Sure, designers of embedded computer systems know digital techniques. But they may not be intimately familiar with the digital management of the power supplies used by their systems. Two methods reign when it comes to implementing digital power management. Yet it's not clear which method will win the favor of system designers—or the favor of the courts.
Power-One's proprietary Z-One system was the first method to arrive. Then came the open-standard Power Management Bus, or PMBus. More than 30 companies have adopted PMBus, including power-supply companies and IC manufacturers. Meanwhile, Power-One has sued Artesyn Technologies, now part of Emerson Network Power, claiming that the company infringed on its valid digital power-management patents.
As part of a Markman hearing, a U.S. district court ruled in March 2007 in favor of Power-One for most of the important issues related to digital power management. (A Markman hearing refers to the Supreme Court ruling on Markman v. Westview Instruments Inc., which states that a trial judge will decide on the asserted patent claims.) Next, a jury will deliver its decision. Power-One's jury trial is set for this month, and the outcome could determine the future of power-supply digital management.
A recent Supreme Court decision casts a cloud over this patent litigation. The Court adopted a new standard that makes it easier for patents to be denied or challenged on the grounds of being too obvious for patent protection (see "Patent Law: Who Knows What's Obvious?"). The patent ruling could also subject existing patent holders to litigation over obviousness. Some experts say the ruling protects the country's competitiveness, whereas others wonder whether the ruling will hurt innovative firms, such as startups and small companies.
A jury decision in favor of Power-One could prevent anyone from using the company's patents that describe the digital management of power supplies. Ideally, Power-One would like to license the technology, which would expand its applications. If the jury rules against Power-One, then it would appear that the PMBus can be used by anyone that meets its documented specifications (see "The Patents In Question" at www.electronicdesign.com, Drill Deeper 16122).
Digital power management is set up on a basic principle: Power-supply hardware would include links that allow the setting of a converter's output voltage and other operating parameters and then monitor operation to ensure it's functioning properly. Thus, similar power-converter hardware could be used throughout a system.
Initially, a graphical user interface (GUI) would set the operating parameters of all power converters, and the digital power-management system would function as programmed. Then, the digital power-management system would monitor all power converters and notify the host of any failures or performance degradation.
THE Z-ONE SYSTEM
Power-One's Z-One architecture integrates a power system's management and power conversion functions. According to the company, this cuts overall system-level costs by 20% to 50% compared with more conventional approaches.
In addition, it allows up to 32 point-of-load (POL) converters to fully communicate with each other under the control of a digital power manager (DPM). Each of these digital Z-Point-of-Load (Z-POL) converters operates with a 3- to 14-V input (except for the ZY8160, which is 8 to 14 V) and provides a programmable 0.5- to 5.5-V dc output.
The Z-One system employs a single-wire Z-One Digital Bus controlled by the DPM (Fig. 1). This high-speed bi-directional bus, which provides both frequency synchronization and data transfer, can access all Z-POL converters in a single communication cycle. The bus carries all of the information to and from the Z-POL converters and DPM, including all operating parameters for each POL converter.
Operational parameters, such as the output voltage, sequencing, tracking, monitoring, interleaving, and protection thresholds, are user-programmed via the GUI and stored in the DPM. At system startup, this stored information programs the Z-POL converters.
After system programming, ongoing communications between the DPM, Z-POLs, and host system support intelligent operation. Designers can repeat this point-and-click process for power-system optimization. The GUI need only be interfaced with the power system during programming, and it can be easily reconnected to support power-system design changes.
Communications with the host system use the industry-standard I2C bus communication protocol. Besides remote programming of each POL, the latest status information of each POL (output voltage, output current, and temperature) is stored in the DPM and can be transmitted to the system.
Hardware for the Z-One system integrates two ASICs, the ZM7300 DPM, and the maXyz ZY7000 series of Z-POL converters. The ZM7300 DPM programs and controls sequencing, tracking, supply supervision, and intelligence with the aid of analog-to-digital converters (ADCs). C&D Technologies is a second source for the Z-One system components.
DIGITAL POWER MANAGER
The ZM7300 DPM family eliminates the need for external components for power management, programming, and monitoring Z-One POL converters and other industry-standard power and peripheral devices. Designers can program the ZM7300 series via the I2C bus and change them during product development and deployment. These products communicate with host systems via standard I2C interfaces to support 100- and 400-kbit/s operating modes.
ZM7300 DPMs come in industry-standard 9- by 9-mm quad flat no-lead (QFN) packages. Users can program them via an IEEE 1149.1-compliant JTAG port during board assembly, or by using the wizard-driven Z-Series GUI and the I2C port. They also provide "on-the-fly" reprogramming capabilities without having to remove or replace board components.
