User manual

Circuit Analysis and Explanations
INJECTING LINE VOLTAGE INTO FILTER-2 (ACHIEVING
PFC > 0.94)
If a small portion (750mV to 1.00V) of line voltage is injected
at FLTR2 of the LM3445, the circuit is essentially turned into
a constant power flyback as shown in Figure 5.
30126935
FIGURE 5. Line Voltage Injection Circuit
The LM3445 works as a constant off-time controller normally,
but by injecting the 1.0V
Pk
rectified AC voltage into the FLTR2
pin, the on-time can be made to be constant. With a DCM
Flyback, Δi needs to increase as the input voltage line in-
creases. Therefore a constant on-time (since inductor L is
constant) can be obtained.
By using the line voltage injection technique, the FLTR2 pin
has the voltage wave shape shown in Figure 6 on it with no
triac dimmer in-line. Voltage at V
FLTR2
peak should be kept
below 1.25V. At 1.25V current limit is tripped. C11 is small
enough not to distort the AC signal but adds a little filtering.
Although the on-time is probably never truly constant, it can
be observed in Figure 7 how (by adding the rectified voltage)
the on-time is adjusted.
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FIGURE 6. FLTR2 Waveform with No Dimmer
For this evaluation board, the following resistor values are
used:
R3 = R8 = 309 kΩ
R20 = 1.91 kΩ
Therefore the voltages observed on the FLTR2 pin will be as
follows for listed input voltages:
For VIN = 180V
RMS
, V
FLTR2, Pk
= 0.78V
For VIN = 230V
RMS
, V
FLTR2, Pk
= 1.00V
For VIN = 265V
RMS
, V
FLTR2, Pk
= 1.15V
Using this technique, a power factor greater than 0.94 can be
achieved without additional passive active power factor con-
trol (PFC) circuitry.
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FIGURE 7. Typical Operation of FLTR2 Pin
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