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TECHNOLOGY REPORTPOL Power Supplies Come In Many Flavors

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An influx of novel point-of-load converterswrangles with tougher semiconductor demands.

Sam Davis, CONTRIBUTING EDITOR

As per usual, the power-supply industryis feeling the heat from the semiconduc-tor industry to meet upgraded require-ments for voltage, current, power, andswitching speed. That pressure is nomore apparent than with point-of-load(POL) converters, which power mostlow-voltage loads.

On the bright side, this challenge has spun out sever-al new trends:

• Power-supply companies are developing their ownPOL ICs.• New POL architectures promise better performance.• Digital control is elevating POL converters to a newlevel.• Improved power MOSFET packages boost POL effi-ciency.• Monolithic IC regulators with higher switching fre-quency allow for smaller POL capacitors and induc-tors.

Ultimately, system designers and integrators mustinvestigate whether to design and build POLs or buy

OEM POLs. Acompany with lim-ited technicalresources wouldprobably buy test-ed and qualifiedPOLs, whereas acompany withadequateresources maydesign and buildits own POLs. Regardless of the approach used,designers have many different ways to achieve POLpower conversion.

To encourage in-house designs, semiconductor com-panies now provide more “hand-holding.” For exam-ple, Carl Smith, marketing manager for InternationalRectifier (IR), points out that his company has devel-oped simulation tools and “demo” boards to assist cir-cuit and system designers who may not have the neces-sary design resources in-house. IR provides support forpower MOSFETs as well as its power-oriented ICs thatdrive and control the MOSFETs. Thus, IR’s support isat the POL system-design level.

WHAT’S A POL? • Why exactly are these trends takingplace? To get that answer, designers first must under-stand POLs. These non-isolated converters acceptpower input from an isolated dc-dc converter and pro-vide dc power to close-proximity loads. This configu-ration costs less than an isolated dc-dc converterapproach, because the non-isolated POL convertersdon’t have a transformer for I/O isolation. (For a lookat some groups that have formed in this area, see“POLA And The Power-Management Bus,” p. 50.)

POL dc-dc converters are usually found in small,nonbrick packages, such as a single inline package(SIP) or surface-mount-technology (SMT) module.POL converter modules must be located as close as

POL POWER SUPPLIESCOME IN MANY

FLAVORS

TECHNOLOGY REPORT: POINT-OF-LOAD CONVERTERS+ + +

9.6 to

14 V48 V

Front-end power supply POL

System pc

boardBus

converter

POL

System pc

board12 V

Front-end power supply

1. Typically, system power management involves an isolat-ed bus converter operating from 48 V (nominal) that deliv-ers a nominal 12 V dc to a non-isolated POL converter.

2. IBM doesn’t use a bus converter.Instead, it employs a 12-V intermedi-ate bus produced directly by an ac-dc front-end power supply.

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possible to the circuits they power,which aids voltage regulation. This hasbecome increasingly important asprocessors operate at lower voltagesand higher currents. Also, higher clockfrequencies mean that POLs must pos-sess fast dynamic response.

Today, most POL converters are partof a distributed power architecture(DPA). Many but not all distributedpower systems employ an intermediatebus architecture (IBA) that inserts a busconverter in front of the POL (Fig. 1).

Randy Malik, a member of IBM’ssenior technical staff, is responsible forcomputer power-system design. There-fore, he’s involved with power-systemdesign of desktop and industrial computer systems. Hepoints out that the company’s POL goals include atleast 90% efficiency (45�C to 55�C) with minimumspace requirements. The company purchases POLsbased on its specifications and occasionally designs andbuilds some of its own.

These POL converters operate from a 12-V-input busderived from an ac-dc front-end power supply (Fig. 2).Because the 12-V supply is required for fans and harddrives, it’s also used directly for the POLs that provideoutput voltages in the 1- to 2-V region. No intermedi-ate bus converters are necessary. Virtually all circuitsuse synchronous rectification and multiphase conver-sion. Most multiphase circuits employ four phases,while a handful use six or eight phases. A typical multi-phase POL for a microprocessor might handle over100 A at about 1 V.

