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EE462L, Spring 2014 DC−DC Boost Converter 1 Buck converter + vL – iL iin Iout L Vin Boost converter C iin + vL – iL iC Iout L Vin + Vout – C iC + Vout – 2 ! Boost converter iin + vL – iL iD Iout L Vin C iC + Vout – This is a much more unforgiving circuit than the buck converter • If 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 3 Boost converter iin + vL – iL iD Iout L Vin C iC + Vout – • Modify your MOSFET firing circuit for Boost Converter operation (see the MOSFET Firing Circuit document) • Limit your output voltage to 120V 4 Boost converter iin + vL – iL iD Iout L Vin C iC + Vout – Using KVL and KCL in the average sense, the average values are Iin +0V– Iout L Vin C Iout + Vout 0A – Find the input/output equation by examining the voltage across the inductor 5 Switch closed for DT seconds iin + Vin − iL Iout L Vin C diL Vin dt L Iout + Vout – Reverse biased, thus the diode is open for DT seconds Note – if the switch stays closed, the input is short circuited! 6 Switch open for (1 − D)T seconds + (Vin − Vout ) − iL iin Iout L Vin C diL Vin Vout dt L + Vout (iL – Iout) – Diode closed. Assume continuous conduction. for (1−D)T seconds 7 ! Since the average voltage across L is zero VLavg D Vin 1 D Vin Vout 0 Vout (1 D) Vin D Vin D Vin The input/output equation becomes Vin Vout 1 D A realistic upper limit on boost is 5 times 8 Examine the inductor current Switch closed, diL Vin vL Vin , dt L Switch open, diL Vin Vout vL Vin Vout , dt L Vin Vout A / sec L iL Imax Iavg = Iin Vin A / sec L Imin DT Iavg = Iin is half way between Imax and Imin ΔI (1 − D)T T 9 Inductor current rating 2 2 I Lrms I avg 1 2 1 2 I pp I in I 2 12 12 Max impact of ΔI on the rms current occurs at the boundary of continuous/discontinuous conduction, where ΔI =2Iin 2Iin iL Iavg = Iin ΔI 0 2 2 I Lrms I in I Lrms 1 2Iin 2 4 Iin2 12 3 2 I in 3 Use max 10 MOSFET and diode currents and current ratings iin + vL – iL iD Iout L Vin C iC + Vout – 2Iin 0 2Iin 0 Use max Take worst case D for each I rms 2 I in 3 11 Capacitor current and current rating iin iL iD Iout L Vin C iC + Vout – iC = (iD – Iout) 2Iin −Iout 0 −Iout Max rms current occurs at the boundary of continuous/discontinuous conduction, where ΔI =2Iout Use max I Crms I out See the lab document for the derivation 12 Worst-case load ripple voltage iC = (iD – Iout) 0 −Iout The worst case is where C provides Iout for most of the period. Then, Q I out T I out V C C Cf 13 Voltage ratings Diode sees Vout iin iL Iout C sees Vout + Vout – L Vin C iin iL Iout L Vin C + Vout – MOSFET sees Vout • Diode and MOSFET, use 2Vout • Capacitor, use 1.5Vout 14 Continuous current in L Vin Vout A / sec L iL 2Iin Iavg = Iin 0 (1 − D)T 1 Vin Vin 11 D Vin V V 1 D 2 I in out in 1 D T 1 D 1 D T Lboundary Lboundary Lboundary f 2 I in Vin D Lboundary f , V D Lboundary in 2 I in f Then, considering the worst case (i.e., D → 1), V L in 2 I in f use max guarantees continuous conduction 15 use min Impedance matching I out 1 DIin Iin + + Source DC−DC Boost Converter Vin − Vin 1 D − Vout V Rload out I out Iin + Vin Equivalent from source perspective Requiv − 1 D Vout 1 D 2 Vout 1 D 2 R V Requiv in load I out I in I out 1 D 16 Example of drawing maximum power from solar panel PV Station 13, Bright Sun, Dec. 6, 2002 6 Isc Pmax is approx. 130W (occurs at 29V, 4.5A) 5 I - amps 4 For max power from panels, attach 3 Rload 2 1 0 0 5 10 15 20 25 V(panel) - volts 30 35 40 Voc I-V characteristic of 6.44Ω resistor 45 29V 6.44 4.5 A But as the sun conditions change, the “max power resistance” must also change 17 Connect a 100Ω resistor directly, extract only 14W PV Station 13, Bright Sun, Dec. 6, 2002 6 130W 5 4 I - amps So, the boost converter reflects a high load resistance to a low resistance on the source side 3 2 14W 1 0 0 5 10 15 20 25 30 35 40 45 V(panel) - volts To 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 transferred Requiv 1 D Rload , D 1 2 Requiv Rload 1 6.44 0.75 100 18 BOOST DESIGN Worst-Case Component Ratings Comparisons for DC-DC Converters Our components 9A Converter Type Boost Input Inductor Current (Arms) 2 I in 3 10A 250V 5.66A Output Capacitor Voltage Output Capacitor Current (Arms) 1.5 Vout I out 200V, 250V Diode and MOSFET Voltage 2 Vout 120V 5A 120V Likely worst-case boost situation 16A, 20A Diode and MOSFET Current (Arms) 2 I in 3 10A L. 100µH, 9A C. 1500µF, 250V, 5.66A p-p Diode. 200V, 16A MOSFET. 250V, 20A 19 BOOST DESIGN Comparisons of Output Capacitor Ripple Voltage Converter Type Boost Volts (peak-to-peak) I out 5A Cf 0.067V 1500µF 50kHz L. 100µH, 9A C. 1500µF, 250V, 5.66A p-p Diode. 200V, 16A MOSFET. 250V, 20A 20 BOOST DESIGN Minimum Inductance Values Needed to Guarantee Continuous Current Converter Type Boost For Continuous For Continuous Current in the Input Current in L2 Inductor V 40V L in – 2 I in f 200µH 2A 50kHz L. 100µH, 9A C. 1500µF, 250V, 5.66A p-p Diode. 200V, 16A MOSFET. 250V, 20A 21