DC-DC power processing

Power Electronics
EE 368 Lecture-5
These slides are compiled from the material collected from the text book and web resources:
DC to DC Power
processing DC Choppers

Step-down operation.
Step-up operation.
Generation of timing pulses for PWM and FM.
One, two and four quadrant converters
DC Choppers

DC Choppers
A chopper is a device that converts fixed DC input to a
variable DC output voltage directly.
DC-dc converters are used to convert unregulated
dc voltage to regulated or variable dc voltage at the
output.
In other words
There are several dc-dc converter topologies,
the most common ones being buck converter,
boost converter, and buck-boost converter.
Application
Used in switch-mode dc power supplies
Used in dc motor drive.
In dc motor control applications, they are called chopper-
controlled drives
TIDA-00349 Uniquely Efficient Isolated DC/DC Converter for Ultra-
Low Power and Low Power Applications Reference Design Board
Image

DC Choppers
Want to change the motor
RPM
Efficient DC to DC power
conversion
Will it reduce
RPM Efficiently ?
Want to reduce intensity of light

A standalone photovoltaic power system

System Block Diagram
Power Processing

What is the efficient method
to control Traction Motor
RPM ??
Hybrid Automobile
Ground Power

Traction Motor
Diesel Locomotive
Generator which
produce power
through Dynamo and
then supply power to
Traction motors.

Diesel Locomotive
Engine with Generator

Problem
Electrical Motor is a varying electrical load
• A motor needs a lot more power at startup that it does when running.
• Strip motor Winding create almost a short circuit to the source
• A series resistance is used in starter circuits at starter-up and gradually removed
when it reaches to max RPM at the load.
• Motors also draw a lot more power when it run at full load.
Solution: A variable DC power supply

DC Choppers
Manual in series resistance
to control current by
resducing voltages Using PWM to
control
Voltage/Current

DC Choppers
Need a process that convert
FIXED DC source power
efficiently into VARIABLE DC
power that is required by
the LOAD.
Answer: DC-DC choppers circuits
will perform the job.

DC Choppers
The chopper will produce higher voltage at the load than the input supply voltage.
Two types of DC to DC conversions:
The chopper will produce lower voltages from the input supply voltages:
Step down or BUCK CONVERTER
Step up or BOOST CONVERTER
BUCK CONVERTER
BOOST CONVERTER
BUCK-BOOST CONVERTER
The chopper will produce lower/higher voltage at the load than the supply voltage.

DC Choppers
buck-boost converter
buck converter boost converter
Step down Chopper , Step Up Chopper and Step Down/Up Chopper

DC Choppers
Step Down Chopper with Resistive Load (Buck Converter)
V
i0
V0
Chopper
R
+

When Switch is ON, the source voltages appear across the load
When Switch is OFF zero voltages appear across the load
SW = ON Vo = Vs
SW = OFF Vo = zero
SW
Because of DC input to the switch, forced commutation is
required to turn OFF the power supply to the load
IGBT can be used as a SWITCH with forced commutation
circuitry

DC Choppers
Vdc
v0
V
V/R
i0
Idc
t
t
tON
T
tOFF
Switching of IGBT
ON
OFF
verage value of output or load voltage.
verage value of output or load current.
Time interval for which SCR conducts.
Time interval for which SCR is OFF.
Period of switching
dc
dc
ON
OFF
ON OFF
V A
I A
t
t
T t t




   or chopping period.
1
Freq. of chopper switching or chopping freq.f
T
 
Circuit Analysis

DC Choppers
Vdc
v0
V
V/R
i0
Idc
t
t
tON
T
tOFF
Switching of IGBT
ON
OFF
Average Output Voltage
.
duty cycle
ON
dc
ON OFF
ON
dc
ON
t
V V
t t
t
V V V d
T
t
but d
t
 
  
 
 
  
 
 
  
 T
Circuit Analysis
Average Output Current

DC Choppers
2
0
2
But during ,
Therefore RMS output voltage
1
.
.
ON
ON o
t
O
ON
O ON
O
t v V
V V dt
T
tV
V t V
T T
V d V


 

