Stresses Wide Input DC-DC Converters Experienced power supply des
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Stresses Wide Input DC-DC Converters
Stresses Wide Input DC-DC Converters
Experienced power supply designers previously working with topology turn their attention another, know they must shift mental gears quite dramatically. "rules game" change therefore major design issues will arise this this fact recognized very outset. "Equations topologies available, just needs them" correct? fact though things this Application Note also provides such design equations, that enough. Equations nature "single-point" computations. example input given operating condition: 15V, VOUT appropriate Buck converter equation calculate input capacitor current valid this specific condition. under more practical scenario suppose could vary between 22V, with just representing vague "nominal" value? What then most appropriate input voltage calculate worst-case input capacitor current? will find that equations, however complete, guides that information directly. Designer "picks" lowest input voltage Designer picks highest input voltage 22V, Designer picks nominal value 15V, them actualy wrong. correct answer here 10V. Take another example: should inductor designed highest input lowest input voltage range? Buck didn't seem matter much what input voltage design inductor, applies same nonchalance Boost Buck-Boost, there power supply through further testing. important point even while delivering this constant maximum load, internal currents power supply change their shape, peak values, average values considerably response changes input voltage. purpose this Note figure these values vary each topology thereby "worst case" design test condition each them. logical design procedure finally emerges based topology hand. mentioned, comprehensive table design information (Table provided three main topologies: Buck, Buck-Boost Boost. Unlike most other references literature, this table cast terms 'r'. This actually makes table very designer-friendly because recognizes that only real degree design freedom available inductor current ripple ratio 'r'. criterion selection fact actual value too) happens same topology, application condition, switching frequency. Once fixed (usually between 0.3-0.5), everthing else more less predetermined. only have heed appropriate input voltage which this change from
National Semiconductor Application Note 1246 Sanjaya Maniktala September 2002
topology another. will also noticed that design table includes drops across switch diode topologies, something that commonly available related literature. must realize that because evershrinking output voltages, these 'negligible' forward "drops" have actually become increasingly important today. design procedure based design table considers power supply operating constant (maximum) load with fixed output voltage, whose input voltage varied. predict response resulting variation duty cycle, thereby figure worst case input test design condition. Conclusions summarized Table equations essentially cast terms output voltage (VO), load ("IO") Duty Cycle ("D"), inductor current ripple ratio ("r"). input voltage "VIN" included directly stress formulae, intended reflect input voltage variation. most important fact keep mind this article when relating that topologies, corresponds high high (since output voltage considered fixed).
Inductor Current Waveforms
inductor current waveform consists AC/ramp component "I", DC/average component "IDC", latter being geometric center ramp. Note that literature 'AC' value usually taken half ramp, here just equating them convenience. essential difference between topologies that Buck, average inductor current equals load current times, Boost Buck-Boost, average diode current that equals load current. from Table shown that constant BUCK
clear that average inductor current Boost Buck-Boost becomes very high approaches Remember that this corresponds decreasing input voltage "VIN_MIN". Therefore inductor design must conducted lowest input voltage these topologies. Buck there hardly dependency inductor current input voltage since average current depends only load (which considered fixed analysis). Buck regulator, first pass selection, often simply pick inductor with current rating equal load, irrespective input voltage. These variations included plots shown Figure Read these consultation with listing provided Table
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2002 National Semiconductor Corporation
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Inductor Current Waveforms
(Continued)
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FIGURE TABLE Worst Input Voltage Condition Design/Test Given Parameter Parameter (IAC Inductor) Core Loss Inductor Energy/Core Saturation Average Current Inductor Current Inductor Copper Loss/Temperature Inductor Current Input Capacitor Input Voltage Ripple Current Output Capacitor Output Voltage Ripple Current Switch Average Current Switch Buck VIN_MAX VIN_MAX VIN_MAX/VIN VIN_MAX/VIN VIN_MAX/VIN VIN_50 VIN_MAX/VIN VIN_MAX VIN_MAX VIN_MIN VIN_MIN Boost VIN_50 VIN_50 VIN_MIN VIN_MIN VIN_MIN VIN_MIN VIN_50 VIN_MAX VIN_MIN VIN_MIN VIN_MIN VIN_MIN Buck-Boost VIN_MAX VIN_MAX VIN_MIN VIN_MIN VIN_MIN VIN_MIN VIN_MIN VIN_MIN VIN_MIN VIN_MIN VIN_MIN VIN_MIN
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Inductor Current Waveforms
Parameter Peak Current Switch/Diode/Inductor Average Current Diode Temperature Diode Worst Case Efficiency
(Continued)
TABLE Worst Input Voltage Condition Design/Test Given Parameter (Continued) Buck VIN_MAX VIN_MAX VIN_MAX VIN_MAX Boost VIN_MIN VIN_MIN Buck-Boost VIN_MIN VIN_MIN
Numbers columns refer corresponding numbered curves Figure means input voltage appropriate VIN_50 input voltage which D=0.5
component inductor current, "IAC", cannot fully ignored even Buck. This parameter important, firstly, because along with IDC, determines peak value inductor current. This peak value needs known accurately evaluate energy handling requirement inductor (defined 1/2*L*I2PEAK). size inductor accordingly, core saturate. more importantly, topologies, this component completely responsible core loss. Core loss does depend IDC, long inductor saturating). Now, topologies, there applied voltage "VON" across inductor when switch This causes certain resulting ramp component across inductor based fundamental equation L*I/(D/f) VON*D/(L*f), where frequency. input voltage falls, VONdecreases helping lower ramp component, same time increases this helps promote ramp. interesting question asked: what eventually happens input voltage falls? equation provided Table that BUCK/BUCK-BOOST plotting these Figure that maximum highest input voltage Buck/Buck-Boost maximum VIN_50 closest voltage) Boost where VIN_50 defined here input voltage which D=50% topology under consideration. This value also provided Table input voltage range does include VIN_50, must choose either VIN_MIN VIN_MAX, whichever happens closer VIN_50. also define useful parameter called current ripple ratio which ratio value inductor current, with converter delivering maximum load.
