FAN4800AS Datasheet, PDF (14/19 Pages) Fairchild Semiconductor – PFC/PWM Controller Combination

TriFault Detect™
To improve power supply reliability, reduce system
component count, and simplify compliance to UL1950
safety standards; the FAN4800AS/CS/01S/02S includes
TriFault Detect™ technology. This feature monitors
FBPFC for certain PFC fault conditions.
In a feedback path failure, the output of the PFC could
exceed safe operating limits. With such a failure, FBPFC
exceeds its normal operating area. Should FBPFC go too
low, too high, or open; TriFault Detect™ senses the error
and terminates the PFC output drive.
TriFault Detect is an entirely internal circuit. It requires no
external components to serve its protective function.
PFC Over-Voltage Protection
In the FAN4800AS/CS/01S/02S, the PFC OVP
comparator serves to protect the power circuit from being
subjected to excessive voltages if the load changes
suddenly. A resistor divider from the high-voltage DC
output of the PFC is fed to FBPFC. When the voltage on
FBPFC exceeds 2.75V, the PFC output driver is shut
down. The PWM section continues to operate. The OVP
comparator has 250mV of hysteresis and the PFC does
not restart until the voltage at FBPFC drops below 2.5V.
VDD OVP can also serve as a redundant PFC OVP
protection. VDD OVP threshold is 28V with 1V hysteresis.
Selecting PFC R_sense
Rsense is the sensing resistor of the PFC boost converter.
During the steady state, line input current x Rsense equals
IGAINMOD x 5.7kΩ.
At full load, the average VVEA needs to around 4.5V and
ripple on the VEA pin needs to be less than 400mV.
Choose the resistance of the sensing resistor:
R_SENSE
=
(4.5 − 0.7) × 5.7KΩ × IAC × Gain ×V_IN ×
2 × (5.6 − 0.7) × Line _ Input _ Power
2
(2)
where 5.6 is VVEA maximum output voltage.
Error Amplifier Compensation
The PWM loading of the PFC can be modeled as a
negative resistor because an increase in the input
voltage to the PWM causes a decrease in the input
current. This response dictates the proper compensation
of the two transconductance error amplifiers. Figure 26
shows the types of compensation networks most
commonly used for the voltage and current error
amplifiers, along with their respective return points. The
current-loop compensation is returned to VREF to
produce a soft-start characteristic on the PFC. As the
reference voltage increases from 0V, it creates a
differentiated voltage on IEA, which prevents the PFC
from immediately demanding a full duty cycle on its
boost converter. Complete design is discussed in
application note AN-6078SC.
There is an RC filter between Rsense and ISENSE pin.
There are two reasons to add a filter at the ISENSE pin:
1. Protection: During startup or inrush current conditions,
there is a large voltage across Rsense, the sensing
resistor of the PFC boost converter. It requires the
ISENSE filter to attenuate the energy.
2. To reduce inductance, L, the boost inductor. The
ISENSE filter also can reduce the boost inductor value
since the ISENSE filter behaves like an integrator before
the ISENSE pin, which is the input of the current error
amplifier, IEA.
The ISENSE filter is an RC filter. The resistor value of the
ISENSE filter is between 100Ω and 50Ω because IOFFSET x
RFILTER can generate a negative offset voltage of IEA.
Selecting an RFILTER equal to 50Ω keeps the offset of the
IEA less than 3mV. Design the pole of the ISENSE filter at
fPFC/6, one sixth of the PFC switching frequency, so the
boost inductor can be reduced six times without
disturbing the stability. The capacitor of the ISENSE filter,
CFILTER, is approximately 100nF.
PFC Soft-Start
PFC startup is controlled by VVEA level. Before the
FBPFC voltage reaches 2.4V, the VVEA level is around
2.8V. At 90VAC, the PFC soft-start time is 90ms.
PFC Brownout
The AC UVP comparator monitors the AC input voltage.
The PFC is disabled as AC input lowers, causing VRMS to
be less than 1.05V.
Figure 26. Compensation Network Connection for
the Voltage and Current Error Amplifiers
© 2010 Fairchild Semiconductor Corporation
FAN4800AS/CS/01S/02S • Rev. 1.0.1
14
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