Power Electronics-DC-DC Converters/Choppers.pptx

POWER
ELECTRONICS
DC-DC Converters/
Choppers
D.Poornima,AP(Sr.Gr)/EEE,SRIT 1
D.Poornima,
Assistant Professor (Sr.Gr),
Department of EEE,
Sri Ramakrishna Institute of Technology,
Coimbatore

DC-DC Converters/Choppers
 A static device used to obtain a variable DC voltage from a constant DC voltage
 Variable voltage supply is used in electric tractions, trolley cars, golf carts, electric
vehicles, SMPS, PV cell based power generation etc.
 Offers greater efficiency, smooth control, lower maintenance and smaller size
 Choppers are classified into
 Step down choppers
 Step up choppers
 If the output voltage of a chopper is less than the input voltage it is called step down
chopper
 If the output voltage of a chopper is greater than the input voltage it is called step up
chopper

Basic Operating Principle
• Input is fixed DC voltage
• Output is regulated by chopping the supply voltage
• Chopping is done by switching ON and OFF any of the switch
• When thyristor is ON, supply voltage appears across the load
• When thyristor is OFF, voltage across the load is zero
• Thyristors, power BJT, power MosFET, IGBTs are used
• Thyristors are for higher power applications

STEP-DOWN CHOPPER
• Circuit consists of a semiconductor switch (SW), a diode (D), a R-L load.
• When the switch is ON, the output voltage is equal to input voltage.
• When switch is OFF, the power diode operates in the freewheeling mode and the inductor L filters
the ripples in load current.
• Operates in two different modes such as
1. Mode – I during 0 ≤ t ≤ TON and
2. Mode – II during TON ≤ t ≤ T
• Input voltage is fixed
• Average output voltage can be controlled by
varying the ON time and OFF time of
semiconductor switch

WORKING..
Mode – I (0 ≤ t ≤ TON )
• The gating signal of semiconductor
switch S is as shown .
• When the switch SW is closed (ON),
the output load current Io start to build
up exponentially due to inductance L
Mode – II (TON ≤ t ≤ T)
• Switch SW is open (OFF), diode D is ON
and provides a free wheeling path and
current Io starts to decrease.

• The average output voltage is
𝑉
𝑜 =
𝑇𝑂𝑁
𝑇𝑂𝑁 + 𝑇𝑂𝐹𝐹
𝑉𝑖
where, TON is ON time period of switch, TOFF is OFF time period of the switch, Vi is the input voltage = V
𝑉
𝑜 =
𝑇𝑂𝑁
𝑇
𝑉
where, T = TON + TOFF is total time period and
𝑇𝑜𝑛
𝑇
is called the Duty Cycle or Duty Ratio
𝑽𝒐 = 𝑫𝑽
 D varies between 0 and 1.
 If D = 1 then the ratio of output voltage to input voltage at steady state goes to infinity, which is not physic
ally possible.
 Boost converter is a non-linear circuit
 In a practical Boost converter the duty cycle, D, if kept at a value greater than 0.7 will lead to instability.

STEP-UP CHOPPER
• Circuit consists of a semiconductor switch (CH), a diode (D),an inductor L a load.
• When the switch is ON, inductor is connected to the supply Vs and stores energy.
• When switch is OFF, inductor current is forced to flow through the diode
• Operates in two different modes such as
1. Mode – I during 0 ≤ t ≤ TON and
2. Mode – II during TON ≤ t ≤ T
• Input voltage is fixed
• Average output voltage can be controlled by varying the ON
time and OFF time of semiconductor switch

WORKING..
Mode – I (0 ≤ t ≤ TON )
• When the switch S is closed (ON), the current th
rough inductance increases from Imin to Imax an
d energy is stored in the inductor.
Mode – II (TON ≤ t ≤ T)
• Switch S is open (OFF), the current through in
ductance L decreases from Imax to Imin
• Stored energy in inductor L is released to load
• Current tends to decrease, polarity of emf induced
in L is opposite to that in Fig
• Output voltage across load is given by
𝑉0 = 𝑉 + 𝐿
𝑑𝑖
𝑑𝑡

