MP6003 – MONOLITHIC FLYBACK/SEPIC DC-DC CONVERTER

MP6003 Rev. 1.01

www.MonolithicPower.com

12

1/26/2014

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Duty Cycle Range

The duty cycle range is determined once N is

selected. In general, the optimum operating

duty cycle should be smaller for high input/low

output than low input/high output applications.

Except for high output voltage or wide input

range applications, the maximum D usually

does not exceed 60%.

Voltage Stress of the Internal Power Switch

& External Schottky Diode

For the internal power switch, the voltage stress

is given by:

P

O

IN

DS

V

N

V

V

V

+

×

+

=

lower the Vp. Smaller R can reduce Vp, but

power loss will increase. See Snubber Design

for details.

--

+

--

+

C2

C

R

D

0

C1

S

D2

Figure 4—Key Operation Waveform

For the rectifier, D2, the voltage stress is given

by:

2

PD

IN

O

2

D

V

N

V

V

V

+

+

=

Use of a R-C or R-C-D type snubber circuit for

D2 is recommended.

2

PD

V

can

be selected as

40~100%

of

)

NV

V

(

K

V

0

)

MAX

(

IN

s

)

MAX

(

DS

+

×

=

)

N

V

V

(

K

V

)

MAX

(

IN

0

2

D

)

MAX

(

2

D

+

⋅

=

For example,

V

23

)

8

V

75

V

5

(

6

.

1

V

V

144

)

V

5

8

V

75

(

25

.

1

V

V

5

V

,

6

.

1

K

,

25

.

1

K

,

8

N

,

V

75

V

2

D

DS

O

2

D

S

)

MAX

(

IN

=

÷

+

×

=

=

×

+

×

=

=

=

=

=

=

So

the power switch rating should be higher than

144V,

and

the

rated

voltage

for

the

synchronous rectifier or Schottky diode should

be higher than 23V.

Snubber Design (Passive)

Snubber for Power Switch

Figure 5 shows four different ways to clamp the

voltage on the power device. RCD type of

snubber circuit is widely used in many

applications.

S

(A)

(B)

(C)

(D)

S

S

S

Figure 5—Snubber Designs