IEEC - DC/DC Converter Packaging

DC/DC Converter Packaging

Worldwide power supply consumption will eclipse $17 billion during 1996 with North America and Europe accounting for close to 75% of the market. The move to distributed power in the communications industry remains the single most important driver to growth in the DC/DC converter markets. Although there is some movement towards modular AC/DC supplies, the majority will remain packaged as enclosed or open-frame systems with little fundamental change in packaging configuration. However, on average, the growth in distributed processing in the communications and computer market will reinforce the migration to lower output power of AC/DC power supplies. By the end of the decade, more than half of all AC/DC supplies produced will be less than 200 watts.

The DC/DC converter market is also driving towards higher power densities at lower cost while average power output declines with smaller modules located closer to the actual driven module or device. By the end of the decade, we expect that fully 70-75% of the DC/DC converter market will be in some form of modular package, whether PCB mountable, encapsulated, or other modular configuration.

As the computer and communications industries continue their migration to 3.3V or lower voltage logic, the low voltage and higher efficiency supply markets will become increasingly important. It is estimated that, by the year 2000, the 3.3V and below DC/DC converter market will represent close to 40% of the total market in value terms and a higher percentage in unit quantity. For an equivalent power supply, reducing the voltage from 3.3V to 1V reduces the power consumption by a factor of ten! Texas Instruments has recently developed a 1.65V converter that is 93% efficient.

The growth in unit consumption and increasing power density of modular DC/DC converters has resulted in the proliferation of use of ceramic hybrid, direct bond copper, and insulated metal substrates bonded to metal plates which form the base of the module. The move to miniaturize and cost reduce DC/DC modules has forced a number of leading suppliers such as AT&T, VICOR, ASTEC, NEMIC-LAMBDA, and others to optimize the interconnect and thermal management functions at the substrate level.

DC/DC Bricks

Fifteen years ago, Vicor Corporation revolutionized the DC/DC converter market with the introduction of its unique high frequency zero current switching technology along with a modular form factor now commonly referred to as a brick -- 4.6" x 2.4" x 0.5". Today, most power supply manufacturers offer the brick (or half-brick) form factor to ensure interoperability with power supplies from other manufacturers.

Traditional power supply packaging includes packaged discrete power components and/or thick film hybrids typically mounted on a conventional printed circuit board. Modular converters combine the needs of reduced volumetrics, low cost and increased power density. There is a fundamental debate over whether the designer should isolate the power devices in a separate module on a "high performance" thermally enhanced substrate or to build the complete assembly on a new material. Regardless of the choice of packaging, modular supplies depend on thermal conduction from device to the case base.

Substrate technologies in use or under consideration include:

Conventional PCBs with thermal vias

Plated copper-on-ceramic

Insulated Metal Substrate (IMS)

Direct Bond Copper (DBC)

Flex-on-metal

Two representative modular DC/DC converters have been analyzed to highlight examples of alternate technology solutions, and are discussed here in abbreviated form. For a detailed overview of power supply packaging, and thermal management approaches in general, please contact Prismark Partners or the IEEC.

AT&T JW 150A DC/DC Converter

AT&T Microelectronics Power Systems (Mesquite, TX) has become a major player in the modular power market by serving Lucent's internal needs along with a substantial merchant presence. The JW 150A 150 watt converter is typical of AT&T's brick product line. The following points are of interest:

The converter measures 2.4" x 2.4" x 0.5" and weighs a total of 95 grams. The external hard plastic cover is easily removed and reveals a completely potted assembly based on two substrates.

A conventional FR-4 multilayer board with two-sided SMT assembly is stacked on top of a base IMS substrate with one-sided surface mount. The two boards are connected through a series of nine soldered pins. The top PCB contains the bulk of control and switching devices while the IMS substrate is used to mount the power components with a direct conduction path through the base.

The conventional PCB uses 2 ounce copper, whereas the IMS substrate uses 4 ounce copper, to thermally cope with the high power SMT devices mounted on it.

Vicor J31-CW DC/DC Converter

Vicor pioneered the development of high frequency modular converters in the early 1980's and is today a leading global merchant supplier with an installed base of over 3 million units worldwide. The J31-CW is a 100 watt converter in half-brick format (2.3" x 2.4" x 0.5").

Although delivered in the same form factor and weight, the packaging approach of the Vicor module is radically different from the AT&T converter. The entire assembly is potted in a solid mass of cured epoxy which made disassembly impossible and our tear down job exceedingly difficult! By crossectioning, the following differences can be highlighted:

The converter consists of one thin multilayer PCB with 2 ounce copper.

The majority of components are surface mounted on both sides of the PCB. Several components are very bulky (in contrast to custom surface mount magnetics used in the AT&T module) and had to be hand soldered.

The highest power devices are bare die mounted on two separate thick copper-on-ceramic substrates which are mounted directly to the aluminum base plate for thermal conduction.

The base plate itself is a two-shelf machined cavity approximately .125" thick. The various machined shelves are used to bond the copper/ceramic modules.

Vicor's approach to high power and magnetic device packaging is obviously quite different from AT&T's. Given that Vicor's zero current switching technology is operating at high frequency, concern over capacitive coupling and resultant noise and EMI problems that could arise using IMS (thin high dielectric constant insulator between copper and aluminum base plate) may have pushed them towards a ceramic module approach.

Summary

The AT&T JW 150A and Vicor J31-CW DC/DC converters are representative of the growing volume of modular "brick" power modules in use today.

Even though the power supply industry has standardized the overall form factors for these products, there are clearly unique approaches being employed to package the combination of power components, magnetics, and switching electronics.

The AT&T module uses conventional PCB, IMS and 100% packaged surface mount components in its designs with a very clean result. Vicor has developed a unique substrate for bare power device assembly which thermally and electrically isolates the power IC from the rest of the assembly.

In summary, the fast growing modular power supply market will be a battleground for innovative substrate and assembly technologies which address a complex set of thermal, electrical and reliability requirements. No one approach has won out as yet, but clear opportunities lie ahead.

Copyright 1996 Integrated Electronics Engineering Center and Prismark Partners LLC.

5/96

Warranty Disclaimer -- All information used in the preparation of this report was obtained from sources believed to be reliable at the time the information was collected. Prismark Partners LLC, its employees, its agents, and assignees have exercised their best efforts in preparing this report. Prismark Partners LLC extends no warranties with respect to this information and shall bear no liability whatsoever to the report recipient or to any other party as a result of the use of this report or the information contained herein.