Question

The inner workings of most electronic devices for diagnostic imaging are temperature-sensitive, in that they will not function properly if safe operating temperature is exceeded. However, these devices also include internal components that emit considerable heat within the device.

Provide TWO specific strategies that an equipment designer could employ to reduce heating of the temperature-sensitive components in an electronic device,  explain how physics concepts are applied in each strategy.

Answer 1

Joule's Law states that the power of heating generated by an electrical conductor is proportional to the product of its resistance and the square of the current (P = IV = I² R). In a demanding power consuming device, a doubling of load current actually quadruples the thermal output, and therefore in applications where heating is an unwanted by-product of current use it's necessary to develop effective cooling solutions.

Two strategies that are commonly used by the designers of such electronic devices are cooling by Convection (via forced air flow or passive natural convection) and cooling by Conduction.

1) Convection is the transfer of heat from one place to another by the movement of fluids. In forced-air convection, a fan directs airflow onto the hot surface of the component. The heat loss due to the forced convection is given by an equation called Newton's Law of Cooling:

dt

Where the rate at which the heat is transferred depends on the convection heat transfer coefficient h, the exposed surface area A and the temperature difference between the object and the convecting fluid. Since h grows along with the speed of the air around the object, then propeling air into the object with a fan would increase the heat transference.

This makes sense in many applications when the device’s enclosure has enough room for adequate airflow. Also another challenge here would be that the noise generated by a fan motor is unacceptable for patients and staff. Fan speed control techniques that actively slow down the fan under conditions of lower output power and/or lower ambient temperatures can help in such situations.

In lieu of a fan, passive natural convection cooling using an open-air rack to promote the natural movement of air across the electronic component can sometimes adequately remove excess heat. Spontaneous convection is driven by buoyancy for the most part and surface tension to a lesser extent.

Another approach to convection cooling is an ionic wind cooling system. In this method, increased airflow is generated based on the principle that air moves between two charged electrodes. The advantage of an ionic wind system over fan-based convection cooling systems is that an ionic wind system has no moving parts to wear out.

2) Conduction is the flow of internal energy from a region of higher temperature to one of lower temperature by the interaction of the adjacent particles (atoms, molecules, ions, electrons, etc.) in the intervening space. Conduction is governed by Fourier's Law:

$\frac{\mathrm{d} Q}{\mathrm{d} t}=\frac{kA(\Delta T)}{l}$

Where the factors affecting the rate of heat transfer are a constant k that depends on the material, the cross sectional area A, the temperature difference delta T and the length l.

When conductivity is high, there is no need for convection. We can dissipate heat by placing the component in direct contact with heat sinks, cold plates or other similar thermal transfer approaches. In this configuration, the heat is conducted through the colder material and away from the electronic component. The removal medium employed in a conduction cooling subsystem can be air, water, oil, metal or special liquids.