Switch Mode Power Supplies

Use of Transformers for Electrical (DC) Isolation• Safety – Isolate output voltages from

Electrical Grid• Multiple Reference ( “ground”) Potentials• Voltage Matching – Large difference

between voltage levels of power source and converter outputs.

• Feedback must traverse isolation boundary1. Separate winding for feedback2. Optical isolation

Gate Drive

Vdd

Transformer Converter Types:

• Flyback Converter – Derived from Buck-Boost• Forward Converter - Derived from Buck• Full Bridge – Derived from Buck

Flyback Converter

p sn n (t)

ip(t)

is(t)

1p p in sp

p pT

n I V DTv t dt

n n

R

2in s

p in sp pp

V DTI V DT

n Ln

R R

ip(t)

+

Vin

-

+

Vout

-

iL(t)

Buck-Boost Converter

(t)

is(t)

(Assume CMM)(Assume CCM)

Flyback Converter (cont)

1out sin s

p s

V D TV DT

n n

1

sout in

p

n DV V

n D

ˆ ˆˆ p p s s

n I n I

R Rs

p sp

nI I

n

ˆ ˆ sp s

p

nI I

n

Same as Buck-Boost

Flyback Converter (cont)

ˆ2

pin p

II I D

1

2

ˆ ˆ1 12 2

psout s p

II nI I D I D

n

p s pp

p p s

V DT V DI

L L f

1

21out inI In

D n D

2

11in out

nDI I

D n

Same as Buck-Boost

2

1 1in

n DV

n D

1 2 n n+

Vin

-

+

Vout

-

2 2

1 1out out in out in

n nV DV DV D V V

n n

2

1

out

out in

VD

nV V

n

2

1

1out in

nV D DV

n

2

1

out

out in

VD

nV V

n

2

1 1in

n DV

n D

2

1

1out in

nV D DV

n

2 2

1 1out out in out in

n nV DV DV D V V

n n

1 p sn n +

Vout,1 > 0

-

+

Vout,2 < 0

-

2sn

Feedback

Multiple Secondaries

i i

dv n

dt

1 21 2

ˆ ˆ ˆs sp s s

p p

n nI I I

n n

1 21 2

s sp s s

p p

n nI I I

n n

1 21 2

s sp s s

p p

n nI I I

n n