NCP1606
http://onsemi.com
14
To prevent negative voltages on the ZCD pin, the pin is
internally clamped to V
CL(NEG)
(600 mV typ) when the
ZCD winding is negative. Similarly, the ZCD pin is
clamped to V
CL(POS)
(5.7 V typical), when the voltage rises
too high. Because of these clamps, a resistor (R
ZCD
in
Figure 29) is necessary to limit the current from the ZCD
winding to the ZCD pin.
At startup, there is no energy in the ZCD winding and
therefore no voltage signal to activate the ZCD
comparators. This means that the driver could never turn
on. Therefore, to enable the PFC stage to startup under
these conditions, an internal watchdog timer is integrated
into the controller. This timer turns the drive on if the driver
has been off for more than 180 ms (typical). Obviously, this
feature is deactivated during a fault mode (OVP, UVP, or
Shutdown), and reactivated when the fault is removed.
STARTUP
Generally, a resistor connected between the ac input and
V
CC
(pin 8) charges the V
CC
capacitor to the V
CC(on)
level
(12 V typical). Because of the very low consumption of the
NCP1606 during this stage (< 40 mA), most of the current
goes directly to charging up the V
CC
capacitor. This
provides faster startup times and reduced standby power
dissipation. When the V
CC
voltage exceeds the V
CC(on)
level, the internal references and logic of the NCP1606 turn
on. The controller has an undervoltage lockout (UVLO)
feature which keeps the part active until V
CC
drops below
V
CC(off)
(9.5 V typical). This hysteresis allows ample time
for the auxiliary winding to take over and supply the
necessary power to V
CC
(Figure 30).
Figure 30. Typical V
CC
Startup Waveform
V
CC
V
CC(on)
V
CC(off)
When the PFC pre−converter is loaded by a switch mode
power supply (SMPS), then it is often preferable to have the
SMPS controller startup first. The SMPS can then supply
the NCP1606 V
CC
directly. Advanced controllers, such as
the NCP1230 or NCP1381, can control when to turn on the
PFC stage (see Figure 31) leading to optimal system
performance. This setup also eliminates the startup
resistors and therefore improves the no load power
dissipation of the system.
Figure 31. NCP1606 Supplied by a Downstream SMPS Controller (NCP1230)
1
7
6
5
2
3
4
NCP1606
+
+
+
+
1
7
6
5
2
3
4
NCP1230
PFC_Vcc
88
V
CC
+
C
bulk
D
boost
QUICK START and SOFT START
At startup, the error amplifier is enabled and Control is
pulled up to V
EAL
(typically 2.1 V). This is the lowest level
of control voltage which produces output drives. This
feature, called “quick start,” eliminates the delay at startup
associated with charging the compensation network to its
minimum level. This also produces a natural “soft start”
mode where the controller’s power ramps up from zero to
the required power (see Figure 32).
