FAN4800A Datasheet, PDF (19/24 Pages) Fairchild Semiconductor – PFC/PWM Controller Combination

TriFault Detect™
To improve power supply reliability, reduce system
component count, and simplify compliance to UL 1950
safety standards, the FAN4800A/C, FAN4801/1S/2/2L
includes TriFault Detect. 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 FAN4800A/C, FAN4801/1S/2/2L, 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 2.50V. 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 VEA needs to around 4.5V and
ripple on the VEA needs to be less than 400mV.
Choose the resistance of the sensing resistor:
R_sense
=
(4.5 − 0.7) × 5.7K Ω × IAC ×Gain ×V_IN ×
2 × (5.6 − 0.7) × Line input Power
2
(2)
where 5.6 is VEA maximum output.
PFC Soft-Start
PFC startup is controlled by VEA level. Before FBPFC
voltage reaches 2.4V, the VEA 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 FAN4800A/C, FAN4801/1S/2 disables
PFC as lower AC input that the VRMS is less than
1.05V. The brownout voltage of FAN4802L is lower
than FAN4801/1S/2 that the VRMS is less than 0.9V.
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 45 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
referred 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,
which is the sensing resistor of the PFC boost
converter. It requires the ISENSE filter to attenuate
the energy.
2. To reduce 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
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.
Figure 45. Compensation Network Connection for the
Voltage and Current Error Amplifiers
© 2008 Fairchild Semiconductor Corporation
FAN4800A/C, FAN4801/1S/2/2L • Rev. 1.0.1
19
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