FAN102 Fairchild Semiconductor, FAN102 Datasheet - Page 12

• Manufacturer Part Number: FAN102
• Description: The primary-side PWM controller significantly simplifies power supply design that requires CV and CC regulation capabilities
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN102 Rev. 1.0.3
Leading-Edge Blanking (LEB)
At the instant the MOSFET is turned on, a high-current
spike occurs through the MOSFET, caused by primary-
side capacitance and secondary-side rectifier reverse
recovery. Excessive voltage across the R
lead to premature turn-off of MOSFET. FAN102
employs an internal leading-edge blanking (LEB) circuit.
To inhibit the PWM comparator for a short time after the
MOSFET is turned on. Thus, external RC filtering is not
required.
Startup
Figure 29 shows the typical startup circuit and
transformer auxiliary winding for a FAN102 application.
Before FAN102 begins switching, it consumes only
startup current (typically 10µA) and the current supplied
through the startup resistor charges the V
(C
ON
consumed by FAN102 increases to 3.5mA. Then, the
power required for FAN102 is supplied from the
transformer auxiliary winding. The large hysteresis of
V
small capacitor for V
Protections
The FAN102 has several self-protective functions, such
as Over-Voltage Protection (OVP), Over-Temperature
Protection (OTP) and brownout protection. All the
protections are implemented as auto-restart mode.
Once the fault condition occurs, switching is terminated
and the MOSFET remains off. This causes V
When V
current consumed by FAN102 reduces to the startup
current (typically 10µA) and the current supplied startup
resistor charges the V
the turn-on voltage of 16V, FAN102 resumes normal
DD
),
DD
AC Line
). When V
1
2
3
4
provides more holdup time, which allows using a
FAN102
CS
COMR
COMI
COMV
DD
FAN102
reaches the V
C
Figure 29. Startup Circuit
GATE
SGND
DD
D L
VDD
begins
VS
reaches turn-on voltage of 16V (V
DD
8
7
6
5
V
+
-
.
D L
DD
switching
capacitor. When V
DD
turn-off voltage of 5V, the
C
R
D D
STAR T
D
D D
and
CS
the
DD
resistor can
DD
DD
capacitor
reaches
current
N
to fall.
R
R
Np
A
S1
S2
DD-
12
operation. In this manner, the auto-restart alternately
enables and disables the switching of the MOSFET until
the fault condition is eliminated (see Figure 30 ).
V
V
voltage conditions. If the V
open feedback condition, OVP is triggered. The OVP
has a de-bounce time (typcal 250µs) to prevent false
trigger by switching noise. It also protects other
switching devices from over voltage.
Over-Temperature Protection (OTP)
A built-in temperature-sensing circuit shuts down PWM
output if the junction temperature exceeds 140°C.
Brownout Protection
FAN102 detects the line voltage using auxiliary winding
voltage since the auxiliary winding voltage reflects the
input voltage when the MOSFET is turned on. The VS
pin is clamped at 1.15V while the MOSFET is turned on
and brownout protection is triggered if the current out of
the VS pin is less than I
MOSFET conduction.
Pulse-by-Pulse Current Limit
When the sensing voltage across the current sense
resistor exceeds the internal threshold of 1.4V, the
MOSFET is turned off for the remainder of switching
cycle. In normal operation, the pulse-by-pulse current
limit is not triggered since the peak current is limited by
the control loop.
3.5mA
DD
10µA
DD
V
V
16V
5V
DS
DD
Operating Current
over-voltage protection prevents damage from over-
Over-Voltage Protection (OVP)
Power
Figure 30. Auto-Restart Operation
On
Operation
Normal
Occurs
Fault
VS-UVP
Situation
DD
Fault
(typical 180µA) during the
voltage exceeds 28V by
Fault Removed
Operation
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Normal