Multiphase Technique Is Flexible and Scalable

Mar 1, 2003 12:00 PM
Edited by Power Electronics Technology Staff

Multiphase power conversion is virtually the only viable approach to handle today's multi-gigahertz microprocessors that devour 60A to 80A at 1.3V to 1.5V, and are moving toward 100A and higher with supplies of 1 V and below. Multiphase techniques distribute the total current across interleaved, phase-shifted PWM channels and associated output MOSFETs and inductors. This spreads heat, lowers stress on components, and reduces the input and output current ripple to minimize the input and output filter capacitors.

Moving from 60A to 100A with a multiphase converter requires more than increasing the number of phases. While this offers lower current per phase, reduced ripple current in input/output filters, improved transient response, it also creates an increase in cost and p.c. board layout problems.

The multiphase solution is a scalable architecture that adapts to load requirements. One answer is International Rectifier's XPhase technology, which supports 1 to x-phases with the option of choosing number of phases, current per phase, and switching frequency. Phases can be added (one at a time) to increase the output current capability.

XPhase architecture (figure) implements feed-forward voltage mode control with modified type 3 compensation and trailing edge modulation with adaptive voltage positioning. It lets the controller automatically adapt to variations in the input voltage at each phase, and reduces voltage variations during load transients with minimal power loss at heavy loads. Also, you can tailor the controller for a 200-ns loop response. The first XPhase chip set consists of the IR3081 control IC and IR3086 phase IC, whose performance exceeds Intel's VRM or VRD/EVRD 10.0 guidelines.

One IR3081 drives all the phase ICs. It includes voltage identification (VID) coding, PWM ramp oscillator, error amplifier, bias voltage, and fault detection. The oscillator frequency is programmable from 100 kHz to 1 MHz. The IR3081's 6-bit VID has 0.5% overall system accuracy, programmable dynamic VID slew rate, programmable VID offset and load line output impedance, programmable soft start, programmable hiccup overcurrent protection with delay to prevent false triggering, power good indication, and thermal monitoring. It operates from a 12-V supply and comes in a 28-lead MLPQ package.

There's one IR3086 phase IC for each of the converter phases. It contains synchronous 2.5 A gate drivers, PWM comparator and latch, overvoltage protection, and current sensing and sharing. Other features include inductor temperature compensation, programmable phase delay, programmable feed-forward voltage mode ramp, up to 1 MHz per phase operation, single-wire average current share bus, and overvoltage protection. It communicates with other phase ICs via a 5-wire analog bus that consists of bias voltage, phase timing, average current, error amplifier output, and VID voltage. By eliminating the need for point-to-point wiring between the controller and the phase ICs, the 5-wire bus shortens interconnects to cut parasitic inductance and noise. The IR3086 operates from a 12V supply and comes in a 20-lead MLPQ package.

Because accurate current sharing is critical to multiphase designs, XPhase implements lossless average inductor current sensing technique, which offers better accuracy at lower cost than other approaches.

A feature unique to XPhase control is body braking, which turns off the synchronous rectifier MOSFET in response to a significant decrease in load current. When this occurs, the synchronous rectifier's lossy body diode conducts the inductor current, thereby increasing the voltage across the inductor. The result is nearly two times improvement in the inductor slew rate. In addition to permitting the use of larger inductor values, body braking also lowers output capacitor requirements.

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