Power Supply Thermal Issues are More Important than Ever

August 24, 2015
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It’s an old engineering story, sometimes told in jest, but often not: the power supply is the last item specified. “We’ll just have to find a supply that fits, once we know our power needs” is the thinking. With the ever-increasing demands for greater energy efficiency, higher reliability and faster design cycles, as well as increasing regulation, this approach is no longer acceptable.

An effective engineering team will consider all aspects of power needs early in the project, including thermal dissipation and packaging. Thinking about the power system early in the project is essential if unpleasant surprises of needing a bigger supply, or a new cooling strategy are to be avoided: no one wants to hear, “I think we might have to add fans here, after all,” as the projects gets close to completion.

Vicor’s Power Component Design Methodology is an approach to power system design that enables engineers to architect the power chain, while retaining flexibility. If requirements change, it is straightforward to replace the selected power components with devices that will meet the new specification. This is considerably easier than using a large centralized power supply, where the entire power system may have to be changed to meet the revised specification.

There are now a range of tools for simulating and linking electrical and thermal performance, such as the PowerBench Whiteboard, making it easier to create optimized thermal designs. Sometimes it is even possible to squeeze greater performance from the system in terms of available power, reduce the need for a “just in case” tolerance cushion, or increase reliability due to cooler running.

Choosing the right power components is critical. In the past open frame power supplies were used because of their convenience, but they provide some challenges. Due to its irregular ‘skyline’ profile, the airflow may be uneven and potentially blocked (” shadowed”) by neighboring components. A heat sink could reduce the problem, but requires using an interposed conforming flexible pad between the converter and the heat sink, or dedicated pre-heat sunk implementations.

At the same time, available heat conduction to the system PC board copper via the converter’s pins ranges from negligible to merely modest, at best. The result is both poor cooling, as well as localized hot spots where individual components may be thermally stressed, especially under “corner conditions”, even if the overall average temperature is within allowed bounds.

More recently overmolded (encapsulated) converter package have been introduced such as Vicor ChiP platforms, overcoming these limitations. The smooth, flat top surface means that a heat sink is easy to attach and effective in use.

Similarly, the updated design and increased number of leads provides a simple, effective, low-impedance thermal path to the case or cold plate. So it’s a thermal win in both convection and conduction – see Figure 1. (There are even a few extra percentage points of cooling via convection through the sides, an extra benefit.)

VI Chip Heat Dissipation

Figure 1: Modeling and analysis of the overmolded package shows the multiple, effective thermal paths it offers.

 

The ChiP package offers two-sided cooling, which doubles the ability to remove heat from the package, but does increase the complexity of the heat sink design. Optimizing the thermal design presents the biggest challenge at the front end of the power chain, where power levels tend to be highest.

The new Vicor Integrated Adapter (VIA) Packaging Technology greatly simplifies the design of the front end of power chains, particularly thermal design. By delivering a new level of power density, efficiency and thermal performance in an easy-to-use package that also integrated some peripheral circuitry, such as filtering, the VIA package makes it even easier for engineers to develop thermally-efficient power chains.

VIA Packaging

Figure 2: Vicor Integrated Adapter (VIA) Packaging Technology

 

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