Current Mode Control-Functional Basics and

Classical Analysis

Pulsewidth Modulated Dc-to-Dc Power Conversion. By Byungcho ChoiCopyright © 2013 IEEE, Published by John Wiley & Sons, Inc

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2

22

Chapter Outline

1) Current Mode Control Basics

2) Classical Analysis and Control Design Procedures

3) Closed-Loop Performance of Peak Current Mode Control

4) Current Mode Control for Boost and Buck/Boost Converters

3

33

Voltage Mode ControlC

urre

nt M

ode

Con

trol B

asic

s

2

1

Switch turn-on at beginning of switching periodControl law: turn-off the switch when intersects with .

| ( 0) || ( 0) |

ramp con

O ref

V v

Z j V VZ j

xx

x f �

Ov�

�

Sv

conv

rampV

refV

rampV

conv

2Z

1Z( )q t

( )q t

4

44

Current Mode ControlC

urre

nt M

ode

Con

trol B

asic

s

2

1

Switch turn-on at beginning of switching periodCSN: current sensing networkControl law: turn-off the switch when intersects with .| ( 0) || ( 0) |

I con

O ref

v vZ j v VZ j

xxx

x f �

Li

Sv

refV

CSN

Iv

conv

2Z

1Z

Ov�

�

conv

Iv

( )q t

( )q t

5

55

Propagation of Current Feedback DisturbanceC

urre

nt M

ode

Con

trol B

asic

s

Sub-harmonic oscillation when 0.5D >x

conv

conv

IvIvc

IvIvc

0.5D >

0.5D �

6

66

Addition of Compensation RampC

urre

nt M

ode

Con

trol B

asic

s

SvLi

Ov

�

�

CSN

Iv ��

refV

rampV

conv

2Z

1Z( )q t

conv

Iv

I rampv V�

rampV

( )q t

Control waveformsx

7

77

Addition of Compensation RampC

urre

nt M

ode

Con

trol B

asic

s

SvLi

Ov

�

�

CSN

Iv ��

refV

rampV

conv

2Z

1Z

conv

I rampv V�

I rampv Vc �

Propagation of current feedback disturbancex

8

88

Waveforms of Current Mode ControlC

urre

nt M

ode

Con

trol B

asic

s

Current mode control with unity inductor current sensingx

Equivalent representationx

I Lv i

conv

Li

L rampi +V

rampVrampV( )q t

( )q t

conv

rampV

I Lv i

conv

con rampv V�

( )q t

( )q t

Li

conv

9

99

Waveforms of Current Mode ControlC

urre

nt M

ode

Con

trol B

asic

s

PWM waveforms

Compensation ramp slope

con rampv V�

( )Li k'( 1)Li k' �

sdT'

Li

Lic

conv

nSeS

fS

n sS dT'

f sS dT'

Li

sdT'

e sS dT'

Lic

con rampv V�

( )Li k'

( 1)Li k' �

| ( ) ( )| ( )=

| ( 1) ( 1)| ( 1)=

( 1) 1 for stablity( )

2

L L L n s e s

L L L f s e s

f eL

L n e

f ne

i k i k = i k S dT S dT

i k i k = i k S dT S dT

S Si ki k S S

S SS

c� ' ' � '

c� � � ' � ' � '

�' �� �

' �

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10

1010

Peak Current Mode ControlC

urre

nt M

ode

Con

trol B

asic

s

Sv Ov

�

�

CSN

Iv ��

refV

rampV

conv

2Z

1Z( )q t

rampV

IvI rampv V�conv

( )q t

Block diagram Control waveforms

Peak current mode control senses the switch current.Switch current sensing is functionally identical to the inductor current sensing.Advantages of switch current sensing

i) simple CSN design ii) dc and

xxx

ac sensing for switch current iii) overcurrent protection for switch current

11

1111

Sub-harmonic Oscillation and Compensation RampC

urre

nt M

ode

Con

trol B

asic

s

Sub-harmonic oscillationwithout compensation ramp Effects of compensation ramp

conv

( )d t 0.25

0.57

conv

( )d t 0.25

0.57

12

1212

Current Sensing NetworkC

urre

nt M

ode

Con

trol B

asic

s

1

1CSN gain :

11( ) 1 1

resets the transformer when the switch is open.

and form a noise filter.

R R

Ii sense

Q

tR C H

ramp H HR R R R

z

f f

vR Ri n

VtV t V e V tR C R C

D Q

R C

�

x

§ ·§ ·§ ·¨ ¸ �x � | � ¨ ¸¨ ¸¨ ¸ © ¹© ¹¨ ¸

© ¹

x

x

senseR

pwmV

RRRC

conv

ramp IV v� HV

0

Iv

�

�

rampV�

�

1 : n

Qi

QzD fR

fC

rampV

IvI rampv V�conv

( )q t

Iv

13

1313

Benefits and Issues of Current Mode ControlC

urre

nt M

ode

Con

trol B

asic

s

z Benefits of current mode control

Improved dynamic performance: current mode control is suited for boost and

buck/boost converters with an RHP zero in duty-ratio-to-output

transfer function

Robustness of converter dynamics: current mode control offers good performance

for both CCM and DCM operations

Simple compensation design: current mode control employs a standard two-pole

one-zero compensation for all three basic PWM converters

z Issues of current mode control

Dynamic modeling and analysis of the multi-loop control system

Sampled-data feature (sampling effects) of the control scheme