MC33368_11 Datasheet, PDF (10/16 Pages) ON Semiconductor – High Voltage GreenLine Power Factor Controller

MC33368
Table 1. Design Equations
Calculation
Formula
Notes
Converter Output Power
Peak Indicator Current
Inductance
Switch On−Time
Switch Off−Time
PO + VO IO
IL(pk) +
2 ^Ǹ2 PO
h Vac(LL)
ǒ Ǔ t
VO
Ǹ₂
–Vac(LL)
h Vac(LL)2
LP +
Ǹ_{2 VO PO}
t(on) +
2 PO LP
h Vac2
t(off) +
t(on)
VO
Ǹ_{2 Vac ŤSin qŤ}
–1
Calculate the maximum required output power.
Calculated at the minimum required ac line voltage for
output regulation. Let the efficiency η = 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 C5. 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.
Minimum Switch
Off−Time
Delay Time
Switching Frequency
t(off)min +
LP IL(pk)
VO
ǒ Ǔ td + – R10 C7 ln
VCC – 2
VCC
f+
1
t(on) ) t(off)
The off−time is at a minimum at ac line crossings. This
equation is used to calculate t_(off) as Theta approaches
zero.
The delay time is used to override the minimum
off−time at the ac line zero crossings by programming
the Frequency Clamp with C7 and R10.
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.
Peak Switch Current
Multiplier Input Voltage
R7 + ^VCS
IL(pk)
ǒ Ǔ ^{VM +}
Vac ^Ǹ2
R5
R3
)
1
Set the current sense threshold V_CS to 1.0 V for
universal input (85 to 265 Vac) operation and to 0.5 V
for fixed input (92 to 138 Vac, or 184 to 276 Vac)
operation. Note that V_CS must be less than 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.
Converter Output
Voltage
Converter Output
Peak−to−Peak
Ripple Voltage
ǒ Ǔ VO + Vref
R2
R1
)
1
– IIB R1
Ǹǒ Ǔ _{DVO(pp) + IL(pk)}
2
1
2 p fac C3
) ESR2
The I_IB R1 error term can be minimized with a divider
current in excess of 100 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 C3.
Error Amplifier
Bandwidth
BW +
gm
2 p C1
The bandwidth is typically set to 20 Hz. When operating
at high ac line, the value of C1 may need to be
increased.
NOTE:
The following converter characteristics must be chosen:
V_O = Desired output voltage.
Vac_(LL) = AC RMS minimum required operating line voltage for output regulation.
I_O = Desired output current.
DV_O = Converter output peak−to−peak ripple voltage.
Vac = AC RMS operating line voltage.
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