ML4813
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
L
q =
RFI
FILTER
RL1
RH1
L1
–
C1
VOUT-
ML4813
CONTROLLER
+
VS
Q1
D1
RH2
VOUT+
RS
RL2
Figure 7. Simplified Application Circuit
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:
I
L
q =
I
P
sin
q
05
V
IN
q
t
ON
L
05
(4)
Where:
I
L
(q) = instantaneous peak inductor current
t
ON
= Power MOSFET "ON" time
V
IN
(q) = V
P
sin
q
= Instantaneous input voltage
V
P
= Input peak voltage
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
05
(5)
By combining (4) and (5), the following useful
relationship is obtained:
t
ON
=
L
I
P
2
V
RMS
(6)
INDUCTOR CURRENT
SWITCH CURRENT
SINUSOIDAL PEAK ENVELOPE
AVERAGE CURRENT
Figure 8. Switch and Line Currents in the Flyback PFC Circuit
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
