Power Electronics Laboratory. THE UNIVERSITY OF NEW SOUTH WALES School of Electrical Engineering & Telecommunications

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1 .0 Objective THE UNIVERSITY OF NEW SOUTH WALES School of Electrical Engineering & Telecommunication ELEC464 Experiment : C-C Step-own (Buck) Converter Thi experiment introduce you to a C-C tep-down (buck) converter circuit. Thee are found in many power upplie where a C upply at a voltage which i lower than the available C upply V d i required. Operation, and teady-tate characteritic of the tep-down (buck) with continuou and dicontinuou conduction mode (CCM & CM) will be tudied experimentally. ynamic characteritic of the buck converter in the CCM will alo be tudied..0 Background The buck converter circuit i hown in figure (a) in which the witch T i operated from a pulewidth modulator operating at a carrier frequency f. The witch i alo operated with a duty cycle which range from 0 to. Figure (b) how variou waveform in the circuit when the witch turn ON and OFF with a certain duty cycle. The duty cycle i defined a the ratio of ON time (t on ) of the witch to the period T / f of it witching. i d T L i L i o V d + v oi + v L C v o V o R (Load) (a) v L V d - V o 0 V o t V d v oi 0 t on T t off t i L I L = I o I d i d (b) Figure Experiment C-C Buck Converter F. Rahman/March 003

2 We will aume that all the device and component of the circuit are ideal, the inductor current i continuou (alway poitive) and that the output voltage i held contant at V o during the witching interval by the large output capacitor. Under thee circumtance the inductor voltage v L, and inductor the capacitor current i L and i C, repectively, are a indicated in figure (b). The following equation then hold for the buck converter. V o V d... () ( ) I max Vo..() R Lf ( ) I min Vo.(3) R Lf where V o = output C voltage, V V d = upply C voltage, V L = inductance of the inductor, H R = Load reitance,. Vo I L,average I R ; and V d I d VoI o.(4) R For a given witching frequency, the deired minimum inductance L min for continuou conduction of inductor current i given by, ( )R L min.(4) f Note that the proportional relationhip between V o and implied by equation (), which i deirable for control of the converter, i poible only when the inductor current remain continuou over the witching period. Hence the importance of a minimum value of L for a given witching frequency. The output voltage ripple a a fraction of the output voltage i given by Vo Vo 8LCf where C i output filter capacitance. (5) 3. Non ideal effect on converter performance 3. evice voltage drop When the forward (on-tate) voltage drop, V T of the tranitor witch T and V for the diode, are taken into account, Vo Vd VT V ( )..(6) for the buck converter. Note that the effect of device drop i more ignificant on the buck converter than on the boot converter. Experiment C-C Buck Converter F. Rahman/June 0

3 3. Equivalent erie reitance (ESR) of the filter capacitor The ESR of the filter capacitor may have a ignificant effect on the output voltage ripple, often producing ripple greater than that of the capacitor. Thi ripple i given by V.(7) o,esr icrc where R c i the ESR of the filter capacitor and ic i the capacitor current ripple. Note that thi voltage doe not add algebraically to the ripple of the capacitor voltage. 3.3 Inductor Reitance The erie reitance of the inductor affect the power lo of the dc-dc converter. It effect i more ignificant in the boot converter than in the buck converter. The following equation apply Vd Vo (8) RL R RL R( ).(9) where R L i the reitance of the inductor. 3.4 Switching loe Loe in the device due to overlap of the voltage and current tranient at turn off and turn on affect the efficiency of the converter. The witching frequency f i carefully elected to avoid thee loe becoming ignificant. f alo affect the phyical ize of the inductor. 3.5 icontinuou conduction icontinuou conduction in the inductor of a C-C buck converter make the otherwie direct relationhip between the output and input voltage rather complex, making it load and converter parameter dependant, a indicated below. V o V d 8Lf R for the buck converter..(0) Thi make the deign of the cloed loop voltage regulator much more complex. Becaue of thi, continuou conduction i normally aimed for. Experiment C-C Buck Converter 3 F. Rahman/June 0

4 4. Equipment power IGBT witch 3 phae diode bridge rectifier module L-C moothing filter for the C upply 5 load reitor for buck converter tapped inductor board; -0 H for buck converter; four-channel ocillocope C voltmeter and ammeter module Iolated tranducer board with V/A and V/V enor PC with SP board and interface card 5. Experiment Familiarie yourelf with the experimental et-up, epecially the IGBT, the rectifier diode bridge, the input C filter, the buck inductor L, the buck diode, the output filter capacitor C, and the load reitor. The inductor board ha inductor in the range of -0 H which are connected to tap. A SP board reident in a PC generate the PWM witching pule with which to turn the buck witch ON and OFF. The frequency of witching can be changed only by running the appropriate Buck Converter for x khz program from the ektop where x tand for the witching frequency. For each of thee program, the appropriate operator interface mut alo be run from the menu. The duty cycle for each of thee witching frequencie can be elected by a lider on the PC monitor. The C upply V d to the buck converter i obtained from an AC upply via an auto-tranformer (variac), a rectifier diode bridge followed by an LC filter. Thee are located on the left-hand ide of the equipment panel. For the whole of thi experiment, V d mut be et to 50V. When the converter duty cycle or load increae, V d will drop becaue of voltage drop in the variac. For each etting of or load, you will need to adjut the variac o that V d i alway 50V. PRECAUTIONS!!. The experiment i pre-wired on the equipment panel. o not attempt to alter any connection of the power circuit while the C upply i on. The only alteration you will need to make to the power circuit i the election of taping of the buck inductor (uing the blue wire) and connection/diconnection of the load (uing the red wire). Make ure that all power connection (crewed terminal block) are firm (tight) before the C upply i witched on. Any intermittent connection in any part of the circuit will detroy the IGBT.. Alo, make ure to adjut the variac to zero before you change the buck inductor tap. You mut not manually diconnect the buck inductor while it i till carrying current. Experiment C-C Buck Converter 4 F. Rahman/June 0

