MC34262, MC33262
Table 1. Design Equations
Notes
Calculate the maximum required output power.
Calculated at the minimum required ac line voltage
for output regulation. Let the efficiency
h
= 0.92 for
low line operation.
Let the switching cycle t = 40
ms
for universal input
(85 to 265 Vac) operation and 20
ms
for fixed input
(92 to 138 Vac, or 184 to 276 Vac) operation.
In theory the on−time t
on
is constant. In practice t
on
tends to increase at the ac line zero crossings due
to the charge on capacitor C
5
. Let Vac = Vac
(LL)
for initial
t
on
and t
off
calculations.
The off−time t
off
is greatest at the peak of the ac line
voltage and approaches zero at the ac line zero
crossings. Theta (q) represents the angle of the ac
line voltage.
The minimum switching frequency occurs at the peak
of the ac line voltage. As the ac line voltage traverses
from peak to zero, t
off
approaches zero producing an
increase in switching frequency.
Set the current sense threshold V
CS
to 1.0 V for
universal input (85 Vac to 265 Vac) operation and
to 0.5 V for fixed input (92 Vac to 138 Vac, or
184 Vac to 276 Vac) operation. Note that V
CS
must
be <1.4 V.
Set the multiplier input voltage V
M
to 3.0 V at high
line. Empirically adjust V
M
for the lowest distortion
over the ac line voltage range while guaranteeing
startup at minimum line.
The I
IB
R
1
error term can be minimized with a divider
current in excess of 50
mA.
The calculated peak−to−peak ripple must be less than
16% of the average dc output voltage to prevent false
tripping of the Overvoltage Comparator. Refer to the
Overvoltage Comparator text. ESR is the equivalent
series resistance of C
3
.
The bandwidth is typically set to 20 Hz. When operating
at high ac line, the value of C
1
may need to be
increased. (See Figure 25)
Calculation
Required Converter Output Power
Peak Inductor Current
Formula
P
O
= V
O
I
O
I
L(pk)
=
2
2 P
O
hVac
(LL)
h
Vac
(LL)2
Inductance
t
L
P
=
V
O
2
− Vac
(LL)
2 V
O
P
O
Switch On−Time
t
on
=
2 P
O
L
P
h
Vac
2
t
on
V
O
Switch Off−Time
t
off
=
2 Vac
⎪Sin
q⎜
1
t
on
+ t
off
−1
Switching Frequency
f=
Peak Switch Current
R
7
=
V
CS
I
L(pk)
Multiplier Input Voltage
V
M
=
Vac
R
5
R
3
2
+1
Converter Output Voltage
V
O
= V
ref
R
2
R
1
+1
− I
IB
R
2
Converter Output
Peak to Peak
Ripple Voltage
DV
O(pp)
= I
O
1
2pf
ac
C
3
2
+ ESR
2
Error Amplifier Bandwidth
BW =
gm
2
p
C
1
The following converter characteristics must be chosen:
V
O
− Desired output voltage
Vac − AC RMS line voltage
Vac
(LL)
− AC RMS low line voltage
I
O
− Desired output current
DV
O
− Converter output peak−to−peak ripple voltage
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