At the system level, the major problems Malik hasencountered are those related to thermal management.Problem heat sources emerge primarily from inductorsand multilayer pc boards.

FACTORIZED POWER ARCHITECTURE • Vicor is one ofthe power-supply manufacturers that has developed itsown POL ICs, called V·I Chips (VICs). Vicor’s factorizedpower architecture (FPA) uses these chips, which pro-

vide a fundamentally different approach to POL conver-sion. FPA separates dc-dc power conversion into its twoessential elements—isolation and transformation on theone hand and output voltage control and regulation onthe other—providing these functions in the form of cost-effective modules with very high density.

Figure 3 shows the FPA approach. Voltage transfor-mation modules (VTMs) put isolated current multiplica-tion and voltage division directly at the POL, providinga constant voltage division ratio, or “K factor,” as highas 32. Upstream, a pre-regulator module (PRM) con-trols the factorized bus voltage applied to the VTM.Because VTMs provide true voltage and current trans-formation, the factorized bus voltage can be relativelyhigh. Narrower copper traces can minimize distributionlosses. And, the PRM may be located at any convenientlocation, either adjacent to or remote from the VTM.

The VTM’s very high bandwidth, unimpeded byseries inductance, allows support for highly dynamicloads using efficient energy storage at the relativelyhigh factorized bus voltage. Without bulk capacitors atthe POL, board real estate is reclaimed for its essentialfunctions. Thus, more advanced, more competitiveproducts can be powered by the density, efficiency, per-formance, and cost-effectiveness of the FPA—the high-est performance and least intrusive form of distributed

power.V·I Chip VTM models can efficiently

supply up to 100 A at regulated outputvoltages of as low as 0.8 V dc or higher,when needed. Compatible V·I ChipPRMs are available for use with a widevariety of input sources. Available noware 48-VIN PRM models for telecom, IT,and distributed power systems. In devel-opment are 24 VIN (telecom and indus-trial); a narrow-range 48 VIN (PoE); 18VIN (ac adapter); 12 VIN (silver box,intermediate bus); and 28 VIN (militaryand aerospace).

Initially available in a “Full-VIC”package, V·I Chips occupy only 1 in.2of pc-board area at power densities of

●●● TECHNOLOGY REPORT / POINT-OF-LOAD CONVERTERS

Factorized bus(e.g. 48 V)

Regulated load voltageVOUT

(e.g. 1.5 V)Pre-regulator module

Isolation and transformation

Voltage transformation module

Wide-rangedc bus

(e.g. 36to 75 V dc)

NP NS VOUT

+

–

+

–

+

–

K = VOUT/VF

Transformation factorK = VOUT/VF

Regulation

L VF Load

ZIOS GUI

I2C bus

Pc board

Z-Series digital power manager Z-One digital bus

Intermediatebus

3 to 13 V Up to 32Z-POLs

VOUT

48 VHostsystem

DC-DCbrick

3. The FPA approach uses unique ICs and a unique architecture that offersadvantages over the conventional bus converter-POL approach.

4. Power-One’s Z-One system includes a digital power manager, Z-One digitalbus, 48-V bus converter, and up to 32 Z-POSs that integrate digital PWMs.

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up to 1000 W/in.3 Mounting options include BGA in-board mounting and above-board J-lead configura-tions. To address lower power applications, future“Fractional VICs” will occupy just a fraction of a Full-VIC package’s area.

DIGITALLY CONTROLLED POL CONVERTERS • Power-One invested in silicon-based technology to developits Z-One architecture, which integrates a power sys-tem’s management and power-conversion functions.The company claims this cuts overall system-levelcosts by 20% to 50% compared with more conven-tional approaches. It enables 32 POL converters tofully communicate with each other under the controlof a digital power manager (DPM). Each digital Z-Point-of-Load (Z-POL) converter can operate with a3- to 13-V input and provide a programmable 0.5- to5.5-V dc output.

