Experiment 2 - DC-DC Step-up (Boost) Converter

ELEC4614

Power Electronics Laboratory

THE UNIVERSITY OF NEW SOUTH WALES School of Electrical Engineering & Telecommunications ELEC4614 Power Electronics Laboratory Experiment 2: DC-DC Step-up (Boost) Converter 1.0

Objectives

This

experiment introduces you to a step-up DC-DC converter circuits. These are found in many power supplies where a DC supply at a voltage which is higher than the available DC supply Vd is required. Operations and steady-state characteristics of the step-up (boost) converter circuits will be studied experimentally. 2.0

Background

The boost converter circuit is shown in figure 1(a) below in which the switch T is operated from a pulse-width modulator operating at a carrier frequency f s. The switch T is also operated with a duty cycle D which ranges from 0 to 1. Figure 1(b) indicates a few waveforms of the circuit when the switch T is turned ON and OFF at frequency f s, with a duty cycle D. D

iL

id +

vL

iD



+

ic

Vd

C

T

Io

Vo

R (Load)

Figure 1(a)

vL Vd

Vd -Vo Imax

i L

iL

ic V - o R

Experiment 2 DC-DC Boost Converter

DT Figure 1(b) 1

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We will assume that all the devices and components of the circuit are ideal, the inductor current is continuous (always positive) and the output voltage is held constant at Vo by the large output capacitor during the switching intervals. Under these circumstances the inductor voltage v L, and inductor the capacitor currents iL and iC, respectively, are as indicated in figure 1(b). The following equations then hold for the boost converter. Vd 1 D

Vo 

I max 

Vd V D  d 2 (1  D ) R 2 Lf s

…………………………………….(2)

I min 

Vd V D  d 2 (1  D ) R 2 Lf s

…………………………………..(3)

I L,average 

where

…………………………………………..(1)

Vd (1  D) 2 R

…………………………………….(4)

Vo = output DC voltage, V Vd = supply DC voltage, V L = inductance of the inductor, H R = Load resistance, . Io 

Vo I ; I d  o ; and Vd I d Vo I o R 1 D

………………………….(5)

For a given switching frequency, the minimum inductance Lmin for continuous conduction is given by,

Lmin 

D( 1  D ) 2 R 2 fs

…………………………………(6)

The output voltage ripple across the filter capacitance C is given by

Vo Vo

3.0



D RCf s

………………………………….(7)

Switching losses

Losses in the devices due to overlap of the voltage and current transients at turn off and turn on affect the efficiency of the converter. The switching frequency f s is carefully selected to avoid these losses becoming significant. f s also affects the physical sizes of the inductor and capacitor.


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4.0


Discontinuous conduction

The relationship between the output voltage Vo and D, although simple when the inductor current is continuous during the whole of switching period, it is very steep when D approaches 1. The dynamics of this converter is also not suitable for operation in continuous conduction mode. For the discontinuous conduction mode, it can be shown that

1  2D 2 R  Vo  Vd 1  1   2  Lf s 

…………………….(8)

Although this is a non-linear function, nevertheless, discontinuous conduction mode is the preferred mode of operation of a boost converter. 4.

Equipment 1 power IGBT switch 1 3 phase diode bridge rectifier module 1 L-C smoothing filter for the DC supply 1 50  load resistor for the boost converter 1 47-235 H inductance board for boost converter 1 four-channel oscilloscope 1 DC voltmeter and ammeter modules 2 Isolated transducer boards with 1V/1A and 1V/V sensors PC with DSP board and interface card

5.

Experiment

Familiarise yourself with the experimental set-up, especially the IGBT, the rectifier diode bridge, the input DC filter, the buck inductor L, the buck diode, the output filter capacitor C, and the load resistor. The inductor board has inductors in the range of 22-110 H which are connected to taps. A DSP board resident in a PC generates the PWM switching pulses with which to turn the buck switch ON and OFF. The frequency of switching can be changed only by running the appropriate Buck Converter for x kHz programs from the Desktop where x stands for the switching frequency. For each of these programs, the appropriate operator interface must also be run from the menu. The duty cycle D for each of these switching frequencies can be selected by a slider on the PC monitor. The DC supply Vd to the buck converter is obtained from an AC supply via an auto-transformer (variac), a rectifier diode bridge followed by an LC filter. These are located on the left-hand side of the equipment panel. For the whole of this experiment, Vd must be set to 50V. When the converter duty cycle or load increases, Vd will drop because of voltage drop in the variac. For each setting of D or load, you will need to adjust the variac so that Vd is always 50V. PRECAUTIONS!! 1.

The experiment is pre-wired on the equipment panel. Do not attempt to alter any connection of the power circuit while the DC supply is on. The only alterations you will need to make to the power circuit is the selection of tapings of the buck inductor (using the blue wire) and connection/disconnection of the load (using the red wire). Make sure that all power connections (screwed terminal blocks) are firm (tight) before the DC supply is switched on. Any intermittent connection in any part of the circuit will destroy the IGBT. Experiment 2 DC-DC Boost Converter 3 F. Rahman/Feb, 2011

ELEC4614

2.


