ml4813 Micro Electronics Corporation, ml4813 Datasheet - Page 11

• Manufacturer Part Number: ml4813
• Description: Flyback Power Factor Controller
• Manufacturer: Micro Electronics Corporation

APPLICATIONS
The output voltage "rides" on the input voltage when the
(+) output is measured with respect to PWR GND as
shown in Figure 10.
The extra op amp provided in the ML4813 can be used to
sense the output voltage for regulation and overvoltage
conditions. This op amp is connected as a difference
amplifier with its output referenced to PWR GND.
Resistors RH1, RH2, RL1, RL2 are used to scale down the
voltage.
Normally, RH1 = RH2 = RH and RL1 = RL2 = RL. The
voltage designated as V
The output capacitance should be calculated such that it
has the required output ripple at the worst case operating
point. In addition, the ESR should be sufficiently low to
prevent excessive dissipation due to RMS currents. The
first criterion can be met by choosing the value of the
output capacitor based on the following:
Where:
The second criterion for the selection of the output
capacitor can be satisfied by choosing a component with
adequately low ESR value that can safely bypass the RMS
currents.
OUTPUT DIODE
The output diode can be a "fast" or ultrafast' type
depending on the operating frequency. Reverse recovery
losses are low since under normal operating conditions,
the regulator operates in discontinuous current mode. The
diode should be rated to handle the maximum output
current. The resulting power dissipation will be the
forward drop of the diode times the output current.
POWER SWITCH
If a power MOSFET is used, it should be sized for the
required efficiency. Lower R
lower losses, but if they are operated at high frequencies
(100kHz), higher charge dumping losses will be
experienced. The RMS current value through the power
FET and the sensing resistor is:
C
P
f
L
IN
OUT
V
I
V
C
RMS
= Line frequency times 2 (120 for 60Hz line)
R
S
OUT
= Total input power
=
= Peak output capacitor ripple voltage
=
= Total output capacitance
V
˜
OUT
2p
4 24
L I
.
f
™
™
L
™
P
™
RH RL
3
D
V
™
RL
P
V
RMS
+
IN
R
f
L
(Continued)
S
™
in Figure 7 is given by:
™
V
OUT
DS(ON)
Ê
k
r
=
1
sin
2
devices will yield
k
r
p
(16)
(17)
(15)
Where:
Table 1 is provided to assist in calculating (18). When the
power switch is a bipolar transistor (constant V
then the power dissipation produced can be calculated
by:
Where:
Figure 10. Output Voltage with Respect to PWR GND
Table 1. Constants for Calculating IRMS (Equation 18)
I
f
r = f
P
V
V
RMS
L
D
P
RMS
CE
= Line frequency times 2 (120 for 60Hz line)
D
f
= Power dissipation in the transistor
SWITCH
(kHz)
SWITCH
= Forward drop of the power transistor
=
100
110
120
130
140
150
160
170
180
190
200
= Total RMS current through the power MOSFET
20
30
40
50
60
70
80
90
= RMS value of the minimum input voltage
0 9 .
PWR GND
V
RMS
™
200V
/f
P
L
IN
™
V
CE
V OUT
1000
1083
1167
1250
1333
1417
1500
1583
1667
167
250
333
417
500
583
667
750
833
917
r
TIME
ML4813
Ê
k
r
=
1
11.2
12.9
14.4
25.8
17.1
18.3
19.4
20.4
21.4
22.4
23.3
24.2
25.0
25.7
26.5
27.3
28.0
28.9
CE
9.1
sin
drop),
V OUT+
V OUT-
2
k
r
p
(19)
11