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MC33262 Datasheet PDF : 16 Pages
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MC34262 MC33262
APPLICATIONS INFORMATION
The application circuits shown in Figures 19, 20 and 21
reveal that few external components are required for a
complete power factor preconverter. Each circuit is a peak
detecting current–mode boost converter that operates in
critical conduction mode with a fixed on–time and variable
off–time. A major benefit of critical conduction operation is
that the current loop is inherently stable, thus eliminating the
need for ramp compensation. The application in Figure 19
operates over an input voltage range of 90 Vac to 138 Vac
and provides an output power of 80 W (230 V at 350 mA) with
an associated power factor of approximately 0.998 at
nominal line. Figures 20 and 21 are universal input
preconverter examples that operate over a continuous input
voltage range of 90 Vac to 268 Vac. Figure 20 provides an
output power of 175 W (400 V at 440 mA) while Figure 21
provides 450 W (400 V at 1.125 A). Both circuits have an
observed worst–case power factor of approximately 0.989.
The input current and voltage waveforms of Figure 20 are
shown in Figure 22 with operation at 115 Vac and 230 Vac.
The data for each of the applications was generated with the
test set–up shown in Figure 24.
Table 1. Design Equations
Notes
Calculate the maximum required output power.
Calculation
Required Converter Output Power
Calculated at the minimum required ac line voltage
for output regulation. Let the efficiency η = 0.92 for
low line operation.
Peak Inductor Current
Let the switching cycle t = 40 µs for universal input
(85 to 265 Vac) operation and 20 µs for fixed input
(92 to 138 Vac, or 184 to 276 Vac) operation.
Inductance
In theory the on–time ton is constant. In practice ton
tends to increase at the ac line zero crossings due
to the charge on capacitor C5. Let Vac = Vac(LL) for initial
ton and toff calculations.
The off–time toff is greatest at the peak of the ac line
voltage and approaches zero at the ac line zero
crossings. Theta (θ) represents the angle of the ac
line voltage.
Switch On–Time
Switch Off–Time
The minimum switching frequency occurs at the peak
of the ac line voltage. As the ac line voltage traverses
from peak to zero, toff approaches zero producing an
increase in switching frequency.
Set the current sense threshold VCS 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 VCS must
be <1.4 V.
Set the multiplier input voltage VM to 3.0 V at high
line. Empirically adjust VM for the lowest distortion
over the ac line voltage range while guaranteeing
startup at minimum line.
Switching Frequency
Peak Switch Current
Multiplier Input Voltage
The IIB R1 error term can be minimized with a divider
current in excess of 50 µA.
Converter Output Voltage
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 C3
The bandwidth is typically set to 20 Hz. When operating
at high ac line, the value of C1 may need to be
increased. (See Figure 25)
Converter Output
Peak to Peak
Ripple Voltage
Error Amplifier Bandwidth
The following converter characteristics must be chosen:
VO — Desired output voltage
Vac — AC RMS line voltage
IO — Desired output current
Vac(LL) — AC RMS low line voltage
∆VO — Converter output peak–to–peak ripple voltage
Formula
PO = VO IO
IL(pk) =
2 2 PO
ηVac(LL)
t
ǒ VO
2
–
Vac(LL)Ǔ η Vac(LL)2
LP =
2 VO PO
2 PO LP
ton = η Vac2
toff =
ton
VO
–1
2 Vac Sin θ
f= 1
ton + toff
R7 =
VCS
IL(pk)
VM = Vac 2
ǒ R5 + 1Ǔ
R3
VO
=
Vref
ǒ
R2
R1
+ 1Ǔ
– IIB R2
∆VO(pp) = IO
ǒ 1 Ǔ2+ ESR2
2πfac C3
gm
BW =
2 π C1
MOTOROLA ANALOG IC DEVICE DATA
9
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