*EE462L, Spring 2014DCDC SEPIC (Converter)

*Boost converter + Vout Iout Vin iin L1+ v L1 SEPIC converter

*SEPIC 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 90VSEPIC = single ended primary inductor converter

*+ 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(so C1 is discharging). C is also 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 constantVoltage can be stepped-up or stepped-down

*Voltage can be stepped-up or stepped-down The buck-boost converterInverse polarity with respect to inputCompared with SEPIC:+ Fewer energy storage components+ Capacitor does not carry load current+ In both converters isolation can be easily implemented- Polarity is reversed

*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 SEPIC 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.

*SEPIC 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

_1221762425.unknown

_1221796271.unknown

_1221796297.unknown

_1221796206.unknown

_1150520753.unknown

*5A1500F50kHz0.067VMOSFET M. 250V, 20AL1. 100H, 9AC. 1500F, 250V, 5.66A p-pDiode D. 200V, 16AL2. 100H, 9AC1. 33F, 50V, 14A p-pSEPIC DESIGN

Comparisons of Output Capacitor Ripple Voltage

Converter Type

Volts (peak-to-peak)

Buck/Boost

_1221762523.unknown

*40V2A50kHz200H90V2A50kHz450HMOSFET M. 250V, 20AL1. 100H, 9AC. 1500F, 250V, 5.66A p-pDiode D. 200V, 16AL2. 100H, 9AC1. 33F, 50V, 14A p-pSEPIC 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

_1221797668.unknown

_1221797692.unknown

*MOSFET M. 250V, 20AL1. 100H, 9AC. 1500F, 250V, 5.66A p-pDiode D. 200V, 16AL2. 100H, 9AC1. 33F, 50V, 14A p-pSEPIC DESIGNConclusion - 50kHz may be too low for SEPIC converterSEPIC 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

_1150520774.unknown

_1150520776.unknown

_1191217316.unknown

_1150520773.unknown