EE462L, Spring 2013DCDC Boost Converter

Vin + Vout C iC Iout iinBuck converter Boost converter Vin + Vout C iC Iout iin

Boost converter This is a much more unforgiving circuit than the buck converterVin + Vout C iC Iout iin iL LiDIf the MOSFET gate driver sticks in the on position, then there is a short circuit through the MOSFET blow MOSFET!If the load is disconnected during operation, so that Iout = 0, then L continues to push power to the right and very quickly charges C up to a high value (250V) blow diode and MOSFET!Before applying power, make sure that your D is at the minimum, and that a load is solidly connected

Boost converter Vin + Vout C iC Iout iin iL LiDModify your MOSFET firing circuit for Boost Converter operation (see the MOSFET Firing Circuit document)Limit your output voltage to 120V

Boost converter Using KVL and KCL in the average sense, the average values are Find the input/output equation by examining the voltage across the inductor

Switch closed for DT seconds Reverse biased, thus the diode is openfor DT secondsVin + Vout C iin iL LNote if the switch stays closed, the input is short circuited!+ Vin

Switch open for (1 D)T secondsDiode closed. Assume continuous conduction.Vin + Vout C iin iL Lfor (1D)T seconds(iL Iout)+ (Vin Vout )

Since the average voltage across L is zeroThe input/output equation becomesA realistic upper limit on boost is 5 times

Examine the inductor currentSwitch closed,Switch open,DT(1 D)TTImaxIminIavg = IinIavg = Iin is half way betweenImax and IminIiL

Inductor current ratingMax impact of I on the rms current occurs at the boundary of continuous/discontinuous conduction, where I =2Iin2Iin0Iavg = IinIiLUse max

MOSFET and diode currents and current ratingsUse max2Iin02Iin0Take worst case D for eachVin + Vout C iC Iout iin iL LiD

Capacitor current and current rating2Iin IoutIout0Max rms current occurs at the boundary of continuous/discontinuous conduction, where I =2IoutUse maxiC = (iD Iout)Vin + Vout C iC Iout iin iL LiDSee the lab document for the derivation

Worst-case load ripple voltageThe worst case is where C provides Iout for most of the period. Then,Iout0iC = (iD Iout)

Voltage ratingsDiode sees VoutMOSFET sees VoutC sees VoutDiode and MOSFET, use 2VoutCapacitor, use 1.5VoutVin + Vout C iin iL LVin + Vout C iin iL L

Continuous current in L2Iin0Iavg = IiniL(1 D)Tguarantees continuous conductionThen, considering the worst case (i.e., D 1),use maxuse min

Impedance matching

DCDC Boost Converter

+Vin+

Iin+VinIinEquivalent from source perspectiveSource

Example of drawing maximum power from solar panelIscVocPmax is approx. 130W (occurs at 29V, 4.5A)For max power from panels, attachI-V characteristic of 6.44 resistorBut as the sun conditions change, the max power resistance must also change

Connect a 100 resistor directly, extract only 14W130W6.44 resistor100 resistor14WTo extract maximum power (130W), connect a boost converter between the panel and the load resistor, and use D to modify the equivalent load resistance seen by the source so that maximum power is transferredSo, the boost converter reflects a high load resistance to a low resistance on the source side

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)

Boost

1.5

2

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5A1500F50kHz0.067VBOOST DESIGN

Comparisons of Output Capacitor Ripple Voltage

Converter Type

Volts (peak-to-peak)

Boost

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40V2A50kHz200HBOOST DESIGN

Minimum Inductance Values Needed to

Guarantee Continuous Current

Converter Type

For Continuous Current in the Input Inductor

For Continuous Current in L2

Boost

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