Advanced Power Modules forTelecom ApplicationsDe-regulation and competition in wireline and wireless infrastructure telecommunications systems hasaccelerated the need for lower cost equipment solutions with ever-increasing bandwidth. Increasingly,designers are asked to provide more voltage rails for a variety of digital signal processors (DSP), fieldprogrammable gate array (FPGA), application Specific integrated circuit (ASIC) and microprocessors. Inshort, they are required to generate more voltages, at higher currents, more efficiently, with less noise, in asmaller space. And, if that wasn’t challenge enough, the solution has to cost less, too! This article provides abrief understanding of the evolution of board-mounted power systems, and how the latest generation canachieve higher performance and lower cost – in a smaller footprint. Brian Narveson and Adrian Harris,Texas Instruments Inc., USA

Deploying access equipment closer tothe subscriber requires smaller enclosures(pad and pole mounting) that must survivein a tougher environment. Infrastructureequipment is being designed for smallerfootprints as central office space comes ata premium. Driving power managementfactors are size, thermal management, costand electrical performance (regulation,transient response and noise generation).

Addressing various issuesThe need to address size, efficiency and

cost simultaneously has ignited a need totake another look at power architectures.The first generation of board-mountedpower used a power architecture known asa distributed power architecture (DPA) (seeFigure 1). This architecture used an isolated(brick) power module for every voltage rail.It worked well when there were limitedrails, but cost and printed circuit boardspace increased significantly with eachadded voltage rail. Sequencing of thevoltage rails was also difficult and requiredadding external circuitry which, in turn,added cost and board space.

To deal with the size and cost constraintsof DPA, second generation systems movedto a fixed voltage intermediate busarchitecture (IBA). An IBA (Figure 2) uses asingle, isolated-brick power module andmany non-isolated, point-of-load (POL)DC/DC converters. The POLs can be eitherpower modules, such as TI PTH series, ordiscrete buck converters. The isolatedconverter works over the same input voltagerange as the first generation, either 36 to75V or 18 to 36V. It creates an intermediatebus that is regulated to 3.3, 5 or 12V. Thevoltage choice is up to the system designer.This design results in less board space, lowercost and easier sequencing of the voltages,

due to features such as Auto Track. The onlydrawback of this architecture is reducedefficiency due to the double conversionrequired for each voltage.

Today, most telecom systems use afixed-voltage IBA. But, as small pad andpole-mounted access equipment designsevolve to sealed enclosures, with no forced

Figure 1: Typical DPA architecture

Figure 2: Fixedvoltage IBA

www.ti.com POWER MANAGEMENT 31

Power Electronics Europe Issue 2 2007

air cooling, it creates a need for a higherefficiency and smaller footprint solution. Asevery designer knows, the best way to getrid of heat in a system is not to create it.Since all of the power goes through thefront-end isolated converter, it is the mainfocus when looking for efficiencyimprovement. The proven way to increaseisolated converter efficiency is to run it at afixed duty cycle and not regulate theoutput. That led to the unregulatedintermediate bus architecture (Figure 3).

This architecture uses an unregulatedbus converter where the output voltage is aratio of the input voltage. In the example,an ALD17 5:1 converter creates an outputvoltage that is one-fifth of the input. Thistechnique creates a design where a 150Wsystem/board now can be designed with aone-sixteenth brick and achieve 96%efficiency for the first conversion stage.Unregulated voltage buses becamepossible when wide input (4.5 to 14V)PWMs and power modules such as TI’s T2products were introduced. This architectureis limited by the maximum input range ofthe bus converters, which is 36 to 55V. Thisis necessary to keep the input voltage to

POLs less that 12V. The 12V maximum isnecessary because for POLs to generate 1Vor less output voltages, the input voltagecannot exceed 10 to 12 times the output.However, an increasing number of telecomOEMs are considering a move to thislimited input range. That is due to thesignificant cost savings, size reduction andefficiency improvements obtained with thisarchitecture.

Some telecom OEMs insist onmaintaining the traditional wider inputvoltage specification of 36 to 75V withinput transients to 100V. For theserequirements, the power industry hasresponded with the quasi-regulated IBA(Figure 4). The main difference from thisand the unregulated IBA is that if the inputvoltage exceeds 55 to 60V, the outputvoltage is regulated to around 10V. Thedrawback of this approach is that theisolated power module must increase insize to accommodate the regulationcircuitry, and its efficiency is reduced above55V. An example of this kind of product isthe TI PTQB series.

To provide a meaningful comparison, theexample in each figure has identical voltage

and current requirements. It is based on atheoretical networking card utilising multiplehigh performance DSPs with associatedanalog and digital circuits. The outputvoltages are 3.3V at 5A, 2.5V at 6.5A, 1.8Vat 11A and 1.2V at 20A. For a comparisonof the architectures described above, seeFigure 5. The graphs indicate that theultimate dream is, indeed, possible. Aquasi-regulated or unregulated powersystem can provide higher efficiency, in lessspace – and at lower cost. The mostnotable improvement from the secondgeneration fixed-IBA to the quasi/unregulated-IBA is efficiency. As shown inFigure 5, power conversion efficiencyincreased almost 7%. This translates to athermal load reduction of 14W for a 200Wsystem.

