NCP1605, NCP1605A, NCP1605B
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3. Place a resistor R
DRV
between the drive pin and
Pin 6 to ease the circuit detection by creating
some overriding at the turn on instant. R
DRV
should be selected in the range of 3 times R
ZCD
.
For instance, if R
ZCD
is 6.2 kW, a 22 kW resistor
can be used for R
DRV
.
Overvoltage Protection
While PFC circuits often use one single pin for both the
Overvoltage Protection (OVP) and the feedback, the
NCP1605 dedicates one specific pin for the undervoltage
and overvoltage protections. The NCP1605 configuration
allows the implementation of two separate feedback
networks (see Figure 62):
  One for regulation applied to Pin 4.
  Another one for the OVP function.
Figure 61. Configuration with One Feedback
Network for Both OVP and Regulation
FB
HV
OVP
Vout (bulk voltage)
Figure 62. Configuration with Two Separate
Feedback Networks
1
2
3
4
13
16
14
15
5
6
7
8   9
12
10
11
R
out1
R
out3
R
out2
FB
HV
OVP
Vout (bulk voltage)
1
2
3
4
13
16
14
15
5
6
7
8   9
12
10
11
R
out1
R
out2
R
ovp1
R
ovp2
The   double   feedback   configuration   offers   some
upgraded safety level as it protects the PFC stage even if
there is a failure of one of the two feedback arrangements.
However, if wished, one single feedback arrangement is
possible as portrayed by Figure 61. The regulation and
OVP blocks having the same reference voltage, the
resistance ratio Rout2 over Rout3 adjusts the OVP
threshold. More specifically,
The bulk regulation voltage is:
V
out
+
R
out1
) R
out2
) R
out3
R
out2
) R
out3
@ V
ref
The OVP level is:
V
ovp
+
R
out1
) R
out2
) R
out3
R
out2
@ V
ref
The ratio OVP level over regulation level is:
V
ovp
V
out
+ 1 )
R
out3
R
out2
For instance, (V
OVP
= 105% * V
out
) leads to the
following constraint: (R
out3
= 5% * R
out2
).
As soon and as long as the circuit detects that the output
voltage exceeds the OVP level, the power switch is turned
off to stop the power delivery.
Remark: Like in the NCP1601, the V
TON
processing
circuit is informed when there is an OVP condition, not
to overdimension V
TON
in that conditions. Otherwise, an
OVP sequence would be viewed as a deadtime phase by
the circuit and V
TON
would inappropriately increase to
compensate it (refer to Figure 56).
PfcOK / REF5V Signal
The NCP1605 can communicate with the downstream
converter. The signal pfcOK/REF5V is high (5 V) when
the PFC stage is in normal operation (its output voltage is
stabilized at the nominal level) and low otherwise.
More specifically, pfcOK/REF5V is low:
" During the PFC stage startup, that is, as long as the
output voltage has not yet stabilized at the right level.
The startup phase is detected by the latch L
STUP
of the
block diagram. L
STUP
is set during each off phase so
that its output (STUP) is high when the circuit enters
an active phase. The latch is reset when the error
amplifier stops charging its output capacitor, that is,
when the output voltage of the PFC stage has reached its
desired regulation level. At that moment, STUP falls
down to indicate the end of the startup phase.
" In case of a condition preventing the circuit from
operating properly, i.e., during the V
CC
charge by the
high voltage startup current source, in a Brownout
case or when one of the following major faults turns
off the circuit:
  Incorrect feeding of the circuit (UVLO high when
V
CC
<V
CC
OFF, V
CC
OFF equating 9 V typically).
  Excessive die temperature detected by the thermal
shutdown.
  Undervoltage Protection
  Latched off of the circuit (when the STDWN pin,
V
Pin13
, exceeds 2.5 V).