[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.
Synchronous rectification replaces the flywheel diode with a power FET. In more detail, a basic buck or boost converter requires only a single switching transistor, plus a rectifying diode. The problem with that, though, is the losses related to current times forward drop and reverse recovery time. Schottky diodes help, with their relatively low forward-voltage drop and good reverse-recovery characteristics. But what’s really needed for efficiency is another MOSFET switch instead of a silicon diode.
The tradeoff is added complexity, both in terms of parts count and timing control. The need for added timing control arises because there must be a certain amount of dead time between one switch opening and the other closing.
Also, that dead time requires some sort of diode to conduct between the time the top switch opens and the bottom switch closes. This can be handled by the intrinsic body diode in the MOSFET or by an external Schottky.
Let’s take synchronous switching one step further. In highpower ac-dc flyback-topology converters, the quasi-resonant or valley switching power supply varies switching frequency with input voltage changes to switch the MOSFET at the lowest point, or the valley, of the switching-MOSFET drain voltage.
Cycle skipping improves efficiency at light loads. In skip mode, a new cycle is initiated only when the output voltage drops below the regulating threshold. The switching frequency is proportional to the load current. With synchronous rectification, care must be taken to open the switch when the current through the inductor reverses so the MOSFET’s body diode blocks the reverse current.
Multiple phases and phase shedding involve ganging multiple low-current switching converters. These run at a common switching frequency, but with their clock phases shifted.
Paralleling the output of several switching regulators provides increased current capacity, along with other advantages. Each parallel switcher in a multiphase dc-dc converter operates at a relatively low frequency. However, when combined, they produce the responsiveness and regulation performance of a single-phase very high switching-frequency converter without the switching losses associated with higher frequencies. Also, by staggering the phases, the inherent output ripple is smoothed out.
So far, so good. Things get really interesting with multiphase switching regulators, though, when shedding phases to handle light loads with higher efficiency. At light loads, it makes sense to shut down some phases, because the efficiency of individual converters is greater at higher loads. As the total load increases, dormant phases can be brought back on line. The tricky part here lies in phase synchronization and balancing—adjusting the relative phase angles on the fly.
For certain situations, it’s better to drive all clock phases in sync. In some of its dc-dc regulators, Primarion (now part of Infineon) does just that. The chips can switch between two modes: one “normal,” the other called Active Transient Response (ATR).
In normal mode, phase pulses are evenly distributed to minimize the combined ripple. In ATR, the clocks to all phases are time-aligned, effectively paralleling the inductors to reduce total inductance and increase transient ramp time. This technique has been applied to POLs with eight phases to deliver di/dt rates in excess of 800 A/µs at the inductors and over 1500 A/µs at the output capacitor.
DIGITAL POWER Five or so years ago, there was a curious debate about whether it was better to close the control loop in switch-mode power supplies in the analog domain or the digital domain. Eventually, everyone realized that was the wrong debate, that “digital” ought to mean telemetry and a programmable bus (with maybe the parallel possibility of programming some parameters with external resistors or by pin-strapping).
Those goals could be achieved regardless of the implementation of the control loop. It’s perfectly possible to put an analog loop inside a digital “wrapper.” (There’s still a debate on the digital side as to whether control is better implemented via a microcontroller or a state machine.)
There are a number of advantages to a two-way control and monitoring bus. Downstream, it enables rapid reconfiguration. Used bidirectionally and coupled with a graphical user interface (GUI), it provides a way to manage the control loop in-circuit (Fig. 4). Upstream, the two-way control and monitoring bus facilitates system diagnostics and prediction of potential failures. Through temperature monitoring, it provides a way to manage multiple server fans to avoid hot spots in cabinets.
With the market clearly in favor of some kind of control bus, the question came down to implementation—whether an industrystandard bus was better, or whether proprietary buses would provide more innovation. It’s the classic opensource, closed-source debate. As it worked out, the debate led to a legal situation that stalemated development for over a year.
I’ve been told privately that those issues will resolve themselves shortly, perhaps as soon as the 2009 APEC conference, or shortly after. Unfortunately, that news will be too late for this report. But Electronic Design will report on it online as soon as it happens.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
