Figure 1. DC-DC Boost Converter. i. (1)

Transcription

1 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 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., L s never zero) I L d I out n Varac (Optonal, see Step 7) 1/5Vac Transformer DBR Remember never connect a varac drectly toadbr! V n Fgure 1. DC-DC Boost Converter L C Leave 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 Lavg Vout Iout, where Lavg In. (1) Assumng contnuous conducton, the crcut has two topologes swtch closed, and swtch open. Both are shown n Fgures a and b. v L v L L I out L I out V n L C C + V out V n L 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 L ncreases at the rate of dl dt v V L n, t DT, () L L and the nductor s chargng. When the swtch s open, the dode s forward based, and L decreases at the rate of Page 1 of 1

2 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 dl dt vl Vout, DT t T, (3) L L and the nductor s dschargng. The nductor voltage s shown n Fgure 3. V n V out Fgure 3. Inductor Voltage n Contnuous Conducton Because of the steady-state nductor prncple, the average voltage v L across L s zero. Snce v L has two states, both havng constant voltage, the average value s DT ( Vout T )(1 D) T, so that D Vout D Vout D. Smplfyng the above yelds the fnal nput-output voltage expresson Vout. (4) 1 D The graph of L s shown n Fgure 4. Inductor Current n Contnuous Conducton I T L max Lavg L mn Lavg Lavg I I DT (1 D)T Fgure 4. Inductor Current Waveform for Contnuous Conducton Page of 1

3 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 From (), dl dt I, L DT so that D I DT, (5) L Lf where f s the swtchng frequency. The boundary of contnuous conducton s when T L mn =, as shown n Fgure 5. I L max I Lavg Lavg L 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 D I I L f L f L f boundary boundary boundary Lavg, (6) L boundary D. (7) I f Lavg From (1), I Lavg I n. (8) Substtutng nto (8) nto (7) yelds Page 3 of 1

4 Lboundary EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 D. (9) In f Because the maxmum value of D s 1, then L. (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 L 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 L 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 V out Fgure 7. Inductor Voltage n Dscontnuous Conducton Page 4 of 1

5 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 Current Waveforms n Contnuous Conducton Contnuous current waveforms are shown below. I T Inductor L max Lavg Lavg I n L mn Lavg I I DT (1 D)T I I MOSFET L max Lavg Lavg I n I L mn Lavg I I Dode L max Lavg Lavg I n I L mn Lavg Capactor ( C I ) L out I Lavg I out I Lavg Iout In Iout I Lavg I out I out (Note compared to the other waveforms shown above, I 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 1

6 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 Current Ratngs n Contnuous Conducton Conservatve current ratngs for the nductor n contnuous conducton correspond to the stuaton where Imax I n, (11) whch, as explaned n the Buck Converter experment, yelds 1 1 I Lrms, max In In In 1, (1) 1 3 so that I Lrms, 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 L 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 ( In Iout ) In. The tme-weghted average 1 of the two gves the squared rms current I out Page 6 of 1

7 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 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, 3 1 3D 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 3 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 1

8 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 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 Q 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 V max. (16) C Cf Page 8 of 1

9 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 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.. Modfy the MOSFET D-control crcut accordng to the MOSFET Frng Crcut document for Boost Converter Operaton. Check your range of D wth the expected D range gven n Fgure b n that document. 3. Double-check that the polarty of your output capactor s correct. 4. Locate one of the 15W lght bulb test load assembles. Check the lght bulb wth an ohmmeter to make sure t s not burned out. 5. Connect the lght bulb test load to your crcut. 6. 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. 7. Do not connect a DBR yet. Connect the 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. 8. Set D to the mnmum settng, and F 9kHz. Page 9 of 1

10 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 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 SLOWLY 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, n whch case 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 and F. 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 (an optonal varac and) 5Vac transformer to a DBR. Connect the DBR to your boost converter, keepng the wres short. Then, energze the 5Vac transformer and DBR. If usng a varac, adjust the varac so that Vac of the transformer s approxmately 7-8V. 1. Wth F 9kHz, slowly rase D to where the output voltage s about 1V. Measure D,, In, Vout, and Iout. Save a screen snapshot that shows the peak value of VDS. Let the crcut run at ths condton for 1 or more mnutes, (optonal use an nfrared thermometer to measure the MOSFET surface temperature), and then turn off your crcut. VDS for the 9kHz, 1V Condton Save screen snapshot #1 Page 1 of 1

11 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., Wth your crcut turned off, quckly and carefully use your hand to check MOSFET heat snk temperature and to check for other hot components. 1. Compare Vout/ to theory. Multply voltages and currents to compute nput and output powers, and then compute the effcency of your crcut. 13. Turn on your crcut, and slowly lower D n steps of approxmately.1, to the lowest value (.e., D.1), measurng D,, In, and Vout as you go. Does the crcut reman n contnuous conducton over the range of D? If not, compare boundary equaton (9) wth your experence at the actual boundary pont. 14. Lower D to the mnmum settng. Repeat Steps 9-1, usng F 3kHz. If temperature measurements were taken, compare MOSFET temperatures for the 1V, 9kHz and 1V, 3kHz cases. Otherwse, comment on the dfference between hand-checked MOSFET heat snk temperatures for the two cases. 15. Usng the measurements taken for the 3kHz, 1V condton, employ (5) to compute I, and then use 1 I Lrms, max In I to compute nductor rms current. 1 1 I Crms, max I to compute capactor rms current Use In addton to descrbng what you dd n the above steps, be sure to nclude the followng n your report: a plot of measured and theoretcal Vout/ versus D for 9kHz on one graph, and a plot of measured and theoretcal Vout/ versus D for 3kHz on a separate graph. The followng optonal steps are to be performed wth solar panels as the power source and wth good sun (.e., panel short crcut current of 3.5A or more). The panel voltage that you measure should be at the panel (.e., the left-most analog voltmeter) 17. Note the sky condtons. Connect a solar panel par drectly to a 1V, 15W lght bulb. Measure panel voltage, panel current, and compute solar panel output power. 18. Next, nsert the boost converter between the panel par and 1V lght bulb. Wth F 9kHz, sweep D over ts range to measure and plot the I-V and P-V characterstcs of the panel par. Record the maxmum power value. Page 11 of 1

12 EE46L, Power Electroncs, DC-DC Boost Converter Verson Feb., 13 Parts Lst 8 nylon cable te (n student parts bn) Extra parts For the student parts bn and screw cabnet, at least 5 of the 5V MOSFETs (ndvdually bagged) 5 of the V, 16A ultrafast rectfers 5 of the DC jacks 5 of the 1kΩ audo taper and lnear taper potentometers 5 of the PWM modulator chps 5 of the nvertng drver chps 5 of the 14-pn sockets 5 of the 8-pn DIP sockets 5 of the green plugs 1 of the #4-4 x 1 flat slotted nylon screws and lock nuts Plastc bags for parts 6 x6, 4ml Page 1 of 1