Figure 1. DC-DC Boost Converter

Transcription

1 EE46, Power Electroncs, DC-DC Boost Converter Verson Oct. 3, 11 Overvew Boost converters make t possble to effcently convert a DC voltage from a lower level to a hgher level. Theory of Operaton Relaton Between V out and V n n Contnuous Conducton The dealzed boost converter crcut s shown below n Fgure 1. Under normal operaton, the crcut s n contnuous conducton (.e., s never zero) I d I out n Varac 1/5Vac Transformer DBR Remember never connect a varac drectly toadbr! V n Fgure 1. DC-DC Boost Converter C eave the 1µF cap across the nput termnals C.1Ω V out The crcut s assumed to be lossless so that P n = P out, or V n avg = Vout Iout, where avg In =. (1) Assumng contnuous conducton, the crcut has two topologes swtch closed, and swtch open. Both are shown n Fgures a and b. v v I out I out V n C C V out V n C C V out Fgure a. Swtch Closed for DT Seconds Fgure b. Swtch Open for (1-D)T Seconds (Contnuous Conducton) When the swtch s closed, the dode s reverse based and open, and ncreases at the rate of d v V = = n, t DT, () dt and the nductor s chargng. When the swtch s open, the dode s forward based, and decreases at the rate of Page 1 of 11

2 EE46, Power Electroncs, DC-DC Boost Converter Verson Oct. 3, 11 d dt v Vn Vout = =, DT < t < T, (3) and the nductor s dschargng. The nductor voltage s shown n Fgure 3. V n Vn V out Fgure 3. Inductor Voltage n Contnuous Conducton Because of the steady-state nductor prncple, the average voltage v across s zero. Snce v has two states, both havng constant voltage, the average value s ( V ) n DT ( Vn Vout T )(1 D) T =, so that Vn D Vn Vout VnD Vout D =. Smplfyng the above yelds the fnal nput-output voltage expresson Vout Vn =. (4) 1 D The graph of s shown n Fgure 4. Inductor Current n Contnuous Conducton T max avg mn = = avg avg DT (1 D)T Fgure 4. Inductor Current Waveform for Contnuous Conducton Page of 11

3 EE46, Power Electroncs, DC-DC Boost Converter Verson Oct. 3, 11 From (), d dt Vn = =, DT so that Vn VnD = DT =, (5) f where f s the swtchng frequency. The boundary of contnuous conducton s when T mn =, as shown n Fgure 5. max = I avg avg mn = DT (1 D)T Fgure 5. Inductor Current at the Boundary of Contnuous Conducton Usng Fgure 5 and the nductor dschargng slope from (3), so that ( V V )( 1 D) V V ( 1 D) out n n n VnD = = = = I f f f boundary boundary boundary avg, (6) boundary VnD =. (7) I f avg From (1), I avg = I n. (8) Substtutng nto (8) nto (7) yelds Page 3 of 11

4 EE46, Power Electroncs, DC-DC Boost Converter Verson Oct. 3, 11 boundary VnD =. (9) In f Because the maxmum value of D s 1, then Vn >. (1) In f wll guarantee contnuous conducton for all D. Note n (9) and (1) that contnuous conducton can be acheved more easly when I out and f are large. Dscontnuous Conducton At low load, the converter may slp nto the dscontnuous conducton mode. Referrng back to Fgure b, ths occurs when the nductor current coasts to zero. At that moment, the capactor attempts to reverse and backfeed the nductor, but the dode prevents current reversal. Thus, the dode opens, and the crcut assumes the topology shown n Fgure 6 untl the swtch closes agan. Durng ths thrd state, all load power s provded by the capactor. I out V n C V out Fgure 6. Thrd State for Dscontnuous Conducton Once dscontnuous, the voltage across the nductor s zero. The correspondng voltage waveform s shown n Fgure 7. V n Dscontnuous Vn V out Fgure 7. Inductor Voltage n Dscontnuous Conducton Page 4 of 11

5 EE46, Power Electroncs, DC-DC Boost Converter Verson Oct. 3, 11 Current Waveforms n Contnuous Conducton Contnuous current waveforms are shown below. T Inductor = max avg avg = I n mn = avg DT (1 D)T MOSFET max = avg avg = I n mn = avg Dode max = avg avg = I n mn = avg Capactor ( C = I ) out avg I out avg Iout = In Iout avg I out I out (Note compared to the other waveforms shown above, s exaggerated n the fgure to llustrate how the capactor current can be negatve n both DT and (1 D)T regons) Fgure 8. Current Waveforms for Contnuous Conducton Page 5 of 11

6 EE46, Power Electroncs, DC-DC Boost Converter Verson Oct. 3, 11 Current Ratngs n Contnuous Conducton Conservatve current ratngs for the nductor n contnuous conducton correspond to the stuaton where max = I n, (11) whch, as explaned n the Buck Converter experment, yelds 1 = 1 I rms, max In ( In ) = In 1, (1) 1 3 so that I rms, max = I n. (13) 3 Conservatve current ratngs for the MOSFET and dode are when D s large, so that (13) apples for them also. To determne the rms current through C, consder the capactor current n Fgure 8, and the worst-case scenaro n Fgure 9. I n I out In I out DT (1 D)T Fgure 9. Maxmum Rpple Current Case for Capactor Current When the swtch s closed, the capactor current s Iout. When the swtch s open, the capactor current s Iout. If the swtch closed nterval lasted for the entre T, the squared rms value would be I out. If the swtch open nterval lasted for the entre T, the rms value would be, for the maxmum rpple case, 1 ( I n Iout ) ( In ). The tme-weghted average 1 of the two gves the squared rms current I out Page 6 of 11

