[Technology Report]
Optimized Power Supplies Beget Superior Data-Center Efficiency
Data centers are conspicuous consumers of power. Power-supply design that maximizes efficiency can deliver big payoffs.
In the meantime, the best way to explain the situation is by looking first at PMBus, the open-standard digital power-management protocol for communication between the power converter and other devices. PMBus defines the transport and physical interface. It additionally provides a command language. It doesn’t, though, address communication between one power source and another.
The transport layer is based on the SMBus (System Management Bus) extension of the I2C serial bus with packet error checking and host notification features. In lieu of polling, it adds a third signal line that enables slave devices such as POL converters to interrupt the system host/bus master. In addition, there are hardwired signals for turning slave devices on and off and for write-protecting memory-held data. Also included are packet specifications and a power-control-specific command set.
For a more detailed explanation of PMBus and its relation to power in the data center, see “PMBus Takes Command of Data Center Power Issues” by Intel’s Brian Griffith in Electronic Design’s sister publication, Power Electronics Technology.
A strongly competitive proprietary approach has been Power-One’s Z-Bus. PMBus places a certain coding and computational overhead on the system controller. Z-Bus uses a separate controller that can handle many POLs.
The proprietary Z-system preceded PMBus and operates differently. It relies on an external controller that gets its commands via an I2C interface and communicates with Z-POLs over a single synchronization/ data line called the Z-Bus. The Z-Bus synchronizes multiple Z-POLs to a master clock in the manager chip and carries bidirectional data between POLs and the dynamic power management (DPM). The Z-system offers multi-POL control over output voltage, output tracking and sequencing, switching frequency, interleaving, and active digital current sharing.
Beyond voltage regulation, programmable parameters include output tracking and sequencing, switching frequency, interleaving, feedback-loop compensation, and active digital current sharing with multiple Z-POL converters. Among the programmable protection features are output overcurrent and overvoltage, input undervoltage, power-good signal limits, and fault management. Real-time reporting includes output voltage and current and POL temperature. This can all be programmed via a highly intuitive GUI.
In late 2007, the race between Z-Bus and PMBus entered a new phase when a jury agreed with Power-One that any use of PMBus to control POLs infringed Power- One’s patents. Abruptly, the number of new product announcements about digital POLs for IBA applications almost dried up—but not completely.
Last year, it appeared that Ericsson Power Modules might have sidestepped the patent problem by moving PMBus control to the formerly “dumb” bricks and tightening up their regulation, relieving the POLs of that burden. Specifically, Emerson introduced new fractional bricks in the standard quarter-brick footprint that could be set via PMBus for output levels between 8.5 and 13.5 V, to ±2% precision, with a current capacity up to 33 A at 12 V. Thanks to the digital control loop, typical efficiency is 96% at half load, and that efficiency is nearly flat out to full load.
Physically, Ericsson added a separate header for the communication bus at the opposite end from a brick’s standard input/ output pins. The bus includes pins that allow two of these bricks to load-share while automatically phase-interleaving their switching signals to minimize conducted interference. Whether that still infringes on Power-One’s patent claims is still moot, but it was the only interesting new product development on the PMBus front during the past year.
OTHER PLAYERS Companies that introduced PMBusbased POLs before the Texas jury reached its decision for Power-One in November 2007 included Intersil, Linear Technology, Maxim Integrated Products, Primarion/ Infineon, and Texas Instruments. Additional players in the field took a hard look at the broadness of Power-One’s claims.
In 2005, Zilker Labs (recently acquired by Intersil) introduced a line of POLs that use state-machine control, rather than a microcontroller. (For a complete discussion of the methodology’s merits, see “PMBus Controller Takes A State-Machine Approach.”) For designers, the real appeal of the Zilker/Intersil approach may be ease of use—in particular, how simple it is to use multiple Zilker controllers in the same circuit.
The controllers can be individually programmed (pin-strap, resistor, or SMBus) for regulated output voltage, turn-on delay, and output-voltage ramp rate. SMBus programming provides the highest precision, but simply using a pair of resistors allows the output to be positioned between 0.6 and 5.5 V in 10-mV steps. Alternatively, simple pin-strapping (three pins: low, high, or open) enables the output voltage to be set to any of nine values between 0.6 and 5 V.
For power sequencing, a group of Zilker’s POL devices might be configured to power up in a predetermined order by issuing PMBus commands to assign preceding and succeeding devices in the sequencing chain. Yet a simpler approach would use Zilker’s patented autonomous sequencing mode, in which case no I2C/SMBus host would be needed.
All that’s necessary is that the I2C pins be interconnected for each device. In that case, sequence order is determined on the basis of each device’s bus address. Phasespreading is possible when all converters are synchronized to the same switching clock. In that case, the phase offset for each chip is determined by its device address, where phase offset = device address × 45°.
Radically different from all other players, Vicor’s Factorized Power approach, introduced in 2003, is based on very efficient nonregulating isolated POL-like chips while providing upstream regulation. Advantages include a higher bus voltage, with low I2R losses and voltage drop.
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