The power supply generates heat during operation, and the continuous temperature rise will cause changes in performance, which may eventually lead to system failure; in addition, the heat will also shorten the life of components and affect long-term reliability.
A heat-generating component, even if the temperature rise exceeds its allowable limit, causes the entire system to heat up, it does not necessarily mean that the entire system is overheated, but the excess heat generated by the component must be dissipated.
So where does the heat go?
Dissipate to a cooler location, either adjacent to the system and the case, or outside the case (only possible if the outside is cooler than the inside).
Thermal Management Solutions
Thermal management follows the basic principles of physics, and there are three ways of conducting heat: radiation, conduction, and convection.
For most electronic systems, the required cooling is achieved by conducting heat away from the source of heat and then transferring it elsewhere by convection.
Thermal design requires a combination of various thermal management hardware to efficiently achieve the required conduction and convection.
There are three most commonly used cooling elements: heat sinks, heat pipes, and fans.
Heat sinks and heat pipes are passive cooling systems that do not require a power supply, while fans are an active forced air cooling system.
heat sink
A heat sink is an aluminum or copper structure that captures heat from a heat source by conduction and transfers the heat into an airflow (in some cases, water or other liquids) for convection.
Various types of radiators
Heat sinks come in thousands of sizes and shapes, from small stamped metal fins that connect individual transistors to large extrusions with many fins (fingers) that intercept convective airflow and transfer heat to that airflow.
Heatsinks have the advantage of having no moving parts, running costs, failure modes, and more.
Once the heat sink is connected to the heat source, convection occurs naturally as the warm air rises, starting and continuing to create airflow.
Although heat sinks are easy to use, there are some drawbacks:
1. Heat sinks that transmit large amounts of heat are bulky, costly, and heavy, and must be placed correctly, which will affect or limit the physical layout of the circuit board;
2. The fins may be blocked by dust in the airflow, reducing efficiency;
3. It must be properly connected to the heat source so that the heat can flow smoothly from the heat source to the radiator.
Heat pipe
It is another important component of the thermal management kit, transferring heat from point A to point B without any form of active forcing mechanism.
It contains a sintered core and a sealed metal tube of working fluid that does not act as a heat sink itself, but rather absorbs heat from a heat source and transfers it to a cooler area.
Heat pipes can be used when there is not enough space for a heat sink near the heat source or when there is insufficient airflow.
Heat pipes work efficiently and transfer heat from the source to a more manageable location.
working principle:
The heat source converts the working fluid into steam within the sealed tube, and the steam carries heat to the cooler end of the heat pipe. At this end, the vapor condenses into a liquid and releases heat, and the fluid returns to the hotter end.
This gas-liquid state transition process runs continuously and is driven only by the temperature difference between the cold and hot ends.
Connecting a heat sink or other cooling device on the cold end can solve the problem of heat dissipation from localized hot spots where airflow is blocked.
fan
It's the first step toward a forced-air-cooled active cooling device, ditching passive radiators and heat pipes, but fans also have their own headaches:
1. Increase the cost, require space, and increase the system noise;
2. It is prone to failure, consumes energy and affects the efficiency of the entire system.
But in many cases, especially when the airflow path is curved, vertical, or obstructed, they are often the only way to get adequate airflow.
The key parameter that defines a fan's capacity is the unit length or unit volume flow of air per minute.
Physical size is an issue though: a large fan at a low speed can produce the same airflow as a small fan at a high speed, so there's a size-speed trade-off.
Modeling and Comprehensive Simulation
Standalone passive systems are larger in size but are more reliable and efficient, while fans can function in situations where passive cooling alone cannot be used.
Which system to choose for cooling can often be a difficult decision.
This is where modeling and simulation are needed to determine how much cooling is needed and how to achieve it, which is critical to an efficient thermal management strategy.
For miniature models, heat sources and their paths for heat flow are characterized by their thermal resistance, which is determined by the material, mass, and size used.
Modeling showing how heat flows from the heat source is also the first step in evaluating components that cause thermal failure due to their own heat dissipation.
Device suppliers such as high heat dissipation ICs, MOSFETs, and IGBTs often provide thermal models that provide details of the thermal path from the heat source to the device surface.
Once the thermal loads of the various components are known, the next step is to model at the macro level, which is as simple as it is complex:
Airflow from various heat sources is sized to keep its temperature below allowable limits; basic calculations are performed using air temperature, unforced airflow available, fan airflow, and other factors to get a rough idea of temperature conditions.
Next comes a more complex modeling of the entire product and its packaging using the model and location of each heat source, the PC board, the surface of the case, and other factors.
Finally, modeling has to solve two problems:
1. The problem of peak and average dissipation. For example a steady state component that dissipates 1W of heat continuously has a different thermal impact than a device that dissipates 10W of heat but has a 10% intermittent duty cycle.
This means that the average heat dissipation is the same, and the associated thermal mass and heat flow will produce different heat distributions. Most CFD applications can be analyzed with a combination of static and dynamic.
2. Imperfections in physical connections between components and micromodel surfaces, such as the physical connection between the top of the IC package and the heat sink.
If the connection has a small pitch, the thermal resistance of this path will increase, and it is necessary to fill the thermal pad on the contact surface to enhance the thermal conductivity of the path.
Thermal management can reduce the temperature of the components in the power supply and the internal environment, which can prolong the service life of the product and improve the reliability.
