ML4813
APPLICATIONS (Continued)
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.
V
V
OUT+
OUT
200V
V
OUT-
PWR GND
Normally, RH1 = RH2 = RH and RL1 = RL2 = RL. The
TIME
voltage designated as V in Figure 7 is given by:
S
RL
RH + RL
Figure 10. Output Voltage with Respect to PWR GND
VS = VOUT
(16)
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:
I
= Total RMS current through the power MOSFET
RMS
f = Line frequency times 2 (120 for 60Hz line)
L
r = f
/f
SWITCH L
Table 1 is provided to assist in calculating (18). When the
power switch is a bipolar transistor (constant V drop),
then the power dissipation produced can be calculated
by:
CE
P
IN
COUT
(17)
2pfL DVR VOUT
0.9 P
VRMS
Where:
IN
PD =
VCE
(19)
C
= Total output capacitance
OUT
P
= Total input power
IN
DV = Peak output capacitor ripple voltage
Where:
P = Power dissipation in the transistor
R
f = Line frequency times 2 (120 for 60Hz line)
L
D
V
V
= RMS value of the minimum input voltage
= Forward drop of the power transistor
RMS
CE
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.
r
kp
r
sin2
f
SWITCH
Ê
k
1
(kHz)
r
OUTPUT DIODE
20
30
167
250
9.1
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.
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
40
333
50
417
60
500
70
583
80
667
90
750
100
110
120
130
140
150
160
170
180
190
200
833
POWER SWITCH
917
1000
1083
1167
1250
1333
1417
1500
1583
1667
If a power MOSFET is used, it should be sized for the
required efficiency. Lower R
devices will yield
DS(ON)
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:
r
3
L IP fL
kp
r
IRMS
=
sin2
(15)
Ê
4.24 VRMS
k
1
Table 1. Constants for Calculating IRMS (Equation 18)
11
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
