Innovators Tackle R, L, And C To Enhance The Fundamental Building Blocks

Oct. 22, 2010
New Developments in capacitors, resistors, and balun transformers

Ragone charts


TC offset

It seems there are so many new developments in active circuits, it’s tempting to dismiss basic passive components as boring. Package sizes may shrink, but the basics of resistors, capacitors, and inductors change little from what was familiar to Ohm, Faraday, and Henry. For the most part, that assumption turns out to be true.

Every once in a while, though, some engineers can’t leave well enough alone and come up with ideas that sometimes modestly refine the basic components of electronic circuits—and sometimes radically alter the ways new designs are conceived.

Commercial ultracapacitors are one example of a game-changing breakthrough. In fact, capacitors still provide the most fertile environment for innovation, particularly in the form of ultracaps based on carbon nanotubes. Ratiometric resistor combinations (at a macro scale) offer new levels of instrumentation precision on the circuit board. And coils? Well, we’ll see about coils.

“Ultracapacitors,” or “supercapacitors,” are more properly termed “electric double-layer capacitors.” They can be fabricated with many orders of magnitude greater capacitance than conventional capacitors.

For example, some radial-lead board-mount devices are rated for 5 to 10 F at 2.5 V, flashlight-battery size units rated for 120 to 150 F at 5 V, and larger single-capacitor cans good for 650 to 3000 F at 2.7 V. They can be combined into modules with capacitances from 20 to 500 F, with voltage ratings from 15 to 390 V. Properly balanced in series/parallel combinations, they are superior to batteries for storing and discharging bursts of power.

W/kg and Wh/kg
Before getting into how ultracaps are made, it’s useful to compare them to batteries in terms of energy density and power density. It’s sometimes said that while batteries store energy, ultracaps store power. That can start arguments, because fundamentally, both devices store energy.

To resolve that issue, Ragone (Fig. 1) charts are useful tools. A Ragone chart is a log/log cloud plot with energy density (Wh/kg) on the Y axis and power density (W/kg) on the X axis. Diagonals depict constant discharge rates. As you look at Figure 1, keep in mind the log/log scale of both axes. The spreads, in linear terms, are much bigger than they look in the figure.

As one can see, both kinds of devices store energy. But it’s not unreasonable to distinguish between batteries and ultracaps in terms of their relative strengths. To better understand the complementary application of batteries and ultracaps, think about a Tesla Roadster and a municipal bus, both driven by electric motors (see “Ultracaps ‘Brake’ Wasteful Energy Habits”).

The lightweight (approximately 2700-lb curb-weight) Tesla is designed to provide plenty of torque for acceleration. Torque is energy, so Teslas rely primarily on big battery packs. On the other hand, the heavy (circa 25,000-lb) bus needs a certain amount of acceleration to get moving and to deal with stop-and-go traffic. But mostly, it needs modest acceleration rates applied in relatively short bursts.

For those occasions, if the bus can call on a power source like a bank of ultracaps from time to time, its motor will be able to turn the ultracaps’ watts into foot-pounds of torque for short-term acceleration. (Note the diagonal line on the Ragone plot.) Meanwhile, on level ground, battery energy keeps the heavy bus rolling. For hill-climbing, a slower discharge from the ultracap bank can assist the bus’ battery bank and maintain speed against the pull of gravity.

Continue on next page

The Inside Story
How are ultracaps built? Like traditional capacitors, they store power as an electrical charge between two electrodes. Unlike conventional electrolytics, those electrode plates are not based on tightly rolled sandwiches of metal foil. The secret of the ultracap’s extraordinary charge-holding capacity is the enormous area across which charge can be collected, made possible by taking advantage of that “double-layer” concept and aided by technology for creating divided activated carbon particles (see “The Latest: Commercializing Carbon Nanotubes”).

An understanding of how ultracaps store charge starts with conventional electrolytic capacitors. Chemically, the actual anode “plate” is a nonmetallic electrolyte on the surface of one of the physical “plates.” It is formed by applying a voltage to the rolled-up anode and cathode during manufacturing. The result is the growth of an insulating metal oxide on the surface of the anode.

