A 900W, 300V to 50V Dc-dc Power Converter with a 30MHz ......
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A 900W, 300V to 50V Dc-dc Power Converterwith a 30MHz Switching Frequency
John S. Glaser (firstname.lastname@example.org)Jeffrey Nasadoski
Electronic Power Conversion LabGeneral Electric Global Research
Richard HeinrichNaval Electronics & Surveillance Systems
Lockheed Martin Corporation
AbstractDesigners of power conversion circuits areunder relentless pressure to increase power density whilemaintaining high efficiency. A primary path to higherpower density is the use of increased switching frequency. Inthis paper it is argued that the use of switching frequenciesin the VHF band (30MHz-300MHz) are a viable path to theachievement of substantive gains in power density. Evidencefor this viewpoint is presented in the form of an unregulated900W prototype dc-dc converter with a 30MHz switchingfrequency, an input voltage range of 270VDC to 330VDC,and an output voltage of 50VDC. This converter uses a quadmodule architecture with series input and parallel outputto provide acceptable efficiency with the specified inputvoltage range. This converter operates with peak outputpower of 1kW at 330VDC input, and has an efficiency of>78% under nominal conditions, with maximum efficiencynear 80%.
Designers of DC-DC power converters are under re-lentless pressure to increase power density, efficiency,and reliability, reduce cost, and improve transient re-sponse, preferably achieving all these goals simulta-neously. In reality, all these parameters must be bal-anced in a manner that best meets the needs of theapplication, and certain goals will have priority basedon that application. For example, aerospace applicationsoften have restrictions on overall system mass, whichresults in high power density as a primary goal. Thispaper promotes the use of switching frequencies in theVHF (very high frequency, 30MHz-300MHz) band as apromising approach to provide substantial gains in power
density.A key contributor to power converter volume is the
required energy storage, normally implemented with ca-pacitors and inductors . For a given energy storagetechnology, the size of the storage elements is usually amonotonically increasing function of the energy stored.Increased power density requires reduced stored energyand/or increased storage density. The latter is subject tofundamental material limitations such as breakdown volt-age and permittivity for capacitors, and saturation fluxdensity and permeability for inductors . Improvementin the properties of magnetic and dielectric materials is aslow process. The alternative to increased energy densityis the reduction of the stored energy per cycle. Thisis accomplished by increasing the switching frequencyFSW .
This works up to a point, but as FSW continues toincrease, increased switching losses, proximity lossesand core losses in magnetic components, and problemswith parasitic reactances diminish these gains. Whilethese issues can be mitigated to some extent, at a highenough frequency they dominate the converter design.Further increases in FSW eventually lead to reducedpower density [1, 3, 4]. It has been proposed that alarge jump in switching frequency to the VHF band anda move to a new set of topologies and architectures canprovide a way to move beyond the limitations of presenttechnology.
The paper gives a short overview of approaches toVHF dc-dc power conversion. It shows an approach to
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increase the practical bus voltage for such convertersby a factor of two or more. Experimental evidence forthis approach is presented in the form of an unregu-lated 900W prototype dc-dc converter with a 30MHzswitching frequency, an input voltage range of 270VDCto 330VDC, and an output voltage of 50VDC. Thisconverter uses a quad-module architecture with seriesinput and parallel output to provide acceptable efficiencywith the specified input voltage range. This converter hasan efficiency of >78% under nominal conditions, withmaximum efficiency near 80%.
II. MOTIVATION FOR VHF SWITCHING FREQUENCIES
The point was raised that increased power density viaincreased switching frequency has practical limitations.Thus, how does a >10X jump in FSW provide a patharound these limitations?
Figure 1 shows an estimated plot of power densityversus FSW . Some caveats regarding this plot are inorder: 1) This plot is meant to illustrate general trends,so please consider the numbers to be factor-of-twoestimates, 2) the numbers are representative of kilowatt-scale converters with bus voltages of 300V, and 3) powerdensity numbers are fuzzy by nature.
For conventional PWM switching converter technol-ogy, increased FSW leads to increased power densityat lower frequencies, but the slope flattens out in therange of several hundred kilohertz. The main causes aremagnetic component losses and semiconductor switchinglosses. Magnetic cores show increased losses and mustbe operated at lower flux densities, increasing componentsizes. Winding eddy current losses, especially proximitylosses, become large and difficult to mitigate. Switchinglosses become large, and circuit parasitic componentscontribute greatly to switching device stress.
At a few MHz, one needs fully soft-switched convert-ers, with a large penalty in device stress and conductionloss, along with single-layer magnetics with minimalproximity losses, and these requirements result in asignificant size penalty. The result is that power densityis actually reduced.
Beyond a few MHz, one has already incurred theconduction loss penalty of full soft-switching, so to firstorder, switch losses no longer increase with FSW . Wecan then bound the problem by assuming the use of
Figure 1. Estimate of dc-dc converter power density entitlement versusFSW for conventional and fully soft-switched converter technologies.
single layer air core inductors. It was proposed that fora constant heat flux, a constant Q, and a solenoidalair core winding, the inductor volume varies inverselywith
FSW . Using some estimated inductor sizes
based on , and inverter topologies like [7, 8], whichare in turn based on Class E/DE [9, 10, 11], we canvery roughly estimate the power density of convertersbased on such magnetics, allowing one to position aline representing the limitation presented by air coreinductors. Recent developments in magnetic materialsmay allow a reduction in inductor volume . Forthis reason and based on available semiconductors, FSWnear 30MHz is a promising area to investigate. Differentvoltages and powers will affect the breakpoints in thechart, but the general concept remains valid.
III. APPROACHES TO VHF POWER CONVERSION
Dc-dc converters can in general be modeled as aninverter which generates an AC power signal, followedby a rectifier and filter to convert the AC power signalback to DC. In the VHF range, inverters and rectifiersemploy soft switching for both turn-on and turn-off, sothat switching losses are kept at acceptable levels. Themost common inverter topologies used in the HF or VHFband are either based on class D, E, or DE topologies[9, 3, 13, 7, 10, 11, 14]. Class E and DE topologiesare distinguished by the use of resonant waveforms andswitch transition timing such that all switching transitionsare soft, and that any anti-parallel diodes of switches donot conduct. The latter means that bidirectional switchesare not required, and implies the absence of reverserecovery losses. In Class DE, peak voltage stresses on theswitches are advantageously limited to the bus voltage,but driving a high-side switch with the precise timing
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required becomes difficult as FSW and the bus voltageincreases.
Class E avoids the high-side drive issue via the use ofa single-ended ground-referenced switch, but the trade-off is high device voltage stress. Furthermore, classE inverters are characterized by a fixed relationshipbetween FSW , switch capacitance CSW , dc bus voltageVDC , and power P :
P = K1FSW CSW V 2DC (1)
where K1 is a constant determined by the converterpassive component values. It can further be shown thatthe class E inverter loss due to the switch resistance RSWcan be approximated by
Ploss = K2P 2
V 2DCRSW (2)
where K2 is another constant determined by the con-verter passive component values. Finally, the normalizedloss is
= K1K2FSW RSW CSW (3)
The RSW CSW product is a figure of merit for a givenswitch technology. Thus, the maximum class E converterefficiency is rigidly tied to the converter specificationsand the switch technology. An illustrative example canbe given by considering a 1kW high-Q class E inverterwith a 300VDC nominal input, and a 90% drain effi-ciency with all losses in the switch. The resulting switchrequirements translate to a 1200V switch with CSW