Power Electronics

Data Points

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Modular Power Supplies Enable Quick Configuration

From XP Power, fleXPower is a modular, configurable power supply series that allows the company to produce multi-output supplies with a range of output-voltage and power options, and deliver sample quantities of these units in 24 hours. The supplies are offered in 400-W to 1100-W chassis, which can be stacked to achieve power levels up to 2200 W. The company provides samples with full agency approvals at every output voltage ranging from 2 V to 60 V. Product labeling reflects that the approvals apply to the actual voltage specified by the customer.

Each output module delivers up to 200 W, 300 W or 400 W, depending on the selected module size and voltage. Modules may be series connected for outputs up to 150 V and paralleled for outputs up to 240 A. As many as five output modules may be installed per chassis.

The fleXPower power supplies employ several electrical and mechanical design modifications that improve performance over that of the company's previous offerings.

These new supplies improve power density by greater than 10%, while reducing the length of the units from 11 in. to 10 in. Power density for the 700-W chassis is 7.3 W/in3.

One of the changes on the electrical side was to replace a conventional silicon diode and associated snubber components (seven parts in all) with a SiC Schottky diode in the front-end boost circuit. This saved space and improved power supply efficiency by 1%. Additional space savings were achieved by “floating” the units' heatsinks to reduce EMI. Although these heat-sinks are electrically connected to the chassis through capacitors, there's no direct connection. This step led to the elimination of two gas tubes and three MOVs previously required to meet EN-61000 surge and electrical fast transient requirements.

On the mechanical side, updated packaging techniques simplified construction and reduced part counts. Output-voltage modules — the dc-dc converters that stepdown the output of the ac-dc front end — were reduced from three pc boards per module to one. In addition, the use of a Berquist thermal clad material in the output voltage modules reduced the thermal resistance between the power transistors and the substrates. The new material enabled the package tabs of the transistors to be soldered directly to the substrate. This, in turn, allowed an increase in power delivery while reducing transistor junction temperatures by 5°C.

Another mechanical improvement was the replacement of a standard fan guard that mounted to the chassis with one that was punched into the chassis. In redesigning the fan guard, the company also increased the spacing between the fan guard and the fan to reduce fan noise by 4 dB. At the same time, the construction of the fan assembly was changed to make it field replaceable.

Until recently, XP Power was known as XP in Europe and as XPiQ and XP ForeSight in the United States. For more information, see www.xppower.com.

Digital Controller Makes Power Factor Correction Feasible in AC Adapters

Using adaptive digital control techniques, the iW2202 power control IC from iWatt (Los Gatos, Calif.) enables the design of smaller, lower cost and higher-efficiency ac adapters that provide power factor correction (PFC). The controllers target adapters such as those used in battery-charging applications and is best suited for designs in the 50-W to 200-W range.

The iW2202 has a digital core that combines both the PFC converter and the output regulation stage. iWatt's single-stage PFC is made possible by simultaneously controlling multiple operating parameters, a feat that has been previously unachievable using analog control techniques. The controller, which employs primary-side control, enables accurate regulation of the power supply's output without optocouplers, references and external compensation circuits. By removing this output-sensing circuitry, the iWatt solution achieves improved reliability and reduced cost.

“iWatt technology makes power factor conversion affordable in applications that were previously reluctant to implement costly energy-saving measures,” said Curtis Davis, president and CEO of iWatt.

A white paper describing operation of the digital, single-stage active PFC controller is available online at www.iwatt.com/pdf/WhitePaper.pdf.

Other features of the iW2202 include the ability to achieve greater than 90% conversion efficiency, less than 300-mW standby power consumption, built-in cable drop compensation, and protection against overvoltage, overcurrent, overtemperature and short circuits. The iW2202 is available now in a 14-lead SOP with a starting price of $1.29 in 1000-piece quantities. For more information, see www.iwatt.com.

Fuel Cell Steps Toward Commercialization

A recent fuel cell demonstration illustrates the progress being made by one company seeking to develop a commercially viable micro fuel cell. Neah Power Systems (Bothell, Wash.) has demonstrated a fully integrated, multicell “stack” that functions as the core engine of a fuel cell. The demonstration, which was performed in Neah's laboratory, verifies the feasibility of the company's silicon-based architecture.

Based on this proof-of-concept, Neah Power Systems expects to complete an actual fuel-cell prototype by the end of 2005. That prototype will integrate the fuel-cell stack with the required pump-and-power conversion circuitry. The company has completed the majority of what it calls the “research and invention” phase of development, and describes the work that now lies ahead in developing fuel-cell prototypes and products as “optimization and engineering.”

The fuel-cell stack represents another milestone in Neah's efforts to commercialize its technology, because the demo unit shrinks the space required by the main fuel-cell component. The current fuel-cell stack is less than one-third the size of the company's original fuel-cell stack, which was demonstrated in 2003. To reduce the size of the stack, the company optimized the electrode structure by doping the electrodes with new catalysts and conductive materials.

