Application Note 1612

3

AN1612.1

November 28, 2011

The RMS current through the MOSFET can be calculated from:

Selecting the conduction loss in the MOSFET to 1% of total output

required conduction loss is shown in Equation 10.

Vishay’s SI4436DY is selected in this design.

Output Diode Selection

Schottky diodes are recommended for the output diode due to

their low forward voltage drop. The voltage stress across the

output diode can calculated by:

Diodes Inc’s B180 are employed in this design.

Output Filter

The output capacitance needs to meet the ripple and noise

requirements, and also be able to handle the ripple current.

Assuming ceramic capacitors are used as the output filter, the

voltage ripple from the capacitor’s ESR is negligible. The

minimum capacitance required to meet specifications can be

approximately calculated from Equation 12.

10µF ceramic capacitors are selected for each output. Design

margin has been provided to account for noise spikes.

Snubber Circuit

When the MOSFET switches off, it interrupts the current that

flows through the transformer leakage inductance. An RCD

snubber circuit is typically used in flyback converters to clamp

voltage spikes on the MOSFET.

Assuming that the transformer leakage inductance is 2% of the

magnitizing inductance, the energy stored in the leakage

inductance during MOSFET’s on-time is:

Average power transferred to the snubber circuit is:

To limit peak voltage spikes across the MOSFET to 50V, the

snubber voltage is set to:

The average power transferred to the snubber circuit in

determined by:

constant is substantially longer than the switching period to keep

low ripple voltage on the snubber circuit. A time constant of 10

times the switching period is used for calculation:

Feedback Network

The feedback is being tapped off of the primary auxiliary

winding. This is one of the advantages of selecting the flyback

topology, since the auxiliary winding voltage follows the output.

This scheme was fully exploited, since the load fluctuation is

minimal, and that load regulation does not suffer much at these

power levels. For tighter regulation requirements, an

opto-coupled solution would need to be used, which leads to

additional cost.

Referring to the schematic on page 8, the output voltage can be

set by:

The control-to-output transfer function of the DCM flyback

converter is [1]:

Where:

I

rms FET

,

I

pk

d

3

---

⋅

=

(EQ. 9)

1.06

0.35

3

-----------

0.362A

=

⋅

=

r

DS ON

()

P

FET cond loss

–

,

I

FET rms

,

2

-------------------------------------------

=

(EQ. 10)

0.03

0.362

2

------------------

0.229

Ω

=

=

V

Diode

nV

×

=

IN MAX

,

V

OUT

+

(EQ. 11)

1

26.4

×

15

41.4V

=

+

=

C

OUT

ΔV

PP

2

---------------

1d

2

–

() T

SW

⋅

I

OUT

-------------------------------------

⋅

>

(EQ. 12)

0.42

μF

>

50

3

–

×10

2

----------------------

10.5

–

()

0.1 300

3

×10

⋅

-----------------------------------

⋅

>

W

L

1

2

--- L

2

⋅⋅

=

(EQ. 13)

1

2

--- 0.02 23.8

6

–

×10

1.06

⋅⋅

⋅

267.4nJ

=

=

P

L

W

⋅

=

(EQ. 14)

267.4

9

–

×10

300

3

×10

0.08W

=

⋅

=

V

S

peakV

MOSFET

V

IN MIN

,

–

=

(EQ. 15)

50

21.6

–

28.4V

=

=

R

S

V

S

2

P

l

-------

=

(EQ. 16)

28.4

2

0.08

--------------

10.08k

Ω

=

=

C

S

10

T

SW

R

S

------------

⋅

≈

(EQ. 17)

10

=

3.33

6

–

×10

10

3

×10

---------------------------

3.33nF

=

⋅

R

22

R

23

----------

V

OUT

V

F

+

V

ref

-----------------------------

1

–

=

(EQ. 18)

15

0.6

+

2.514

---------------------

15.2

=

–

=

G

vc

K

R

⋅⋅

2

-------------------------------------

1s ESR C

⋅⋅

+

1s 0.5 R

⋅⋅

⋅

+

()

-------------------------------------------------------

⋅⋅

=

(EQ. 19)