5 Goal: In thi experiment, you will run the buck converter with witching frequencie f = 5, 0 and 0kHz; you will elect buck inductance L =, 66 and 0µH for each of thee frequencie; for each of thee combination, you will vary the duty cycle from 0. to 0.9 in tep of 0.. Finally you will take frequency repone tet data by varying inuoidally, from 00Hz to 4kHz while the converter operate with f = 0kHz and L = 0µH. ata obtained from thee tet will allow you undertand the role of L and f on the continuou and dicontinuou mode of operation of the converter, and it control characteritic. 5. The power circuit for the buck converter i hown in figure. Before witching the C upply to the converter ON, run the SP program Buck Converter - 5kHz in directory Elec464_lab_buck on the dektop. Run the Space control dek program and the Open experiment under the file menu to run the correponding experiment file. and oberve the PWM witching pule for a witching frequency of f = 5 khz. Adjut the duty cycle by uing the lider on the PC monitor. Note down the range over which can be adjuted. Set initially to minimum. Connect the witching ignal from the SP to the gate of the IGBT uing a BNC cable. Connect thi ignal alo to channel 4 of the CRO and ue it to edge trigger the CRO at all time. Select L = 0 H, and = minimum initially. Connect the load (R 5 ). The C upply, V d, to the converter i obtained from rectifier-filter circuit. Switch AC power to the variac (auto-tranformer) upplying the rectifier and adjut it to obtain a C upply of 50V. The C upply mut be maintained at 50V throughout the ret of thi experiment by adjutment of the variac for each etting of or load current. 5. Oberve the inductor and the capacitor current, and voltage acro thee on CRO channel, and 3 of the CRO uing the iolated enor or clip-on probe. Blue wire LC filter I d T I o Red wire 3- phae 45V 50Hz Supply Variac V d BNC L (-0mH) C 000mF V o 5 Ohm Load Rectifier IGBT gate river SP PC CRO Iolated enor board Experiment C-C Buck Converter 5 F. Rahman/June 0

6 Figure The peak-peak ripple on the output voltage can be oberved by uing AC coupling on the CRO. 5.3 iplay the witching waveform at the top of the CRO. Uing a uitable time bae, diplay i L, v L, i c, v o, i T, and i, three at a time, below the trace of the witching waveform. Adjut from 0. to 0.9 and identify value of for which continuou and dicontinuou conduction of inductor current i L take place. 5.4 Tabulate the C value of V o, I d, I L and the inductor current ripple ΔI L veru. Vary from 0. to 0.9 in tep of 0.. Maintain V d = 50V throughout, by adjuting the variac. Note down the value of for which conduction of the inductor current i at the boundary between continuou and dicontinuou conduction. Print the waveform of i L, i d, i c and v o for thi condition of operation only. Mark the value of in the printout. Reduce V d to zero by adjuting the variac. 5.5 Select L = 66 H. Set V d = 50 V by adjuting the variac. Repeat 5.3 and Select L = uh, Set V d = 50V by adjuting the variac. Repeat 5.3 and Run SP program Buck Converter - 0kHz and repeat for L = 66 and µh. 5.8 Run SP program Buck Converter - 0kHz and repeat for L = 66 and µh. 5.9 Run SP program Buck Converter Frequency Repone Tet. In order to carry out a frequency repone tet on the inductor current and output voltage repone, you will vary the duty cycle inuoidally at ome frequency and meaure the variation in I L and V o. A the frequency of variation of increae, the variation of I L and V o will eventually fail to follow it. Thi i given by a cut-off frequency at which the amplitude of I L and V o fall to of it amplitude when the frequency of variation i well below the cut off frequency. Set the frequency of initially at 00Hz on the PC monitor. Set L = 0µH and f = 0 khz. Note down the amplitude of the inductor current and load output voltage V o. Increae the frequency of and take a few reading of amplitude of I L and V o v frequency of. You are expected to take data well beyond the cut-off frequency. 6.0 Report 6. Plot graph of V o veru for ection and dicu with reference to theory. 6. ecribe how witching frequency f and the value of L affect the current ripple in the buck inductor uing your experimental reult to verify theory. 6.3 ecribe how V o change with when the converter operate with dicontinuou conduction uing your experimental reult to verify theory. 6.4 Plot the frequency repone data for inductor current I L and output voltage V o veru frequency of found in 5.9. Comment on thee reult and dicu with reference to theory. 6.5 You may ubtantiate your comment by uing the imulation reult. A model of the buck converter could be built uing PSIM in the School computing lab or in room 30. Such model will give you all waveform you oberved during your experiment. Experiment C-C Buck Converter 6 F. Rahman/June 0