FAN4800A Datasheet, PDF (18/24 Pages) Fairchild Semiconductor – PFC/PWM Controller Combination

Functional Description
The FAN4800A/C and FAN4801/1S/2/2L consist of an
average current controlled, continuous boost Power
Factor Correction (PFC) front-end and a synchronized
Pulse Width Modulator (PWM) back-end. The PWM
can be used in current or voltage mode. In voltage
mode, feed forward from the PFC output bus can be
used to improve the line regulation of PWM. In either
mode, the PWM stage uses conventional trailing-edge,
duty-cycle modulation. This patented leading/trailing
edge modulation results in a higher usable PFC error
amplifier bandwidth and can significantly reduce the
size of the PFC DC bus capacitor.
The synchronization of the PWM with the PFC
simplifies the PWM compensation due to the controlled
ripple on the PFC output capacitor (the PWM input
capacitor). The PWM section of the FAN4800A,
FAN4801/1S operates at the same frequency as the
PFC; and FAN4800C, FAN4802/2L operates at double
with PFC.
In addition to power factor correction, a number of
protection features are built into this series. They
include soft-start, PFC over-voltage protection, peak
current limiting, brownout protection, duty cycle limiting,
and under-voltage lockout (UVLO).
I_GAINMOD
=
IAC
× (VEA −
VRMS²
0.7)
×
K
(1)
Note that the output current of the gain modulator is
limited around 159μA and the maximum output voltage
of the gain modulator is limited to 159μA x
5.7K=0.906V. This 0.906V also determines the
maximum input power.
However, IGAINMOD cannot be measured directly from
ISENSE. ISENSE=IGAINMOD – IOFFSET and IOFFSET can
only be measured when VEA is less than 0.5V and
IGAINMOD is 0A. Typical IOFFSET is around 31μA ~ 48μA.
Selecting R_AC for IAC Pin
The IAC pin is the input of the gain modulator and also
a current mirror input and requires current input.
Selecting a proper resistor RAC provides a good sine
wave current derived from the line voltage and helps
program the maximum input power and minimum input
line voltage. RAC=VIN peak x 56KΩ. For example, if the
minimum line voltage is 75VAC, the RAC=75 x 1.414 x
56KΩ=6MΩ.
Gain Modulator
The gain modulator is the heart of the PFC, as the
circuit block controls the response of the current loop to
line voltage waveform and frequency, RMS line voltage,
and PFC output voltages. There are three inputs to the
gain modulator:
1. A current representing the instantaneous input
voltage (amplitude and wave shape) to the PFC.
The rectified AC input sine wave is converted to a
proportional current via a resistor and is fed into the
gain modulator at IAC. Sampling current in this way
minimizes ground noise, required in high-power,
switching-power conversion environments. The gain
modulator responds linearly to this current.
2. A voltage proportional to the long-term RMS AC line
voltage, derived from the rectified line voltage after
scaling and filtering. This signal is presented to the
gain modulator at VRMS. The output of the gain
modulator is inversely proportional to VRMS (except
at unusually low values of VRMS, where special gain
contouring takes over to limit power dissipation of
the circuit components under brownout conditions).
Current Amplifier Error, IEA
The current error amplifier’s output controls the PFC
duty cycle to keep the average current through the
boost inductor a linear function of the line voltage. At
the inverting input to the current error amplifier, the
output current of the gain modulator is summed with a
current, which results in a negative voltage being
impressed upon the ISENSE pin.
The negative voltage on ISENSE represents the sum of
all currents flowing in the PFC circuit and is typically
derived from a current sense resistor in series with the
negative terminal of the input bridge rectifier.
The inverting input of the current error amplifier is a
virtual ground. Given this fact, and the arrangement of
the duty cycle modulator polarities internal to the PFC,
an increase in positive current from the gain modulator
causes the output stage to increase its duty cycle until
the voltage on ISENSE is adequately negative to
cancel this increased current. Similarly, if the gain
modulator’s output decreases, the output duty cycle
decreases to achieve a less negative voltage on the
ISENSE pin.
3. The output of the voltage error amplifier, VEA. The
gain modulator responds linearly to variations in this
voltage.
The output of the gain modulator is a current signal, in
the form of a full wave rectified sinusoid at twice the
line frequency. This current is applied to the virtual
ground (negative) input of the current error amplifier. In
this way, the gain modulator forms the reference for the
current error loop and ultimately controls the
instantaneous current draw of the PFC from the power
line. The general form of the output of the gain
modulator is:
PFC Cycle-By-Cycle Current Limiter
As well as being a part of the current feedback loop,
the ISENSE pin is a direct input to the cycle-by-cycle
current limiter for the PFC section. If the input voltage
at this pin is less than -1.15V, the output of the PFC is
disabled until the protection flip-flop is reset by the
clock pulse at the start of the next PFC power cycle.
© 2008 Fairchild Semiconductor Corporation
FAN4800A/C, FAN4801/1S/2/2L • Rev. 1.0.1
18
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