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Around here we sit through many presentations about miniaturization of components and performance efficiencies realized at the same time. We also hear about the effort spent on reducing the size of passives used for energy conversion as inductors, capacitors, and transformers typically take up a large portion of the solution size.
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For engineers, all of this means more attention must be paid to thermal management and the junction temperature of semiconductor devices, which handle the conversion of power to a usable form. After all, when a device is running, it consumes electrical energy that’s transformed into heat. And if this heat isn’t properly controlled, it can negatively affect efficiency or even damage the board.
Simply shrinking a power supply, however, usually means you will be dealing with a challenging set of thermal issues. It becomes harder to obtain a sufficient surface area in contact with air that’s necessary to achieve effective heat dissipation.
On the plus side, shrinking systems and components leads to higher power density. Therefore, in addition to improving thermal resistances, if we can develop more efficient components and systems, power dissipation can be reduced. And that can directly result in lower temperatures.
Mitigating Heat Effectively from the IC Through the PCB
What stands in the way of achieving higher power density? Thermal performance has been one of the limiting factors. Proper circuit packaging is important for the heat to exit the system easily. This allows the system to avoid experiencing unwanted temperature rises.
Thermal performance is an electrical byproduct of power-management ICs, which you can’t ignore or eliminate via filtering components at the system level. The mitigation of thermals requires small adjustments throughout every step in the development process so that the design can achieve its system requirements for a given size constraint.
Let’s examine a few of Texas Instruments’ approaches to the problem. Most electronic systems have printed circuit boards (PCBs) with various components that dissipate power under different operating conditions. A bond-wire-type quad flat no-lead package (QFN) with an exposed pad is a popular package from a power density point of view.
Typical QFN packages are built using bond-wire interconnects between silicon and frame, which usually have very high electrical resistance. In a typical scenario, a QFN package with an exposed pad connected to the PCB results in more than 90% of power conducted from the package through the PCB to ambient air.
The HotRod QFN
TI has developed a clever approach to improve package thermal performance to a degree beyond that of the previous generation. Called a HotRod package, it replaces QFNs with a flip-chip style package (Fig. 1). The electrical connections are made via lands on the bottom side of the component to the surface of the connecting substrate (PCB).
