NCP1239
http://onsemi.com
21
The PFC controller connection is really straightforward as
testified by Figure 39: simply connect to Pin 1, the base of
a pnp transistor that connects the PFC’s V
CC
to the NCP1239
one (perhaps add a small decoupling capacitor like a 0.1
F
on the PFC) and this is all! The PFC startup network goes
away as it is fully supplied by the PWM auxiliary winding
and even high quiescent current devices do not hamper the
standby power since they are completely disconnected in
standby.
Figure 39.
The NCP1239 turns off the pnp Q1 during the standby so that the PFC controller is no longer supplied in this mode.
1
16
2
3
4
15
14
13
NCP1239
5
12
6
7
8
11
10
9
1
8
2
3
4
7
6
5
PFC Controller
PFC_V
CC
V
CC
+
+
PFC stage
Q1
Rectified
AC line
Short−Circuit or Overload Condition
The NCP1239 differs from other controllers in the sense
that a fault condition is detected independently of the
auxiliary voltage level. In auxiliary supply−based power
supplies, it is necessary that the (isolated) secondary output
conditions properly reflects on the (non−isolated) auxiliary
winding in order to instruct the controller on what is
happening on the other side of the transformer. For the
following reasons, it sometimes becomes extremely
difficult to build an efficient short−circuit protection
circuitry and even more difficult to implement over power
detection (e.g. the output load is 25% above the nominal
value but Vout is still present).
The primary leakage inductance is high: this is probably
the main reason why building efficient short−circuit
detection is difficult. When the power switch opens, the
leakage inductance superimposes a large overvoltage spike
on the drain voltage. This spike is seen on the secondary side
but also on the auxiliary winding. Unfortunately, since the
V
CC
capacitor and the auxiliary diode form a peak rectifier,
the auxiliary V
CC
often depends on this peak value rather
than the true plateau which corresponds to the output level.
