EE462L, Fall 2011DCDC Buck/Boost Converter
Boost converter + Vout Iout Vin iin L1+ v L1 Buck/Boost converter
Buck/Boost converter This circuit is more unforgiving than the boost converter, because the MOSFET and diode voltages and currents are higherBefore applying power, make sure that your D is at the minimum, and that a load is solidly connectedLimit your output voltage to 90V
+ Vout Iout CVinIin L1+ 0 + 0 KVL and KCL in the average sense00IoutIinC1L2Iout+ Vin KVL shows that VC1 = VinInterestingly, no average current passes from the source side, through C1, to the load side, and yet this is a DC - DC converter
Switch closedVin iin L1+ Vin + v L2 C1+ Vin L2assume constant+ v D KVL shows that vD = (Vin + Vout),so the diode is openThus, C is providing the load power when the switch is closedVin iin L1 Vin +C1+ Vin L2+ Vout Iout C (Vin + Vout) +IoutiL1 and iL2 are ramping up (charging). C1 is charging L2.C is discharging.+ Vin
Switch open (assume the diode is conducting because, otherwise, the circuit cannot work)Vin iin L1 Vout +C1+ Vin L2+ Vout Iout CC1 and C are charging. L1 and L2 are discharging.+ Vout KVL shows that VL1 = VoutThe input/output equation comes from recognizing that the average voltage across L1 is zeroassume constant
Inductor L1 current ratingUse maxDuring the on state, L1 operates under the same conditions as the boost converter L, so the results are the same
Inductor L2 current rating2Iout0Iavg = IoutIiL2Use max+ Vout Iout CVinIin L1+ 0 + 0 00IoutIinC1L2Iout+ Vin Average values
MOSFET and diode currents and current ratings02(Iin + Iout)0Take worst case D for eachVin iin L1+ v L1 + Vout Iout CMOSFETDiodeiL1 + iL2Use maxswitchclosedswitchopen2(Iin + Iout)iL1 + iL2
- Output capacitor C current and current rating2Iin + IoutIout0As D 1, Iin >> Iout , soiC = (iD Iout)As D 0, Iin
Series capacitor C1 current and current ratingSwitch closed, IC1 = IL2Vin iin L1 Vin +C1+ Vin L2+ Vout Iout C (Vin + Vout) +Iout+ Vin Vin iin L1 Vout +C1+ Vin L2+ Vout Iout C+ Vout Switch open, IC1 = IL1
- Series capacitor C1 current and current rating2Iin2Iout0As D 1, Iin >> Iout , soiC1As D 0, Iin
Worst-case load ripple voltageThe worst case is where D 1, where output capacitor C provides Iout for most of the period. Then,Iout0iC = (iD Iout)
Worst case ripple voltage on series capacitor C12Iin2Iout0iC1switch closedswitch openThen, considering the worst case (i.e., D = 1)
Voltage ratingsMOSFET and diode see (Vin + Vout)Diode and MOSFET, use 2(Vin + Vout)Capacitor C1, use 1.5VinCapacitor C, use 1.5VoutVin L1C1+ Vin L2+ Vout C (Vin + Vout) +Vin L1 Vout +C1+ Vin L2+ Vout C
Continuous current in L12Iin0Iavg = IiniL(1 D)Tguarantees continuous conductionThen, considering the worst case (i.e., D 1),use maxuse min
Continuous current in L22Iout0Iavg = IoutiL(1 D)Tguarantees continuous conductionThen, considering the worst case (i.e., D 0),use maxuse min
Impedance matching
DCDC Boost Converter
+Vin+
Iin+VinIinEquivalent from source perspectiveSource
Impedance matchingFor any Rload, as D 0, then Requiv (i.e., an open circuit)For any Rload, as D 1, then Requiv 0 (i.e., a short circuit)Thus, the buck/boost converter can sweep the entire I-V curve of a solar panel
Example - connect a 100 load resistorD = 0.806.44 equiv.100 equiv.D = 0.50D = 0.882 equiv.With a 100 load resistor attached, raising D from 0 to 1 moves the solar panel load from the open circuit condition to the short circuit condition
Example - connect a 5 load resistorD = 0.476.44 equiv.100 equiv.D = 0.18D = 0.612 equiv.
BUCK/BOOST DESIGN
Worst-Case Component Ratings Comparisons
for DC-DC Converters
Converter Type
Input Inductor Current (Arms)
Output Capacitor Voltage
Output Capacitor Current (Arms)
Diode and MOSFET Voltage
Diode and MOSFET Current (Arms)
Buck/Boost
1.5
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5A1500F50kHz0.067VMOSFET M. 250V, 20AL1. 100H, 9AC. 1500F, 250V, 5.66A p-pDiode D. 200V, 16AL2. 100H, 9AC1. 33F, 50V, 14A p-pBUCK/BOOST DESIGN
Comparisons of Output Capacitor Ripple Voltage
Converter Type
Volts (peak-to-peak)
Buck/Boost
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40V2A50kHz200H90V2A50kHz450HMOSFET M. 250V, 20AL1. 100H, 9AC. 1500F, 250V, 5.66A p-pDiode D. 200V, 16AL2. 100H, 9AC1. 33F, 50V, 14A p-pBUCK/BOOST DESIGN
Minimum Inductance Values Needed to
Guarantee Continuous Current
Converter Type
For Continuous Current in the Input Inductor
For Continuous Current in L2
Buck/Boost
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MOSFET M. 250V, 20AL1. 100H, 9AC. 1500F, 250V, 5.66A p-pDiode D. 200V, 16AL2. 100H, 9AC1. 33F, 50V, 14A p-pBUCK/BOOST DESIGNConclusion - 50kHz may be too low for buck/boost converter
Additional Components for Buck/Boost Converter
Series Capacitor Voltage
Series Capacitor (C1) Current (Arms)
Series Capacitor (C1) Ripple Voltage (peak-to-peak)
Second Inductor (L2) Current (Arms)
1.5
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