MC34262_11 Datasheet, PDF (10/19 Pages) ON Semiconductor – Power Factor Controllers

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.
Calculation
Required Converter Output Power
Peak Inductor Current
Inductance
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.
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 R₁ error term can be minimized with a divider
current in excess of 50 mA.
Switch On−Time
Switch Off−Time
Switching Frequency
Peak Switch Current
Multiplier Input Voltage
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.
Converter Output
Peak to Peak
Ripple Voltage
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 26)
Error Amplifier Bandwidth
The following converter characteristics must be chosen:
V_O − Desired output voltage
Vac − AC RMS line voltage
I_O − Desired output current
Vac _(LL) − AC RMS low line voltage
DV_O − Converter output peak−to−peak ripple voltage
Formula
P_O = V_O I_O
I_L(pk) =
2 2 P_O
hVac_(LL)
ǒ Ǔ _t
V_O
2
− Vac_(LL)
h Vac_(LL)2
L_P =
2 V_O P_O
2 P_O L_P
t_on = _{h Vac2}
t_off =
t_on
V_O
−1
2 Vac ⎪Sin q⎜
f= ¹
t_on + t_off
R₇ =
V_CS
I_L(pk)
ǒ Ǔ ^{VM =}
Vac 2
R_{5 + 1}
R₃
ǒ Ǔ _{VO = Vref}
R_{2 + 1}
R₁
− I_IB R₂
DV_O(pp) = I_O
ǒ Ǔ ¹
2
+ ESR²
2pf_ac C₃
gm
BW =
2 p C₁
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