ML4819
0
–4.0
–8.0
–12
–16
–20
–24
Where D
is the duty cycle [T /(T
+ T )]. The
ON
ON ON OFF
V
= 15V
CC
input boost inductor will dry out when the following
condition is satisfied:
(2)
(3)
V (t) < V
× 1−D
[
]
IN
OUT
ON(MAX)
or
V
= 1−D
[
× V
OUT
]
INDRY
ON(MAX)
V
V
: Voltage where the inductor dries out.
INDRY
: Output dc voltage.
OUT
T
= 25 C
A
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.
0
20
40
60
80
100
120
I
, REFERENCE SOURCE CURRENT (mA)
REF
Figure 11. Reference Load Regulation
The recommended maximum duty cycle is 95% at
100KHz to allow time for the input inductor to dump its
energy to the output capacitors.
APPLICATIONS
For example:
POWER FACTOR SECTION
if: V
= 380V and
ON(MAX)
OUT
D
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.
= 0.95
then substituting in (3) yields V
drying out is an increase in distortion at low input voltages.
= 20V. The effect of
INDRY
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:
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.
R = R16, C = C6.
T
T
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.
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.
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.
1.414 ×P
IN(MIN)
I
=
(4)
IN
(MIN)PEAK
V
IN(MAX)
V
= 260V
= 50W
IN(MAX)
P
IN(MIN)
One more condition where the inductor can dry out is
analyzed below where it is shown to be maximum duty
cycle dependent.
then:
I
= 0.272A
IN(MIN)PEAK
For the boost converter at steady state:
Step 2: Choose a minimum current at which point the
V
IN
inductor current will be on the verge of drying
out. For this example 40% of the peak current
found in step 1 was chosen.
VOUT
=
(1)
1−DON
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
