Supplying the juice

March 6, 2003
When it comes to power supplies, off-the-shelf products save time and money over custom designs.
One family of configurable ac-dc switching power supplies offers one, two, or three-slot packages giving up to six user-specifiable isolated outputs.

Jeffrey Hord
Configurable Products and Accessories
Vicor Corporation
Andover, Mass.

For most electronic-system OEMs, the standard approach to providing power for a new system is to design a power supply from the ground up, with discrete components that uniquely satisfy the system requirements. Problem is, this usually requires experienced design engineers and consumes a lot of time and resources. But the approach tends to be economical especially if the manufacturer anticipates high-volume production.

However, with the ready availability of off-the-shelf power supplies, designers should think twice before embarking on a custom design. For one thing, designing a reliable custom power supply that meets the requisite agency standards costs more than an off-the-shelf model, especially if changes or modifications need to be made. Also, reducing time to market is virtually unachievable with a custom supply.

Finally, power supplies are subject to the design pressures of smaller size and higher density. Consequently, designers must be evermore experienced to manage increasingly difficult packaging, EMI, and thermal problems.

Power-supply manufacturers meet many OEM requirements and avoid the downsides of custom supplies by offering a wide range of component-based products. In addition, they offer OEMs flexibility in that system changes can often be made simply by substituting other components. Advances in semiconductor manufacturing have reduced chip size and allowed designers to pack more circuitry into smaller spaces, ultimately saving expensive board real estate and rack space. In general, power supplies based on modular components fall into three classes: configurable power supplies, power modules, and quasi-custom power supplies.

Dc-dc converters come in a number of physical sizes and a wide range of input and output voltages and power levels. Combining them with discrete components lets them satisfy a multitude of requirements.

Configurable power supplies have a preengineered chassis and can satisfy many different requirements. They are usually the quickest solution because preconfigured chassis are on the shelf and modular components added to the chassis complete the design. Also, in contrast to a custom power supply, there are no nonrecurring engineering expenses. Complete systems can be configured at the factory or, in many cases, on site by the user.

In many cases, a configurable power supply is tantamount to having a custom power supply because it can be used in so many different applications. Users specify inputs, outputs, power levels, and other parameters. Each chassis is a standardized front end with slots to accept dc output assemblies, and is available with single or three-phase ac or dc inputs.

Some power systems offer added flexibility. Power requirements for these field-configurable systems can be changed on the fly without additional development time or returning units to the supplier.

Power modules, such as dc-dc converter modules, are available in a range of sizes and formats for designers willing to complete the power-supply design. These modular dc-dc converters, in combination with accessories such as ac-dc front ends, can satisfy many unique power requirements. Because each module has prequalified agency approvals, designers enjoy an advanced starting point toward a finished power supply.

Typical input voltages include 12, 24, 28, 36, 48, 50, 72, 150, 270, 300, and 375 Vdc, although only a few manufacturers address such a wide range. Typical output voltages include 0.8, 2, 2.5, 3.3, 5, 12, 15, 24, 28, and 48 Vdc. Most dc-dc converters can even be trimmed, some as much as -90% to 10% of the nominal output.

Traditionally, designers had to use discrete components to supplement modular dc-dc converter capabilities with features such as ac-dc rectification, filtering, and power factor correction. Now, specialized accessory components are increasingly available. Compatible front-end accessories provide a number of features such as input transient protection, EMI filtering, and inrush-current limiting. In addition, they have international agency approvals and can accommodate the wide range of input source voltages worldwide markets require. Together with the power components, these accessories let users quickly assemble complete power systems by selecting and interconnecting standard, modular parts to meet design requirements.

This quasi-custom, ac-dc power supply mounts in the head assembly of a device that scans and radiates cancer cells. It provides four outputs (5.0, 15.0, and -15.0 Vdc) from a 90 to 132-Vac input.

Quasi-custom power supplies are a viable alternative for the designer unwilling or unable to devote resources to designing a power supply. Manufacturers and value-added resellers design, develop, and manufacture complete turnkey systems and power supplies using dc-dc converters. These manufacturers use power components in a modular, building-block design approach that offers low cost, quick turnaround, and reliable performance.

Using standard high-density dc-dc converters and other modular components satisfies the basic power-system functions. Then, power-design specialists address system level requirements such as electrical, mechanical, thermal, and packaging issues of the unique application. Typically, these custom system manufacturers manage each power project from early system definition through volume production and product delivery.

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