 2
2
Output power
But
Output power
O O O
O
O
O
O
O
P V I
V
I
R
V
P
R
dV
P
R





Circuit Analysis
RMS value of output voltage:

DC Choppers
Vdc
v0
V
V/R
i0
Idc
t
t
tON
T
tOFF
Example:
A Chopper circuit is operating on Time Ratio Control (TRC)
at a frequency of 2 kHz on a 460 V supply. If the load
voltage is 350 volts, calculate the conduction period of the
thyristor in each cycle.
Given: Vin =460 v, Vout = 360 v, f = 2000 Hz
T = 1/f = 1/2000 = 0.5 m sec
Vout = T on/ Time period x Vin
Conduction period of thyristor
Ton = (T x Vout)/Vin
(0.5 x 350) / 460 x 1000 = 0.38 m sec

DC Choppers
A dc chopper has a resistive load of 20 Ω and input voltage VS = 220V.
When chopper is ON, its voltage drop is 1.5 volts and chopping
frequency is 10 kHz. If the duty cycle is 80%, determine the average
output voltage and the chopper on time.
Example:
Find:
1. Output average voltage and current
2. Output RMS voltage and current
3. Output power

DC Choppers
Step Down Chopper with Resistive Load
(Buck Converter)
Transistor Switch ‘on’ Period
Transistor Switch ‘off’ Period

DC Choppers
when the switching transistor is switched on, it is
supplying the load with current.
Initially current flow to the load is restricted as
energy is also being stored in L1, therefore the
current in the load and the charge on C1 builds up
gradually during the ‘on’ period.
Throughout the ON period, there will be a large
positive voltage on D1 cathode and so the diode will
be reverse biased and play no role during ON period
Transistor Switch ‘on’ Period

Transistor switched off: the energy stored in the magnetic field
around L1 is released back into the circuit.
The voltage across the inductor (the back e.m.f.) is now in reverse
polarity to the voltage across L1 during the ‘on’ period, and sufficient
stored energy is available in the collapsing magnetic field to keep
current flowing for at least part of the time when the transistor switch
is OFF.:
The back e.m.f. from L1 now causes current to flow around the circuit via the load and D1, which is now
forward biased.
Once the inductor has returned a large part of its stored energy to the circuit and the load voltage begins to
fall, the charge stored in C1 becomes the main source of current, keeping current flowing through the load
until the next ‘ON’ period begins.
The overall effect of this is that, instead of a large square wave appearing across the load, there remains
only a ripple waveform, i.e. a small amplitude, high frequency triangular wave with a DC level of
VOUT = VIN x (On time of switching waveform (tON) /time periodic of switching
waveform( T))

DC Choppers

DC Choppers
STEP-UP OR BOOST CONVERTER (Circuit converts Low input voltage to High output Voltages)
• Electrical motors used in driving electric automobiles require much higher voltages,
in the region of 500V, that could be supplied by a battery alone
• Voltage keep dropping with time as it is consumed by the load over longer
period of time.
Sources:
Batteries Rectified DC from AC mains supply
DC from solar panels Fuel cells *
Dynamos DC generators.
Input Sources for boost Converter
* A fuel cell produces electricity through a chemical reaction

DC Choppers
Boost Converter Basic Circuit:
Same components as they were
used in Buck converter except that
their positions have been rearranged.
MOSFET or BJT can be used as
a SWITCH
Basic Principle
1. High frequency applied to the
gate of the MOSFET, it will
behave as short path.
2. With no gate pulse applied to
MOSFET it will become open
path. Behavior of an Inductor
Step-up DC Choppers

DC Choppers
Boost Converter
• There is virtually no current flowing in the remainder of the circuit as the combination of D1, C1
and the load represent a much higher impedance than the path directly through the heavily
conducting MOSFET.
Boost converter Operation (At Switch ON)
• At start high frequency square wave are applied to the
MOSFET gate (Virtual ON condition for the switch).
• During this time MOSFET conducts, placing a short
circuit from the right hand side of L1 to the negative
input supply terminal.
• During this period current flows between the positive
and negative supply terminals through L1, which stores
energy in its magnetic field.

