Miniature solutions for voltage isolation - TI.com

1 13 Analog Applications JournalMiniature solutions for voltage isolationThe hot growth in distributed-power architecture hasfueled the development of Miniature low-power (<2-W)DC/DC converters. As their name implies, these devicesminimize the impact of the converter onboard space. Theyprovide point-of-use isolated power conversion for analogcircuitry in industrial applications and safety-critical appli-cations such as telecommunications and medical , Miniature DC/DC converters benefit designerswho need galvanically isolated output power or noisereduction in analog most modern noise-sensitive circuitry, system designersoften have a small number of components that requiretotal isolation from their input power supply. It is neces-sary to isolate the load and noise presented to the localpower-supply rails from the main-supply rails of the entiresystem. Mixed-signal integrated circuit (IC) design, forexample, frequently leads to nonfunctional devicesbecause of noise problems.

2 Large amounts of digital noisecombined with sensitive analog circuitry often results ininterference DC/DC converters with galvanic isolation offervery low output noise and high accuracy. Galvanic isolationhelps to reduce system noise by providing a floatingground on the secondary side of the isolation can then be used to provide asimple, isolated-output power source; or, it could be usedto generate different voltage rails, dual-polarity rails,and/or nonstandard voltages. Also, as a noise reductiontechnique in analog circuitry, the isolation barrier preventsnoise from the digital ground bus from affecting the sensi-tive analog standard brick-type converter is not well suited forthis requirement. In addition to its higher cost, the brick soutput power is overkill for applications requiring <2 s more, the installed size of a brick is often require miniaturization to save valuable boardreal estate.

3 Because they are significantly smaller thaneven the new sixteenth-brick format ( in2vs. in2), Miniature DC/DC converters are particularly suited to gen-erating onboard voltages in space-constrained why not roll your own DC/DC power supply? Discretecomponent onboard converter designs are a low-cost alterna-tive to bricks and off-the-shelf Miniature DC/DC , fewer designers are developing discrete designsbecause of their disadvantages. Extra functionality,such asdevice protection and module-to-module synchronization,are difficult to implement; reliability is often poor; andachieving a small-sized power supply is difficult. In addi-tion, many designers of higher-level products of whichthe DC/DC converters are only one component lack thetime to become expert enough in converter design tobuild their own devices. As a result of these challenges,more and more designers are specifying Miniature DC/DCconverters for their applications.

4 The converters offer significantly lower risk, much faster time to market, anddevelopment-cost their advantages over bricks and discrete designs, Miniature DC/DC converters also provide a compact-sizedpower solution for point-of-load (POL) power converters enable designers to overcome the chal-lenges caused by the high peak-current demands and low-noise margins of the latest high-performance semicon-ductor devices. The converters can be placed close totheir loads. This minimizes losses caused by voltage drops,helps overcome noise sensitivity and EMI emission issues,and ensures tight regulation under dynamic load s available? Miniature DC/DC converters offered by C&D Technologies,Texas Instruments, Wall Industries, and others benefitdesigners who require isolation and output current in the20- to 500-mA range. The converters are available with 5-, 12-, 15-, or 24-V inputs. Single-output converters areavailable with voltages from to 24 VDC.

5 Dual-outputconverters with voltages from to 24 VDC are DC/DC converters offer regulated or unregulatedoutputs with input-to-output isolation ratings of 1000,1500, or 3000 VDC. Operating temperature extends from 40 C to +100 C. Due to their high switching frequencies(up to 400 MHz), the converters deliver efficiencies of upto 85%.Built-in featuresFor system designers seeking to implement miniatureDC/DC converters, the devices offer various built-in featuresthat ease system integration and simplify design. Many ofthe available products incorporate thermal and short-circuitprotection and internal filtering. Some of the more advancedconverters allow device-to-device synchronization. If anapplication uses more than one converter on a PC board,Texas Instruments IncorporatedPower ManagementBy Geoff Jones (Email: Manager3Q and Mixed-Signal ProductsTexas Instruments IncorporatedPower Management14 Analog Applications JournalAnalog and Mixed-Signal 2005beat frequencies and other electrical interference can begenerated.)