We used power modules in theseexamples because they provide thegreatest power density, and are the solutionof choice at many telecom OEMs (pricingfor POLs is based on 1K distributorquantities). OEM pricing would be less.Discrete POLs can be used in all systems,but the board space increases by at a factorof 2, although cost is reduced.

Electrical performanceThe remaining challenge for the designer

is to meet the increasing electricaldemands at the heart of each system –high performance DSPs and ASICs. Primaryperformance issues are voltage regulation,current transient response and noise.

Regulation and current transientresponse are closely linked. In order to gethigher performance, with lower power in asmaller size, digital semiconductors arefabricated with smaller geometry transistorsthat require ever decreasing voltages. Sub-1V core voltage requirements are nowbecoming the standard. Along with this lowvoltage have come increasingly tightertolerances. It is now common practice tospecify a total voltage tolerance of 3%including line (variations in input voltage),load (small deviation in load current), time,temperature and current transients. Thisleaves the power designer with only 30mVof head room to accommodate everythingthe digital world throw at him. About half ofthe tolerance budget (15mV) is usuallyabsorbed for the DC parameters of line,load, time and temperature. The remaining15mV is then available to deal with sudden(1 to 3 clock cycles) changes in currentdue to computational or data transmissionsloads.

This creates power system designchallenges to minimise voltage deviation inthe presence of these current transients. Ifthe core voltage (VCC) exceeds the specifiedtolerance limits, the digital IC may initiate areset or have logic errors. To prevent this

32 POWER MANAGEMENT www.ti.com

Issue 2 2007 Power Electronics Europe

Figure 3: UnregulatedIBA

Figure 4: Quasi-regulated IBA

occurrence, designers need to pay closeattention to the transient performance ofthe POL modules being used. Digital loads,such as the latest Gigahertz DSPs, requireextremely fast transient responses with verylow voltage deviation. To achieve thesetargets, many additional output capacitorsare usually added to the DC/DC converterto provide hold-up time until its feedbackloop can respond. The power module,including this added capacitance to meettransient voltage tolerances, represents thecomplete power solution.

Capacitors have been evolving over theyears, with volumetric efficiencies gettingbetter all the time. Even with highervolumetric efficiency, the overall powersolution can be over twice the size of thepower module alone. This requires a large

PCB allocation that is usually not availablein today’s physically smaller systems.What’s more, the bill of material (BOM)costs of VCC power can be more thandouble the cost of the power modulewhen adding in the cost of capacitors.

With innovations in DC/DC powermodule technology, system designers arenow able to achieve faster transientresponse and less voltage deviation usingless output capacitance. An example is theT2 series next generation PTH modules(Figure 6). These devices incorporate anew feature called TurboTrans technology.This patented technology allows thedesigner to custom tune the module tomeet a specific transient load requirement.Tuning is accomplished with a singleexternal resistor.

TurboTrans can achieve up to an eight-times reduction in output capacitance. Thisfeature saves the cost of capacitors andPCB space. Another benefit of thistechnology is enhanced stability with ultra-low equivalent series resistance (ESR)capacitors. Designers can use newerOscon, polymer tantalums or all ceramicoutput capacitors without stability concerns.This allows the designer to use capacitortechnologies capable of withstanding hightemperature, lead-free solder profiles.

The final performance hurdle for isolatedand POL converters is noise. Whenswitching POLs run at different frequenciesand share a common input bus,frequencies resulting from the sum anddifference of those frequencies can createbeat frequencies that make EMI filteringdifficult. As an example, if a system has twoPOLs with one operating at 300kHz and asecond at 301kHz, the beat frequency is1kHz. This can require larger, morecomplex system filters. T2 power moduleshave a SmartSync feature that lets thedesigner synchronise the switchingfrequency of multiple T2 modules to aspecific frequency. Synchronised moduleseliminate beat frequencies and make EMIfiltering easier. SmartSync can be used toset the frequency, so switching noise is outof a particular frequency band (i.e audiofrequencies). TurboTrans and Smart Syncare standard features on T2 powermodules that add no additional system costto the systems described earlier.

ConclusionA telecom system built with state-of-the-

art power modules allows the systemdesigner to reduce system size, decreasedissipated power, meet the powerdemands of high performance digitalcircuits and reduce the cost of powercompared to regulated voltage IBAsystems.

www.ti.com POWER MANAGEMENT 33

Power Electronics Europe Issue 2 2007

Figure 6: T2 Series Power Modules withTurboTrans

Figure 5: Comparisonof architectures interms of cost (upper),real estate (middle),and efficiency (lower)