MP44011 – BOUNDARY-MODE PFC CONTROLLER WITH HARMONIC INJECTION
MP44011 Rev. 1.01 www.MonolithicPower.com 10
1/12/2012 MPS Proprietary Information. Patent Protected.
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© 2012 MPS. All Rights Reserved.
When the inductor current reaches zero, the
inductor voltage reverses. Then the ZCS
generates the turn-on signal for the MOSFET by
sensing the falling edge of the voltage on the
auxiliary winding coupled with the inductor. If the
ZCS voltage goes higher than 2.1V, the
comparator waits until the voltage falls below
1.35V. If the voltage falls below 1.35V, the
MP44011 turns on the MOSFET. The 7.8V high
clamp and 0.55V low clamp protect the ZCS pin.
The internal timer generates a MOSFET turn-on
signal if the driver signal is low for more than
175µs, and can turn on the MOSFET during
start-up since there is no ZCD signal is
generated during this period.
Zero-Crossing Compensation
The MP44011 offers a 30mV voltage offset for
the multiplier output near the line votlage’s zero-
crossing that can force the circuit to process
more energy at the bottom of the line voltage.
This function reduces the total harmonic
distortion (THD) of the current.
To prevent excess energy consumption, this
offset reduces as the instantaneous line voltage
increases so that the offset is negligible near the
top of the line voltage.
Harmonic Injection Function
Vac
T1
D1
Rs
Vbus
C1
Cbus
D2
Q1
Figure 4: Flyback PFC Main Circuit
Peak Pri mar y Cur rent
Primary C urr ent
Average Primar y Cur r ent
Figure 5: Primary Current Waveform of
Traditional BCM Flyback PFC
The flyback PFC application exemplifies the
harmonic injection function for this document. For
a traditional BCM flyback PFC, the transformer
acts like two coupled inductors where the current
will not flow through both windings at the same
time; the current is discontinuous. The primary
current rises from the zero crossing and the
secondary current returns to zero for every
switching cycle. The average input current is
related to both the peak primary current and the
duty cycle.
Peak Pr im ary C urr ent
Primary Current
Average Pr im ar y Cur rent
Figure 6: Primary Current Waveform of
Flyback PFC with the MP44011
For the MP44011, the MULT pin senses the AC
input line and injects constant ratio harmonics
into sensed line voltage, so the peak primary
current reference has a choppy sinusoidal signal.
As a result, the peak primary current flowing
through the transformer is sharply reduced, and
the circuit can use a smaller transformer core
that will not saturate.
In addition, harmonic injection into the input
current reduces the input power fluctuation: This
results in a smaller output voltage ripple on the
output capacitor, and allows for the use of
smaller capacitors. Therefore, the MP44011’s
internal harmonic injection function can reduce
both bus capacitor values and transformer size.
However, there is a trade-off between smaller
core and performance: a smaller core can have
more winding layers and therefore greater
leakage inductance. This leakage reduces
efficiency and increase device voltage stress..
