NCP1397A/B, NCV1397A/B
www.onsemi.com
20
Figure 42. Adding a Comparator on the BO Pin Offers a way to Latch−off the Controller
−
+
20 ms
RC
To permanent
latch
+
V
latch
V
DD
−
+
BO
+
VBO
BO
Rlower
Rupper
VbulkV
CC
Q1
NTC
Vout
IBO
On Figure 42, Q1 is blocked and does not bother the BO
measurement as long as the NTC and the optocoupler are not
activated. As soon as the secondary optocoupler senses an
OVP condition, or the NTC reacts to a high ambient
temperature, Q1 base is brought to ground and the BO pin
goes up, permanently latching off the controller.
Protection Circuitry
This resonant controller offers a dedicated input (Fault
input) to detect primary overcurrent conditions and protect
power stage from damage.
Once the voltage on the Fault input exceeds 1.04 V
threshold the external timer capacitor starts charging by
I
timer1
current. Simultaneously the Soft−Start discharge
switch is activated to shift operating frequency up to keep
primary current at acceptable level. In case the overload
disappears fast enough the Soft−Start discharge switch is
open, I
timer1
current turned−off and timer capacitor
discharges via an external parallel resistor. In case the
overload lasts for more than timer duration (given by I
timer
,
V
timer
, C
timer
and R
timer
) the IC stops the operation and waits
until the C
timer
will discharge to 1 V. The application then
restarts via Soft−Start.
In case of heavy overload, like transformer short circuit,
the primary current grows very fast and thus could reach
danger level prior the fault timer elapses. The NCP1397B
therefore features additional comparator (1.55 V) on the
Fault input to permanently latch the application and protect
against destruction. Figure 44 depicts the architecture of the
fault circuitry for NCP1397B controller.
The NCP1397A features second fault comparator as well
but in this case it doesn’t latches off the IC but speeds up the
Fault timer capacitor charging by turning on additional
current source I
timer2
– refer to Figure 43. The NCP1397A
can thus be used in applications that have to recover
automatically from any fault conditions.
