FAN4801 FAIRCHILD [Fairchild Semiconductor], FAN4801 Datasheet - Page 19

• Manufacturer Part Number: FAN4801
• Description: PFC/PWM Controller Combination
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2008 Fairchild Semiconductor Corporation
FAN4800A/C, FAN4801/1S/2/2L • Rev. 1.0.2
R
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. V
a redundant PFC OVP protection. V
28V with 1V hysteresis.
Selecting PFC R
R
converter. During the steady state, line input current x
R
At full load, the average V
ripple on the V
the resistance of the sensing resistor:
where 5.6 is V
PFC Soft-Start
PFC startup is controlled by V
voltage reaches 2.4V, the V
90V
PFC Brownout
The AC UVP comparator monitors the AC input voltage.
The FAN4800A/C, FAN4801/1S/2 disables OPFC when
the VRMS is less than 1.05V and continues 500ms. The
VRMS threshold low voltage of FAN4802L is 0.9V, which
is different from the FAN4802.
SENSE
SENSE
SENSE
AC
, the PFC soft-start time is 90ms.
equals I
=
is the sensing resistor of the PFC boost
4 (
5 .
−
2
EA
EA
GAINMOD
. 0
×
5 (
maximum output.
) 7
needs to be less than 400mV. Choose
6 .
×
5
−
sense
7 .
. 0
x 5.7KΩ.
K
) 7
Ω
×
EA
×
Line
I
AC
EA
needs to around 4.5V and
DD
Input
×
EA
level is around 2.8V. At
OVP can also serve as
Gain
level. Before FBPFC
DD
Power
×
OVP threshold is
V
IN
×
2
(2)
19
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.
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 V
on the PFC: As the reference voltage increases from
0V, it creates a differentiated voltage on I
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 R
There are two reasons to add a filter at the ISENSE pin:
1. Protection: During startup or inrush current conditions,
2. To reduce L, the boost inductor: The I
The I
I
R
Selecting an R
the I
f
boost inductor can be reduced six times without
disturbing the stability. The capacitor of the I
C
SENSE
PFC
R
FILTER
FILTER
SENSE
there is a large voltage across R
sensing resistor of the PFC boost converter. It
requires the I
can reduce the boost inductor value since the I
filter behaves like an integrator before the ISENSE pin,
which is the input of the current error amplifier, I
/6, one sixth of the PFC switching frequency, so the
EA
Figure 45.
SENSE
PFC Output
filter is between 100Ω and 50Ω because I
R
R
, is approximately 100nF.
less than 3mV. Design the pole of I
R
can generate a negative offset voltage of IEA.
F1
F2
for the Voltage and Current Error Amplifiers
FILTER
C
filter is an RC filter. The resistor value of the
This
FILTER
FBPFC
ISENSE
REF
VRMS
IAC
FILTER
SENSE
to produce a soft-start characteristic
15
response
2
4
3
Compensation Network Connection
filter to attenuate the energy.
equal to 50Ω keeps the offset of
2.5V
VEA
C
GMv
R
V2
V1
dictates
Modulator
16
SENSE
Gain
SENSE
C
and ISENSE pin.
V1
I
Rmul
MO
SENSE
, which is the
the
SENSE
www.fairchildsemi.com
V(t)
Rmul
SENSE
EA
GMi
IEA
filter also
C
, which
OFFSET
R
filter at
I2
proper
EA
I1
V
1
SENSE
filter,
.
REF
x
C
I1