• The average output voltage is
𝑉. 𝑇𝑂𝑁 = (𝑉0 − 𝑉)𝑇𝑂𝐹𝐹
where, TON is ON time period of switch, TOFF is OFF time period of the switch, Vi is the input voltage = V
𝑉
𝑜 =
𝑇
𝑇𝑂𝐹𝐹
𝑉
where, T = TON + TOFF is total time period
𝑽𝒐 =
𝟏
𝟏 − 𝑫
𝑽
𝑓 =
1
𝑇
is frequency of switching or chopping frequency

Control Strategies of a Chopper
 The output voltage of a chopper is directly proportional to the duty cycle.
 This can be controlled using
 Time ratio control
 Current limit control

Time Ratio Control
Can be achieved in two ways
• On period of the waveform is controlled keeping the total time period constant
– called Pulse Width Modulation
• Either Ton or Toff is kept constant and the total time period T is varied- called
Frequency Modulation
PULSE WIDTH MODULATION (Constant Frequency System)
• The time period is kept constant, but the ‘On Time’ or the ‘OFF Time’ is varied.
• Thus duty cycle ratio can be varied.
• Since the ON time or the ‘pulse width’ is getting changed in this method, so it is
popularly known as Pulse width modulation.

FREQUENCY MODULATION (Variable Frequency System)
• The ‘Time Period’ is varied while keeping either of ‘On Time’ or ‘OFF time’ as
constant.
• Since the time period gets changed, so the frequency also changes accordingly
• This method is known as frequency modulation control.

Disadvantages of FM over PWM
• In FM, chopping frequency must be varied in wide range to control the output voltage
– filter design is relatively difficult
• OFF time of the switch is more in FM, so load current may become discontinuous
• Due to the wide range frequency control, interference in radio lines may occur.

CURRENT LIMIT CONTROL
• The on time and off time of the switch is done in such a way as to keep the load current constant
• Current is allowed to fluctuate or change only between 2 values i.e. maximum current (I max) and
minimum current (I min).
• When the current is at minimum value, the chopper is switched ON.
• After this instance, the current starts increasing, and when it reaches up to maximum value, the
chopper is switched off allowing the current to fall back to minimum value.

NON ISOLATED CONVERTERS/CHOPPERS
• The load impedance is directly connected with the input dc voltage.
• Load is not electrically isolated from input dc supply
• Simple in construction and commonly used in different industrial applications as variable voltage dc
supply.
• Also called switch mode dc supply or switch mode dc to dc converters.
• There are different circuit configurations available for dc-to-dc converter without isolation,
• Most commonly used configurations are:
1. Buck converter
2. Boost converter
3. Buck-Boost converters
4. Cuk converters

Buck Converter
• Circuit topology consists of a switch S, a diode DF,
inductor L, capacitor C and load.
• Output voltage can be controlled from zero to the
maximum input dc voltage V by varying the duty
cycle of switch S.
• Also called stepdown chopper.
• High frequency semiconductor switches such as MOSFET and IGBT are generally used
• When the switch S is ON, energy is stored with in the inductor L.
• When the switch S is OFF, the stored energy of inductor is transferred to capacitor C through
freewheeling diode DF.
• This circuit operates in two different operating modes such as
1. Continuous conduction mode
2. Discontinuous conduction mode

Buck Converter
• In continuous conduction mode of operation, the switch S must be turned on before the
inductor current iL reaches zero.
• In case of discontinuous conduction mode of operation, the switch S must be turned ON
after the inductor current iL becomes zero.

Buck Converter- Continuous conduction
• When the switch S is ON, the voltage
across free wheeling diode is V and the
output voltage is Vo.
• During the ON-period TON, inductor
current iL increases from Imin to Imax
linearly.
• When the switch S is OFF, the free
wheeling diode conducts and voltage
across DF is zero and negative output
voltage appears across inductor.
• During the OFF-period TOFF, inductor
current iL decreases from Imax to Imin
linearly

Important parameters of Buck Converter

Boost Converter
• Circuit topology consists of a switch S, a diode DF,
inductor L, capacitor C and load.
• Output voltage can be above the input dc voltage V
by varying the duty cycle of switch S.
• Also called stepup chopper.
• This circuit operates in two different operating modes such as
1. Continuous conduction mode
2. Discontinuous conduction mode

Boost Converter
• In continuous conduction mode of operation, the switch S must be turned on before the
inductor current iL reaches zero.
• In case of discontinuous conduction mode of operation, the switch S must be turned ON
after the inductor current iL becomes zero.