When an ordinary electrolytic capacitor is charged by applying a voltage between anode and cathode, the voltage causes charges in the electrolyte to separate and accumulate between the two plates. The accumulation of oppositely charged ions in the solution compensates for excess charge on the electrode surfaces. The interface is called the Helmholtz layer.

Ultracaps create Helmholtz layers on both the anode and cathode and increase the surface area over which they operate by many orders of magnitude, compared to electrolytics, by building the electrodes out of finely divided carbon particles.

In an ultracap, when a dc voltage is applied across the electrodes, compensating accumulations of cations or anions develop in the solution around each. If no electron transfer can occur across the interface, a “double layer” of separated charges (electrons or electron deficiency at the metal side and cations or anions at the solution side of the interface boundary) exists across the interface.

The Helmholtz-region capacitance depends on the electrode area and the size of the ions in solution. The separation of charges in double layers is about 0.3 to 0.5 nm, compared 10 to 100 nm in electrolytics (and as much as 1000 nm in mica or polystyrene caps). That means the capacitance per square centimeter in the ultracap is on the order of 104 times larger than in the electrolytic.

However, the double-layer configuration reduces the potential capacitance of a practical device because the ultracap consists of a pair of electrodes, each with half the total mass. In addition, the ultracapacitor is effectively two capacitors in series, halving the effective capacitance.

More Differences
Ragone charts emphasize the difference between ultracapacitors and batteries in terms of energy density versus power density, but there are more differences than that. Briefly:

• Batteries depend on chemical reactions with long time constants. Charging is a relatively slow process, and the optimum charging profile is nonlinear. Conversely, capacitors charge and discharge according to the familiar exponential relationships based on the RC time constant.

• Batteries provide a more or less constant voltage over long time periods. Capacitors discharge rapidly, and their output voltage decays exponentially.

• Batteries are good for only a limited number of charge/discharge cycles, and the number of cycles depends on how deeply they are discharged. Capacitors, especially ultracapacitors, can be charged and discharged repeatedly for tens of millions of cycles. (Unlike electrolytics, ultracaps aren’t cycle-limited.)

• Batteries are big and heavy. Capacitors are small and light.

Continue on next page

Designing with UltraCaps
Combining ultracaps, batteries, and external energy sources makes for an interesting design exercise. The circuit designer must connect those components in a way that will limit charge current and continually recharge the capacitor between load events.

One solution for charging is to use a dc-dc buck regulator for charging, with a voltage feedback mechanism for determining when the charge is complete. For discharge, the complementary approach is to use a dc-dc boost converter to maintain the capacitor output voltage at a constant level. Those are the basics. Application differentiation comes from additional monitoring and control circuitry.

Wound Transformers Unwound
Baluns are balanced-to-unbalanced transformers that can be implemented either as wound transformers or as assemblies of transmission lines. However, the BD0205F5050A00 “Xinger” from Anaren Inc. replaces wound baluns with a novel strip-line balun.

The 50-Ω unbalanced to 50-Ω balanced strip-line transformer is designed to be the final stage in front ends for high-speed analog-to-digital converters (ADCs). The BD0205F5050A00 (Fig. 2) also exhibits performance advantages over its wound-balun competition. It has a very small (e.g., 1608-size surface-mount package) footprint on the circuit board as well.

In this kind of application, the balun should provide an impedance transformation and convert a source’s single-ended signal into a differential signal at the ADC input. High-performance ADCs use differential inputs to reject even-order harmonics, along with ground and power noise, all of which can affect ADC performance in terms of spurious-free dynamic range (SFDR), signal-to-noise ratio (SNR), and total harmonic distortion (THD).

Applications arise in wireless infrastructure and in high-speed instrumentation—specifically the fast and accurate resolution of very high intermediate frequencies (IFs) and signal bandwidths greater than 100 MHz. These apps include basestations and test equipment that use data converters that operate at 200 Msamples/s and higher, with resolutions of 12 to 16 bits. Baluns are used in the “front ends” that connect the last active stage of the signal chain to the converter’s analog inputs.

