Electrical design is just the beginning of electronics, and heat dissipation is an important part of any reliable design. This article on thermal design is a new knowledge for experienced engineers and a new concept for rookie engineers.
Semiconductors do not achieve 100% efficiency, and power consumption is dissipated and transferred in the form of heat. Since semiconductors are devices that rely on electrical energy to operate, heat dissipation plays an important role both in performance and in product life. The same is true for analog or digital devices. Switching power consumption is frequency dependent, and as semiconductor and electronics speeds increase, power consumption increases. Therefore, it is helpful to know how much heat the device will generate and how to dissipate it effectively.
Before we begin to discuss semiconductor junction temperature issues, we need to understand the two concepts of thermal resistance and thermal resistance. Thermal resistance is related to time, and thermal resistance is related to steady state operation. Imagine heating the pan on an electric oven. It will take a while to get hot, right? The same is true for semiconductor junction temperatures. It takes a little time for heat to escape from the semiconductor junction. Understand this principle is the key to avoiding the device being burned out.
Power consumption is mainly due to the simultaneous conduction of voltage and current, switching and transient action. The unit of power consumption is watts. The voltage multiplied by the current is equal to the power (watts = volts x amps). Calculating the power for a short period of time gives the transient thermal temperature. Then calculate the average value over a period of time to obtain the steady-state temperature rise value of the semiconductor junction.
The calculated power consumption is in watts, and the thermal resistance is measured in degrees Celsius per watt (°C/W). Using the factor labeling method, borrowing the methods and mathematical formulas proposed by Diodes, we can get:
Rth(JX Θ ) = (Tj –Tx) / P
Where P is the dissipated power (heat) from the junction of the semiconductor to the "X" point. Ideally, nearly 100% of the power consumption should go from the junction to the "X" point during this measurement. This value depends only on the physical properties of the heat flow path, regardless of the power consumption and the board size at which the device is located.
Note that the Greek character "theta(θ)" is the thermal resistance expressed in °C/W. Using Wikipedia to verify, we can see that the thermal resistance of the thermal pathway is nothing more than a series of resistors in series, as shown in Figure 1.
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Now we should have a basic concept of temperature rise or temperature drop on the path. Of course, there are also situations that are affected by the external environment, such as high ambient temperature, air flow or congestion, and even the heating of surrounding devices. However, this formula allows us to understand the temperature of semiconductor devices at certain points, which explains why surface mount devices are not soldered directly to printed circuit boards (PCBs).
In electrical analysis, a resistor is a component that does not have time-varying characteristics. However, from the above analysis, we know that the impedance of the semiconductor device is varied. To achieve the goal of equalizing the resistance, the electrical engineer parallels the capacitor to the resistor, thereby producing an exponential curve to counteract the change in resistance calculated by a simple mathematical formula. In a sense, I think this is an electrical engineer's revenge on mechanical engineers for not being able to learn the annoying thermodynamics courses.
As the opening and introduction to the thermal design, this article talks so much. As always, feedback is welcome to stimulate more discussion on the topic of heat/heating. I also provided some valuable reference materials on the design of semiconductor junction heat dissipation. TI's application notes also explain packaging, surface mount, and other good things. In addition, references from Diodes, the University of Colorado, and Cheggs are more in-depth knowledge of device physics.
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