AN-8024

APPLICATION NOTE

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.1 • 9/18/09

6

The typical voltage and current margins for the rectifier

diode are:

1.3

RRM

DO

VV

>⋅

(18)

1.5

RMS

FDO

II

>⋅

(19)

current rating of the diode.

(Design Example)

The diode voltage and current are

calculated as:

373

5

25.5

18.18

MAX

IN

DO

O

V

VV

V

n

=+

= +

=

1

10.47

18.18 0.36

6.9

0.47

RMS

RMS

MAX

DO

DS

MAX

D

In I

D

A

−

=⋅

−

=⋅

=

Two 5A and 40V diodes in parallel are selected for the

rectifier diode.

[STEP-10] Feedback Circuit Configuration

Since FSBH-series employs current-mode control, the

feedback loop can be implemented with a one-pole and one-

zero compensation circuit.

The current control factor of FPS, K is defined as:

3.2

LIM

LIM

SAT

FB

I

I

K

V

==

(20)

the feedback saturation voltage. which is typically 3.2V.

As described in step 4, it is typical to design the flyback

converter to operate in CCM for heavy load condition. For

CCM operation, the control-to-output transfer function of a

flyback converter using current mode control is given by:

ˆ

ˆ

(/

) (1

/

)(1

/

)

2(1

/

)

ωω

ω

=

⋅⋅

+−

=⋅

++

o

vc

FB

LIN

P

S

Z

RZ

RO

IN

P

v

G

v

KR V

N

N

ss

VV

s

(21)

Equation (21) are obtained as:

2

2

(1

)

1(1

)

,

(/

)

L

ZRZ

P

CO

M

S

P

L

O

RD

D

and

R CDL

N

N

R C

ωω

ω

−

+

==

=

control-to-output

transfer

function

of

Equation

(21).

Because the RHP zero reduces the phase by 90 degrees, the

crossover frequency should be placed below the RHP zero.

Figure 7 shows the variation of a CCM flyback converter

control-to-output

transfer

function

for

different

input

voltages. This figure shows the system poles and zeros

together with the DC gain change for different input

voltages. The gain is highest at the high input voltage

condition and the RHP zero is lowest at the low input

voltage condition.

Figure 7.

CC

M Flyback Converter Control-to Output Trans-

fer Function Variation for Different

Input Voltages

Figure 8 shows the variation of a CCM flyback converter

control-to-output transfer function for different loads. This

figure shows that the low frequency gain does not change for

different loads and the RHP zero is lowest at the full load

condition.

Figure 8. CCM Flyback Converter Control-to Output

Transfer Function Variation for Different

Loads

When the input voltage and the load current vary over a wide

range, it is not easy to determine the worst case for the

feedback loop design. The gain, together with zeros and poles,

vary according to the operating conditions. Moreover, even

though the converter is designed to operate in CCM or at the

boundary of DCM and CCM in the minimum input voltage

and full load condition, the converter enters into DCM,