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 2005 Semtech Corp.

www.semtech.com

SC4809A/B/C

POWER MANAGEMENT

Application Information

The flyback power stage is very popular in 48V input

telecom applications for output power levels up to

approximately 50 watts. The exact power rating of the

flyback power stage, of course, is dependent on the input

voltage/output voltage combination, its operating

environment and many other factors. Additional output

voltages can be generated easily by simply adding another

winding to the coupled inductor along with an output

diode and output capacitor. Obtaining multiple output

voltages from a single power stage is another advantage

of the flyback power stage.

A simplified schematic of the flyback power stage with a

drive circuit block included is shown in Figure 1. In the

schematic shown, the secondary winding of the coupled

inductor is connected to produce output voltage. The

power switch, Q1, is an N-channel MOSFET. The

secondary inductance, L

output filter, The resistor R, represents the load seen by

the power supply output.

Figure 1: Flyback Power Converter

The important waveforms of the flyback power stage

operating in DCM are shown in Figure 2.

Figure 2: Discontinuous Mode Flyback Waveforms

The simplified voltage conversion relationship for the

flyback power stage operating in CCM is given by:

D

1

D

N

N

V

V

P

S

I

O

−

•

•

=

The simplified voltage conversion relationship for the

flyback power stage operating in DCM is given by:

K

D

N

N

V

V

P

S

I

O

•

•

=

Where K is defined as:

S

SEC

T

R

L

2

K

•

•

=