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In part 3 of this series, we used the concept of thermal resistors to develop a thermal equivalent network of a system and determined its equivalent junction to ambient thermal resistance. With this approach, we were able to link thermal resistances to physical properties of the system and intuitively understand the dominant heat transfer mechanisms with the equivalent thermal resistor equations. In this blog we’ll take a look at several popular cooling techniques that are commonly employed in electronic systems and discuss how they work.
Heat sinks are passive heat transfer devices that transfers heat from an IC package to the ambient environment with a much smaller thermal resistance than the parallel thermal resistance from the package to the environment due to convection and radiation. In order for the heat sink to be effective, its equivalent thermal resistance must satisfy the following equation:
where is the effective thermal resistance of the heat sink, is the thermal resistance of the package top due to convection and , is the thermal resistance of the package top due to radiation.
Figure 6. Thermal resistance model of a N-fin heat sink with a TIM connected to the top of a package.
Figure 6 shows a thermal resistance model of a N-fin heat sink (N is the number of fins) with a thermal interface material (TIM) connected to the top of a package. The TIM is needed to improve the contact between the package and the heat sink and thus the effective thermal resistance of the heat sink needs to include the resistance of the TIM. Using the techniques we learned from the previous blog, we can find:
which tells us the effective resistance is equal to the resistance of the TIM plus some resistance from the base of the heat sink and the parallel resistance of the N-fins. If we assume the fin resistances are equal, then the equation can be further simplified to:
The equivalent resistance of the heat sink simplifies to the resistance of the TIM plus some resistance from the base of the heat sink and the resistance of a heat sink fin divided by the N number of fins. Since the area of the heat sink fins can be larger than the top surface area of the package, its convection and radiation resistance can be smaller than that of the package top surface. Furthermore, this resistance is divided by the number of fins of the heat sink leading to an N times improvement. However, for a given heat sink base area, increasing the number of fins above a certain number will eventually lead to an increase in the thermal resistance of each fin since the fins will start to approach each other dropping the effective heat transfer coefficient. It is also important to choose high thermal conductive materials for the heat sink and TIM to improve the overall performance of the heat sink since these thermal resistances add directly to the effective thermal resistance of the heat sink.
Another technique to cool an electronic system is to spread more of the heat from the IC to the back side of the PCB using thermal Vias and heat spreaders. Thermal Vias placed under the IC can significantly reduce the thermal conduction resistance of the PCB and help guide the heat to the heat spreader plates placed on the bottom side of a PCB. Heat spreaders are made from high thermal conductive material like graphite and have large surface areas to improve heat dissipation.
Electronic fans are routinely used in consumer electronic systems such as desktop computers, laptops, and projectors, etc. when the use of passive heat sinks and heat spreaders may not be sufficient to remove the heat. A fan uses a motor and requires power to actively move airflow around the system to remove heat. It can also be a source of audio noise, so you would need consider noise generation as well as reliability issues when choosing a fan. Many fans today allow you to control the speed with a pulse width modulated (PWM) signal so you can design a thermal management system that allows you to dynamically adjust the fan speed as a function of your system temperature.
A heat pipe is a heat transfer device that uses the principles of thermal conductivity and phase change to transfer heat between solid components. Phase change for a heat pipe general refers to a liquid changing to a gas once the liquid reaches its boiling point at the hot spot and then propagates as a gas down the pipe and condenses back to a liquid when it reaches the lower temperature interface. The liquid is then wicked back up to the hot spot usually through capillary action and the process repeats as it removes the heat from the hot spot to the cooler interface. Heat pipes are also widely used in consumer electronic systems and can be found in computers, tablets, and even smart phones.
Finally, as electrical engineers, we do have the ability to control the power dissipation of our system with various power throttling techniques but usually at the cost of degrading our system’s performance. The goal here would be to trade-off performance as gracefully as possible so our customers can appreciate that you have done everything possible to maintain the best user experience. Many electronic systems now employ thermal sensors throughout the PCB such that an onboard processor can make monitor the temperature in the system and make dynamic throttling decisions as the temperature increases. As electrical engineers, we intimately understand the various different power profiles of our system and can start to turn on fans, reduce features, disable different parts of the system, and/or throttle clock speeds as the temperature in our system reaches different temperature thresholds.
Congratulations on making it through our EE thermal 101 blog series. We hope you were able to learn some basics of heat transfer. With a foreseeable future of higher power density electronics, electrical engineers will play a critical role in the thermal management design of products. Cadence® offers SigrityTM PowerDCTM as a proven electrical and thermal technology that has been used in the design, analysis, and sign-off of real-world packages and PCBs for many years. PowerDC enables electrical engineers to extend power integrity with fast and accurate thermal analysis for IC packages and PCBs. It includes an integrated electrical/thermal co-simulation environment that considers the effect of increasing electrical resistance that occurs at higher temperatures to help you confirm the design has met specified DC voltage and temperature margins. Check out our PowerDC page for more information.
Read more blogs of thermal topic:
EE Thermal 101 – Thermal Basics for Electrical Engineers (Part 3 of 4)
EE Thermal 101 – Thermal Basics for Electrical Engineers (Part 2 of 4)
EE Thermal 101 – Thermal Basics for Electrical Engineers (Part 1 of 4)
Why is Power Integrity Hot (or is it Cool)?
Some Don't Like It Hot: Thermal Model Exchange