TECHNOLOGY LEADER SERIES

The tradeoffs are clear betweenthese two alternatives. Linear sup-plies, especially at the kilowatt level,need large, expensive transformers—making them bulky, heavy and expen-sive. As a result, they typically costsignificantly more than comparableswitching supplies. High-power linearsupplies also achieve an energy con-version efficiency that is typically lessthan 60%, adding to the cost of run-ning the supply. Shipping costs forthis supply option adds to the ex-pense, since large linear power sup-plies can weigh more than 65 pounds.

You do get cost benefits fromusing switching power supplies,which operate by interrupting(switching) the rectified line powerto create a high-frequency source forvoltage conversion. The high fre-quency makes the step-down trans-former stages more efficient, allow-ing the use of smaller ferritetransformers, while typically achiev-ing power conversion efficiencies of80% or greater. But the transientsgenerated in the switching processare normally difficult to remove, re-sulting in noise levels that are typi-cally much higher than in linearpower supplies.

Therein lies the dilemma. "Manycommunication and medical systems,especially analog, need clean power,"says Greg Von Rehder, product mar-keting manager for B+K Precision,

Yorba Linda, CA. He adds that, assystem designs have become more dig-ital, they are more tolerant of powernoise. Even so, the noise levels of con-ventional switchers are still too largefor many applications, says Von Re-hder. That has forced many engineersto remain with linear power designs,which have noise levels around 1 mil-livolt RMS, despite their cost, size andweight disadvantage.

B+K's third optionNow, thanks to B+K Preci-

sion's new VSP series high-power DC supplies, engi-neers are gettinganother option."Using newer switchmode technology, we canoffer low-noise switchingsupplies that cost around 25%less than linear supplies with thesame power," explains Von Rehder.

The new power supplies pack asmuch as 1.2 kilowatts into a 19-inch, rack-mounted box that mea-sures just 1U (1.75 inches) in height.What's more, the new productachieves an energy conversion effi-

ciency of 80%, while keeping noiselevels under 20 millivolts. B+K, whichsells the power supply through a widenetwork of distributors, sees a broadrange of applications—ranging fromautomatic test equipment to R&Dlabs to telecommunications.

Behind the performance advan-tages of the VSP series are advances inswitching techniques. Two are partic-ularly significant. One is soft switch-ing; the other is a two-device asym-metric half-bridge DC to DCconverter design.

As shown in Figure 1, the suppliesfirst generate a power-factor-cor-rected, high-voltage DC source fromthe line voltage. It then goes to the

DC to DC converter to obtain avoltage near the desired output

level. The converter can op-erate in constant-voltage

or constant-currentmode. It also pro-vides a fast loadtransient response,

moving from 60% to90% of full load in 100

microseconds.The converter's soft switch-

ing technique is a vital step for re-ducing switching noise. This tech-nique ensures that the switchingaction will occur when the voltageacross the switching device is at aminimum. By turning the switchingdevice in the converter on and off

Now you can have power—Without all the noise

When engineers go shopping for power supplies, the classic choice fac-ing them has always been either linear or switching designs. Linear sup-plies produce a clean DC ouput—but the downside is lower efficiencyand relatively high cost. On the other hand, switching supplies that pro-duce the same wattage are more efficient and less expensive, but theyalso deliver high-frequency noise along with the DC power.

Packing a lot into a small box, the new VSP power supplies offer 1.2 kW of DCpower in less than two inches.

B+K Precision's new DC power supplies generate kilowatts of power with only millivolts of noise

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A D V E R T O R I A L

T E C H N O L O G Y L E A D E R S E R I E S

TECHNOLOGY LEADER SERIES

when there is little voltage across it,the transformer load does not seesharp voltage transients. Eliminatingthat transient gets rid of much of thehigh-frequency system noise thatwould otherwise propagate throughthe transformer to the output stage. Italso helps reduce the noise that wouldtypically feed back to the source. Abuilt-in RFI filter further reduces con-ducted power line noise emissions, aswell as susceptibility, allowing thesupplies to meet EN55022 Class Astandards.

The VSP series further reducesthe converter's noise by using a"piggy pack" linear regulator to fol-low the conversion stage. The totaleffect is to improve the transient re-sponse to the changes in load and toreduce output noise and ripple fromthe DC converter to a maximum of20 millivolts, with 5 millivolts typi-cal at full load.

Full voltage controlAlong with controlling output

noise, the converter and regulatorallow the VSP series devices to offerprecise output voltage control. This ismanaged several ways:• Ten-turn potentiometers on thefront panel, along with three-digit me-ters, give users the ability to manually

tune the output voltage and current asneeded.• An RS-232 or optional GPIB inter-face allows that control to be per-formed remotely.• Opto-isolated analog programmingalso allows remote control of the out-put voltage/current from external ana-log voltage control signal, while keep-ing the control voltage galvanically

isolated from the power output.• Finally, the analog remote sensingfeature automatically maintains thedesired voltage at the load end of thepower cable.

The applications that the VSP se-ries targets typically need high DCvoltages at very high current levels.Supplies provide 1.2 kilowatts at20V, 40V, 60V, and 120V outputvoltages. For applications requiring

even more power,supplies can be run inparallel or series toboost current or volt-age as needed.

However, connect-ing power supplies inparallel to boost cur-rent can create somerisks. If the voltagesare not perfectlymatched, the lower-

voltage outputs look like loads toother supplies. The resultant push andpull among the supplies makes cur-rent regulation difficult.

The VSP series avoids this prob-lem. As shown in Figure 1, the unitsinclude a current share control bus.Connecting the current share con-troller bus of all parallel connectedVSPs automatically adjusts the out-

put stage so that each cascaded sup-ply equally shares the load con-nected to the paralleled VSPs. Asmany as nine units can be cascaded,providing more than 10 kilowatts ofDC power.

The small size of the VSP seriessupplies, as well as their power effi-ciency, makes them easy to cascadein a rack-mount environment. Ventsat the front and back of the enclo-sures give the fans enough airflow tokeep the units cool, so they can sim-ply be stacked on top of one another.No clearance is needed between themwhile mounted in a rack.

In short, B+K's alternative to thelinear versus switching debate inpower supplies is a convincing one.Says Von Rehder: "The outputfrom the VSP series is clean, yet theunits are small and inexpensive, es-pecially considering how muchpower they provide." �

T E C H N O L O G Y L E A D E R S E R I E S

FOR MORE INFORMATIONTo learn more about the VSP series,contact:B+K Precision Corporation22820 Savi Ranch ParkwayYorba Linda, CA 92887 Phone: (714) 921-9095Fax: (714) 921-6422www.bkprecision.com

When one is not enough, VSP units can be stackedtogether to generate as much as 10 kW.

VARIABLEDC

OUTPUT

LINEINPUT

CONTROLPOWERSUPPLY LINEAR

REGULATOR

TWO DEVICESOFT

SWITCHINGDC - DC

CONVERTER

FRONTPANEL

CONTROLS

ISOLATED/NON ISOLATED

ANALOGINTERFACE

INPUTRF

FILTER

POWER FACTORCORRECTED

HVDC GENERATION CURRENTSHARE

CONTROLLER

DIGITALINTERFACE

Figure 1: The VSP series controls switching noise by using soft-switching tech-niques in its converter and a linear regulator at the output.