DC to DC Synchronous Converter Design

Abdus Sattar, IXYS Corporation

IXAN0068

10

The power dissipation in the driver is defined by,

SW

DD

total

G

thJC

a

J

DRIVER

f

V

Q

R

T

T

P

•

•

=

−

=

)

(

The estimated driver power dissipation,

mW

P

D

84

10

200

10

10

42

3

9

=

•

•

•

•

≈

−

Dead-Time Power Loss: [3]

The dead-time is defined as the time required when both the MOSFETs are off in order to

prevent shoot-through. In this period, the Schottky diode (or integral body diode is

forward-biased and provided a power loss defined by,

Driver IXS839 provides delay time in the datasheet,

)

/

5

.

0

(

)

(

)

(

_

pF

nS

pF

C

nS

t

Delay

Time

Delay

•

=

(22)

Assume,

nS

t

t

d

d

100

2

1

=

=

, which provides

pF

pF

C

Delay

200

)

(

=

The Delay-Time loss is define in (11-13) as

S

rr

rr

in

S

L

O

f

rr

cd

td

f

t

I

V

f

td

I

I

V

P

P

P

•

•

•

•

+

•

•

⎟

⎠

⎞

⎜

⎝

⎛

Δ

−

•

=

+

=

2

1

1

2

1

1

1

3

9

3

9

10

200

10

37

2

.

2

12

2

1

10

200

10

100

2

528

.

0

12

85

.

0

x

x

x

x

•

•

•

•

+

•

•

⎟

⎠

⎞

⎜

⎝

⎛ −

•

=

−

−

= 0.19905+0.09768= 0.297W= 297mW

()

S

L

O

f

rr

cd

td

f

td

I

I

V

P

P

P

•

•

⎟

⎠

⎞

⎜

⎝

⎛

Δ

+

•

=

=

=

2

2

0

2

2

2

3

9

10

200

10

100

2

528

.

0

12

85

.

0

x

x

•

•

⎟

⎠

⎞

⎜

⎝

⎛ +

•

=

−

=0.208W= 208mW

mW

mW

mW

P

P

P

td

td

BD

505

208

297

2

1

=

+

=

+

=

Synchronous Converter Efficiency:

If we neglect inductor’s DC power loss and capacitor’s ESR loss then the total

Given output power, Po = Vo*Io = 3.3*12= 39.6W

Estimated input power, Pin= 39.6+2.933=42.6W

The efficiency is defined as,

Pin

Po

Iin

Vin

VoIo

=

•

=

η

(23)