Also, make sure to adjust the variac to zero before you change the boost inductor taps. You must not manually disconnect the boost inductor while it still carrying current.

Goals: In this experiment, you will run the boost converter with switching frequencies f s = 5, 10 and 20kHz; you will select boost inductance L = 47, 141 and 235µH for each of these frequencies; you will vary the duty cycle D from 0.1 to 0.9 in steps of 0.1 for each of these combinations, Finally you will take frequency response test data by varying D sinusoidally, from 100Hz to 4kHz while the converter operates with f s = 20kHz and L = 141µH. Data obtained from these tests will allow you understand the roles of L and f s on the continuous and discontinuous modes of operation of the converter, and its control characteristics. 5.1

The power circuit for the boost converter is shown in figure 2. Before switching the DC supply to the converter ON, run the DSP program Boost Converter - 5kHz in directory “Elec4614_labs_boost” on the desktop and run the DSpace control desk program using “Open experiment” under the file menu to run the corresponding experiment file. Observe the PWM switching pulses for a switching frequency of f s = 5 kHz. Adjust the duty cycle D. Note down the range over which D can be adjusted. Set D initially to minimum. Connect the switching signal from the DSP to the gate of the IGBT using a BNC cable. Connect this signal also to channel 4 of the CRO and use it to trigger the CRO at all times. Set L to 235 H, and D to minimum initially. Connect the boost converter load (R  5 ). The DC supply, Vd , to the converter is obtained from rectifier-filter circuit. Switch AC power to the variac (auto-transformer) supplying the rectifier and adjust it to obtain a DC supply of 50V. The DC supply must be maintained at 50V throughout the rest of this experiment by adjustment of the variac for each setting of D or load current.

5.2

Observe the inductor and the capacitor currents, and voltages across these on CRO channels 1, 2 and 3 of the CRO using the isolated sensors or clip-on probes. Blue wire LC filter

3phase 415V 50Hz Supply

Id

D

IL

L (47-235H)

Variac

ID

Io Red wire

iT

Vd

Vo

C

Io 5 Ohm Load

1000F BNC Rectifier

IGBT gate Driver


DSP

PC

4

CRO

Isolated sensor boards

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Figure 2 The peak-peak ripple on the output voltage can be observed by using AC coupling on the CRO. 5.3

Display the switching waveform at the top of the CRO. Using a suitable time base, display iL, v L, ic, v o , iT, and iD, three at a time, below the trace of the switching waveform. Adjust D from 0.1 to 0.9 and identify vales of D for which continuous and discontinuous conduction of inductor current iL take place. Note that the input current of the boost converter and the output DC voltage V o can become disastrously high when D approaches unity. You may find it necessary to maintain D below about 0.7 in order to prevent such situations.

5.4

Tabulate the DC values of Vo, Id , IL and the inductor current ripple ΔIL versus D. Vary D from 0.1 to 0.9 in steps of 0.2. Maintain Vd = 50V throughout, by adjusting the variac. Note down the value of D for which conduction of the inductor current is at the boundary between continuous and discontinuous conduction. Print the waveforms of iL, id , ic and v o for this condition of operation only. Mark the value of D in the printout. Reduce Vd to zero by adjusting the variac.

5.5

Select L = 141 H. Set Vd = 50V by adjusting the variac. Repeat 5.3 and 5.4.

5.6

Select L = 47 µH, Set Vd = 50V by adjusting the variac. Repeat 5.3 and 5.4.

5.7

Run DSP program Boost Converter - 10kHz and repeat 5.3 – 5.6 for L = 141 and 47 µH.

5.8

Run DSP program Boost Converter - 20kHz and repeat 5.3 – 5.6 for L = 141 and 47 µH.

5.9

Run DSP program Boost Converter Frequency Response Test. In order to carry out a frequency response test on the inductor current and output voltage responses, you will vary the duty cycle D sinusoidally at some frequency and measure the variations in IL and Vo . As the frequency of variation of D increases, the variation of IL and Vo will eventually fail to follow it. This is given by a cut-off frequency at which the amplitude of IL and Vo falls to 0.707 of its amplitude when the frequency of D variation is well below the cut off frequency. Set the frequency of D initially at 100Hz on the PC monitor. Set L = 235µH and f s = 20 kHz. Note down the amplitude of the inductor current and load output voltage Vo . Increase the frequency of D and take a few readings of amplitude of IL and Vo vs frequency of D. You are expected to data well beyond the cut-off frequency.

6.0

Report

6.1

Plot graphs of Vo versus D from the results of sections 5.2 and 5.8 and discuss with reference to theory and experimental results.

6.2

Describe how switching frequency f s and the values of L affect the current ripple in the boost inductor using your experimental results to verify theory.

6.3

Describe how Vo changes with D when the converter operates with discontinuous conduction using your experimental results to verify theory.

6.4

Plot the frequency response data for inductor current IL and Vo versus frequency of D found in 5.9. Comment on these results.


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6.5


You may substantiate your comments by using the simulation results. Computer model of the buck converter could be built using PSIM in the School computing lab or in room 130. Such models will give you all waveforms you observed during your experiment.


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Experiment 2 - DC-DC Step-up (Boost) Converter

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