There's almost nothing as good as a design contest to get engineer's attention, especially if the core of the contest is an analog component which can be the core of a wide variety of applications from DC to RF. The recently concluded "Rejustor Application Design Contest" from Microbridge Technologies, Inc. (developers of the Rejustor) once again demonstrates this reality. (Note: The winners and links are at the end.)
For readers who are unfamiliar with this proprietary component, the Rejustor is an adjustable, precision, analog resistor. The contest judges, including people from Microbridge as well as outside consultants, judged the entries with this weighting: design and engineering analysis: 30%; use of unique features of the Rejustor: 40%; creativity: 20%; and the final writeup: 10%.
The winning entry, by Cheng-Yang Tan, was a programmable RF attenuator which took advantage of the 100 MHz+ bandwidth of the Rejustor (in contrast to the under 1 MHz bandwidth of an electrically variable chip resistor. The attenuator design based on the Rejustor is not only small, but it is continuously variable (compared to the 0.5 dB steps of most fixed-step designs, and retains setting without power, and is far simpler than a continuously variable RF attenuator based on a servo motor controlling an attenuator (nonvolatile but with obvious cost and size issues); or a voltage-variable attenuator which requires a large number of individual components.
The design is based on a pi-network filter topology. Input and output coupling transformers with calculated turn-ratios were added to match the Rejustor impedances to the 50-ohm impedance commonly used in RF designs.
The two second-place winners also showed what designers can do when facing a challenge and opportunity. One used a collection of Rejustors to build a precision milliohm meter, capable of 0.01% precision. Here, the Rejustors replaced alternatives such high-cost fixed resistors and laser-trimmed resistors, and mechanical trimming potentiometers. The resultant design was less costly, easier to calibrate (and recalibrate, via software), and mechanically rugged.
The other second-place winner used Rejustors to provide a current-balancing mechanism among the phases of a low-voltage, high-current (100 A) switching-mode power supply/regulator. Imbalances due to parasitics, component variations, and other unpredictable factors, results in the more some phases delivering the bulk of the current, while the others provide far less. The excess loading results in additional electrical and thermals tress, and thus shortened life, for those phases. One solution is to over-design to provide extra margin, but this is costly and increases bulk, as well. With the approach used by this winner, supply vendors can implement phase-current balancing at final test.
The quartet of third-place winners confirmed that clever engineers find a variety of problem-solving uses for components. In this grouping were:
- an adjustable biquad filter which allowed for use of wider-tolerance (thus cheaper) capacitors on the BOM, as well as calibration and compensation for any changes in component parasitic values which occur as a result of final production encapsulation (potting) of the filter module;
- all-analog optical isolation, for electrically harsh signal environments. The "obvious" approach of per-channel digitization, followed by optical, magnetic, or capacitive isolation, is very expensive for multiple channels. A lower-cost, analog channel, however, requires careful calibration to correct for nonlinearities and other imperfection of both the transit (LED) end and the receive (photodiode) ends. Rejustors in the LED-driver circuit and the photodiode end allow for use of low-cost optical components and links, and they can also be tuned to be electrically compatible with the standard 4-to-20 mA industrial loop interface;
- A correction and compensation circuit for a robot-navigation gyroscope rate sensor. The sensor has inherent, asymmetrical drifts which results in navigation data error. Software-based algorithms can correct some of the error, but have limitations and are confused by noise spikes which are due to rough road-induced data spikes. Rejustors were central to a low-pass, 5th-order filter which has both fine-tunable parameters and needed non-standard resistance values which could otherwise only be attained using complex combinations of serial and parallel standard-value devices;
- Finally, monitoring of a multiple IGBT series stack for "graceful degradation" as an early indicator of component failure. The gate-drive current of each insulated gate bipolar transistor must be monitored for an increase in value, which results from the IGBT shorting (the most common failure mode), but each circuit has significant response deviations due to component differences, manufacturing issues, and temperature. Existing monitoring circuits use some combination of discrete resistors, digital potentiometers, mechanical potentiometers, laser trimming, temperature- and strain-sensitive sensors, and fusible passive-resistor arrays, but these all have limitations. By using Rejustors, the target threshold limits for the monitoring circuit can be set to greater precision during final test, and better temperature compensation and matching is possible, while manual adjustments are eliminated.
To see these winning entries, click on the appropriate link:
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