ML4819

8

REV. 1.0 10/10/2000

APPLICATIONS

POWER FACTOR SECTION

The power factor section in the ML4819 is similar to the

power factor section in the ML4812 with the exception of

the operation of the slope compensation circuit. Please

refer to the ML4812 data sheet for more information.

The following calculations refer to Figure 12 in this data

sheet. The component designators in the equations below

refer to the following components in Figure 12:

INPUT INDUCTOR (L1) SELECTION

The central component in the regulator is the input boost

inductor. The value of this inductor controls various

critical operational aspects of the regulator. If the value is

too low, the input current distortion will be high and will

result in low power factor and increased noise at the

input. This will require more input filtering. In addition,

when the value of the inductor is low the inductor dries

out (runs out of current) at low currents. Thus the power

factor will decrease at lower power levels and/or higher

line voltages. If the inductor value is too high, then for a

given operating current the required size of the inductor

core will be large and/or the required number of turns will

be high. So a balance must be reached between distortion

and core size.

One more condition where the inductor can dry out is

analyzed below where it is shown to be maximum duty

cycle dependent.

For the boost converter at steady state:

V

V

D

OUT

IN

ON

=

−

1

(1)

input boost inductor will dry out when the following

condition is satisfied:

(2)

or

VD

V

INDRY

ON MAX

OUT

=−

1

()

(3)

Effectively, the above relationship shows that the resetting

volt-seconds are more than setting volt-seconds. In energy

transfer terms this means that less energy is stored in the

inductor during the ON time than it is asked to deliver during

the OFF time. The net result is that the inductor dries out.

The recommended maximum duty cycle is 95% at

100KHz to allow time for the input inductor to dump its

energy to the output capacitors.

For example:

drying out is an increase in distortion at low input voltages.

For a given output power, the instantaneous value of the

input current is a function of the input sinusoidal voltage

waveform. As the input voltage sweeps from zero volts to

its maximum value and back, so does the current.

The load of the power factor regulator is usually a

switching power supply which is essentially a constant

power load. As a result, an increase in the input voltage

will be offset by a decrease in the input current.

By combining the ideas set forth above, some ground

rules can be obtained for the selection and design of the

input inductor:

Step 1: Find minimum operating current.

I

P

V

IN MIN PEAK

IN MIN

IN MAX

()

()

()

.

=

×

1 414

(4)

then:

Step 2: Choose a minimum current at which point the

inductor current will be on the verge of drying

out. For this example 40% of the peak current

found in step 1 was chosen.

0

–4.0

–8.0

–12

–16

–20

–24

0

20

40

60

80

100

120

Figure 11. Reference Load Regulation

Vt

V

D

IN

OUT

ON MAX

()

()

<× −

[]

1