Boost Converter- Continuous conduction
• When the switch S is ON, energy is stored
with in the inductor L
• Inductor current iL increases from minim
um value to maximum value.
• The voltage across inductor L is equal to
input voltage.
• When the switch S is OFF, -ve voltage
𝑉𝐿 = 𝐿
𝑑𝑖
𝑑𝑡
is developed across inductor L
• Voltage across switch will be V + VL which
is greater than input voltage V.
• Stored energy of inductor is transferred
to capacitor C through diode D
• Inductor current decreases linearly.
• During this period, not only the stored
energy of inductor flows but also the
energy flows from dc source to load.

Important parameters of Boost Converter

Buck Boost Converter
 Input voltage source is connected to a solid state device.
 The second switch used is a diode.
 The diode is connected, in reverse to the direction of power flow from source, to a capacitor
 Load and the capacitor are connected in parallel
 Output voltage polarity is opposite to that of the input
 Two modes of operation
 Switch is on
 Switch is off

Buck Boost Converter….
Mode I : Switch is ON, Diode is OFF
 The Switch is ON and therefore represents a short circuit ideally offering zero resistance to the flow
of current
 all the current will flow through the switch and the inductor and back to the DC input source.
 The inductor stores charge during the time the switch is ON
Mode II : Switch is OFF, Diode is ON
 The polarity of the inductor is reversed and the energy stored in the inductor is released and is ulti
mately dissipated in the load resistance
 This helps to maintain the flow of current in the same direction through the load
 Also step-up the output voltage as the inductor is now also acting as a source in conjunction with th
e input source.

Types of Converters
• In choppers, unidirectional power semiconductors devices are used.
• There is a restriction for the polarities of output voltages Vo and the direction of output current Io
• Chopper can be used in any four quadrants by arranging the semiconductor devices appropriately.
• Four-quadrant operational characteristics are classified as :
1. First-quadrant, or Type-A Chopper
2. Second-quadrant, or Type-B Chopper
3. Two-quadrant type-a, or Type-C Chopper
4. Two-quadrant type-b, or Type-D Chopper
5. four-quadrant Chopper, or Type-E Chopper

First-quadrant, or Type-A Chopper
• when chopper is ON ,
 Vo=Vs
 the direction of flow of current (io) is given by the arrow.
• when Chopper is OFF ,
 Vo=0 , but if the load contains inductance, it will discharge
 Direction of current (io) remains unchanged i.e., it will flow throug
h the freewheeling diode, FD.
• Average voltage (Vo) and load current (io) are always positive.
• Power flow in type-A is always from source to load.
• Average output voltage (Vo) is lesser than that of the input voltage (Vs).
• It is a step-down chopper –used for motoring operation of DC motor.

First-quadrant, or Type-A Chopper
• For a motor load, the representation will be
RLE load
• when chopper is ON ,
𝑉0 = 𝐼0𝑅 + 𝐿
𝑑𝑖
𝑑𝑡
+ 𝐸
• when Chopper is OFF ,
0 = 𝐼0𝑅 + 𝐿
𝑑𝑖
𝑑𝑡
+ 𝐸

Second-quadrant, or Type-B Chopper
• For this type of chopper, the load must contain the dc source ‘E’, like
a battery (or a dc motor) .
• When chopper (CH2) is ON
 Vo=0.
 Load voltage ‘E’ will drive a current through ‘L’ and chopper (CH2)
 The inductance(L) will store energy
• When chopper is OFF,
 Vo=E + L di/dt.
 Value of Vo exceeds the supply voltage (Vs).
 Diode D2 will be forward biased and will be in conduction which will allo
w the power to flow to the source
• Whether chopper is ON or OFF, the current flow (io) will be out of the
load and will be treated as negative.
• Since Vo is always positive and io is negative, the power flow will be
from load to source.
• Called a step-up chopper