ogy. reader refer AN-1197 deeper understanding current ripple ratio relates optimization. That particular Application Note based Buck converter, same principles apply topologies. case, allowing greater current ripple than optimum 0.3-0.5 reducing inductance) does appreciably reduce size inductor, does increase size/requirements either both input/output capacitors. Now, having designed inductor given value appropriate input voltage end, discussed earlier, vary input voltage over expected range, changes accordingly. equations Table cast essentially terms these main parameters that vary with input voltage. variation with also provided, thus making only actual variable analysis. value required inductance (based chosen "r") found Table physical size this inductor also calculated from required energy handling capability listed. More inductor design later.
Input Capacitor Currents
parameter current, "IIN", through input electrolytic capacitor. determines basic/minimum selection criterion since capacitor must rated least worst case current that pass through capacitor operated with current higher than rated value, guaranteed have specific life most manufacturers. Life expectancy temperature curves/ equations provided, then considered valid. From Table small "r", that this goes
This parameter important determines among other things, inductance "L", physical size most power components. shown that size inductor reduced increasing "r". However 0.3-0.4 represents most optimum choice topol3
Plotting these Figure that maximum VIN_50 closest voltage) Buck/Boost maximum lowest input voltage Buck-Boost temperature output capacitor must also evaluated above input voltages. input voltage
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Input Capacitor Currents
(Continued)
IPP_OUT maximum lowest Boost/Buck-Boost
input
voltage
range does include VIN_50, must choose either VIN_MIN VIN_MAX, whichever happens closer VIN_50. also concerned with peak peak current, IPP_IN through input capacitor this determines input voltage ripple VIN=IPP_IN*ESRIN, where ESRIN Equivalent Series Resistance input capacitor. This input ripple major component spectrum input power supply. From Table small "r", that this goes IPP_IN constant BUCK
Switch RMS/Avg Current
MOSFET Switch need calculate conduction loss given I2RMS*rds. crossover losses lowest minimum input voltage. since they usually small fraction conduction losses, thus ignored here. IRMS switch varies following manner. From Table small "r", that this goes
Plotting these Figure that IRMS maximum lowest input voltage Buck/Boost/Buck-Boost Plotting these Figure that IPP_IN constant/maximum highest input voltage Buck IPP_IN maximum highest input voltage Boost IPP_IN maximum lowest input voltage Buck-Boost Buck stage, input voltage ripple almost constant with respect input voltage variations, provided very small. However since does increase somewhat high input voltages, preferable evaluate this parameter highest input voltage.
Output Capacitor Currents
Output Capacitor also needs least enough handle worst case current through "IOUT". From Table small "r", that this goes IOUT BUCK
should however noted that Buck, dissipation Switch input voltages goes only slightly, remaining topologies, this dissipation expected steeply input voltages, leading large drop efficiency. Table average switch current also provided, calculation dissipation bipolar switches. shown that above conclusions also valid average value switch current (which required calculate conduction loss bipolar switch). Talking about efficiency leads other main component loss power supply, diode loss. will this varies, what implies effect input variations efficiency power supply.
Average Diode Current/Efficiency
diode need calculate forward loss given IAVG*VD, where "VD" drop across diode when conducts. Boost Buck-Boost, average diode current load current, going change with duty cycle. Buck does vary. From Table that this goes IAVG BUCK IAVG constant BOOST/BUCK-BOOST Plotting these Figure that IAVG maximum highest input voltage Buck IAVG constant Boost/Buck-Boost that dissipation switch Buck remains almost constant input voltage increases, that diode dissipation however increases expect efficiency Buck regulator fall high input voltages account increased diode dissipation. Boost Buck-Boost, diode dissipation does change input voltage falls, switch dissipation increases dramatically. expect efficiency Boost Buck-Boost fall input voltages account increased switch dissipation (unless crossover losses very large, which case reverse occasionally found true).
