a novel control method of dc-dc converters dr.m.nandakumar professor department of electrical...

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A NOVEL CONTROL METHOD OF DC-DC CONVERTERS

• Dr.M.Nandakumar• Professor

• Department of Electrical engineering• Govt. Engineering College

• Thrissur

Dept. of EEE, GEC, Thrissur

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Outline• Introduction• DC-DC converter topologies• Buck converter• Closed loop control of buck converter using PI controller• One cycle control• Buck converter using OCC• Boost converter• Boost converter using PI controller• Boost converter using OCC• One Cycle Control of Buck Boost converter• Performance comparison of PI and OCC controller• conclusion

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Introduction• DC-DC converters are subjected to variable input/

variable output conditions

• Regulation of converter operation is an essential requirement

• Closed loop controller is used for the regulation of out put voltage

• 1. Line Regulation• 2. Load regulation

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DC-DC conversion techniques –an introduction

5 V o = 50V dc -dcc onve r te r

10A

V dc = 100V

5 V o = 50V

10A

V dc = 100V

50V+ -

P lo ss= 500W 5 V o = 50V

10A

V dc = 100V

P lo ss= 500W L A & B D - L ine a r A m plif ie r & ba s e dr ive r

L A &B D V r e f

C a s e 1 : V olta ge d iv ide r C a s e 2 : L ine a r s e r ie s re gula tor

V dcV o

C

LS1

2

DC- DC CONVERTER TOPOLOGIES

• Buck converter or step - down converter• Boost converter or step - up converter• Buck-Boost converter or step-down/up converter• Cuk converter• Full Bridge converter

Only step-down and step-up are the basic converter topologiesBoth buck-boost and cuk converters are combination of these basic topologiesFull bridge converter is derived from step-down converter

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Switch-mode dc-dc converter

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Drawbacks and modifications of the circuit

In practical circuits, load will be inductive (even for resistive load due to stray inductance) leading to dissipate or absorb the inductive energy which may destroy the switch

Output voltage fluctuates between 0 and Vd

Modifications

Problem of stored inductive energy is overcome using freewheeling diode

Output voltage fluctuation are very much diminished using Low pass Filter

Drawbacks

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Buck converter (Step-down converter)

iL

+

_

vd

+

vo=V0

_

Low pass filterio

RC

L

id

+

voi

_

+ vL -

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Sep-down dc-dc converter

iL

+

_

vd

+

vo=V0

_

Low pass filterio

RC

L

id

+

voi

_

+ vL -

1Cut off frequency of low pass filter,

2cf

LC

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Step-down converter circuit states(Continuous Conduction Mode)

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Volt-sec balance(cont.)

Under steady state operation the integral of the inductor voltage vL over one time period must be zero

0 0

0s on s

on

T t T

L L L

t

v dt v dt v dt

0( ) ( )( ) 0d o on s onV V t V T t

o on

d s

V tD

V T

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Buck converter (Step-down converter) in CCM

In Continuous Conduction Mode (CCM), neglecting power losses associated with all circuit elements, the input power Pd is equal to output power Po

d d o oV I V I

o on

d s

V tD

V T

0

1o d

d

I V

I V D

Io is the average output current and Id is the average input current Hence in CCM step – down converter is equivalent to a dc transformer (step down)

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Closed loop control of buck converter

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Closed loop control of Buck Converter(with fixed input)

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Closed loop control of Buck Converter(with fixed input)-output voltage

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Buck converter using PI controller

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Transient performance of PI controller

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Closed loop control of Buck Converterwith input voltage perturbations - line

regulation

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Closed loop control of Buck converterInput (changes form 14 V to 20V) and output voltage wave forms using PI controller

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• In PWM control, the duty ratio is modulated in a direction that reduces the error.

• When the input voltage is perturbed, that must be sensed as an output voltage change and error produced in the output voltage is used to change the duty ratio to keep the output voltage to the reference value.

• This means it has slow dynamic response in regulating the output in response to the change in input voltage.

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One cycle control (OCC)

One cycle control• Non linear control technique.• Uses the concept of control of average value of switching

variable.

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Buck converter using One Cycle control (OCC)

Controls the duty ratio of switch such that the average value of switched variable is equal to or proportional to the control reference in each cycle

The output voltage of the buck converter is the average value of the switched variable vs.

int1

1

1( ) ( )in

f

in

f

v t v t dtRC

Vt

RC

K. M. Smedley, “ Control Art of Switching Converters,”Ph.D. Thesis, California Institute of Technology, 1990.

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Buck converter using One Cycle control (cont.)

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Power Source Perturbation RejectionV

in Vref

VrefV

int

d(t)

t

t

tTs

Here, the input perturbation will immediately cause a change in slope of the integration within one switching period. As a result duty ratio changes and output voltage do not change even if power a source having a disturbance.Ie if input suddenly increases the slope of integrator output (= ) increases and it reaches the reference voltage Vref early and ON period reduces and OFF period increases leading to reduction of duty ratio D

01

1

1( ) ( )in

f

in

f

v t v t dtRC

Vt

RC

1

in

f

V

RC

Dept. of EEE, GEC, Thrissur 25

Change in Reference VoltageV

ref

VrefV

int

d(t)

t

t

tTs

Vin

When the control reference is perturbed by a large step up, the time taken to reach the new control reference increase (slope of integration remains the same since Vin is not changing)); therefore the duty ratio is larger. When the control reference is lower, the duty ratio is smaller.

