Power Electronics

Synchronous Rectifier Module Eyes Isolated Supplies

To gain a few more points in conversion efficiency, power supply designers have been exploiting the benefits of synchronous rectification (SR) for some time now. Primarily, the focus has been on nonisolated dc-dc converters employed in distributed power architectures or on high performance voltage regulator modules (VRMs) — where every bit counts, and the volumes are high. Well, it's time this proven technique is extended to the isolated front, thereby helping designers take full advantage of SR techniques.

Semiconductor player Microsemi has taken that step. In collaboration with Seattle, Wash.-based SRMOS Inc., Microsemi's Colorado Division has developed an SR module for the secondary side of the isolation transformer. Demonstrated at this year's APEC 2003, designers had focused on many issues in developing this module, some of which include matching MOSFETs to driver ICs, alleviating MOSFET body diode losses — especially above 300 kHz — eliminating problems due to cross conduction, ensuring high performance during rapid load changes, easing complex timing and signal coupling problems with nonself-driven approaches, and handling light load or discontinuous current operation adequately.

This compact surface-mount module, SRM4010, comprises two power MOSFETs that function as low loss rectifiers (Fig.1). Driven by dedicated ICs that provide effective gate drive for these ultralow RDS(ON) MOSFETs, it provides rectification efficiency of more than 95% from 200 kHz to 400 kHz. It connects to transformer secondary of a forward converter to provide high current rectification at output voltages ranging from 5V down to 1V. Rated to handle 40A current, the module is a complete standalone SR solution that includes power MOSFETs, proprietary control ICs, auxiliary circuits, and a 5V regulator. The 5V linear regulator serves as a source for powering other secondary side circuitry, useful in low-voltage outputs where the output is not high enough to power secondary side control circuits.

To prevent cross conduction and minimize MOSFET body diode conduction, the SRM4010 uses a patented predictive timing plus control scheme. This intelligent prediction in the control circuit permits the module to handle sudden changes in pulse widths without cross conduction. As a result, it adapts quickly to continuous and discontinuous load current conditions, allowing the module to operate under any condition such as current mode and voltage mode control. Predictive timing plus offers automatic adjustment to discontinuous current condition, fault protection, and automatic current sharing. Also, the on-board driver and the power MOSFETs are optimized to keep the gate drive losses to a minimum. Thus, the module offers high gate drive current capability, which lends itself to driving 40A-plus MOSFETs at 400 kHz.

The SR4010 comes in a 0.8 in. × 0.8 in. × 0.28 in. current isolated surface-mount package featuring low thermal resistance of 3.5°C/W and ultralow inductance/resistance connections. Designed for high regulation and low noise, this robust package includes a copper baseplate for effective heat distribution (Fig. 2, on page 58). Because the isolated baseplate can be directly mounted on printed circuit boards (PCBs), the SR module provides high thermal efficiency to handle more power dissipation.

The SRM4010 is the first in a family of high efficiency SR modules for low-voltage isolated power supplies with 48V input. Internal tests conducted on the SRM4010 in a forward converter topology with 3.3V output shows that the SR module can achieve 95% and higher efficiency for output currents over 40A at switching frequencies in the 250 kHz to 400 kHz (Fig.3) range. Presently, this part is being ramped up for production. Demo boards are readied for those who prefer to evaluate the part before taking it to production. While the maker has plans to expand this family, higher voltage models capable of delivering 12V, 24V, and 28V outputs at wider current ranges are also in the works.

More Choices for Ultra-Low RDS(ON) MOSFETs

Independently, power MOSFET suppliers continue to innovate on the power MOSFET front to further wring every ounce out of discrete devices so as to satisfy the ongoing efficiency demands of the dc-dc converters. Lately, Philips Semiconductor, Silicon Semiconductor, and Vishay Siliconix have all announced ultralow on-resistance parts with reduced gate-drain capacitance to enable efficient switching at higher frequencies. While Philips and Vishay Siliconix are banking on improved Trench processes, nascent Silicon Semiconductor has developed a rugged planar process to boost the performance of 30V MOSFETs. Using its patented planar process, Silicon Semiconductor has released ultralow RDS(ON) parts with a substantial cut in gate and switching charges to enable 2 MHz switching operation. Plus, to clamp off the body diode, Silicon Semiconductor's MOSFET incorporates high-current junction barrier Schottky. This results in negligible conduction and reverse recovery losses.

On the trench side, Vishay Siliconix has exploited its innovative trench technology, called WFET, to offer record-breaking figure of merit performance. According to Vishay Siliconix, the newest members derived from its WFET technology achieve unprecedented RDS(ON) × Qgd figure of merit. The WFET process uses a thicker gate oxide at the bottom of the silicon trench to enable a two-thirds reduction in gate-drain capacitance with minimal impact on on-resistance. Patent-pending techniques permit further increase in cell density, which lowers on-resistance. Based on the improved trench process, the supplier has released four parts for efficient high-side operation.

Microsemi, Colorado Division, Broomfield, Colo.; www.microsemi.com
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Philips Semiconductors, San Jose, Calif.; www.philips.com
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Silicon Semiconductor, Research Trianlge Park, N.C.; www.siliconsemi.com
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Vishay Siliconix, Santa Clara, Calif., www.vishay.com
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