Chapter 3 - Choppers

7/31/2019 Chapter 3 - Choppers

1/74

UTMUNIVERSITI TEKNOLOGI MALAYSIA

1


SEE 4433Power Electronics

And Drives


2/74


2

Chapter 3:

DC to DCConversion(Choppers)


3/74


3

Overview

General Buck converter

Boost converter

Buck -Boost converter Switched-mode power supply (SMPS)

- fly back converter

- forward converter

- bridge converter


4/74


4

DEFINITION :

Converting the unregulated DC input to a controlledDC output with a desired voltage level.

General block diagram:

DEFINITION

Applications:High-frequency switched-mode power supply(SMPS), DC motor control (traction, forklift, electric vehicles, trams,battery chargers, capacitor chargers)


5/74


5

Linear Regulator

Transistor is operated in linear

(active) mode. Output voltage:

The transistor can be convenientlymodeled by an equivalent variableresistor, as shown.

Power loss is high at high currentdue to:


6/74


6

Switching Regulator

Transistor is operated in switched-mode:Switch closed: Fully on (saturated)Switch opened: Fully off (cut-off)

When switch is open, no current flow in itWhen switch is closed no voltage drop

across it.

Since P=V.I, no losses occurs in theswitch.

Power is 100% transferred from

source to load.Power loss is zero (for ideal switch):

Switching regulator is the basis of allDC-DC converters


7/74


7

Buck (Step-Down) Converter

Duty cycle D:0 D 1

complement D :D

= 1 - D


8/74


8

Switch is turned on (closed)

Diode is reversed biased. Switch conducts inductor current This results in positive inductor voltage, i.e:

It causes linear increase in the

inductor current


9/74


9

Switch turned off (opened)

Because of inductiveenergy storage, i L continues toflow. Diode is forward biased

Current now flows(freewheeling) through thediode. The inductor voltage can bederived as:


10/74


10

Analysis


11/74


11

Analysis


12/74


12

Steady-state Operation

Realization using

power MOSFET

and diode +-

+ -

V L(t) ic(t)

V g

i L(t)

t DT s T s

+

L

D1 R

Unstable

Steady-state


13/74


13

Average, Maximum andMinimum Inductor Current

Average inductor current = Average current in R L


14/74


14

Continuous Current Mode (CCM)

From previous analysis,

For continuous operation,

This is the minimum inductor current to ensure continousmode of operation. Normally L ischosen to be >> L min


15/74


15

Output Voltage Ripple


16/74


17/74


17

Basic Design Procedures

Calculate D to obtain required output voltage. Select a particular switching frequency (f) and device

preferably f >20 kHz for negligible acoustic noise higher f s results in smaller L and C. But results in higher losses.Reduced efficiency, larger heat sink.

Possible devices: MOSFET, IGBT and BJT. Low power MOSFETcan reach MHz range.


18/74


18

Calculate L min . Choose L >>10 L min Calculate C for ripple factor requirement.

Capacitor ratings: must withstand peak output voltage must carry required RMS current. Note RMS current for

triangular waveform is I p/3, where I p is the peak capacitor current given by i L/2.

Electrolytic Capacitors (E-CAPs) can be used. Wire size consideration:

Normally rated in RMS. But i L is known as peak. RMS valuefor iL is given as:


19/74


19

Question 1.

A buck converter has an input voltage of 12 V. The required averageoutput voltage is 5 V and peak-to-peak output ripple voltage is 20 mV.The switching frequency is 25 kHz. If the peak-to-peak ripple currentof inductor is limited to 0.8 A, determine

i. duty ratio, Dii. filter inductance, Liii. output filter capacitor, C

Question 2. A buck converter is supplied from a 50V battery source. Given L = 400H, C=100 F, R=20 , f =20 kHz and D = 0.4. Calculate:

(a) output voltage(b) maximum and minimum inductor current,(c) output voltage ripple.

Examples

(0.42, 148 mH, 197 uF)


20/74


20

Question 3.

A buck converter has an input voltage of 50V and output of 25 V.The switching frequency is 10KHz. The power output is 125 W.

