MP6003 – MONOLITHIC FLYBACK/SEPIC DC-DC CONVERTER

MP6003 Rev. 1.01

www.MonolithicPower.com

13

1/26/2014

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2014 MPS. All Rights Reserved.

RCD Type of Snubber Design Procedure:

0

Figure 6—Voltage Waveform of Primary

Power Switch Shown in Figure 5(C)

2. Estimated RCD snubber loss is given by:

)

V

V

N

1

(

P

P

P

O

LK

LOSS

_

RCD

×

+

×

=

Where:

C

2

P

LK

LK

f

I

L

2

1

P

×

×

=

at the power switch turn-off.

snubber by:

LOSS

_

RCD

2

P

D

P

V

R

=

S

D

D

f

1

C

R

>>

×

Input Capacitor

The input capacitors (C1) are chosen based

upon the AC voltage ripple on the input

capacitors, RMS current ratings, and voltage

rating of the input capacitors.

be derived from:

PP

_

IN

S

IN

V

T

)

D

1

(

I

1

C

Δ

×

−

×

=

converter stability. C1 RMS current has to be

considered:

D

)

D

1

(

I

I

IN

1

C

_

RMS

−

×

=

C1 has to have enough RMS current rating.

Output Filter

The simplest filter is an output capacitor (C2),

whose capacitance is determined by the output

ripple requirement.

The current waveform in the output capacitor is

mostly in rectangular shape. The full load

current is drawn from the capacitors during the

primary switch on time. The worse case for the

output ripple occurs under low line and full load

conditions. The ripple voltage can be estimated

by:

S

O

C

PP

0

f

2

C

D

I

V

×

×

=

Δ

−

−

ESR also needs to be specified for the output

capacitors. This is due to the step change in D2

current results in a ripple voltage that is

proportional to the ESR. Assuming that the D2

current waveform is in rectangular shape, the

ESR requirement is then obtained by given the

output ripple voltage.

)

D

1

(

ESR

I

V

O

R

_

PP

O

ESR

−

×

=

Δ

−

The total ripple voltage can be estimated by:

ESR

_

PP

O

C

PP

O

PP

O

V

V

V

−

−

−

−

Δ

+

Δ

=

Δ