Monitoring capabilities include output voltage, current, and temperature for each Z-Series converter. A buffer stores this information for host-system communications, facilitating system-level monitoring and remote diagnostics. Z-Series DPMs can also monitor the intermediate voltage bus, accept external interrupts, and initiate crowbar protection.
The newest Z-POL converter is the ZY8160, a programmable, multiphase step-down dc-dc module integrating digital power conversion and intelligent power management. When used with the ZM7300 DPM series, the ZY8160 completely eliminates the need for external components for sequencing, tracking, protection, monitoring, and reporting.
Also, the ZY8160 operates with an 8- to 14-V input and provides a 0.5- to 2.75-V output at a continuous 60 A. All of the ZY8160's parameters are programmable via the standard I2C communication bus and can be changed by a user at any time during product development and service.
Power-One also has what the company calls a "No-Bus" family of intelligent, fully programmable step-down point-of-load dc-dc modules. These also integrate digital power conversion and intelligent power management. One member of the family, the ZY1207, completely eliminates any need for external components for sequencing, tracking, protection, monitoring, and reporting.
The ZY1207 accepts a 3- to 14-V input. It has a 0.5- to 5.5-V output at a continuous 7 A. Its performance parameters are programmable by pin strapping an external resistor and capacitor. Designers can change the parameters at any time during product development and service without a communication bus.
The open-standard PMBus specification defines a digital communications protocol for controlling power conversion and management devices. It's a collaborative effort involving power-supply and semiconductor companies (see "The History Of PMBus Products" at Drill Deeper 16121).
With the PMBus, power converters can be configured, monitored, and maintained according to a standard set of commands. Using PMBus commands, designers can set a power supply's operating parameters, monitor its operation, and perform corrective measures in response to faults or operational warnings.
The ability to set a power supply's output voltage allows the same hardware to provide different output voltages by merely reprogramming. The ability to monitor and maintain a PMBus system enhances its reliability and availability.
Implementing the PMBus spec requires the design of power supplies and their associated ICs to adhere to the required interface and commands. The SMBus provides serial communication between the host computer or system manager and the PMBus-compliant devices (Fig. 2). A variation of the widely used I2C bus, the industry-standard SMBus is a two-wire bus modified several years ago for smart-battery applications.
The PMBus protocol would permit multisourced power-management products. Also, OEMs will be able to control compliant power converters through a standard set of commands. The PMBus specification has two parts. Part I includes the general requirements. It also defines the transport and electrical interface and timing requirements of hardwired signals. Part II defines the command language used with the PMBus.
The most recent version of the specification, 1.1, is dated February 5, 2007. The original version, 1.0, was released March 28, 2005. The most significant change in Version 1.1, Part I, is the increase in permissible bus speed from 100 to 400 kHz. Accomplishing this requires an increase in the drive levels of the I/O pins. To comply with the PMBus specification, devices must meet certain qualifications:
They must meet all requirements of Part I of the PMBus specification.
They must support at least one of the non-manufacturer-specific commands given in Part II of the PMBus specification.
If a device accepts a PMBus command code, it must execute that function as described in Part II of the PMBus specification.
If a device doesn't accept a given PMBus command code, it must respond as described in the Fault Management and Reporting section of Part II of the PMBus specification.
Upon application of power, PMBus devices must start up and begin operation in a controlled manner, as programmed internally or externally, without requiring communication from the serial bus.
The PMBus protocol covers a wide range of power-system architectures and converters. However, not all PMBus devices must support every available feature, function, and command. Here, the term "devices" refers to the specific power-management product; that is, IC, power converter, power supply, etc.
The protocol includes the ability to program fault or warning levels for a power-conversion device. Fault conditions are more serious than warning conditions. They may require the PMBus device to disable the output and stop the transfer of energy to the output. For fault conditions, you can program the PMBus device to respond by shutting down immediately and latching off, shutting down, and retrying, or continuing to operate for a specified delay time before shutting down.
Z-ONE VERSUS PMBUS
A major difference between ZOne and PMBus is the way power-management tasks are partitioned. Z-One splits the power-management responsibilities between a firmware-enabled Digital Power Manager and
a POL-based digital pulse-width-modulator (DPWM) IC. The PMBus requires designers to program the computer according to the PMBus protocol, with much of the intelligence residing in a system controller.
Another difference is that the Z-One system operates only with dc-dc converters, whereas the PMBus operates with dc-dc and ac-dc converters. Plus, many more Z-One products are now available than PMBus products, though more companies support PMBus than the Power-One's Z-Alliance.
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