Plotting these Figure that IOUT maximum highest input voltage Buck lowest input voltage IOUT maximum Boost/Buck-Boost
temperature output capacitor must also evaluated above input voltages. also concerned with peak peak current, IPP_OUT through output capacitor this determines output voltage ripple VOUT IPP_OUT*ESROUT, where ESROUT Equivalent Series Resistance output capacitor. This output ripple major component noise spectrum output power supply. From Table small "r", that this goes IPP_OUT BUCK
Plotting these Figure that IPP_OUT maximum highest input voltage Buck
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Average Diode Current/Efficiency
(Continued) diode temperatures, need test Buck regulator highest input voltage. other topologies, does matter. This shown "VIN" Table implying input voltage. This also tells what input voltage need check efficiency check power supply. clearly varies from topology topology. Table
Inductor Avg/RMS Currents
small, average values inductor current same, "IL". copper loss inductor IL2*R, where winding resistance. copper loss usually very large compared core loss (which depends discussed earlier), largely determines temperature rise inductor. From Table small "r", that RMS/Avg current goes constant BUCK
Inductor Energy
"Energy Handling Capability" e=1/2*L*I2PEAK. This parameter literally "sizes" inductor given application. Note that size determined just inductance, since almost inductance theoretically achieved core, simply winding appropriate number turns complete equations provided Table analysis here, first make approximation rather complicated term involving "r". Assuming small this term becomes that Boost Buck-Boost, large, increases. Therefore when evaluating copper loss temperature rise inductor these, need minimum input voltage. Buck, since does increase with increasing input voltage, value inductor current also higher, should maximum input voltage. constant/maximum highest input voltage Buck maximum lowest input voltage Boost/Buck-Boost
From Table small "r", that Energy handling Capability goes
Peak Switch Current
This parameter important because every controller current limit switch, calculated peak exceeds lowest value possible switch current limit, anywhere input voltage range, required output power cannot delivered. peak current Buck just little higher than load current, example, LM2593HV "Step Down (Buck) regulator" from National Semiconductor, which designed load", minimum value 2.3A switch current limit. Yet, seen Figure from datasheet this device, this Buck operated "positive negative" regulator, which actually standard Buck-Boost topology. this mode, peak current values much higher, seen from Table fact depend only load, duty cycle/input voltage too. peak current values vary topologies, with changes input voltage. From Table small "r", that peak current goes
Plotting these Figure that constant/maximum highest input voltage Buck maximum lowest input voltage Boost/Buck-Boost Note that both Boost Buck-Boost, required energy handling capability increases dramatically duty cycle approaches 0.6. This known designers front-end stages. Such stages typically Boost topology, providing internal 400VDC rail from worldwide input. seen that size required inductor goes sharply minimum input voltage falls, inductor design should carried minimum input voltage. Buck, some designers maximum input voltage, some minimum, some simply nominal input voltage. really does matter much, provided remains, small assumed. reality, does increase input voltage increases (thereby causing slight increase peak value), preferable design inductor buck regulator highest input voltage.
Plotting these Figure that Boost Buck-Boost peak value switch current occurs maxi-
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Peak Switch Current
(Continued)
peak current switch
duty cycle (minimum input voltage), whereas Buck this occurs lowest duty cycle (highest input voltage). Therefore Current Limit must tested minimum input voltage Boost Buck-Boost, Buck must highest input voltage. conclude IPEAK maximum highest input voltage Buck IPEAK maximum lowest input voltage Boost/Buck-Boost Therefore designer Table calculate peak current, must lowest input voltage Boost Buck-Boost, ensure that less than current limit. Buck, peak current must calculated compared current limit highest input voltage.
setting IPEAK 2.3A, solve
0.7A assure ourselves only maximum load 0.7A this configuration. required evaluated from Table
Example
LM2593HV fixed output version) used generate output from input voltage ranging from 4.5V 20V. This Buck Regulator with switch current limit 2.3A (min). What maximum load deliver this positive negative configuration. (Assume 0.5V 1.5V). inductor design must done minimum input i.e. 4.5V Buck-Boost topology according guidelines Table this always represents optimum size inductor. worst-case peak current switch buck-boost (which this corresponds curve from Table Looking this curve Figure shows that this reaches maximum high duty cycle (low input voltage). Therefore proceed with this peak switch current calculation minimum input voltage, which will also perform inductor design. duty cycle calculated from Table
0.65
21.4 This minimum inductance application. inductance higher than this, calculated peak current exceed current limit device, causing foldback. Remaining parameters/ratings calculated similar way, looking Table using guidelines from Table Note that want estimate core losses inductor, which depends swing this maximum highest input voltage, minimum input voltage. would need first lowest input voltage, then calculate required inductance, then finally equations calculate highest input voltage. Basically forms required "bridge" from voltage another, because once value, remains Everything else change.
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FIGURE Buck Regulator Used Buck-Boost Application
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Example
Parameter Duty Cycle
(Continued)
TABLE Design Table: I/IDC, Vµsecs, voltages currents magnitudes. Buck Boost Buck-Boost
VIN_50
Output Voltage, (Vµsec)
(µH)
Current Input
Input Capacitor Current Output
Output Capacitor Energy Handling Capability (µJoules) Current Inductor Average Current Inductor Current Switch
Peak Current Switch/Diode/ Inductor Average Current Switch Average Current Diode
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Stresses Wide Input DC-DC Converters
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