The Z-One system uses a single-wire Z-One DigitalBus controlled by the DPM (Fig. 4). This high-speedbidirectional bus provides both frequency synchroniza-tion and data transfer, and it can access all Z-POL con-verters in one communication cycle. The bus carries allinformation to and from the Z-POL converters andDPM, including all operating parameters for each POLconverter.

Operational parameters, such as the output voltage,sequencing, tracking, monitoring, interleaving, andprotection thresholds, are user-programmed via agraphical user interface (GUI) and stored in the DPM.Control of this power-management system is furnishedby the GUI-based Z-series Intelligent Operating System(ZIOS).

At system startup, this stored information programsthe Z-POL converters. After system programming,ongoing communications between the DPM, Z-POLs,and host system support intelligent operation. Design-ers can repeat this point-and-click process for power-system optimization. The GUI need only interface withthe power system during programming, and it can beeasily reconnected to support power-system changes.

Communications with the host system use the indus-try-standard I2C bus communication protocol. Besidesremote programming of each POL, the latest statusinformation of each POL (output voltage, output cur-rent, and temperature) is stored in the DPM and can betransmitted to the system.

Hardware for the Z-One systemintegrates two ASICs—the ZM7100DPM and the maXyz ZY7000 seriesof Z-POL converters. The ZM7100DPM programs and controls sequenc-ing, tracking, supply supervision, andintelligence with the aid of analog-to-digital converters.

THE TRADITIONAL APPROACH • Oneof the traditional power-supply com-panies is SynQor. The company’sNiQor series comprises non-isolated

buck POL converters. NiQor modules employ syn-chronous rectification for very high efficiency. Dissipa-tion throughout the converter is low, so it doesn’trequire a heatsink or metal baseplate for operation.The NiQor series includes SIPs and SMT using stan-dard footprint and pin-out configurations.

These POLs have a trim adjustment that lets usersadjust the output voltage with an external resistor. Forexample, the NQ12T50SMA16 module accepts a 9.6-to 14.4-V input and provides a nominal 0.725 V at a 16-A output. Thanks to a single external resistor, users canincrease the output voltage above its nominal setpoint.

Among its protection features is undervoltage lock-out, which turns off the converter if the input voltage istoo low and prevents input system instability prob-lems. This family of converters employs foldback cur-rent limiting at about 125% of rated current.

●●● TECHNOLOGY REPORT / POINT-OF-LOAD CONVERTERS

Copper drain clip Passivated die Die attachmaterial

Gate connection Copper track on board

Sourceconnection

5. The DirectFET, with pads that mount to a pc board, can be cooled from boththe top and bottom of its case.

The Point of Load Alliance (POLA) comprises a number ofleading companies, including Artesyn Technologies,Emerson’s Astec Power Division, Ericsson PowerModules AB, and Texas Instruments. The organization’sgoal is to promote pin-compatible, advanced non-isolat-ed, POL plug-in power modules. POLs offer pin-compati-ble footprints that provide the same functionality andform factors, ensuring full interoperability and true sec-ond sourcing.

In December 2004, POLA announced that it will joint-ly design and release products in accordance with thePMBus (Power Management Bus) digital interface andcontrol specification during 2005. PMBus will permitflexibility in the digital control of power-conversion prod-ucts.

The PMBus founding group consists of companiesfrom POLA and the semiconductor industry, includingIntersil Corp., Microchip Technology Inc., SummitMicroelectronics, Volterra Semiconductor, and ZilkerLabs Inc. Its goal is to release an open protocol support-ing communication to power supplies via a digital bus.Implemented over the industry-standard I2C serial bus,this protocol will establish a common command set forconfiguring, controlling, and monitoring dc-dc convert-ers. However, it will not dictate how those commands aregenerated. ED Online 9653

POLA AND THE POWER-MANAGEMENT BUS

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SynQor also produces bus converters, like the 240-W BusQor BQ55090QTA27 with 2000-V input-out-put isolation. Its efficiency is 96% at full rated loadcurrent. This board-mountable, isolated, fixed-switch-ing-frequency dc-dc converter uses synchronous rectifi-cation to achieve its high conversion efficiency. Pack-aged in a standard quarter-brick module, the isolatedBusQor series steps down 48 V to an unregulated 9.6-V (7.2 to 11 V) intermediate bus. BusQor convertersare well-equipped to create the mid-bus voltagerequired to drive POL (non-isolated) converters inintermediate bus architectures.