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Buck converter with one cycle control

Clock frequency =10 kHzOr Clock period = 0.1msecK= 1/Ts = 10000

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Buck converter with one cycle control (cont.)Input voltage and output voltage

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Performance comparison between OCC and PI during input voltage perturbation

• (a)Input voltage perturbation (b) Output voltage using OCC (c) Output voltage using PI controller

b

a

c

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Buck converter using OCC with reference voltage perturbation

Dept. of EEE, GEC, Thrissur 30Performance comparison between OCC and PI during output voltage reference perturbation

• (a)output reference perturbation (b) Output voltage using OCC (c) Output voltage using PI controller

b

a

c

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Step-up (Boost) Converter

+

_

vd

+

vo=V0

_

io

RC

id

L

+vL -

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Volt-sec balance Boost converter

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Volt-sec balance Boost converter (cont.)

Boost converter circuit while the switch is position 1

Boost converter circuit while the switch is position 2

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Boost Converter in Continuous Conduction Mode

+

_

vd

+

vo=V0

_

io

RC

id

L

+vL -

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Inductor voltage in boost converter

Boost Converter in Continuous Conduction Mode

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Boost Converter in Continuous Conduction Mode(cont.)

In steady state the time integral of the inductor voltage over one time period must be zero

0

0

( ) 0

( )(1 ) 0

1

1

d on d o off

d s d s

d

V t V V t

V DT V V D T

V

V D

Assuming a lossless circuit, Pd = Po

0 0

0 (1 )

d d

d

V I V I

ID

I

Io is the average output current and Id is the average input current Hence in CCM step – up converter is equivalent to a dc transformer (step up)

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Closed Loop Control of Boost Converter

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BOOST converter

• In closed loop, output voltage Vo should be equal to reference voltage Vref,

• Hence equation can be rewritten as

0

0

( ) 0

( )(1 ) 0

1

1

d on d o off

d s d s

d

V t V V t

V DT V V D T

V

V D

𝑉 𝑟𝑒𝑓 −𝑉𝑑=𝑉𝑟𝑒𝑓 .𝐷 𝑉 𝑟𝑒𝑓 −𝑉𝑑=1𝑇 𝑆

0

𝑇𝑂𝑁

𝑉 𝑟𝑒𝑓 .𝑑𝑡

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Simulation of Boost converter using OCC

0

1

1 ON

do ref

T

ref d ref refs

VV V

D

V V DV V dtT

Dept. of EEE, GEC, Thrissur 41Performance comparison between OCC and PI during input voltage perturbation

• (a)Input voltage perturbation (b) Output voltage using OCC (c) Output voltage using PI controller

b

a

c

Dept. of EEE, GEC, Thrissur 42Performance comparison between OCC and PI during output voltage reference perturbation

• (a)output reference perturbation (b) Output voltage using OCC (c) Output voltage using PI controller

b

a

c

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BUCK-BOOST Converter

𝑉 𝑟𝑒𝑓 (1−𝐷)=𝑉𝑖𝑛 .𝐷 𝑉 𝑟𝑒𝑓 =1𝑇 𝑆

0

𝑇𝑂𝑁

(𝑉𝑖𝑛+𝑉𝑟𝑒𝑓 ) .𝑑𝑡

( )(1 ) 0

1

d s o s

o

d

V DT V D T

V D

V D

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In closed loop, the output voltage Vo should be equal to reference voltage Vref

Hence by rewriting the equation,

BUCK-BOOST Converter -OCC

0

(1 )

( )

1( )

ON

ref d

ref d ref

T

ref d refs

V D DV

V D V V

V V V dtT

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Closed loop control of Buck boost converter using OCC

0

0

1(1 )

( )

1( )

ON

d ref

ref d

ref d ref

T

ref d refs

DV V V

DV D DV

V D V V

V V V dtT

Dept. of EEE, GEC, Thrissur 46Performance comparison between OCC and PI during input voltage perturbation

• (a)Input voltage perturbation (b) Output voltage using OCC (c) Output voltage using PI controller

b

a

c

Dept. of EEE, GEC, Thrissur 47Performance comparison between OCC and PI during output voltage reference perturbation

• (a)output reference perturbation (b) Output voltage using OCC (c) Output voltage using PI controller

b

a

c

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OCC vs. PI    OCC PI

Buck converter input voltage variation

Settling time 6ms 35ms

Maximum deviation from steady state

0.8V 4.2V

Buck converter reference voltage variation

Settling time 4ms 40ms

Maximum deviation from steady state

0.5V 0.2V

Boost converter input voltage variation

Settling time 1ms 50ms

Maximum deviation from steady state

0.1V 9V

Boost converter reference voltage variation

Settling time 10ms 25ms

Maximum deviation from steady state

1V 1V

Buck Boost converter input voltage variation

Settling time 6ms 25ms

Maximum deviation from steady state

1V 5V

Buck Boost converter reference voltage variation

Settling time 4ms 25ms

Maximum deviation from steady state

2V 2V

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PI Vs. OCC :-Settling time performance

1:- buck input perturbation 2:- buck output reference perturbation

3:- boost input perturbation 4:- boost output reference perturbation

5:- buck boost input perturbation 6:- buck boost output reference perturbation

1 2 3 4 5 60

10

20

30

40

50

60

OCC

PI

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Conclusion• Compared to PI controller, OCC gives a better transient

performance for DC-DC converter.

• Less settling time

• Less maximum deviation from steady state

• Can find wide applications in drives and renewable energy sources.

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