DC Choppers
Boost Converter
Current path during the low period of the switching square
wave cycle.
As the MOSFET is rapidly turned off the sudden drop in
current causes L1 to produce a back e.m.f. in the opposite
polarity to the voltage across L1 during the on period, to
keep current flowing.
This results in two voltages, the supply voltage VIN and the back e.m.f.(VL) across L1 in series with each
other.
This higher voltage (VIN +VL), with no current path through the MOSFET will move forward through D1
and will charges up the capacitor C1
This VIN +VL w[ll both togeater will appear across the load.
This mean the output voltage is Boosted

DC Choppers
Boost Converter
The theoretical DC output voltage is determined by the
input voltage (VIN) divided by 1 minus the duty cycle (D)
of the switching waveform, which will be some figure
between 0 and 1 (corresponding to 0 to 100%) and
therefore can be determined using the following formula:
Vout = Vin / 1-DExample:
If the switching square wave has a period of 10µs, the input voltage is 9V and the ON is half of
the periodic time, i.e. 5µs, then the output voltage will be:
VOUT = 9/(1- 0.5) = 9/0.5 = 18V (minus output diode voltage drop)
if the duty cycle increased from 0.5 to 0.99 the output voltage produced would be:
VOUT = 9/(1- 0.99) = 9/0.01 = 900V

DC Choppers
Frequency Modulation

Angle Modulation is the process in which the frequency or the phase of the
carrier varies according to the message signal.
Frequency Modulation divided into:
Amplitude Modulation
Phase shift modulation.
Time Duration or Pulse Width modulation.
Frequency Modulation is the process of varying the frequency of the carrier
signal linearly with the message signal.
Phase Modulation is the process of varying the phase of the carrier signal
linearly with the message signal.

Amplitude Modulation
Amplitude modulation or AM is the method of
varying the instantaneous amplitude of carrier
signal accordingly with instantaneous
amplitude of message (subject) signal.

Phase:
Phase is the position
of a point in time (an
instant) on a waveform
cycle.
Phase modulation
PM or Phase modulation
is the process of varying
the instantaneous phase
of Carrier signal
accordingly with
instantaneous amplitude
of message signal

Time 
Amplitude

2/18/2004
Introduction to Engineering Electronics
STOLEN FROM K. A. Connor
37
Communicating With Pulses
• PCM: Pulse Code Modulation

2/18/2004
Introduction to Engineering Electronics
STOLEN FROM K. A. Connor
38
PCM: Pulse Code Modulation

Average voltages in one Cycle
%100
Period
TimeOn
CycleDuty
  LHavg VDVDV  1
 *In general analysis, VL is taken as zero volts for
simplicity.
 Duty Cycle is
determined by:
 *Average value of a
signal can be found as:
Period (T)
Duty
Cycle (D)VL
VH
On Off
0
1
( )
T
y f t dt
T
 
Original formula for Vo

Pulse Width Modulation
There are many different ways to control the
speed of DC motors but one very simple and
easy way is to use Pulse Width Modulation.
Power Electronics vision:
Pulse width modulation is
basically a square wave
with a varying width
reference to amplitude of
the original signal.

41
PWM: Pulse Width Modulation with reference
• Signal is compared to a sawtooth wave producing a pulse width
proportional to amplitude
• Reference crossing the data signal above, digital pule goes low.
• Reference crossing the data signal below, digital pule goes high.

  LHavg VDVDV  1
Pulse Width Modulation with no reference signal
Assuming: VL is taken as zero volts for simplicity.

By changing the MOSFET switch turning ON
and OFF time, the stored energy in the
circuit inductor and the charge in the
capacitor iis controlled in-turn the output
average of a BUCK/ BOOST is regulated.
PWM control the output average
voltage of the BUCK/BOOST
converter

Design of PWM Generator:
To generate a 50 percent duty cycle, the 555 timer has to work in a-stable
mode.
By connecting diode, D1 between the trigger input and the discharge input,
the timing capacitor would charge up directly through resistor R1 only, as
resistor R2 is effectively shorted out by the diode.
The capacitor discharges as normal through resistor, R2. The previous
charging time of t1 = 0.693(R1 + R2) C is modified to take account of this
new charging circuit and is given as: 0.693(R1.C). The duty cycle is therefore
given as D = R1 / (R1 + R2). Then to generate a duty cycle of more than 50%,
resistor R1 needs to be equal to resistor R2.
PWM Generator
Ref: P.Sathya1, Dr.R.Natarajan 2

