Understanding Power Resistors And Their Impact On Thermal Management Decisions

As our economy continues to drive the emergence of energy-efficient systems such as hybrid and electric vehicles, composite aircraft, and wind turbines, the need for power resistors has expanded. Electrical controls are replacing many heavy and large mechanical devices that have diminishing life due to frictional wear. But the standard electrical components do not completely optimize the system designs.

Next-generation electrical devices must be improved to reduce weight and size and operate at much safer temperatures. At a constant power level, the peak operating temperature of a resistor increases as the size and mass are reduced, making it difficult to reduce all three parameters simultaneously. These challenges have driven the emergence of new design concepts and considerations to manage and dissipate the heat generated from the power resistor. Thermal management considerations are the key to meeting the design goals within the desired operating conditions while reducing the overall system weight and size.

Reducing System Size: The Effect On Power Resistors

When reducing the size of a system that must be able to handle large amounts of surge or continuous power, considerations include the ultimate mass and space available to house the module. In addition, the operating environment and maximum internal package temperature must be defined to make the appropriate material selections for module construction and other internal device considerations.

The best way to reduce the resistor size and mass is to remove the thermal mass that accompanies the power resistor package. However, doing so increases the operating and exposed temperature of both the resistor element and neighboring components. This is where the new concepts of power resistor designs come into play.

The most effective and efficient way to reduce the size and mass of a power resistor is to utilize the existing material and construction of the module or the system where the module is to be placed as the thermal mass. For example, today’s electric vehicle designs operate on a 330- to 650-V dc high-voltage bus. At these voltage and current levels, traditional relay power switching is replaced with insulated gate bipolar transistors (IGBTs), which already require heatsinks to operate efficiently and extend the component life.

Power resistors are needed to limit the inrush current to the charging system capacitors as part of this circuit, and they are easily mounted to the same heatsinking devices already necessary for the IGBT operation. Planar devices readily mount to the heatsink with negligible size and mass addition to the system, yet offer a tremendous power handling capability for surge conditions (Fig. 1). Even if the system does not have an internal heatsink, the module packaging may already be using an aluminum lid or box where the component can be mounted.

1. Planar resistors offer tremendous power-handling capabilities for safe operation under surge conditions.

Mounting the heat-generating devices to internal heatsinks can solve the initial concern of component heat dissipation. However, the long-term operation effects can push the internal module temperature beyond acceptable limits. Many packaging designs either expose the heatsink fins to the exterior of the package to dissipate the heat to the outside ambient or include liquid-cooled heatsinks. These considerations may not be cost-effective if only required for the power resistor, but the designer can take advantage of these types of features that are already implemented for other internal components of the module.

An Alternate Approach For Heat Dissipation

Some module designs do not allow for these considerations, but power and heat dissipation are still required. In these cases, an optimal approach would take the power to where it can be dissipated, preferably to an existing thermal mass within the system so the weight of the overall construction isn’t increased. Electric and hybrid vehicles do have a cooling system where the heat can be dissipated. The devices in Figure 1 exhibit excellent power-handling capabilities on a static heatsink, or even better on an air-cooled heatsink. The functional operating limits are further increased if the heatsink is liquid cooled. Liquid is far denser than air and carries a much higher thermal capacity to transfer heat.

Heat transfer directly to the coolant flow is possible by depositing the same material systems and circuit designs onto tubular substrates. Replacing a section of the coolant path with the tube component is the most efficient approach to maximize the component power density and limit the operating temperature. Using the same construction as the planar components, the tube transfers the generated heat directly to the coolant flow without a transfer medium (Fig. 2).

2. Thick-film on steel tube components can enable heat transfer directly into coolant flow while maximizing power density and limiting the operating temperature.

This design can operate at power densities well beyond that of a planar device that relies solely on a static aluminum heatsink, and the thermal mass already exists within the vehicle cooling system. Figure 3 plots the average temperature rise of the power circuit against variable flow rates at four different power densities. Designers then can understand the size of the circuit and flow rate required to stay within the operational limits of the design. The product characterized here is a 0.75-in. stainless steel tube with a printed circuit area of 6 in.²Figure 4 shows thermal images at a few points along a 150-W/in.² power density curve.

3. To help understand the requirements of circuit size and flow rate to keep within operational limits, the graph plots the average temperature rise of a power circuit against variable flow rates at four unique power levels.

4. Relating to the graph of Figure 3, the photos show tube thermal imaging at 150 W/in.² at flow rates of 36 LPM (a), 22 LPM (b), and 12 LPM (c).