Second-quadrant, or Type-B Chopper
• Used for regenerative braking of DC motors.
• Chopper control of a subway train

Two-quadrant type-A, or Type-C Chopper
• In this type, Type-A and Type-B choppers are connected in parallel
• Only one chopper works at a time
• Due to the presence of freewheeling diode FD across the load, the outp
ut voltage (Vo) is always positive.
• For first quadrant operation
 Chopper CH1 is turned ON
 Freewheeling diode FD conducts, obtaining the output voltage, Vo=0
 Inductor stores energy.
 load current is positive
• For second quadrant operation
 Chopper CH2 is turned ON
 Energy stored in the inductor flows through CH2-E-L-D2- supply
 Output voltage is given by Vo=Vs.
 load current is negative

Two-quadrant type-A, or Type-C Chopper

Two-quadrant type-B, or Type-D Chopper
• Current polarity is maintained positive, voltage polarity
changes
• Operation is considered for two modes
• Ton>Toff,
 Both the chopper CH1 and CH2 are ON
 Current flows through Vs-CH1-Load-CH2-Vs
 Average output voltage Vo=Vs is positive
• Ton<Toff
 Both choppers are turned OFF
 Diodes D1 and D2 conduct
 Current flows through load-D2-Vs-D1-load
 Average output voltage Vo=-Vs is negative

Two-quadrant type-B, or Type-D Chopper Ton>Toff,

Two-quadrant type-B, or Type-D Chopper Ton<Toff,

Four-quadrant Chopper, or Type-E Chopper
• Consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4 anti-parallel

Working
First-quadrant:-
• When the CH4 is turned ON, CH3 remains
OFF and CH1 gets operated.
• As CH1 and CH4 are at ON condition, the
load voltage is equal to the source voltage
(Vo=Vs)
• Load current begins to flow in the circuit.
• When CH1 is turned off, a positive current
flows through CH4 and D2.
• Thus we can control the load voltage (Vo)
and load current (Io).

Working
Second-quadrant:-
• CH1, CH3, CH4 are kept off and CH2 is being operated.
• As CH2 is turned ON, reverse (or negative) current flows
through L, CH2, D4, and E.
• Inductance (L) is used to store energy when the CH2 is
turned ON.
• When CH2 is turned off, the current is feedback to the
source through diode D1 and D4
• Output voltage is more the supply voltage (Vo=Vs).
• Load voltage is positve and the load current is negative.
• Power feedback is from load to source

Working
Third-quadrant:-
• CH1 is kept off, CH2 is kept ON and CH3 is
being operated
• The polarity of the load emf E is reverse
for working in this quadrant.
• With CH3 being operated, the load gets
connected to the source (Vs) so that both Vo
and Io are -ve leading to third quadrant
operation.
• When CH3 turned OFF, negative current f
reewheels through the CH2 and D4.
• In this way, Vo and Io are controlled

Working
Third-quadrant:-
• CH1 is kept off, CH2 is kept ON and CH3 is
being operated
• The polarity of the load emf E is reverse
for working in this quadrant.
• With CH3 being operated, the load gets
connected to the source (Vs) so that both Vo
and Io are -ve leading to third quadrant
operation.
• When CH3 turned OFF, negative current
freewheels through the CH2 and D4.
• In this way, Vo and Io are controlled

Working
Fourth-quadrant:-
• CH4 is operated and other devices are
kept off.
• Load Emf E must have reverse polarity
for operation in the fourth quadrant
• With CH4 kept ON mode, positive curr
ent flows through CH4, D2, L, and E.
• Inductance ‘L’ stores energy when CH4
is turned ON.
• When CH4 is turned off current is feed
back to the source through diode D2 an
d D3.
• Load voltage is -ve, but the load current
is +ve
• Power is fedback from load to source

Switched Mode Regulators
• In several industrial applications, dc voltage of different voltage levels with respect to ground such
as ±5 V, ± 6 V, ± 9 V, ± 10 V, ± 12 V, are required.
• These different voltage levels should be available from independent of dc supply.
• Figure shows a circuit diagram for isolated dc-to-dc converter.
• This circuit generates two voltages Vo1 and Vo2.
• The amplitude of output voltage depends upon the turn ratio of isolating transformers.
• These converters can be act as step-up, step-down, buck, boost, buck-boost converters.
• This type of isolated dc to dc converter is known as switch mode power supply (SMPS).