6 DCP010512B and DCR010505 converters(Figure 1) overcome this problem with a built-in synchro-nization control that allows multiple converters to be syn-chronized to one another. The feature makes it easy fordesigners to synchronize up to eight devices by connectingthe SYNCIN pins together. This eliminates electrical inter-ference caused by variations in switching s DC/DC converter technologies continue to focuson higher densities and efficiencies as well as on smallerpackages. One of the more innovative converters is manu-factured with the same technology as standard IC packages,including dual-in-line (DIL) and small-outline (SO) styles+5 V+5 V 5 V+12 V+5 VNonisolatedI/O ModulesSupportOp Amp, ADC,DAC, MCU, ModulesSupportOp Amp, ADC,DAC, MCU, to30 VDC5-VPowerSupplyCreate +12-V/1-WOutput with1000-V IsolationCreate Isolated 5-V/1-W Outputfrom +5-V InputSYNCINSYNCINF igure 1. Self-synchronizing DC/DC converters indistributed-power applicationFigure 2.

7 Miniature isolated DC/DC converterin standard IC packageFigure 3. IC lead-frame(Figure 2). They use an IC lead-frame as the medium tointerconnect silicon devices and magnetic componentswithin the IC package (Figure 3). The result is an isolatedDC/DC converter that provides high reliability, excellentthermal management, small size, and compatibility withstandard board-assembly processes. The standard IC formatalso allows tape-and-reel assembly, which helps reducemanufacturing Instruments IncorporatedPower Management15 Analog Applications Journal3Q and Mixed-Signal ProductsDifferent configurationsThe galvanic isolation of the output from a miniatureDC/DC converter allows nonstandard voltage rails to begenerated by connecting multiple converters in series. Thisis accomplished by simply connecting the positive output ofone converterto the negative output of another. This con-figuration allows a wide variety of voltage variations to beproduced.

8 The outputs on some dual-output converterscan also be connected in series to provide two times themagnitude of output voltage . Figure 4 shows a dual 15-Vconverter connected to provide a 30-V converters connected in parallel often providea suitable solution for cases where a single converter isunable to deliver the required output power. When parallelconnection is used, it is always a good design practice touse parallel converters of the same type. Figure 5 showstwo converters connected in voltage isolation in low-power applicationsMiniature DC/DC converters that provide input-to-outputisolation and low output power offer innovative solutionsfor high-density power-supply designs in a variety of appli-cations. Some typical examples include industrial processcontrol, DC motor drive, test and measurement, powertransmission, medical equipment, and data +VOUT VOUT+VIN VIN5V+15 V 15 VGND30 VFigure 4.

9 Connecting dual-output convertersin series2xPowerOutDC/DCConverterDC/DCConve rter+VOUT+VOUT VOUT VOUT+VIN+VIN VIN VINSYNCINSYNCINF igure 5. Connecting multiple DC/DC converters in parallelFigures 6-8 show examples of DC/DC converters and operational amplifiers (op amps) used in common power-supply voltage conversionPositive-to-negative voltage conversion is a popular applica-tion for Miniature DC/DC converters. Precision op amps areoptimized for higher-speed applications. The devices offervery low offset voltage and drift and are commonly foundin data acquisition, telecom equipment, professional audioequipment, and portable applications requiring high preci-sion. Some op amps require complementary power-supplyTexas Instruments IncorporatedPower Management16 Analog Applications JournalAnalog and Mixed-Signal 2005rails. The isolated output of the converter in Figure 6 isused to generate a negative supply voltage .

10 The isolatednegative output is referenced to the op amp s positive and negative voltage from a single supplyMiniature DC/DC converters in standard IC packagingreducethe board space required to create positive andnegative output voltages to an op amp. Figure 7 shows theability of a single isolated converter to create positive-to-negative output voltages efficiently for a signal conditioningcircuit with differential input. The dual-output converter isused to create unregulated output voltages with a magni-tude higher than its +5-V input. Operating from an inputvoltage range of to V, the converter performs aboost function that creates a +12-V output and a buck-boost function with a 12-V isolation for AC or DC instrumentation amplifierThe challenge for system designers working with signalconditioning circuits in AC or DC instrumentation amplifiersis to eliminate ground loops that can affect measurementaccuracy.