EE Thermal 101 – Thermal Basics for Electrical Engineers (Part 1 of 4)

12 Jul 2018 • 5 minute read

Have you ever had a product exceed temperature specifications and wondered what you, as an electrical engineer, can do to solve or mitigate thermal issues in your product? As an electrical engineer in today’s world of electronics, we are facing product designs with much higher power densities than ever before (devices are consuming more power per unit volume) and this is causing our products to become hotter. In the past, mechanical engineers were solely responsible for the thermal design of the product as they were responsible for the mechanical enclosure and other physical aspects of the product but nowadays the thermal responsibility is being shared among the broader team of electrical engineers. With the advent of embedded microcontrollers, firmware, and sensors, electrical engineers are now playing a much greater role in the thermal management of their product and this series of blogs will discuss thermal basics for electrical engineers.

In the first part of the series, we will cover the topic of what is heat and temperature and will relate these concepts to Ohms law. The second part of the series will go over the three fundamental modes of heat transfer and discuss thermal resistances.In the last two parts of the series we will apply the thermal resistance concept to an example and discuss some techniques that electrical engineers can use to cool electronic products.

To start, one of the first things we need to address is, “what is heat and temperature?” Are they the same thing?

The answer is heat and temperature are not the same. Heat is actually a form of energy, also referred to as thermal energy, and it comes from the movement of atoms or molecules. As with other types of energy, heat has SI units of joules [J]:

where kg is a kilogram, m is a meter, s is a second, N is a newton, W is a Watt, C is a coulomb, and V is a Volt. Other units of thermal energy are the calorie and its counterpart the British Therma Unit (BTU) but we’ll stick with the SI units for heat and represent it mathematically by the symbol Q. It is important to remember that heat is a form of energy whereas heat flow, as we will show later, is the flow of thermal energy with respect to time or as we electrical engineers know it as power dissipation.

On the other hand, temperature is a measure of the motion of the particles in an object and is proportional to the average energy in an object. From a more practical standpoint, temperature tells us how hot or cold the object is and for a given material and mass, it tells us the amount of heat energy it has. For example, the hotter an object is, the higher its temperature will be and therefore the greater its thermal energy it will have. The SI units for temperature is kelvins [K] and is related to the Celsius scale by -273.15 °C. We will be using both of these temperature scales interchangeably and represent it symbolically by the letter T.

Now let’s take a look at the duality between the electrical and thermal conduction domains which was briefly discussed in our prior blog. The figure and table below show the fundamental relationships between the two domains.

Figure 2. Fundamental relationships between the electrical and thermal conduction domains

Table 1. Basic relationships between the electrical and thermal conduction domains

Jean-Baptiste Joseph Fourier, yes the same Fourier we know from the Fourier series and transforms, observed Ohm’s law relationship in heat conduction. If we force a temperature difference between two points of a conductor, a heat flow or transfer of energy would flow from the high temperature point to the low temperature point at a rate inversely proportional to the thermal resistance between those two points. This heat flow through the conductor is known as power dissipation in our electrical engineering world. The linkages between the thermal and electrical conduction domains are power dissipation and the physical material and geometrical properties of the circuit.

Another similar concept between the two domains is capacitance. In the thermal domain, thermal capacitance, thermal mass, or heat capacity is the property of a material to store or release heat much like how an electrical capacitor stores or releases charge. Thermal capacitance prevents fast fluctuations in temperature change of an object and is a function of the mass and specific heat of the material which can also be related to the volume of the object. Again, like an electrical capacitor, thermal capacitance can be used as filters to filter fast fluctuations in temperature change.

Although we discussed a lot of similarities between the electrical and thermal domains, it is important to remember there are some key differences as also pointed out in our previous blog:

In the electrical domain, current is constrained to flow within specific circuit elements, but in the thermal domain heat flow emanates from the heat source in three dimensions and by any or all of the three thermal transport mechanisms: conduction, convection and radiation.
Thermal coupling between elements is more prominent and difficult to isolate than electrical coupling.
Thermal time constants are much larger and slower than electrical time constants which means it can take seconds for a PCB to heat up as well as seconds for it to cool down.
Measurement tools are different. For thermal analysis, infrared cameras and thermocouples replace oscilloscopes and voltage probes.

Stay tuned for our next blog about the three different types of thermal transport mechanisms and how we approximate these mechanisms with equivalent thermal resistances. In part 3 of the series, we will use these thermal resistances to build up a thermal network to efficiently analyze the heat transfer and temperature behavior of a system. The final part of the series will cover cooling techniques.

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More blogs from EE Thermal 101:

EE Thermal 101 - Thermal Basics for Electrical Engineers (Part 2 of 4)

EE Thermal 101 - Thermal Basics for Electrical Engineers (Part 3 of 4)

EE Thermal 101 - Thermal Basics for Electrical Engineers (Part 4 of 4)

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