Until Anaren introduced these strip-line baluns, the industry used wire-wound transformers. The difficulty is that the effects of parasitics and from losses in the wound transformers’ ferrite cores compromise the digitization of today’s higher IFs.

Amplitude and phase imbalance are two of the most critical performance characteristics when evaluating a balun. Phase imbalance, which translates to even-order distortions (mainly second harmonics) at the input to the converter, is a particularly critical spec (see “Maximize ADC Performance Through Balance And Symmetry”).

Another advantage to non-ferrite transformer technology is its insensitivity to variations in differential impedances over wider bandwidths, which are common when using unbuffered ADCs that experience a change in input impedance when the converter input switches between the sample and hold functions.

Self-trimming resistors
What can be new about discrete resistors? Take a hint from integrated circuits, where resistance ratios are precisely controlled by geometry and doping. What if something similar could be accomplished on a larger scale? Consider Microbridge Technology and its Rejustors.

Rejustors (electronically readjustable resistors) are passive, adjustable micro-resistors. They do not need any power to hold their settings, since they are re-adjustable many times, bi-directionally, to very high (0.1% to 0.002%) precision using only electrical signals.

Continue on next page

All adjustments can be carried out at low voltage and low current before and/or after packaging—and that’s critical. Unlike laser trimming, Rejustors can be adjusted in the package and resistance values can be lowered, as well as raised, which is impossible with ablative laser trimming.

Rejustors are manufactured on standard CMOS process technologies. At the end of the CMOS process, the Rejustors’ microstructures are released by a bulk-silicon etch process, leaving them suspended over a cavity. This keeps the Rejustors structures, which have a low thermal mass, thermally isolated.

Adjustment is accomplished by repeatedly annealing the resistance element, which changes its properties slightly with each heat cycle. Increments of change are small, but the structures are isolated by the cavity. They also have such low thermal mass that they can be heated and cooled very rapidly.

Actually, two elements are needed to form a Rejustor. First, the poly film resistor is adjusted. And second, an adjacent power resistor is pulsed to apply the heat that does the adjusting.

What makes this interesting is that it’s possible to adjust resistance and temperature constant (TC) independently. Microbridge terms the result an “eTC Rejustor.” Unlike conventional TC-controlled components, no extra temperature sensor is needed, because the eTC Rejustor is its own temperature sensor as well as the adjustment device.

This simplifies a number of vexing production problems for analog engineers. For instance, amplifier offset and TC offset can be compensated in the analog domain, right at the source. No lookup table, ADC, or digital-to-analog converter (DAC) is needed. Also, the lack of a stepwise mixed-signal interface implies zero quantization noise.

Here’s how a designer might make use of an eTC resistive divider with certain TC offset (Fig. 3) versus offset characteristics. Offset, the deviation of the divider output voltage VIN × (R1/(R1+R2)), is expressed in mV/V. TC offset is the temperature coefficient of that divider output voltage, measured in μV per Kelvin (K) per volt of divider input voltage.

Microbridge’s eTC adjustment software makes it possible to measure the actual TC and offset errors, pick target values for offset and TC offset as a point within the region delineated by the parallelogram in Figure 3, and iteratively zero in on the target.

Let’s say that before correction, the performance of the divider in Figure 3 is represented by the dot. In this example, its output voltage is 5% (50 mV/V) below its ideal value, and the temperature variation has a positive 75-μV/VK value. Microbridge’s software and hardware make it possible to move the starting point for offset and TC from its measured position (the dot) to the center of the plot faster than it takes to read about it.

Initially, Microbridge expected to be in the Rejustor business. But when a company has a generic solution and the word gets out and requests start coming in, applications start to cluster and “market forces” dictate application-specific product development. Hence, the company now provides a number of low-pressure and differential gas flow sensors, but its fundamentals have remained the same. Microbridge simply makes very 21st century resistors.


To join the conversation, and become an exclusive member of Electronic Design, create an account today!