13

LTC3806

3806f

For a minimum input voltage of 36V, the largest duty factor

is:

D

V

V

V

+

=

33

33

36

15

0 579

.

.

.

and the minimum percentage ripple is:

X

VD

fL

P

V

kHz

H

W

MIN

IN

MAX

PIN

=

=

µ

=

22

22

36

0 579

250

757

11 34

20 2

•

••

•.

••

.

.%

Transformer Core Selection

High efficiency converters generally cannot afford the core

loss found in low cost powdered iron cores, forcing the use

of more expensive ferrite cores. Actual core loss is inde-

pendent of core size for a fixed inductance, but is very

dependent on the inductance selected. As inductance in-

creases, core losses go down. Unfortunately, increased

inductance requires more turns of wire and therefore cop-

per losses will increase. Generally, there is a tradeoff be-

tween core losses and copper losses that needs to be

balanced. In addition, increased winding resistance will

degrade cross regulation.

Ferrite designs have very low core losses and are preferred

at high switching frequencies, so design goals can con-

centrate on copper losses and preventing saturation.

Ferrite core material saturates “hard,” meaning that the

inductance collapses rapidly when the peak design current

is exceeded. This results in an abrupt increase in inductor

ripple current and consequently, output voltage ripple. Do

not allow the core to saturate! The maximum peak

I

P

VD

X

PK

IN

IN MIN

MAX

MIN

=+









•1

2

Current Sense Resistor Selection

The control circuit limits the maximum voltage drop

across the sense resistor to about 120mV (at low duty

cycle), and only about 70mV at a duty cycle of 92% due to

the maximum allowable drop in the sense resistor. Using

this value calculate:

R

V

I

SENSE

DROP

PK

≤

APPLICATIO S I FOR ATIO

DUTY CYCLE

0

100

150

0.8

3806 F04

50

0

0.2

0.4

0.5

1.0

200

Figure 4. Maximum SENSE Threshold Voltage vs Duty Cycle

Capacitor Selection

In a flyback converter, the input and output current flows

in pulses placing severe demands on the input and output

filter capacitors. The input and output filter capacitors

should be selected based on RMS current ratings and

ripple voltage.

Select an input capacitor with a ripple current rating

greater than:

I

P

V

D

D

RMS

IN

IN MIN

MAX

MAX

=

()

–

1

Continuing the example:

I

W

V

A

RMS

RMS

==

11 34

36

1 0 579

0 579

0 269

.– .

.

.

Low effective series resistance and inductance is also

important in the input capacitor since it affects the electro-

magnetic interference suppression. In some instances

high ESR can also produce stability problems because

flyback converters exhibit a negative input resistance