Electronic Design

DC-DC Chip Set Crafts Compact Bus Converter

Distributed power architectures that use an intermediate voltage bus look for streamlined approaches to implement the bus converter. An International Rectifier chip set offers such a tailored solution.

Built around a new primary-side controller IC, the IR2085S, the set converts the common 48-V dc bus down to an intermediate bus voltage between 5 and 12 V. Combining this controller with IR's primary- and secondary-side MOSFETs yields a dc bus-converter chip set that maximizes bus-converter efficiency and power density while minimizing parts count and size.

As with some other bus-converter products, the bus converter implemented by the chip set doesn't regulate output voltage. Its performance, then, best suits systems that use a regulated 48-V input. However, when the chip set includes the IRF7493 80-V primary-side MOSFETs, the bus converter can accommodate a wide-ranging input of 36 to 60 V. That input range will extend to 75 V with the company's upcoming 100-V primary-side MOSFETs.

A demo bus converter has been created using the IR2085S controller with the IRF7493 primary-side MOSFETs and IR's secondary-side IRF6603 DirectFET MOSFETs. The complete design requires about 20 parts altogether (including magnetics, capacitors, and other passives) and fits on a 1.95- by 0.85-in. double-sided board (see the figure). (That footprint is about 20% smaller than an eighth-brick dc-dc converter.)

This demo delivers 150 W with over 96% efficiency at an output of 8 V—a bus voltage that optimizes the system-level tradeoffs in distribution and switching losses. Its power density is 90 W/in2.

The IR2085S controller integrates a 50% duty-cycle oscillator with a 100-V, 1-A half-bridge driver IC within an eight-pin SOIC. In the past, these two functions required two chips. Moreover, the gate-drive circuit is optimized for use with International Rectifier's primary-side, low-gate-charge MOSFETs.

High- and low-side gate-drive signals are matched within 25 ns to prevent transformer imbalances. Switching frequency and deadtime between low- and high-side pulses can be adjusted via external capacitors to suit application specifics. A soft-start feature gradually raises duty cycle from zero to 50% to limit inrush current. For more details on the controller and chip set, see "Controller Operation" under Techview: The Industry at www.elecdesign.com.

Pricing for the dc bus-converter chip set is $7.67 in lots of 10,000.

International Rectifier

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.