Power in small packages

March 4, 2004
Packaging improvements let dc-to-dc converters and power modules pack in the juice.
Packaging improvements let power modules deliver more punch. This second-generation family of dc-to-dc converters from Vicor delivers up to 600 W of power.
Smaller integrated control devices opened up more space for the main transformer and primary and secondary inductors of the power train. The main transformer design alleviates thermal and noise issues, is more reliable, and costs less.
The interconnect system consists of a pair of header sockets soldered to the PC board using a standard reflow process. The converter is simply inserted later.

Fred Finnemore
Senior Fellow
Vicor Corp.
Andover, Mass.

Modular dc-to-dc converters once came in half-inch-high packages that measured about 2 X 5 in. When first introduced, there was nothing like it in terms of concept, form factor, or topology.

Although the basic elements of concept, form factor, and topology have remained essentially unchanged through a number of generations, the packaging has changed significantly. Continued tweaking together with electrical enhancements have produced families of modules with different electrical performance characteristics and operating ranges, sizes, baseplate and mounting options, pin styles, standoffs, and termination options. More importantly, today's modules boast three times the power and power density and one-third the parts count of the originals. Plus, the price per watt has been dramatically reduced.

Most packaging changes are made in the interest of higher power density. The main objective is to get the maximum volume in those elements that transfer power or magnetic flux. From a packaging standpoint, the challenge translates to physically minimizing the size of elements not in the power train while maintaining structural integrity and dielectric capability. For instance, package wall-section thicknesses that were once 50 thousandths of an inch are now 6 or 7 thousandths.

Packaging improvements show up on baseplates and termination systems, customized transformers and chokes, and interconnect systems. Improvements focus on maximizing performance and minimizing converter volume. The challenges designers face include increasingly higher power demands, greater electromagnetic disturbance, and more heat.

Recent converters have a higher power density which meant finding creative ways to remove heat from the module. Foremost of these was the plated-cavity design.

Also significant is the effectiveness of the thermal interface between the module baseplate and the user's heat-conducting material. Its effectiveness depends on minimizing thermal resistance across the interface which minimizes air in the interface. One way of minimizing the air gap is by controlling the flatness of the baseplate, which is manufactured to a flatness of 5 mils. Another way is to use a tack-free, compliant, thermal-interface material applied to the baseplate to fill surface irregularities. Finally, when the screws are applied with normal torquing of 5 or 6 lb-in., or around 200 lb of clamping force per screw, it excludes the air left in the interstitial space.

Another example of incremental thermal improvement is the introduction of variations in width and thickness. A few substantial section thicknesses are used in the central portion of the baseplate to improve thermal performance. A pocket bottom in an earlier converter version was 0.036 in. It has been increased to 0.060 in. so more heat from the power devices is being distributed to the thicker base.

Ultimately, how effectively the device removes heat limits transformer and module performance. A plated cavity helps conduct heat away from the transformer, increasing power-handling capability. Copper-armor coating on the transformer provides more surface area for heat removal. An open construction also provides a better thermal path to the baseplate and plating helps remove heat from the core.

The chokes, the inductance part of the input and output LC filters, are also customized. Using rectangular wires maximizes the amount of copper and wastes no internal volume. The wires have small corner radii on the edges to maintain an appropriate insulation thickness. The core itself is all copper to minimize the dc resistance of the core.

Design of the chokes, starting with the ferrites and how they are pressed, uses a full-basket approach, an open, five-sided box with return legs for the magnetic flux on three sides. The termination scheme provides paths for the current from the coil ends to the boards to which they are surface mounted. The chokes are constructed using a lead frame soldered to the coil to maintain dimensional control. The lead frame is removed once the coil and terminals are installed and stabilized with an adhesive system. This ensures the terminal dimensions are accurately positioned and coincident with the pad geometry.

Converters with socket options have been around for some time. But because newer converters produce higher current, thicker pins were needed. With more mass, however, they become less flexible. So socket designs had to be reliable after repeated insertions and extractions. Sockets are available for both surface-mount and through-hole designs and provide high current-carrying capability, small size, and low contact resistance.

A pair of header sockets, one for the input and one for the output, is soldered onto the PCB using a standard reflow process. Then the converter is inserted into the header sockets, eliminating additional soldering operations. Standoffs secure the module to the board and electrically connect the baseplate to the ground plane. The standoffs also provide an easy way to secure a heat sink to the module baseplate.

Because converter modules are decoupled from header sockets, they're not reflow soldered. This means the modules are not subject to heat or trauma of soldering and cleaning.

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