Thermal design is pivotal

As part of the electronic controller, a switching bridge is usually implemented using power MOSFETs or IGBTs to drive the motor. When the transistors in the bridge are in the "on" state, I2R losses in the drain-source of the MOSFET or collector-emitter resistance of the IGBT generate heat within the transistor junction. Switching losses also contribute to this internally generated heat, which must be removed to prevent the junction temperature from rising above the maximum recommended by the manufacturer. As the duty cycle of the motor increases, for example to augment speed or torque, the heat generated in the bridge transistors also rises.

Hence any electronic motor controller presents its designer with a thermal budget. The temperature gradient from the heatsink to the junction of the device must be managed so that the worst-case junction temperature is within the manufacturer's specified limit. Therefore, a thermally efficient path - from the surface of the transistor to the heatsink - is essential to couple sufficient thermal energy into the sink, from where it can be efficiently radiated or removed by forced air cooling using a fan if necessary. Inefficiencies in this path frequently have to be compensated by a larger-than-optimal heatsink or increased airflow from a fan, which adds extra cost.

Size and cost constraints

There may also be a physical limit to the allowable size of the heatsink, particularly in space-constrained applications such as automotive ones. One solution may be to choose a sink of a relatively expensive material, such as copper. The thermal conductivity of copper is higher than that of aluminium for a given size or shape. Before considering such a high-cost alternative, it is vital for engineers to ensure that the thermal coupling between the MOSFET and the sink is as efficient as possible. Inserting interface materials of suitable thermal conductivity may allow the electronic-subsystem designer to meet the car maker's specification without resorting to expensive heatsink options. An aluminium sink is also lighter than a copper device of similar dimensions.

Improving the efficiency of the thermal path to the heatsink is mainly achieved by eliminating air gaps in the path. Captive or still air has significantly lower thermal conductivity than transistor-packaging materials, aluminium heatsinks, or even FR4 PCB substrate. Its thermal conductivity is around 0.02W/mK, compared to 237W/mK for aluminium. Filling critical air gaps with a thermally conductive, electrical insulating material is essential in power-electronic equipment, such as motor controllers for a wide variety of applications.

Motor controls under the vehicle hood

One practical example is the current trend to implement electronic motor controls for engine-cooling fans in cars. This allows the fan speed to be varied and thereby enables a more flexible engine-cooling strategy as well as better management of the vehicle's overall electrical budget. The driver switching the motor-drive current must be able to dissipate losses in proportion to the power of the fan motor; for a small vehicle, this power may be in the range of 300 to 500W. The higher drive current drawn by the larger motor results in greater dissipation within the power transistors, although using a larger heatsink or siting the controller elsewhere for improved cooling may be precluded by other vehicle-design constraints. Some recent designs have required a more efficient thermal path from the transistor casing to the sink. This demands gap-filling materials of higher thermal conductivity. The composition and technology roadmap of gap-filling materials are such that there is a trade-off between thermal conductivity and cost. Hence, in a high-volume application such as an automotive project, subsystem designers frequently work with vendors of thermal-interface materials to customise a solution. Working with a European car manufacturers, Bergquist recently developed a gap-pad material (Figure 1) with a conductivity of 1.5W/mK to meet the requirements of a small-car variable-speed cooling-fan design.

Figure 1: A Bergquist gap pad.

Generally speaking, the use of electric motors within vehicles is increasing. They perform a wide range of fundamental and advanced tasks, from seat, mirror or window adjustment to actuators in the engine and transmission systems. Another growing application is steering systems (Figure 2): electronic drives deliver greater controllability, are smoother in operation, and are cost-effective, making the thermal issue an important consideration of motor-control design at many locations throughout a modern vehicle.

Figure 2: Electrical test of a power-steering system.

Motor controls on the factory floor

There are other important parameters to consider when developing a thermal-interface material. In applications such as industrial controls, where drives and controls are often mounted in die-cast housings, metal splinters from machining processes performed on the housing are a common occurrence. These can cut through some interface materials, creating a short circuit. Glass fibre is commonly used to increase the mechanical strength of multi-purpose thermal-interface materials. However, although this yields a strong and cost-effective solution, it does not display resistance to cut-through. New film-type materials are now emerging, which combine high resistance to cut-through with mechanical-strength-enhancing properties.

Motor controls for electric vehicles

In any motor-control application, the maximum duty cycle of the motor defines the worst-case thermal-dissipation conditions. In a traction application such as an electric vehicle, the vehicle's operation is characterised by frequent acceleration, and carrying heavy loads places a heavy demand on the duty cycle of the motor. Motor controllers for electric vehicles have historically needed physically large components, such as through-hole MOSFETs, capacitors and bus bars, for sufficient thermal capacity to maintain temperatures under control. But assembling these large devices is expensive. Throughput is also relatively slow, which reduces productivity and increases the total build-time for each vehicle. Using a thermal substrate, such as Bergquist's Thermal Clad, has allowed manufacturers of electric vehicles to change to surface-mount technology, including in devices such as power semiconductors, enabling automated assembly.

A hot topic

Growing demand for variable-speed operation for electric motors in a wide variety of applications now makes thermal design for power-transistor bridges an important discipline for a greater number of motor-control designers. Eliminating air gaps between power components and the heatsink provided is fundamental to sound thermal design, and a variety of materials are available for this purpose.