1 [9-4] Mor M. Peretz, Switch-Mode Power Supplies Current feedback - - PDF document

SLIDE 1

1

Mor M. Peretz, Switch-Mode Power Supplies

[9-1]

Control of switch-mode converters Current Programmed Mode control

CPM

Mor M. Peretz, Switch-Mode Power Supplies

[9-2]

Problem of voltage mode control

 

dB d Vo dec dB 40

• dec

dB 20

• Second order transfer function = complex compensator

Mor M. Peretz, Switch-Mode Power Supplies

[9-3]

Additional feedback

System order reduction

System

• rder

is reduced for each state variable (inner loop) feedback

SLIDE 2

2

Mor M. Peretz, Switch-Mode Power Supplies

[9-4]

Current feedback loop

N 1 v i

e

• 

AMP MOD N Io RL C D L S Vin



V

e

V Vo D

d io vo ve

For ‘strong’ feedback

e

• ε

v N 1 i v 1 LG   

Mor M. Peretz, Switch-Mode Power Supplies

[9-5]

System representation in CPM

N ve

L

• R

C 1  π 2

L e R

N v

• v

Mor M. Peretz, Switch-Mode Power Supplies

[9-6]

Design of the feedback loops

• e

V V

L e

I V

BW of the inner loop must be well above the outer loop BW Instrumentation limit

SLIDE 3

3

Mor M. Peretz, Switch-Mode Power Supplies

[9-7]

Design of the feedback loops



V

Mor M. Peretz, Switch-Mode Power Supplies

[9-8]

The advantages of current feedback

e

• v

v

dec db

40 

dec db

20 

Typical power stage VM

e

• v

v

dec db

40 

dec db

20 

Same power stage (outer loop) with CM

Mor M. Peretz, Switch-Mode Power Supplies

[9-9]

Average current mode

ref

V

fv

Z

inv

Z

fi

Z

SLIDE 4

4

Mor M. Peretz, Switch-Mode Power Supplies

[9-10]

Peak current mode

L

I c

I

S

T t

Cycle-by-cycle protection

Mor M. Peretz, Switch-Mode Power Supplies

[9-11]

PCM and ACM

• Current feedbacks - reduce the order of

system

• The difference is in BW of the current

feedback loop

• Increase the output impedance

Mor M. Peretz, Switch-Mode Power Supplies

[9-12]

Sub-harmonic oscillations

L

I

L

I

1

I 

2

I 

2

I 

1

I 

e

V

e

V

S

T t t

D>0.5 I2>I1 D<0.5 I2<I1

SLIDE 5

5

Mor M. Peretz, Switch-Mode Power Supplies

[9-13]

Stability analysis of Sub-harmonic

• scillations

   

1 L

• n

L

• n

I t I m t  

   

2 L s L

• n
• ff

I T I t m t  

   

1 2 L s L

• n
• ff

I T I m t m t   

1 2 2 1

• n
• ff
• n
• n
• ff
• ff

m t m t t D m t m D   

Steady-state:    

L s L

I T I 

Mor M. Peretz, Switch-Mode Power Supplies

[9-14]

Stability analysis of Sub-harmonic

• scillations

                 

1 1 1

: :

L s L L s L s L s L s L s

I D d T I i m D d T DC I DT I m DT AC I dT i m dT         

 

1 L s

i m dT  

       

2 2 1 L s s

• n

L s L L

• ff

i T m dT D m i T i i m D                   

Mor M. Peretz, Switch-Mode Power Supplies

[9-15]

Stability analysis of Sub-harmonic

• scillations

                 

2

2 1

• n

L s L

• ff
• n
• n

L s L s L

• ff
• ff

n

• n
• n

L s L s L

• ff
• ff

D i T i D D D i T i T i D D D D i nT i n T i D D                                                   

 

0, 1 0.5 , 1

• n
• ff

L s

• n
• n
• ff

D D i nT Stable when D D D            

   

n

• n

L s L

• ff

D i nT i D          

SLIDE 6

6

Mor M. Peretz, Switch-Mode Power Supplies

[9-16]

Slope compensation

       

L r c L c r

I t I t I I t I I t    

Mor M. Peretz, Switch-Mode Power Supplies

[9-17]

Slope compensation

 

c r

I I t 

   

1 L r s

i m m dT   

   

2 L r s

i T m m dT   

   

2 1 r L s L r

m m i T i m m          

     

2 1 n n r L s L L r

m m i nT i i m m               0, 1 , 1

L s

i nT          

2 2 2 1 1 2 2 2 2

1 1 0.5

r r r r r

• n

r r

• ff

m m m m m m m m m D m m m m m m D m               

Mor M. Peretz, Switch-Mode Power Supplies

[9-18]

SLIDE 7

7

Mor M. Peretz, Switch-Mode Power Supplies

[9-19]

Mor M. Peretz, Switch-Mode Power Supplies

[5-20]