(a) Determine the duty cycle,(b) value of L to limit the peak inductor current to 6.25 A,(c) value of capacitance to limit the output voltage ripple

factor to 0.5 %.

Question 4.Design a buck converter such that the output voltage is 28 V whenthe input is 48V. The load is 8 . Design the converter such that it willbe in continuous current mode.

The output voltage ripple must not be more than 0.5 %.Specify the frequency and the values of each component.Suggest the power switch also.

Examples


21/74


21

Buck Converter Conclusion

The output voltage may be controlled by the duty -ratio, but cannot be larger than input voltage

The voltage conversion ratio depends solely on duty -ratio, and is independent of load condition The capacitor ripple current is independent of loadcurrent

The off -state voltage across device is supply voltage


22/74


22

Boost (step-up) converter


23/74


23

Boost Analysis: Switch Closed


24/74


24

Boost Analysis: Switch Opened

UTM St d t t


25/74


25

Steady-stateOperation

Boost converter produces output voltage that is greater or equalto the input voltage. Alternative explanation:

when switch is closed, diode is reversed. Thus output is

isolated. The input supplies energy to inductor. When switch is opened, the output stage receives energy fromthe input as well as from the inductor. Hence output is large.

Output voltage is maintained constant by virtue of large C. The off-state voltage impressed across power device is V o

UTM


26/74


26

Average, Maximum, MinimumInductor Current

UTM


27/74


27

L and CValues

UTM


28/74


28

The output voltage is always greater or equal to theinput voltage The voltage conversion ratio depends solely on duty -ratio, and always greater than or equal to one

Theoretically the output voltage tends to infinity as Dtends to 1, but in practice the maximum output voltagewill be limited to conduction loss The capacitor ripple current is severe and dependsdirectly on the load current level The off -state voltage impressed across device isoutput voltage

Boost Converter Conclusion

UTM


29/74


29

Example :

A boost converter has an input voltage of 5 V. The average

output voltage is 15 V and the average load current is 0.5 A. If fs = 25kHz, L = 150 H and C = 220 F, determine(a)duty cycle (b) inductor ripple current(c) inductor peak current (d) output ripple voltage

Design a boost converter to provide an output voltage of 36Vfrom a 24 V source. The load is 50 W. The voltage ripple factor must be less than 0.5%. Specify the duty cycle ratio, switchingfrequency, inductor and capacitor size, and power device.

The boost converter has the following parameters:Vd = 20V, D = 0.6, R = 12.5, L = 65 H, C = 200 F, f s= 40 kHz.Determine(a) output voltage, (b) average, maximum and minimuminductor current, (c) output voltage ripple.

UTM


30/74


30

Buck-boost Converter

UTM

B k b A l i


31/74


31

Buck-boost Analysis

UTM

B k b A l i


32/74


32

Buck-boost Analysis

UTM


33/74


33

Output Voltage

Steady state operation : NOTE :

Output of a buck-boostconverter either be higher or lower than input.

If D > 0.5, output is higher than input

If D < 0.5, output is lower input Output voltage is alwaysnegative.

Note that source is never directly connected to load. Energy is stored in inductor when switch is closed andtransferred to load when switch is opened. Off -state voltage across power switch is ( V d Vo )

UTM


34/74


34

Average Inductor Current

Assuming no power loss in theconverter, power absorbed bythe load must equal power supplied the by source, i.e .

But average source current is relatedto average inductor current as :

Substituting for V o ,

UTM L and C


35/74


35

L and Cvalues

Ripple Factor, r

UTM


36/74


36

Example :

A buck-boost converter has input voltage of 12 V. The duty cycle

is 0.25 and the switching frequency is 25 kHz. L = 150 H andC = 220 F. The average load current is 1.25 A.Determine:(a)average output voltage

(b)peak-to-peak output voltage ripple(c)peak-to-peak inductor ripple current(d)peak current transistor (-4 V, 56.8 mV, 0.8 A, 2.07 A)

A buck-boost converter has the specification as follows:

Vd = 18 V, D = 0.6, f s = 40 kHz, R = 10 , L = 50 H, C = 200 FConsider all components are ideal, determine(a)output voltage(b)average, maximum, and minimum inductor current

(-27 V, 6.75 A, 9.45 A, 4.05 A)

UTM


37/74


37

Buck-Boost Converter conclusion

The voltage conversion ratio depends solely on duty -ratio and less than unity for D < 0.5.

