Power Electronics Laboratory

Transcription

1 THE UNERSTY OF NEW SOUTH WALES School of Electrical Engineering & Telecommunications ELEC4614 Experiment : C-C Step-up (Boost) Converter 1.0 Objectives This experiment introuces you to a step-up C-C converter circuits. These are foun in many power supplies where a C supply at a voltage which is higher than the available C supply is require. Operation an steay-state characteristics of the step-up (boost) converter circuit will be stuie experimentally..0 Backgroun The boost converter circuit is shown in figure 1(a) in which the switch T is operate from a pulse-with moulator operating at a carrier frequency f s. The switch T has a uty cycle which ranges from 0 to 1. Figure 1(b) inicates relevant waveforms of the circuit when the switch T is turne ON an OFF at frequency f s, with a uty cycle. i i L i + v L + o i c C T o R (Loa) Figure 1(a) v L - o max i L i L i c - o R T T Figure 1(b) Experiment C-C Boost Converter 1 F. Rahman/March 009

2 We will assume that all the evices an components of the circuit are ieal, the inuctor current is continuous (always positive) an the output voltage is hel constant at o by the large output capacitor uring the switching intervals. Uner these circumstances the inuctor voltage v L, an inuctor an capacitor currents i L an i C, respectively, are as inicate in figure 1(b). The following equations then hol for the boost converter. o 1..(1) max.() (1 ) R Lf s min..(3) (1 ) R Lf s ( 1 ) R.(4) L, average where o = output C voltage, = supply C voltage, L = inuctance of the inuctor, H R = Loa resistance,. o o o ; ; an o o.(5) R 1 For a given switching frequency, the minimum inuctance L min for continuous conuction is given by, Lmin ( 1 ) R f s (6) The output voltage ripple across the filter capacitance C is given by o o RCf s.(7) 3.0 Switching losses Losses in the evices ue to overlap of the voltage an current transients at turn off an turn on affect the efficiency of the converter. The switching frequency f s is carefully selecte to avoi these losses becoming significant. f s also affects the physical sizes of the inuctor an capacitor. Experiment C-C Boost Converter F. Rahman/Feb, 011

3 4.0 iscontinuous conuction The relationship between the output voltage o an, although simple when the inuctor current is continuous uring the whole of switching perio, it is very steep when approaches 1. The ynamics of this converter are also not suitable for operation in continuous conuction moe. For the iscontinuous conuction moe, it can be shown that 1 o 1 R 1 Lf s.(8) Although this is a non-linear function, nevertheless, iscontinuous conuction moe is the preferre moe of operation of a boost converter. 4. Equipment 1 power GBT switch 1 3 phase ioe brige rectifier moule 1 L-C smoothing filter for the C supply 1 50 loa resistor for the boost converter H inuctance boar for boost converter 1 four-channel oscilloscope 1 C voltmeter an ammeter moules solate transucer boars with 1/1A an 1/ sensors PC with SP boar an interface car 5. Experiment Familiarise yourself with the experimental set-up, especially the GBT, the rectifier ioe brige, the input C filter, the buck inuctor L, the buck ioe, the output filter capacitor C, an the loa resistor. The inuctor boar has inuctors in the range of -110 H which are connecte to taps. A SP boar resient in a PC generates the PWM switching pulses with which to turn the buck switch ON an OFF. The frequency of switching can be change only by running the appropriate Buck Converter for x khz programs from the esktop where x stans for the switching frequency. For each of these programs, the appropriate operator interface must also be run from the menu. The uty cycle for each of these switching frequencies can be selecte by a slier on the PC monitor. The C supply to the buck converter is obtaine from an AC supply via an auto-transformer (variac), a rectifier ioe brige followe by an LC filter. These are locate on the left-han sie of the equipment panel. For the whole of this experiment, must be set to 50. When the converter uty cycle or loa increases, will rop because of voltage rop in the variac. For each setting of or loa, you will nee to ajust the variac so that is always 50. PRECAUTONS!! 1. The experiment is pre-wire on the equipment panel. o not attempt to alter any connection of the power circuit while the C supply is on. The only alterations you will nee to make to the power circuit is the selection of tapings of the buck inuctor (using the blue wire) an connection/isconnection of the loa (using the re wire). Make sure that all power connections (screwe terminal blocks) are firm (tight) before the C supply is switche on. Any intermittent connection in any part of the circuit will estroy the GBT. Experiment C-C Boost Converter 3 F. Rahman/Feb, 011