7 EE46, Power Electroncs, DC-DC Boost Converter Verson Oct. 3, 11 1 I Crms = DIout ( 1 D) ( In Iout ) ( In ). (14) 1 Now, substtutng n I = I ( 1 D) yelds out n I Crms 1 = DIn ( 1 D) (1 D) n n n 1 ( I I (1 D) ) ( I ). Smplfyng yelds 1 I Crms = DIn ( 1 D) (1 D) InD In, 3 I I D(1 D) (1 3 ) D Crms = In (1 D) D, D D Crms = In, Settng the partal dervatve wth respect to D shows that the maxmum occurs at yelds I Crms, max = 3 I n. 1 D =, whch 3 Snce D =, then substtutng for I n yelds I I = out Crms, max = Iout. (15) Voltage Ratngs for Contnuous Conducton Referrng to Fgure b, when the MOSFET s open, t s subjected to Vout. Because of the usual double-voltage swtchng transents, the MOSFET should therefore be rated Vout. Referrng to Fgure a, when the MOSFET s closed, the dode s subjected to Vout. The dode should be conservatvely rated Vout. Page 7 of 11

8 EE46, Power Electroncs, DC-DC Boost Converter Verson Oct. 3, 11 Capactor Voltage Rpple Re-examnng the capactor current n Fgure 9, and re-llustrated n Fgure 1, t can be seen that the amount of charge taken from C when the swtch s closed s represented by the dotted area. I n I out In I out I out DT (1 D)T ΔQ Fgure 1. Capactor Charge Gven Up Whle Swtch s Closed As D 1, the wdth of the dotted area ncreases to fll almost the entre cycle, and the maxmum peak-to-peak rpple becomes Iout T Iout ΔV max = =. (16) C Cf The Experment (Important - to avod hgh output voltages, always keep a load attached to the boost converter output when nput power s appled. Use a conventonal 1V, 15W lght bulb as your load. Do not exceed 1V on the output. 1. Convert a buck converter to a boost converter, usng the crcut shown n Fgure 1 of ths document.. Double-check that the polarty of your converter s output capactor s correct. 3. ocate one of the 15W lght bulb test load assembles. Check the lght bulb wth an ohmmeter to make sure t s not burned out. 4. Connect the lght bulb test load to your crcut. 5. Connect an osclloscope Channel #1 to vew V GS, and Channel # to vew V DS. The ground clp of the Channel # probe should not be attached to the crcut, but nstead t should be clpped back onto ts own lead n-cable so that t does not dangle. Page 8 of 11

9 EE46, Power Electroncs, DC-DC Boost Converter Verson Oct. 3, Do not connect a DBR yet. Connect a MOSFET frng crcut to your converter, usng short wres, and then power-up your MOSFET frng crcut. Set the osclloscope to trgger on Channel #1. Observe your osclloscope to confrm that the controls are workng properly. 7. Rase the D potentometer to the max, and then set the duty cycle lmter so D cannot exceed Set D to the mnmum settng, and F 1kHz. Important Note: the frst tme you energze your boost converter, feed the 1/5V transformer through a varac to the DBR, so that you can SOWY ncrease the voltage from zero and read the varac ammeter to detect short crcuts before they become serous. A common problem s to have the MOSFET n backward, so that ts nternal antparallel dode creates a short crcut. The ammeter on the varac s an excellent dagnostc tool. Once you are convnced that your crcut s workng correctly, the varac s then optonal. Remember your boost converter requres DC nput power from a DBR. Does your crcut have a short? If so, do the followng: 1. Make sure that your MOSFET s not connected backwards.. Observe VGS on the MOSFET as you vary D. Does the waveform look correct? 3. Unplug the wall wart. Does the short crcut go away? If not, your MOSFET may be shorted so, dsconnect the MOSFET from the converter, and perform the voltagecontrolled resstance test on the MOSFET, or use the MOSFET tester. 9. Connect a varac to a 5Vac transformer, and 5Vac transformer to a DBR. Connect the DBR to your boost converter, keepng the wres short. Then, energze the 5Vac transformer and DBR. Slowly rase the varac, checkng for a short crcut, so that Vac of the transformer s approxmately 7-8V. 1. Wth F 1kHz, slowly rase D so that Vout ncreases n steps of about 1V up to the maxmum obtaned wth D =.8 or 1V, whchever s less. Measure D, Vn, In, Vout, and Iout as you go. Save a screen snapshot of VDS for Vout = 1V that shows the peak value of VDS. et the crcut run at the 1Vout condton for 1 or more mnutes, and then turn off your crcut. Page 9 of 11

10 EE46, Power Electroncs, DC-DC Boost Converter Verson Oct. 3, 11 Save screen snapshot #1 VDS for 9kHz, 1V 11. Wth your crcut turned off, quckly and carefully use your hand to check MOSFET heat snk temperature and any other hot components. You can also use an nfrared thermometer for ths step. 1. Compare your measured Vout/Vn to theory. Multply voltages and currents to compute nput and output powers, and then compute the effcency of your crcut for each Vout measured. Plot your results n your wrtten report. 13. Take can also take your solar lab data whle you are dong the boost converter. Use a 15W lght bulb as a load. Connect a DBR between a solar panel par and your boost converter to steady the panel current. Sweep D to fnd the maxmum power pont. Compute the maxmum power pont value of Rload = Vout/Iout and the correspondng equvalent max power resstance seen by the solar panels. Does the rato of Rload resstance to equvalent resstance seem by the solar panels check wth D? Page 1 of 11

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