Its protection features include input undervoltagelockout and overvoltage shutdown, which protectsagainst abnormal input voltages. It also includes latch-ing output current limit and short-circuit protection,along with latching thermal shutdown.

The POL converters in Tyco Electronics Power Sys-tems’ Austin MiniLynx series are available with an inputvoltage range of 2.4 to 5.5 V dc or 8.3 to 14 V dc (Fig.6). At 12 VIN, they support intermediate bus voltagearchitectures and POL currents up to 3 A with an outputvoltage range from 0.75 to 5.5 V dc. At 3.3/5 VIN, theysupport output voltages from 0.75 to 3.3 V dc with out-put current capacity up to 3 A without any thermal der-ating at 85°C and zero airflow. Efficiencies of 91% forthe 12-VIN modules and 94% for the 5-VIN modules areachieved when tested at nominal input conditions andfull rated output current.

Offered in SMT packages, both versions are pin-for-pin compatible with the widely used 5-A Austin Micro-Lynx converters. With output voltage programming byan external resistor, the Austin MiniLynx modules haveremote on/off (negative or positive) capability, making iteasier for designers to implement power on/off control.

Additionally, Austin MiniLynx modules provide amigration path to higher-current modules in the samepin-outs. They offer an alternative to discrete designson boards by saving design time and providing low-costsolutions as well. These converters measure 0.80 by0.45 by 0.286 in. (20.3 by 11.4 by 7.27 mm) and areoffered in tape and reel for automated production.

Robust construction enables the modules to operate intemperatures ranging from –40°C to 85 °C. They alsocan withstand industry-standard handling, cleaning, andsoldering processes. Additional protection featuresinclude overcurrent, overtemperature, and undervoltagelockout.

AT THE COMPONENT LEVEL • Power MOSFETs are akey component for virtually all POL converters.They’re usually the semiconductors within a POL con-verter that consume the most power, so their perform-ance has a major impact on circuit efficiency.

There’s also a need to fully optimize the overall pow-er-management solution. This requires selecting thecorrect control and driver ICs to work with the MOS-FETs. Inadequate controllers and drivers can causeadditional power dissipation in the MOSFET as well asin the driver itself. For the past two years, MOSFETmanufacturers have pursued various packaging tech-niques to improve MOSFET performance, such as ballgrid array (BGA), “bottomless” packages, and theDirectFET.

The International Rectifier DirectFET combinesHEXFET power-MOSFET silicon technology withpackaging that provides low on-state resistance in apackage featuring the footprint area of an SO-8 andonly a 0.7-mm profile (Fig. 5). The DirectFET packageis compatible with existing layout geometries used inpower applications; pc-board assembly equipment; andvapor-phase, infrared, or convection soldering tech-niques. The DirectFET product family is also opti-mized to work with IR’s latest-generation controllersand drivers, including X-Phase and the bus converterchip sets.

With DirectFET packaging, dual-sided cooling canmaximize thermal management and improve powerdensity by a factor of two. It also cuts board spaceand component count from 50% to 60%, and itreduces junction temperatures up to 50�C. Reducedlosses and improved thermal performance foster highefficiency and low temperatures, and thereforeenhance system reliability, suiting this device for POLconverters.

Initially, IR introduced DirectFETs with 20- to 30-Vratings. Recently, the company introduced 40- and100-V parts. The higher-voltage MOSFETs employ thesame packaging concept and allow the benefits of theDirectFET technology to be employed in a broaderrange of applications, including 48-V input isolated dc-dc converters.