4 . Also, make sure to ajust the variac to zero before you change the boost inuctor taps. You must not manually isconnect the boost inuctor while it still carrying current. Goals: n this experiment, you will run the boost converter with switching frequencies f s = 5, 10 an 0kHz; you will select boost inuctance L = 47, 141 an 35µH for each of these frequencies; you will vary the uty cycle from 0.1 to 0.9 in steps of 0. for each of these combinations, Finally you will take frequency response test ata by varying sinusoially, from 100Hz to 4kHz while the converter operates with f s = 0kHz an L = 141µH. ata obtaine from these tests will allow you unerstan the roles of L an f s on the continuous an iscontinuous moes of operation of the converter, an its control characteristics. 5.1 The power circuit for the boost converter is shown in figure. Before switching the C supply to the converter ON, run the SP program Boost Converter - 5kHz in irectory Elec4614_labs_boost on the esktop an run the Space control esk program using Open experiment uner the file menu to run the corresponing experiment file. Observe the PWM switching pulses for a switching frequency of f s = 5 khz. Ajust the uty cycle. Note own the range over which can be ajuste. Set initially to minimum. Connect the switching signal from the SP to the gate of the GBT using a BNC cable. Connect this signal also to channel 4 of the CRO an use it to trigger the CRO at all times. Set L to 35 H, an to minimum initially. Connect the boost converter loa (R 50 ). The C supply,, to the converter is obtaine from rectifier-filter circuit. Switch AC power to the variac (auto-transformer) supplying the rectifier an ajust it to obtain a C supply of 50. The C supply must be maintaine at 50 throughout the rest of this experiment by ajustment of the variac for each setting of or loa current. 5. Observe the inuctor an the capacitor currents, an voltages across these on CRO channels 1, an 3 of the CRO using the isolate sensors or clip-on probes. Blue wire LC filter L o Re wire 3- phase Hz Supply ariac L (47-35 H) BNC it C 1000 F o o 5 Ohm Loa Rectifier GBT gate river SP PC CRO solate sensor boars Experiment C-C Boost Converter 4 F. Rahman/Feb, 011

5 Figure The peak-peak ripple on the output voltage can be observe by using AC coupling on the CRO. 5.3 isplay the switching waveform at the top of the CRO. Using a suitable time base, isplay i L, v L, i c, v o, i T, an i, three at a time, below the trace of the switching waveform. Ajust from 0.1 to 0.9 an ientify vales of for which continuous an iscontinuous conuction of inuctor current i L take place. Note that the input current of the boost converter an the output C voltage o can become isastrously high when approaches unity. You may fin it necessary to maintain below about 0.7 in orer to prevent such situations. 5.4 Tabulate the C values of o,, L an the inuctor current ripple Δ L versus. ary from 0.1 to 0.9 in steps of 0.. Maintain = 50 throughout, by ajusting the variac. Note own the value of for which conuction of the inuctor current is at the bounary between continuous an iscontinuous conuction. Print the waveforms of i L, i, i c an v o for this conition of operation only. Mark the value of in the printout. Reuce to zero by ajusting the variac. 5.5 Select L = 141 H. Set = 50 by ajusting the variac. Repeat 5.3 an Select L = 47 µh, Set = 50 by ajusting the variac. Repeat 5.3 an Run SP program Boost Converter - 10kHz an repeat for L = 141 an 47 µh. 5.8 Run SP program Boost Converter - 0kHz an repeat for L = 141 an 47 µh. 5.9 Run SP program Boost Converter Frequency Response Test. n orer to carry out a frequency response test on the inuctor current an output voltage responses, you will vary the uty cycle sinusoially at some frequency an measure the variations in L an o. As the frequency of variation of increases, the variation of L an o will eventually fail to follow it. This is given by a cut-off frequency at which the amplitue of L an o falls to of its amplitue when the frequency of variation is well below the cut off frequency. Set the frequency of initially at 100Hz on the PC monitor. Set L = 35µH an f s = 0 khz. Note own the amplitue of the inuctor current an loa output voltage o. ncrease the frequency of an take a few reaings of amplitue of L an o vs frequency of. You are expecte to ata well beyon the cut-off frequency. 6.0 Report 6.1 Plot graphs of o versus from the results of sections 5. an 5.8 an iscuss with reference to theoretical preictions an experimental results. 6. escribe how switching frequency f s an the values of L affect the current ripple in the boost inuctor using your experimental results to verify theory. 6.3 escribe how o changes with when the converter operates with iscontinuous conuction using your experimental results to verify theory. 6.4 Plot the frequency response ata for inuctor current L an o versus frequency of foun in 5.9. Comment on these results. Experiment C-C Boost Converter 5 F. Rahman/Feb, 011

6 6.5 You may substantiate your comments by using the simulation results. Computer moel of the buck converter coul be built using PSM in the School computing lab or in room 130. Such moels will give you all waveforms you observe uring your experiment. Experiment C-C Boost Converter 6 F. Rahman/Feb, 011