Fly-Back Converter
• Circuit topology consists of a dc supply, switch S,
transformer, diode D, capacitor C and a load.
• When the switch S is ON,
• the input dc voltage V is directly applied to the
primary winding of transformer.
• A voltage is induced in secondary winding but
with polarity opposite of primary voltage.
• Due to reverse polarity, diode D is reverse biased
• Current will not flow through secondary winding
• Primary winding of transformer stores the magne
tic energy

• When the switch S is ON, the voltage across the primary winding of transformer is
V1= V-Vs(ON),
where, V is the dc supply voltage, VS(ON) is the voltage across switch S when it is ON.
• Then the voltage induced in the secondary winding of transformer is
𝑉2 =
𝑁2
𝑁1
𝑉1 =
𝑁2
𝑁1
𝑉 − 𝑉𝑠(𝑜𝑛)
where, N1 and N2 are number of turns of primary and secondary winding respectively

• When the switch S is OFF,
• a negative voltage is induced in the secondary winding as current
decreases.
• Diode D is forward biased and conducts.
• The stored energy is transferred to load.
• Voltage across the secondary winding of transformer is
V2 =VD+VC
• Where, VD is the voltage across diode D, VC is the voltage across
capacitor C and VC = Vo
• Therefore,
V2 = VD + Vo
• and output voltage is equal to
Vo = V2 – VD

RESONANT CONVERTERS
• In PWM technique for switching, switches are made to turn on and off at full
load current at high di/dt
• Switches are subjected to high switching stresses and switching power loss
is high.
• Switching also generates electromagnetic interference (EMI) due to large di/dt
and dv/dt
• For low switching loss, fast switching devices should be used
• If the switching frequency is increased to reduce size and weight of the
converter, switching stress, switching losses and EMI are also increased
• Losses can be minimized if the switch changes its state when voltage across
device becomes zero or current through device is zero.
• This type of switching is called zero voltage switching (ZVS) or zero-current switching (ZCS)
• LC resonance is used to bring voltage and current to zero - circuits which employ this are called Resonant
Converters

Zero-Current Switching Resonant Converters
• Two types of ZCS resonant converters, L-type and M-type.
• Both of these circuit topologies use L and C as a series resonant circuit
• L also limits di/dt of the switching current.
L-type ZCS Resonant Converters
• Inductor L and capacitor C near the dc Source Vs, form a resonant circuit
• L1, C1 near the load constitute a filter circuit.
• Direction of currents and polarities of voltages as marked are treated as positive.
• Working is divided into five modes

Mode 1 (0 ≤ t ≤ t1).
• At t=0, switch is turned on.
• Source voltage Vs, gets applied across L and the switch, current iL begins to
flow through Vs, switch S, L and diode D.
• Load current I0 forward biases the Diode D
• As I0 is freewheeling through diode D, voltage across diode and capacitor is 0
• Inductor rises linearly from its zero initial value.
• The diode current iD is given by
• Due to opposing iL and Io the current through the diode will decrease and
become zero at t1

Mode 2 (t1 ≤ t ≤ t2)
• Switch S remains on, D turns off at t=t1, Current IL, flows through Vs, L,L1
and R.
• A current ic begins to build up through resonant circuit consisting of Vs,
L and C in series.
• Capacitor gets charged, so Vc starts increasing
• Constant current through L1 and R is represented by current source I0
• Inductor current grows to peak value of 𝐼𝑝 = 𝐼0 + 𝐼𝑚
• At t=t2, capacitor voltage reaches peak value =2Vs,ic =0 inductor
current drops from peak value to I0.

Mode 3 (t2 ≤ t ≤ t3)
• Switch S remains on
• Capacitor voltage is 2Vs and tends to discharge , ic tends to reverse
• Two currents in inductor, IL and Ic in opposite directions
• IL tends to decrease and will fall to 0 at t3
• Capacitor voltage is almost 2Vs
• At t=t3, due to the reverse current from capacitor, switch S gets
turned off
• ic = - I0

Mode 4 (t3 ≤ t ≤ t4)
• Switch S turned off at t=t3
• Capacitor begins to supply load current and falls to –I0
• Mode ends when capacitor voltage falls to 0.

