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Evaluate high power density demands on POL converters

26 May 2015  | Jian Yin

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The complexity and performance of the latest FPGAs, processors, ASICs and associated memories for server, telecom, datacom, networking, and industrial equipment continue to intensify the power demands in these applications. Real estate remains at a premium driving the requirements for higher power density point-of-load (POL) solutions due to the conflicting objectives of increasing output capacity while maintaining or even reducing the solution board footprint.

From the power design viewpoint, this introduces three specific challenges: thermal management, compactness of the power solution footprints and improved transient response. Undertaking all three challenges simultaneously can take significant power expertise and design resources; hence, many system designers are turning to modules as a way to quickly and reliably achieve the high current and high density point-of-load (POL) design they need. This article examines how power modules can reliably manage thermal issues, as well as minimise the POL footprint using a 3D stackable inductor structure while improving the transient response performance using a digital control loop.

Packaging matters
One of the keys to achieving high current and high density power converter design is thermal management. Managing thermal issues is one of the greatest challenges in high power (greater than 100W) applications. Footprint and ultimately power rating, especially at high ambient temperature, are dictated by the module's electrical and mechanical design. Using thermally enhanced packages that can efficiently move heat out of the package enables modules to deliver better thermal performance.

A power module is typically built upon a package substrate on which the semiconductor die and other electronic components are mounted and inter-connected. It is therefore imperative for this package substrate to provide good thermal conductivity.

A variety of different types of package substrates are utilised in power modules. A dual layered printed circuit board in a land grid array (LGA) power module provides a substrate with good routing capability and straightforward electrical interconnections. The thermal conductivity, however, is undesirably low and will penalise the module thermal performance. An alternative approach, using a metal lead fame on a quad flat no leads (QFN) package, provides very good thermal conductivity but lacks the ease of routing capability. While the QFN can be modified to improve the routing, this technique usually results in higher package costs.

The best combination of thermal conductivity and routing capability is a single-layer conductive package substrate in a High Density lead-frame Array (HDA). This substrate comprises a peripheral and an interior portion. The peripheral portion includes contact pads used for surface mount on a motherboard while the interior portion includes floating contact pads that are electrically isolated from the peripheral contact pads and are coupled to internal components. The peripheral and interior contact pads of the HDA lead-frame in combination with bond or jumper wires allow the HDA power module to provide excellent routing capabilities, similar to that of a dual-layered PCB, as well as excellent thermal conductivity to dissipate heat because of the single conductive layer of material.

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