Chipsets redefine distributed power designs

A family of DC bus convertor chipsets seeks to redefine the underlying distributed power architecture of 48V-input, 150W board-mounted power convertors for telecomms and networking systems.

International Rectifier has unveiled the first members in a family of DC bus convertor chipsets that redefine the underlying distributed power architecture for 48V-input, 150W board-mounted power (BMP) convertors used in telecomms and networking systems.

IR's DC bus convertor chipset architecture establishes benchmarks for overall efficiency, power density and simplicity, delivering more than 96% efficiency at 20A/150Wout in a circuit footprint less than 1.7in2.

The chipset enables a 53% size reduction when compared with industry-standard quarter-brick form factors and slashes component count in isolated convertors from approximately 50 components to 20.

The new chipset consists of a single IR2085S control IC and one pair each IRF7493 primary- and IRF6603 secondary-side HEXFET power mosfets, plus the IRF7380 for primary-side bias and an IRF9956 for the secondary-side gate clamp.

The chipset is designed for the isolated front-end of two-stage distributed power architectures (DPA) with an intermediate bus voltage feeding nonisolated point-of-load (POL) convertors.

Two-stage DPA schemes do not require a tightly regulated intermediate bus voltage, as the POL will typically accept a relatively wide input voltage, and the POL provides the needed regulation to the load.

In addition, multiple DC bus convertor chipsets can be placed in parallel for higher power requirements.

"IR's DC bus convertor chipset architecture removes the need for a regulated output at the intermediate stage, eliminating expensive feedback circuitry.

Therefore, the overall solution is less complex, the component count is reduced and the board space needed for the conversion is smaller, while achieving over 96% efficiency", said Carl Smith, Marketing Manager for Networking and Telecommunication Products at International Rectifier.

The IR2085S control IC is the heart of the DC bus convertor chipset architecture, based around a 50% fixed duty cycle, self-oscillating control scheme.

The IR2085S replaces two SO-8 packaged devices, and is optimised for DPA applications.

Features include an integrated soft-start capacitor that gradually increases duty cycle from zero to 50% for 5ms to limit inrush current during startup, and maintains equal pulsewidths for the high- and low-side mosfets throughout the startup sequence.

The low- and high-side pulses for the half-bridge are matched to within +/-25ns to prevent transformer imbalances during operation.

Other features include +/-1A gate drive current optimised to work with IR's next generation low charge primary side mosfets, and includes adjustable dead time from 50 to 200ns to protect against shoot-through current.

The dead time can also be adjusted to limit the amount of body diode conduction on the secondary side, therefore maximising efficiency.

The IR2085S has programmable switching frequency up to 500kHz for design flexibility.

Higher switching frequency decreases output voltage ripple and allows use of smaller, lower loss magnetic components.

Circuit designers can control switching frequency and dead time independently with just two external components to customise circuits for their particular application.

A floating channel designed for bootstrap operation up to +100V DC and VCC supply undervoltage lockout is included.

The IR2085S uses a new high voltage, high frequency level shift technology with high dv/dt immunity.

The immunity is in the range of 50V/ns to prevent unwanted turn on of the lower mosfet in the half-bridge, and enables faster switching speed.

On the primary side, two 80V-rated IRF7493 mosfets in the SO-8 package are employed in a half-bridge configuration.

It has low combined on-state resistance and gate charge, at 15mohm (maximum) at 10V gate-to-source voltage, 31nC total gate charge and 12nC gate-to-drain charge for maximum switching performance.

On the secondary side, two 30V-rated IRF6603 mosfets in the DirectFET package are used in a self-driven synchronous rectification topology.

It has low on-state resistance and a thermally enhanced package, at 3.9mohm (maximum) at 10V gate-to-source voltage, and 1C/W thermal resistance to the PCB to deliver maximum current capability and efficiency.

IR's new architecture uses two biasing components.

The IRF7380 dual 80V mosfet has close threshold voltage matching and is used on the primary side in a novel biasing scheme.

The IRF9956 secondary side dual mosfet provides a gate clamp for the secondary-side synchronous rectification mosfets, clamping the drive voltage to 7.5V.

The DC bus convertor chipset samples and evaluation tools are available now.

Production release for the chipset is June 2003.

Pricing is US $7.67 each chipset in 10,000-unit quantities.

Back to top