Mode 5 (t4 ≤ t ≤ t5)
• Capacitor voltage is zero and tends to reverse direction
• Diode D gets forward biased and starts conducting
• Mode comes to an end when switch S is again turned on at t=t5

L Type ZCS Waveforms
• It is seen that during turn-on at t=0 (0≤ t ≤ t1), switch current iL = 0, therefore switching loss VT iL = 0.
• At turn-off at t3 (t2≤ t ≤ t3), iL = 0 and therefore VT iL = 0.
• Switching loss during turn-on and turn-off processes is almost zero.
• The peak resonant current 𝐼𝑚 =
𝑉𝑆
𝑍0
more than the load current I0, otherwise switch current iL will not fall to
zero and switch S will not get turned off.
• The load voltage v0 can be regulated by varying the period t5
• Longer the period t5, lower is the load voltage.

L Type ZCS Waveforms

Zero Voltage Switching Regulators
• ZVS resonant converter consists of diode D1 and capacitor C connected across the switch S.
• Has L,C as the resonant circuit components and L1, C1 as the filter circuit components.
• The function of resonant capacitor C is to produce zero voltage across the switch S.
• Diode D2 provides a free wheeling path to load current I0
• Switch S is turned on and off at zero-voltage across the switch.
• Working can be divided into five modes
• Load current I0 is assumed constant
• Initially, switch S is off
• Inductor current iL = I0 and initial voltage across
capacitor Vco = 0.

Mode 1 (0 ≤ t ≤ t1)
• At t=0, Vc=0, therefore Switch S is turned off at zero
voltage
• Constant current I0, flows through Vs,C and L.
• As a result, voltage across switch S or C builds up lin
early from zero to V, till time t=t1
• Voltage across D2 is Vs at t=0 and 0 at t=t1
• Diode D2 is off.

Mode 2 (t1 ≤ t ≤ t2)
• At t=t1, capacitor is somewhat charged, Vc >Vs
• Diode D2 becomes forward biased
• Resonant current iL is set up in the series circuit Vs,
C,L,D2
• At t=t2, iL=-I0,and capacitor voltage Vc=Vs

Mode 3 (t2 ≤ t ≤ t3)
• At t=t2, iL=-I0,and capacitor voltage Vc=Vs
• At t=t3, capacitor voltage Vc=0
• Reverse bias across D1 vanishes and it starts
conducting
• iL flows through D1
• Switch can be closed at t=t3

Mode 4 (t3 ≤ t ≤ t4)
• At t=t3, capacitor voltage is clamped to 0 by diode
D1 conducting –ve current iL
• Then iL starts to rise to 0.
• As D1 starts conducting, gate pulse is applied to sw
itch
• Switch turns on at zero voltage and zero current
• Current rises linearly to I0 through Vs,S,L, D2
• At t=t4, iD2=0

Mode 5 (t4 ≤ t ≤ t5)
• At t=t4, iL=I0,D2 turns off
• Switch continues to conduct
• Mode 5 ends at t5 when switch is turned off
again at zero voltage

Voltage to Frequency Converters
• Voltage-to-frequency converters (VFCs) accept an analog voltage Vin and generates a pulse train with frequency f.
• The frequency of the output is linearly proportional to its input voltage, that is, f = kVin where k is the sensitivity
of the V-F converter in Hz/V.
• Also called quasi digital converters
• a quasi-digital conversion made by means of a voltage-to-frequency converter (VFC) is going to be used for imple
menting the sensor interface because it exhibits some advantages for the same number of bits
• A VFC is
(1)simpler than a conventional ADC;
(2) it achieves high noise immunity of the transmission signal, and
(3) it achieves high accuracy in the code-to-frequency conversion, with a speed/accuracy compromise that can be
mitigated using efficient conversion techniques.
• Therefore, traditional analog sensors with VFCs provide a well-timed universal solution for the future microelectr
onics in this field.

THANK
YOU

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