For D > 0.5, the conversion ratio greater than unity. Themaximum conversion ratio is limited in practice by circuitlosses

Similar to boost, the capacitor current ripple is severeand depends on load current

The off -state voltage impressed across the device is

the sum of supply and output voltage The average inductor current is the sum of averageinput and output current

UTM


38/74


38

Converters in CCM: Summary

UTM


39/74


39


UTM


40/74


40


UTM Control of DC DC Converter:


41/74


41

Control of DC-DC Converter:Pulse Width Modulation (PWM)

UTM


42/74


42

DC-DC Converter Control

Purpose of control:To regulate theoutput voltage sothat it is

maintained withina specifiedtolerance band(e.g. 2% of outputDC voltage)

Basic blockdiagram for converter control

UTM


43/74


43

Isolated DC-DC Converter

Isolated DC -DC requires isolation transformer

Two types: Linear and Switched -mode Power Supply ( SMPS ) Advantages of switched mode over linear power supply

-Efficient (70 95%)-Weight and size reduction

Disadvantages-Complex design-EMI problems

However above certain ratings, SMPS is the only feasible choice Types of SMPS

-Flyback-forward-Push-pull-Bridge (half and full)

UTM


44/74


44

Linear and SMPS Block Diagram

UTM


45/74


45

Linear and SMPS Block Diagram

UTM


46/74


46

High Frequency Transformer

Basic function:

1) Input - output electrical isolation2) Step up/down time - varying voltageBasic input - output relationship

UTM Fl b k C


47/74


47

Flyback Converter

UTM


48/74


48

Operation: Switch Closed

Flyback Converter

UTM h l d


49/74


Flyback Converter49

Operation: Switch Closed

UTM O i S i h O d


50/74


50

Operation: Switch Opened

Current flows in different direction at the Primary..

Flyback Converter

UTM O i S i h O d


51/74


Flyback Converter51

Operation: Switch Opened

UTM O V l


52/74


52

Output Voltage

Input output relationship is similar to buck-boost converter.Output can be greater of less than input, depending upon D.

Additional term, i.e. transformer ratio is present.Positive output voltage polarity

Flyback Converter

UTM

Flyback Waveforms


53/74


53

Flyback Waveforms

AverageInductor Current

UTM M Mi I d C


54/74


54

Max., Min. Inductor Current

UTM M Mi I d C


55/74


55

Max., Min. Inductor Current

UTM E l


56/74


56

Example :

The Flyback converter has these specifications:

DC input voltage: 40 VOutput voltage: 25 VDuty cycle: 0.5Rated load: 62.5 WMax peak-peak inductor current ripple: 25 % of the

average inductor current.Maximum peak-peak output voltage: 0.1 VSwitching frequency: 75 kHz

Based on the abovementioned specifications, determinea) Transformer turns ratiob) Value of magnetizing inductor L m .c) Maximum and minimum inductor current.d) Value of capacitor C.

UTM E l


57/74


57

Example :

The Flyback converter has the following:

Input voltage: 24 V DCOutput voltage: 5 VTransformer turn-ratio (N1/N2): 3Load resistance: 5Magnetising inductance: 500 HOutput capacitor: 200 FSwitching frequency: 40 kHz

Based on the abovementioned specifications, determinea) duty ratiob) average inductor current in L m .c) maximum and minimum inductor current.d) output voltage ripple.

0.39, 540 mA, 770 mA, 310 mA, 48 mV

UTM E l


58/74


58

Example :

For a Flyback converter, consider all components are ideal.

The parameters are shown as follows:

Based on the abovementioned specifications, calculatea) The range of duty ratios, Db) The minimum value of Lm , to ensure continuous current if the input voltage varies from 10 V to 40 V.c) The required capacitor if output voltage ripple is 0.2 V p-p .