Applications
• Automotive applications
• Power amplifier applications
• Adaptive control applications
• Solar Battery power systems
• Consumer Electronics
• Communication Applications
Advantages
• Gives the high output voltage
• Low operating duty cycles
• Lower voltage on MOSFET

By combining these two regulator designs it is possible to have a regulator
circuit that can cope with a wide range of input voltages both higher or lower
than that needed by the circuit. Fortunately both buck and boost converters
use very similar components; they just need to be re-arranged, depending on
the level of the input voltage.
Buck and Boost Converters Combined
the common components of the buck and boost circuits are combined. A control
unit is added, which senses the level of input voltage, then selects the appropriate
circuit action. (Note that in the examples in this section the transistors are shown
as MOSFETs, commonly used in high frequency power converters, and the diodes
shown as Schottky types. These diodes have a low forward junction voltage when
conducting, and are able to switch at high speeds).

Buck and Boost Converters Combined
Buck Boost Converter

Operation as a Buck Converter
During Tr1 ‘OFF’ Period
Operation as a Buck Converter
During Tr1 ‘ON’ Period
Buck-Boost Converter
Buck Converter
Buck Converter Tr1 Tr2
Supplying source voltages to
load while storing energy in
inductor
ON OFF
Inductor become source to
provide power to the load, while
the main source is disconnected
OFF OFF

Buck-Boost Converter
Operation as a Boost Converter
Operation as a Boost
Converter During Tr2 ‘on’
Period
Operation as a Boost
Converter During Tr2 ‘off’
Period
Boost Converter Tr1 Tr2
Using source voltages to storing
energy in inductor
ON ON
Inductor plus the main source is
used to provide power to the
load
Vo = Vin + VL+ Vc
ON OFF

Type Laminated lithium-ion battery
Voltage 403.2V [1]
Nominal voltage 360V [2]
Total capacity 24 kWh [2] (16 kWh available, 67% DoD [3], 21 kWh
declared [4])
Power output Over 90 kW
EV Battery Specs

Introduction
Converter classification
Electric vehicle (EV) power supply systems to convert high voltage battery power to conventional 12Vdc,
24Vdc, or 28Vdc
Multi Quadrant DC-DC Converters
DC-DC converters in an EV may be classified into unidirectional and bidirectional converters
Unidirectional converters are used to supply power to various onboard loads such as
sensors, controls, entertainment and safety equipment.
Bidirectional DC-DC converters are used where regenerative braking is required.
During regenerative braking the power flows back to the voltage bus to recharge the
batteries.

Four Quadrant Operation of any drives or DC Motor means that the machine operates
in four quadrants. ...
A motor operates in two modes – Motoring and Braking. Amotor drive capable of
operating in both directions of rotation and of producing both motoring and regeneration
is called a Four Quadrant variable speed drive
Multi Quadrant DC-DC Converters

DC MOTOR FACTS
Direction of rotation of a DC Motor can be
reverse by changing supply voltage polarity.
By rotating the rotor of DC motor it produce power

Four Quadrant Operation of any drives or DC Motor means that the machine
operates in four quadrants.
Forward Braking
Forward motoring
Reverse motoring
Reverse braking.
Four Quadrant Operation of DC Motor
A motor operates in two modes –Motoring and Braking. A motor drive capable of operating in
both directions of rotation and of producing both motoring and regeneration is called a Four
Quadrant variable speed drive.

According to direction of
output voltage and current
Semiconductors devices used
in chopper circuit are
unidirectional.

Four quadrant converters. (Forward motoring)
The directions of Io and Vo marked in the figure is taken as positive
direction output voltage (Vo) and output current (Io) follows the direction
as marked in figures then the chopper operation will be restricted in the
first quadrant of Vo - Io plane.

Four quadrant converters. (Forward motoring)

DC-DC power processing

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