FAN7631 Fairchild Semiconductor, FAN7631 Datasheet - Page 13

• Manufacturer Part Number: FAN7631
• Description: The FAN7631 is a pulse-frequency modulation controller for high-efficiency half-bridge resonant converters that includes a high-side gate drive circuit, an accurate current-controlled oscillator, and various protection functions
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FAN7631 • 1.0.2
5. Protection Circuit
The FAN7631 has several self-protective functions:
Overload Protection (OLP), Over-Current Protection
(OCP), level-change OCP, Abnormal Over-Current
Protection (AOCP), Over-Voltage Protection (OVP),
Thermal Shutdown (TSD), Fault Input (FI), and Line
Under-Voltage Lockout (LUVLO or also called brownout).
Level-change OCP, OLP, OCP, OVP, and LUVLO are
Auto-Restart Mode protections while AOCP, TSD, and
fault input are Latch Mode protections.
Once auto-restart protection is triggered, switching is
instantly terminated and the MOSFETs remain off. Then
the FAN7631 keeps attempting to restart after the restart
delay until the protection situation is removed. When a
Latch Mode protection is triggered, the FAN7631 remains
off until LV
LV
5.1. Overload Protection (OLP)
When the sensed voltage on the CS pin drops below
V
(200ns), C
I
below V
SS pin is switched to charging current I
as illustrated in Figure 35. If the CS pin voltage drops
below V
C
triggered and the gate drive signals remain off. Once the
OLP is triggered, FAN7631 repeats charging and
discharging C
t
5.2. Over-Current Protection (OCP)
When the CS pin voltage drops below V
longer than the OCP blanking time, t
triggered,
FAN7631 repeats charging and discharging C
times before restarting.
OLP
OLP
OLP
SS
CC,START
. If the sensed voltage on the CS pin does not drop
, and self auto-restart time, t
t
t
voltage, V
OLP
AR
(-0.37V) for more than OLP blanking time, t
OLP
OLP
 

Figure 35.
(12.5V).
8
SS
C
CC
in the next switching cycle, the current on the
for in next consecutive switching cycle until
terminating
SS
C
SS
starts to be discharged by sinking current
drops to V
SS
3 10
SS
four times, then restarts. The OLP delay,

3.4
3 10
, reaches V

2.6

5
Overload Protection (OLP)

5
LR
switching
(5V) and then rises above
SS_START
AR
, are given as:
BO
operation.
SS1
(200ns), OCP is
(1.6V); OLP is
OCP
, restoring V
(-0.54V) for
SS
Then,
four
(6)
(7)
BOL
SS
13
5.3. Abnormal Over-Current Protection (AOCP)
If the secondary-side rectifier diodes are shorted, a
large current with extremely high di/dt can flow through
the MOSFET before OCP is triggered. AOCP is
triggered with a short blanking time of 50ns, t
the sensed voltage drops below -1.10V, terminating the
switching operation. Once the protection is triggered,
V
Latch Mode protection, the protection is reset when
LV
5.4. Level-Change Over-Current Protection (OCP)
Even with soft-start, there can be large overshoot current
for the initial several switching cycles until the resonant
capacitor voltage reaches its steady-state value. To
prevent the startup failure by OCP, the OCP threshold is
changed to V
disabled during soft-start.
SS
Figure 37.
CC
is discharged by an internal switch. Since it is a
drops to V
Figure 36.
Figure 38.
AOCP
Abnormal Over-Current Protection (AOCP)
LR
(5V).
level while the Latch Mode AOCP is
Over-Current Protection (OCP)
Level-Change OCP
www.fairchildsemi.com
BAO
, when