Implementing the tube products requires standard plumbing fittings to the coolant flow where the tubes are directly formed with proven flares, flanges, beads, or barbs for hose or manifold interconnects (Fig. 5). Another true advantage of these products is the isolation of the electrical circuit where it is deposited and not subjected to fluid exposure. The thin wall construction of the tube itself allows for rapid heat transfer to the fluid flow, and the component does not introduce additional failure modes that would be found with submerged devices such as a glow plug or other liquid-cooled resistor element.

5. A 0.75-in. tube can be characterized for data with bead-formed ends.

Single Device, Dual Function, Multiple Power Levels

The tube product offers many advantages to the system designer. Most vehicles are likely to rely on a lithium-ion (Li-ion) battery pack that requires cooling during energy discharge cycles. Because the coolant is already flowing through the battery pack, the battery charge can be optimized using the same power resistor to heat the fluid flow during the plug-in charging cycle of the batteries.

Li-ion batteries lose their ability to charge as their temperature decreases. Cold climate charging reduces the charging capacity and ultimately results in significant permanent capacity loss, both of which deplete the vehicle range. In contrast, heating the batteries during the charging cycle enables full-capacity charging while using the energy of the wall outlet to provide heat. Once fully charged and unplugged, the vehicle’s coolant system is preheated for instant cabin heat. As the batteries discharge during vehicle operation, they generate heat that is then provided to the cabin for passenger comfort.

Because the planar and tube components are thick-film circuits, the artwork used to deposit the circuit is very flexible to customize performance or provide additional circuits to address specific application requirements. Whether dissipating the heat of a power resistor or proving a specific heated region of the substrate, these products can be balanced to optimize the performance for each specific application while requiring very little space or increasing the overall system weight.

Power Resistor Thermal Management On A Relative Scale

The considerations for power resistors so far have focused on power dissipation in the kilowatt range, but all resistors generate heat and all placement options have limitations. On a control circuit level, the printed-circuit board (PCB) has thermal limitations based on the material chosen as the substrate. While most surface-mount devices (SMDs) on the PCB are well below a 1-W rating, what happens when a 5-W device or more is required? The concern is then raised when the operating environment is defined to be 85°C, 125°C, or even 200°C for certain automotive, down hole drilling, or aerospace applications.

It does not take much localized heat for a resistor to exceed the rating of many PCB substrate materials at these elevated ambient temperatures. For example, FR4 is only UL rated to a 130°C operating temperature. Even if high-temperature boards are used, packaging proximity or other closely placed electronic components can be affected. While the resistor may only be a couple of watts, it can easily surpass the limits of the assembly design or reflow temperature of solder joint construction. It also can impact the lifetime and operating limits of the neighboring components.

The designer has quite a few options. Depending on each unique set of boundaries of a specific application, one or more of the following approaches can help solve the problem of localized heat generated within the circuit:

Raise the resistor off the PCB to keep the heat away from the substrate (Fig. 6).
Choose a metal core PCB or increase the metal content to increase heat dissipation.
Add thermal vias directly below the insulated portion of the resistor to improve the thermally conductive path to the ground plane of the PCB.
Isolate a power resistor from direct contact with the PCB and use a secondary heatsink.
Optimize power-resistor proximity to precision devices to minimize measurement error, tracking drift, or premature aging of these components over a short period of time.

6. Some components raise the resistor element above the PCB to keep heat away from the PCB substrate.

Conclusion

Resistive components play a critical role in electronically controlling system inertia and surge energy. While the resulting creation of heat from a resistive device cannot be avoided, it can be accommodated without adding mass and size to the overall system design. And in a few cases, heat can be directed to replace other components designed specifically to generate heat, when the energy is directed to these areas of the system.

Designers have quite a few options. Depending on each unique set of boundaries of a specific application, one or more of these approaches can help to solve the problem of localized heat generated within the circuit. The use of thick film on metal products offers a wide range of flexible options to solve the power handling and heat dissipation issues. The extremely high power density and low-temperature operation of this technology allow for significant space and weight savings that increase the overall efficiency of the system, and the flexible geometry allows for creative placement options.

Control-level circuits also can incorporate many different designs that remove heat from the PCB and solder joints, allowing for overall circuit size and packaging reductions. At the end of the design cycle, all weight and size reductions lead to overall system cost savings through purchased material costs and/or increased operating efficiencies.