FAN6756 Fairchild Semiconductor, FAN6756 Datasheet - Page 12

• Manufacturer Part Number: FAN6756
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FAN6756 • Rev. 1.0.4
AX-CAP™ Discharge
The EMI filter in the front end of the switched-mode
power supply (SMPS) typically includes capacitor
across
regulations, such as UL 1950 and IEC61010-1, require
the capacitor be discharged to a safe level within a
given time when the AC plug is abruptly removed from
its receptacle. Typically, discharge resisters across the
capacitor are used to make sure that capacitor is
discharged naturally, which introduces power loss as
long as it is connected to the receptacle.
The
discharges the filter capacitor only when the power
supply is unplugged from the power outlet. Since the
AX-CAP discharge circuit is disabled in normal
operation, the power loss in the EMI filter can be
virtually removed.
The discharge of the capacitor is achieved through the
HV pin. Once AC outlet detaching is detected, the PWM
gate remains off and V
charged up, which discharges the filter capacitor.
High/Low Line Compensation for Constant
Power Limit
FAN6756 has pulse-by-pulse current limit as shown in
Figure 29, which limits the maximum input power with a
given input voltage. If the output consumes beyond this
maximum power, the output voltage drops, triggering
the overload protection.
As shown in Figure 29, based on the line voltage,
V
current limit level, V
To maintain the constant output power limit regardless
of line voltage, the cycle-by-cycle current limit level,
V
limit level is proportional to the R
power limit can be tuned using the R
30 shows how the pulse-by-pulse current limit changes
with the line voltage for different R
V
Figure 29. Pulse-by-Pulse Current Limit Circuit
LINE
LIMIT
LIMIT
PK

, decreases as line voltage increases. The current
innovative
; the high/low line compensation block adjusts the
V
LIMIT
AC

H
2

line
V
LIMIT
AX-CAP™

LIMIT
L
connector.

R
R
, defined as:
DD
HV
LS

drops to
V
LINE
PK
technology
Most

HV
HV
3
VDD-OFF

resistors.
V
resistor value and
HV
LMIT
of
resistor. Figure

L
2
. Then V

V
the
intelligently
LIMIT

H
safety
DD
(4)
is
12
Soft-Start
An internal soft-start circuit progressively increases the
pulse-by-pulse current limit level of MOSFET for 8ms
during startup to establish the correct working conditions
for transformers and capacitors.
Protections
FAN6756 provides full protection functions, including
Overload / Open-Loop Protection (OLP), V
Voltage Protection (OVP), Over-Temperature Protection
(OTP), and Current-Sense Short Circuit Protection
(SSCP). SSCP is implemented as Auto-Restart Mode,
while OVP and OTP are implemented as Latch Mode
protections. OLP is Auto-Restart Mode for FAN6756MR
and Latch Mode for FAN6756ML.
When an Auto-Restart Mode protection is triggered,
switching is terminated and the MOSFET remains off,
causing V
(11V), the protection is reset. When V
V
the supply current drawn from HV pin charges the hold-
up capacitor. When V
17V, FAN6756 resumes normal operation. In this
manner, the auto restart alternately enables and
disables the switching of the MOSFET until the
abnormal condition is eliminated.
When a Latch Mode protection is triggered, PWM
switching is terminated and the MOSFET remains off,
causing V
(7V), the internal startup circuit is enabled without
resetting the protection and the supply current drawn
from HV pin charges the hold-up capacitor. Since the
protection is not reset, the IC does not resume PWM
switching even when V
17V, disabling HV startup circuit. Then V
down to 7V. In this manner, the Latch Mode protection
alternately charges and discharges V
more energy in DC link capacitor. The protection is reset
when V
supply is unplugged from the AC line.
DD-OLP
V
LIMIT
0.48
0.46
0.44
0.42
0.38
0.36
0.34
0.32
0.5
0.4
0.3
(V)
100
Figure 30. Current Limit vs. Line voltage
DD
(7V), the internal startup circuit is enabled, and
drops to 4V, which is allowed only after power
DD
DD
to drop. When V
to drop. When V
150
200
DD
DD
V
reaches the turn-on voltage of
reaches the turn-on voltage of
LINE
PK
250
(V)
DD
DD
drops to the V
drops to the V
300
DD
DD
until there is no
drops further to
DD
www.fairchildsemi.com
R
R
R
350
drops again
HV
HV
HV
= 240k
= 200k
= 160k
DD
Over-
DD-OFF
DD-OLP
400