0.123


59/74


59

Forward Converter Derived from Buck Converter, but D max = 0.5 Transformer magnetizing current must be taken into account Assume transformer ideal, when switch ON, D 1forward biased and D 2 reversed biased, V L positive and i Lincreases linearly

When switch OFF, D 1 reversed biased and i L circulatesthrough D 2 . VL negative and i L decreases linearly

UTM Forward Converter


60/74


60

Equating the integral of inductor voltage over one time

period to zero,

It shows that the output voltage is proportional to the duty

ratio, D, similar to buck converter Practically transformer magnetization current must beconsidered in converter operation to avoid converter failuredue to energy stored in its core. A third/reset/demagnetizing winding is required so thatenergy in transformer core can be transferred back to thesupply when switched turned OFF.

Forward Converter



61/74


61

Forward Converter

UTM


62/74


62



63/74


63

Forward Converter When switch ON,

UTM


64/74


64

UTM Full bridge Converter


65/74


65

Full-bridge Converter

UTM


66/74


66

UTM Full Bridge: Basic Operation


67/74


67

Full Bridge: Basic Operation

Switch pair: [S1 & S2]; [S3 & S4].

Each switch pair turn on at a time as shown. The other pair is off. AC voltage is developed across the primary. Thentransferred to secondary via high frequency transformers.

On secondary side, diode pair is high frequency full waverectification. The choke (L) and (C ) acts like the buck convertercircuit.

Output Voltage

UTM Half bridge Converter


68/74


68

Half-bridge Converter

UTM

Half-bridge


69/74


69

Converter

UTM Industry Favorites SMPS


70/74

UUNIVERSITI TEKNOLOGI MALAYSIA

70

Industry Favorites SMPS

UTM Example :


71/74

UNIVERSITI TEKNOLOGI MALAYSIA

71

Q1 Referring to Figure Q1,a) Sketch the waveforms for the inductor voltage and current (per cycle) with f s = 25 k Hz. (5 marks)b) With the assumption that the switching device operates as a fixedfrequency switch with a duty ratio D and the inductor current iscontinuous, show that the output voltage V o = Vd / (1 D).

(10 marks)c) If the switching device operates at a duty ratio of 0.4, calculate theoutput voltage and the average inductor current when the inputvoltage is 60 V and the resistive load is 10 . Determine the inductor value such that peak to peak inductor ripple current is 4 A.

(10 marks)

Example :

Ans: 100 V, 16.67A, 240 u H

UTM


72/74


72

Q2 Referring to Figure Q2, and assuming converter operates in CCMa) Sketch the waveforms for the inductor voltage ,inductor currentand capacitor current(per cycle).b) Prove that the peak-to-peak output voltage ripplec) The converter in Figure Q2 required to provide a 400Vregulated output from a variable DC source, ranges between 150V < Vd < 300 V. The output power varies over the range of 100 Wto 1 kW and the switching frequency is 50 kHz. If the converter operates in CCM,

i) Calculate the range of the duty ratio, Dii) Determine the capacitor value that results in the maximumpeak-to-peak ripple voltage of 3 Viii) Determine the minimum value of inductance to ensure CCMof current over all condition

Example :

Ans: 0.25


73/74


73

Q2 (a)Draw and label a block diagram of a switched mode power supply(SMPS).Briefly state the function of each element in the diagram. (8 marks)

(b) Figure Q3 shows a boost converter with the inductor current waveform insteady state. The key quantities of the waveforms are labeled. The circuit usesa feedback controller to regulate the output voltage at 12V by varying the dutycycle, D. It was found that, for this condition, the peak-peak output voltageripple, vo, is 0.08V. Based on the circuit condition and its waveformdetermine:(i) The duty cycle D and the input voltage, Vs. (3 marks) (ii) The values of R, L and C. (6 marks) (iii) The maximum value of R to keep the converter operation in a continuousconduction mode. All other parameters are kept constant. (3 marks) (iv) Draw the voltages across MOSFET, vm and diode, vd. Suggest theminimum voltage rating for the diode and MOSFET. (3 marks)

Ans: i)0.4, 7.2V, ii)2.5 , 7.2u, 240uH, iii)10

p

UTM


74/74


The End

of Chapter 3

Chapter 3 - Choppers

Documents

Transcript of Chapter 3 - Choppers