The new fuel-cell stack is comprised of three individual fuel cells connected in series to generate approximately 1.2 W to 1.3 W of output. Each cell in the string produces 0.3 V to 0.5 V at about 1 A. Fed by an external bench-top source of methanol, the fuel-cell stack provided stable power output for approximately 1 hr, indicating the robustness of the all-liquid silicon-based system. Power density levels exceeding 80 mW/cm2 at room temperature were achieved.

Neah Power System's silicon-based architecture differs from the approach of the proton exchange membrane (PEM) direct methanol fuel cell (DMFC) system. Unlike the DMFC, the silicon-based architecture does not contain a PEM. Instead, the fuel cell uses liquid electrolyte and silicon-based electrodes. With its finely etched pores, the silicon electrode structure provides a 3-D reaction zone that is said to increase the overall power density and efficiency of the fuel cell when compared with PEM-based DMFC systems.

Initially, the company plans to target military radio applications that rely on a primary lithium battery — the BA-5590. Neah projects it will be able to replace that battery with a fuel cell of the same 883-cc volume and provide about three times the capacity of the lithium battery. The company plans to produce fuel-cell qualification units for this application by 2006 and ship units in volume commercially in 2007. Products for consumer applications, such as for laptop computers, could appear as early as 2008. For more information, see www.neahpower.com.

Rad-Hard SSRs Enhance Reliability in Space

A series of hermetically sealed radiation-hardened solid-state relays (SSRs) from International Rectifier (El Segundo, Calif.) offers a more dependable alternative to electromechanical relays in high-reliability applications, such as communications satellites. The RDHA7x series SSRs are suitable for use in power bus switching, heater control circuits, battery charging and other circuits. Implemented as multichip modules, these relays overcome many of the problems experienced by electromechanical relays (EMRs) in radiation-hardened (rad hard) applications.

Unlike solid-state devices, EMRs can fail when exposed to shock and vibration. They also can freeze up in space when contact lubricant hardens. In addition, the contact bounce that occurs during switching often requires additional components to filter out the contact chatter. And, other components might be needed to drive the EMRs from low-voltage logic circuitry.

In contrast, the new SSRs offer the option of logic-level gate drive, which makes them compatible with 3.3-V to 5-V CMOS and TTL devices. The SSRs also exhibit no contact bounce. When these devices are switched in an uncontrolled manner to achieve the fastest transitions, a small amount of ripple appears on the output. However, most of these models offer a controlled switching rate that eliminates that ripple and makes additional filtering unnecessary.

Initially, the company is releasing four surface-mount devices in this series in single, dual or octal SPST normally open configurations. These SSRs have current ratings ranging from 1.5 A to 20 A, with breakdown voltage ratings ranging from 60 V to 100 V. In addition to surface-mount packaging, most of these devices also are available in alternate package styles for flange mounting.

In the past, SSRs might not have been able to achieve sufficiently low values of on-resistance to be acceptable in high-rel applications. However, improvements in low-voltage MOSFET technology have sufficiently lowered RDS(ON), so that low-voltage MOSFETs can now be used as building blocks in SSRs capable of replacing electromechanical relays. IR's current generation of low-voltage MOSFETs feature RDS(ON) as low as 20 mΩ. To complete the SSRs, the MOSFETs are combined with opto-isolators and other components in wire-bonded, hybrid assemblies (see the figure).

The devices are characterized for dose levels of up to 100 k rad (Si) or higher, and single-event effects (SEE) immunity up to a linear energy transfer (LET) of 37 MeV/(mg.cm2) or higher. Input-to-output isolation is 1000 V. Class “P” (unscreened), and “H” and “K” screened models are available.

Radiation-hardened MOSFETs represent a special niche within the area of discrete semiconductors. Though several companies may have devices that can meet the total dose requirements for rad-hard components, these parts may fall short when it comes to single-event requirements. Single-event specifications are challenging because they represent particularly highly levels of radiation. In addition, existing rad-hard SSRs may not be rated to handle the levels of current specified by the RDHA7x series.

Available now, the SSRs are priced starting at $1122 each in 100-piece quantities for the unscreened version of the RDHA710SE10A2Q, which is a 10-A, dual SPST normally open relay. For more information, see www.irf.com.

Current-Sense Amplifier Responds in 1 µs

Linear Technology's LTC6101 high-side current-sense amplifier combines quick response times with operation from a 4-V to 60-V input voltage range and the ability to withstand voltages up to 70 V. A response time of 1 µs to 2.5 V is specified for a 5-V output step. With this speed, the amplifier is well suited for automatic power shutdown under fault conditions.

The LTC6101 is designed to extract a small differential signal from a high common-mode voltage, and then amplify and translate this into a ground-referenced signal. Achieving this functionality with precision has typically required a combination of discrete components and op amps, differential amplifiers or instrumentation amplifiers. Offered in SOT23s and MSOPs, the IC requires only two external resistors. When precision resistors are used, better than 1% accuracy can be achieved.

The LTC6101 is priced starting at $1.04 each in quantities of 1000. For more information, see www.linear.com.

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