ML4813

8

APPLICATIONS

The ML4813 is used to implement a discontinuous mode

flyback (buck-boost) power factor regulator as shown in

Figure 7. This topology is particularly well suited for low

power applications such as fluorescent ballasts and low

power switching supplies. It is also a useful topology

when there is a requirement for the output voltage to be

lower than the peak input voltage, or where an isolated

output is required. This is not possible with a boost

topology, where the output voltage must always be higher

than the maximum peak of the input voltage range. The

typical input range for the flyback power factor regulator

is from 90VAC to 260VAC.

The regulator operates in the discontinuous current

conduction mode. The inductor energy stored during the

ON time of the power switch Q is completely delivered

to the output capacitance during the OFF time. Under

steady state conditions, the inductor current at the

beginning of the ON time starts to ramp-up from 0 Amps

to a value that is determined by the instantaneous value

of the input full wave rectified voltage; the ON time as it

is set by the error amplifier and the PWM comparator;

and finally by the inductor itself.

The expression for the inductor peak current is given by:

I

Vt

L

L

IN

ON

q

q

05 05

=

™

(4)

Where:

Figure 8 shows the relationship between the low

frequency envelope and the high frequency inductor

current. Note that for clarity the scale between the two

waveforms has not been preserved. Normally for 60Hz

input line and 100kHz switching frequency, each half of

the sine wave contains approximately 833 high frequency

triangular waveforms.

The envelope of the peaks of the switch current, which in

this case represents the current drawn from the input

source, has a sinewave shape. This relationship is shown

as:

II

LP

qq

(5)

By combining (4) and (5), the following useful

relationship is obtained:

t

LI

V

ON

P

RMS

=

™

™

2

(6)

Figure 8. Switch and Line Currents in the Flyback PFC Circuit

RFI

FILTER

ML4813

CONTROLLER

VOUT-

VOUT+

RS

RL2

RH2

C1

D1

Q1

L1

RH1

RL1

VS

–

+

INDUCTOR CURRENT

SWITCH CURRENT

SINUSOIDAL PEAK ENVELOPE

AVERAGE CURRENT

Figure 7. Simplified Application Circuit