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.
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