional comparison of conventional cold plate structure and direct liquid cooling structure. [10]

Vapor Cooling

Evaporative cooling technology increases power densities for high power electronics by more than two times according to Parker, which provides a two-phase evaporative liquid cooling system. The technology uses a noncorrosive, non-conductive fluid which vaporizes and cools hot surfaces on contact. [11]

The system uses a small pump to deliver just enough coolant to the evaporator – usually a series of one or more cold plates optimized to acquire the heat from the device(s). In so doing, the two-phase coolant begins to vaporize, maintaining a cool uniform temperature on the surface of the device. The vaporized coolant is then pumped to a heat exchanger where it rejects the heat to the ambient and condenses back into a liquid, completing the cycle.

Figure 7: High=-performance two-phase evaporative cooling allows twice the density of high-power electronics. [11]

Integrating the high-heat removal of two-phase technology with the reliability of low-flow liquid pumping, Parker’s system is highly modular (hot swappable) and scalable. The cooling process continuously cycles the refrigerant within a sealed, closed-loop system to cool a wide range of systems, including power electronics, motors, transformers, and high-efficiency. It simplifies the plumbing and reduces the overall weight, giving it an excellent thermal performance/cost ratio.

Heat Pipes

Another IGBT cooling method is based on standard heat pipes. A series of pipes are embedded in a metal plate under the power semiconductor and extend from the plate to a remote fin stack. Heat from the semiconductor is absorbed by the heat pipes and transported to the fins, which are cooled by natural or forced (fan) convection.

An example of this system is Therma-Charge from Aavid Thermacore. In this system, the IGBT are mounted on both sides of the plate. [12]

Fig. 8. Heat pipes embedded in the plate carry heat to an air-cooled fin section. [12]

References
1. EPE and ECPE “Position paper on Energy Efficiency – the role of Power Electronics,” Summary of results from European Workshop on Energy Efficiency – the role of Power Electronics, Brussels, Belgium, Feb 2007
2. https://en.wikipedia.org/wiki/Insulated-gate_bipolar_transistor
3. On Electronics, https://www.onsemi.com/pub/Collateral/HBD871-D.PDF
4. Future Electronics, http://www.futureelectronics.com/en/transistors/igbt-transistor.aspx
5. Mentor Graphics, https://www.mentor.com/products/mechanical/engineering-edge/volume4/issue1/showerpower-turbulator-keeps-IGBT-cool
6. Advanced Thermal Solutions, Inc. (ATS), https://www.qats.com/Products/Liquid-Cooling/Cold-Plates
7. Mersen, http://ep-us.mersen.com/us/products/catalog/line/vacuum-brazed-cold-plates-3-igbt-1064x624mm/
8. Wolverine MicroCool, https://www.microcooling.com/our-products/base-plate-products/igbt-base-plate-products/
9. Mentor Graphics, https://www.mentor.com/products/mechanical/engineering-edge/volume4/issue1/showerpower-turbulator-keeps-IGBT-cool
10. Fuji Electric, http://www.fujielectric.com/company/tech/pdf/58-02/FER-58-2-055-2012.pdf
11. Parker, https://www.parker.com/literature/CIC%20Group/Precision%20Cooling/New
%20literature/Two_Phase_Evaporative_Precision_Cooling_Systems.pdf

12. Aavid Thermacore, http://www.thermacore.com/applications/power-electronics-cooling.aspx

For more information about Advanced Thermal Solutions, Inc. (ATS) products and thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.