One of the DirectFET’s major accomplishments isreduced on-state resistance, which includes the siliconitself as well as the means for connecting to externalconnections. For example, an SO-8 power MOSFETusually has about 1.4 mW of resistance for wire bondsand lead resistance. With the DirectFET, the equivalentresistance is only about 0.1 to 0.15 mW. The result is alower overall on-resistance.

Lower on-resistance and improved thermal resist-ance breed higher-efficiency MOSFETs. This letsdesigners operate the MOSFETs at a lower junctiontemperature for a given power level. Or, they can drivethe MOSFET harder without exceeding its maximumrated junction temperature, increasing system-levelpower density.

Currently, there’s no second source for the Direct-

●●● TECHNOLOGY REPORT / POINT-OF-LOAD CONVERTERS

POLs ACCEPT POWER INPUT from an isolated dc-dc converter and provide dc power to close-proximity loads. This costs less than an isolated dc-dc converter appraoch.

0750_ED_VICO 3/4/05 11:14 AM Page 52

FET. However, International Recti-fier has established similar produc-tion facilities in the U.S. andEurope.

ICs SUPPORT POL SYSTEMS • Inputvoltage for POL systems may rangefrom 5 to 12 V. In a system upgrade,a 5-V dc supply may be available,and a specific load could require lessthan 1 V. Linear Technology’sLTC3418, scheduled for introduc-tion in the first quarter of 2005, fitsthis type of application.

This 4-MHz, monolithic, syn-chronous step-down dc-dc con-verter uses a constant-frequency,current-mode architecture. Oper-ating from a 2.25- to 5.5-V inputrange, it provides a regulated out-put voltage from 0.8 to 5 V whiledelivering up to 8-A output.Switching frequency is set by anexternal resistor or synchronizedto an external clock. OPTI-LOOPcompensation optimizes the tran-sient response over a wide range ofloads and output capacitors.

Linear Technology also will intro-duce ICs that fit bus converterapplications, such as the LTC3706,a PolyPhase secondary-side con-troller for synchronous forwardconverters. When used with thecompany’s LTC3705 gate driver

and primary-side controller, thecombination creates a complete iso-lated power supply that combinesthe power of PolyPhase operationwith the speed of secondary-sidecontrol.

The LTC3706 simplifies thedesign of highly efficient, secondary-side forward converters. TheLTC3705 and LTC3706 form arobust, self-starting converter thateliminates the need for the separatebias regulator commonly used insecondary-side control applications.In addition, a proprietary schememultiplexes gate drive signals and dc

bias power across the isolation bar-rier through a single, tiny pulsetransformer.

Another interesting IC for POL-system applications is theLTC3736-1. This two-phase, dualsynchronous step-down switchingcontroller has tracking that drivesexternal complementary powerMOSFETs. Its constant-frequency,current-mode architecture withMOSFET drain-to-source voltage(VDS) sensing eliminates the needfor current-sense resistors, loweringcost and boosting efficiency. Oper-ating the two controllers out ofphase minimizes power loss andnoise caused by the input capaci-tor’s equivalent series resistance.

The LTC3736-1’s unusualspread-spectrum architecture ran-domly varies the switching frequen-cy from 450 to 580 kHz, signifi-cantly reducing the peak radiatedand conducted noise on both theinput and output supplies. Thismakes it easier to comply withinternational electromagnetic-inter-ference standards. Pulse-skippingoperation improves efficiency atlight loads and 100% duty cyclecapability offers low-dropout oper-ation.

ED Online 9654

●●● TECHNOLOGY REPORT / POINT-OF-LOAD CONVERTERS

6. The Austin MiniLynx series from TycoElectronics Power Systems offers 91%and 94% efficiencies in a 0.80- by 0.45-by 0286-in. package. Input voltageranges include 2.4 to 5.5 V dc and 8.3to 14 V dc.

Copyright © 2005 by Penton Media, Inc.

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