Dc-to-dc power-converter modules that operate with input potentials from 60 to 140 V and generate various output potentials up to 15 V are undergoing development. Designed specifically for use aboard planned spacecraft wherein electric power will be supplied at bus potentials from 70 to 120 V, these power-converter modules could also be useful in a variety of terrestrial applications. The fully developed versions of these power-converter modules are expected to operate at power densities about 33 percent greater than those of currently available commercial 28-V-input power-converter modules.

Figure 1. The Multilayer Thick-Film Hybrid approach makes it possible to use package volume more efficiently than does the state-of-the-art single-layer thick-film hybrid approach.

The increase in power density will be achieved by use of multilayer thick-film hybrid packaging conceived expressly for this purpose. This can be explained by reference to Figure 1, which depicts a typical state-of-the-art power converter of single-layer thick-film hybrid design and a proposed electrically equivalent power converter of a two-layer thick-film hybrid design. In addition to large inductors and capacitors for storing energy, there is hybrid circuitry that includes (1) low-power signal-level control devices connected by ball-type wire bonds and (2) high-power devices connected by wedge-type wire bonds. The high-power devices must be mounted with low thermal resistances between themselves and a heat sink; therefore, the high-power devices must be mounted on a substrate in contact with the floor of the converter housing.

Figure 2. This Transformer-Coupled Feedforward/Feedback control scheme offers advantages of greater reliability and functionality, relative to power-converter control schemes based on optocouplers.

The heat-sinking requirement for the low-power devices is less stringent, making it possible to mount these devices on a substrate not in direct contact with the heat sink. Taking advantage of this possibility in the proposed two-layer design, the low-power devices would be placed on a separate substrate. The high-power layer would still be mounted in contact with the heat sink, but the low-power-device layer would be mounted above the high-power layer, so that the floor area occupied by the power-converter circuitry would be less than it is in the single-layer design. Thus, for a given power throughput, the package could be made smaller; in other words, the power density could be larger.

Another notable feature of the developmental power converters is a transformer-coupled feedforward/feedback control scheme (see Figure 2). Hereto-fore, it has been common practice to implement power-converter control circuitry with optocouplers for electrical isolation between input and output sides. However, optocouplers lack the long-term reliability required for the intended spacecraft application. In the developmental control scheme, pulse-width-modulation (PWM) control of the power supplied to the primary winding of a main power transformer is effected on the input side, while sensing of output current and voltage is effected on the output side (the secondary side of the main power transformer).

Power for the secondary-side control circuitry is provided from the primary side through a pulse transformer (distinct from the main power transformer) excited at the switching frequency. An error signal from the control circuitry on the secondary side is fed back to the control circuitry on the primary side via another pulse transformer. This transformer-coupled feedforward/feedback scheme makes it possible to turn on the secondary-side control circuitry before turning on the main power, thereby making it possible to exert control over the output during startup and during recovery from transient short-circuit conditions.

This work was done by Ming Chen of VPT, Inc., for Glenn Research Center. For further information regarding VPT and their standard product line of high-density dc/dc converters, you can contact them at (540) 552-5000 or visit their website at www.vpt-inc.com.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4 - 8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16883.

Electronics Tech Briefs Magazine

This article first appeared in the December, 1999 issue of Electronics Tech Briefs Magazine.

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