The miniaturization of high powered electronics — and the requisite component density that entails — have led engineers to explore cooling methods of increasing complexity. As a result, a growing trend in the thermal management of electronics includes liquid-cooling systems. This has prompted the reintroduction — and reimagining — of cold-plate technology, which has a long history dating back to its inclusion in NASA’s Apollo program.
Thermal management of high powered electronics is a critical component of a design process. Ensuring the proper cooling of a device optimizes its performance and extends the mean time between failures (MTBF). To ensure that a system works properly, engineers must establish the thermal parameters from the system-level down to the junction temperature of the hottest devices.
As temperatures have increased, the use of cold plates in closed-loop liquid-cooling systems has become a common and successful means to manage temperatures. This is true across many industries, including industrial and process applications that typically have higher heat fluxes and higher temperature environments than other electronics systems. Air cooling is insufficient for these high powered components and systems, or it would require flow rates that are unfeasible.
Cold plates are useful for direct, localized cooling of insulated-gate bipolar transistors (IGBT) modules, power semiconductor devices and electronic switches. IGBT modules typically combine high efficiency and fast switching.
Liquid cooling will become increasingly necessary as wide-bandgap materials — notably, silicon carbide (SiC) and gallium nitride (GaN) — become more prevalent in the industry.
The use of cold plates in industrial settings is primarily to cool the electronics and power devices within motors; automation controls; large-scale cooling systems such as refrigeration or air-conditioning units; boilers and dryers; and machine tools.
IBGT devices have voltage and current ratings to meet the demands of new industrial drive inverter designs, according to a recent study out of North Carolina State University. The study also noted that these industrial motors, using IGBT power modules, are used in manufacturing, water and waste management, food processing, plastics processing and chemical processing, among other applications. Cold plates are crucial for cooling IGBT because they are manufactured to connect directly to the power modules. Cold plates provide direct localized cooling for high powered modules and offer higher heat transfer at lower flow rates than air-cooling solutions.
Cold plates and heat exchangers can be used to cool high powered and high temperature systems.
Why is it important to provide thermal management of IGBT? A recent study published by DfR Solutions explained that thermal conditions can accelerate the breakdown of electrical components, which means greater power and switching losses over much shorter lifetimes.
Types of Cold Plates
Cold-plate technology has come a long way since its inception in the 1960s. At their most basic level, liquid cold plates are metal blocks (generally aluminum or copper) that have inlets and outlets, and internal tubing to allow liquid coolant to flow through. Cold plates are placed on top of a component that requires cooling. They absorb and dissipate the heat from the component to the liquid. The heat then is cycled through the system.
Two basic designs for cold plates exist:
- Internal tubing though which the fluid flows.
- Internal micro- or mini-channels across which the fluid flows.
The key to both designs is avoiding high pressure drop, which would necessitate an increase in the flow rate, and consequently, more pumping power to achieve the required level of cooling.
Cold plates typically are made from either aluminum or copper. Recently, developments in thermoplastics offer a potential alternative that is lighter than either metal and with engineered thermal characteristics to boost heat transfer. For the moment, however, cold-plate manufacturers continue to use metal due to its thermal performance as well as its ease for connecting IGBTs and other devices.
Cold plate designs can be customized to fit different IGBT packages.
Tubed cold plates are constructed with the tubes embedded into a single layer of metal, or the tubes are manufactured within two plates that are sealed around the tubing. Most commonly, the tubing is made of copper although some manufacturers offer stainless steel tubing as well. Tubing can be continuous or constructed from straight tubes that have soldered joints. Soldered joints may present a potential source for leaks.
Another factor in a tubed cold plate’s thermal performance is the connection between the tube and the plate. Epoxy or other materials that are used to secure the tubing in place limit the thermal contact between the metals and may even act as an insulator.
Micro- and mini-channel cold plates utilize an internal fin field through which fluid is directed. This vertical fin field increases the surface area of the cold plate, which increases its potential for heat transfer to as much as 6,451.6 W/in2 (1,000 W/cm2), according to some manufacturers. This allows for more compact designs. Fin height and the gap between fins must be optimized to ensure the proper thermal performance and a uniform flow rate.
TABLE 1. Cold-plate manufacturers continue to use metal due to its thermal performance as well as its ease for connecting IGBTs and other devices.
Considerations for Cold Plates in Industrial Settings
One of the primary considerations for incorporating a cold plate into an industrial application is the type of fluid that will be used to carry the heat away from the device. For instance, one natural choice — standard water — may not be the best choice for a micro-channel cold plate. Particle buildup could cause clogging and require more regular maintenance and cleaning. While water is a good conductor of heat, it may not be able to withstand rugged operating environments because of its freezing and boiling temperatures. Additives such as glycol can be used to change the characteristics of a coolant, including water.
In practice, 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.
The most common fluids used with cold plates in liquid-cooling systems include:
- Deionized water.
- Inhibited glycol-and-water solutions
- Dielectric fluids.
In addition, engineered fluids (nanofluids) have been designed to amplify thermal conductivity or perform better in certain working environments. Concerns remain about nanoparticle buildup and possible clogging in the cold plate or the pump, however.
Cold plates provide direct, localized liquid cooling for high powered modules.
To enhance the performance of the cold plate and the liquid-cooling system in general, a recirculating chiller can be added. The chiller is used to condition the coolant, lowering its temperature before the fluid is pumped through the cold plate. Chillers provide precise temperature control and are useful in applications that have demanding requirements for temperature range, reliability and consistency. In the rugged environments of many industrial applications, trying to cool high powered IGBT may necessitate the inclusion of a chiller to optimize the heat transfer capabilities of a cold plate.
The increasing prevalence of IGBT and other high powered devices in industrial electronics requires the thermal performance of liquid-cooling systems, particularly using cold plates placed directly on devices to transfer as much heat as possible. Lowering junction temperatures to normal operating conditions is critical for devices to perform at optimal levels over longer periods, avoiding switching or power losses that would disrupt the overall system.
While cold plates have a long history in thermal management, new designs and technological advancements make them more effective than ever in cooling industrial applications. PC