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]
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.
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.