FAN6920MR Fairchild Semiconductor, FAN6920MR Datasheet - Page 21

• Manufacturer Part Number: FAN6920MR
• Description: The highly integrated FAN6920MR combines Power Factor Correction (PFC) controller and quasi-resonant PWM controller
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN6920MR • Rev. 1.0.5
the DET pin voltage and flows out current I
current I
FAN6920MR depends on this current during t
regulate the current limit level of the PWM switch to
perform high / low line over-power compensation.
As the input voltage increases, the reflected voltage on
the auxiliary winding V
the current I
a lower level.
The R
the DET pin. Engineers can adjust this R
get proper V
characteristic curve of I
CSPWM pin is shown in Figure 44.
where V
winding; and N
Leading-Edge Blanking (LEB)
When the PFC or PWM switches are turned on, a
voltage spike is induced on the current-sense resistor
due to the reciprocal effect by reverse-recovery energy
of the output diode and C
prevent this spike, a leading-edge blanking time is built-
I
DET
Figure 43. Relationship between V
V
V
OPWM
AUX
DET
DET

Figure 44. I
IN


V
DET
resistor is connected from auxiliary winding to
is input voltage; N
IN
DET
LIMIT

is in accordance with V
P

and the controller regulates the V
N N
is turn number of primary winding.
voltage to fit power system needs. The
A
DET
Characteristic Curve
Current vs. V
P
AUX
DET



becomes higher as well as
current vs. V
OSS
R
A
DET
is turn number of auxiliary
of power MOSFET. To
LIMIT
AUX
LIMIT
Voltage
DET
DET
AUX
and V
. Since the
voltage on
resistor to
voltage,
LIMIT
IN
ON
(1)
to
to
in and a small RC filter (e.g. 100Ω, 470pF) is
recommended between the CSPWM pin and GND.
Protection for PWM Stage
VDD Pin Over-Voltage Protection (OVP)
V
once V
voltage. In the case of V
switching operation immediately and enters auto-
recovery protection.
Adjustable Over-Temperature Protection and
Externally Protection Triggering (RT Pin)
Figure 45 is a typical application circuit with an internal
block of RT pin. As shown, a constant current I
out from the RT pin, so the voltage V
can be obtained as I
which consists of NTC resistor and R
pin voltage is lower than 0.8V and lasts for a debounce
time, auto-recovery protection is activated and stops all
PFC and PWM switching.
RT pin is usually used to achieve over-temperature
protection with a NTC resistor and provides external
protection triggering for additional protection. Engineers
can use an external triggering circuit (e.g. transistor) to
pull the RT pin low and activate controller auto-recovery
protection.
Generally, the external protection triggering needs to
activate rapidly since it is usually used to protect the
power system from abnormal conditions. Therefore, the
protection debounce time of the RT pin is set to around
110µs once the RT pin voltage is lower than 0.5V.
For
temperature does not change immediately; the RT pin
voltage is reduced slowly as well. The debounce time
for adjustable OTP should not need a fast reaction. To
prevent improper protection triggering on the RT pin due
to exacting test condition (e.g. lightning test); when the
RT pin triggering voltage is higher than 0.5V, the
protection debounce time is set to around 10ms. To
avoid improper triggering on the RT pin, add a small
value capacitor (e.g. 1000pF) paralleled with NTC and
the R
Figure 45. Adjustable Over-Temperature Protection
21
DD
over-voltage protection prevents device damage
A
over-temperature
resistor.
DD
voltage is higher than device stress rating
RT
current multiplied by the resistor,
DD
protection,
OVP, the controller stops all
A
resistor. If the RT
RT
because
on the RT pin
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RT
flows
the