NCP1230
www.onsemi.com
16
Current Sense Input Pin Latch−Off
The NCP1230 features a fast comparator (Figure 34) that
monitors the current sense pin during the controller off time.
If for any reason the voltage on pin 3 increases above 3.0 V,
the NCP1230 immediately stops the PWM drive pulses and
permanently stays latched off until the bias supply to the
NCP1230 is cycled down (Vcc must drop below 4.0 V, e.g.
when the user unplugs the converter from the mains). This
offers the designer the flexibility to implement an externally
shutdown circuit (for example for overvoltage or
overtemperature conditions). When the controller is latched
off through pin 3 (current sense), SW1 opens and shuts off
PFC_Vcc output.
Figure 37 shows how to implement the external latch via
a Zener diode and a simple PNP transistor. The PNP actually
samples the Zener voltage during the OFF time only, hence
leaving the CS information un−altered during the ON time.
Various component arrangements can be made, e.g. adding
a NTC device for the Over Temperature Protection (OTP).
Figure 37.
Connecting the PNP to the drive only activates the offset
generation during Toff. Here is a solution monitoring the
auziliary Vcc rail.
1
2
3
4
5
8
6
7
Ramp
1k
CVcc
HV
Vz
Drive Output
The NCP1230 provides a Drive Output which can be
connected through a current limiting resistor to the gate of
a MOSFET. The Driver output is capable of delivering drive
pulses with a rise time of 40 ns, and a fall time of 15 ns
through its internal source and sink resistance of 12.3 ohms
(typical), measured with a 1.0 nF capacitive load.
Startup Sequence
The NCP1230 has an internal High Voltage Startup
Circuit (Pin 8) which is connected to the high voltage DC
bus (Refer to Figure 36). When power is applied to the bus,
the NCP1230 internal current source (typically 3.2 mA) is
biased and charges up the external Vcc capacitor on pin 6,
refer to Figure 38. When the voltage on pin 6 (Vcc) reaches
Vccoff (12.6 V typically), the current source is turned off
reducing the amount of power being dissipated in the chip.
The NCP1230 then turns on the drive output to the external
MOSFET in an attempt to increase the output voltage and
charge up the Vcc capacitor through the Vaux winding in the
transformer.
During the startup sequence, the controller pushes for the
maximum peak current, which is reached after the 2.5 ms
soft−start period. As soon as the maximum peak set point is
reached, the internal 1.0 V Zener diode actively limits the
current amplitude to 1.0 V/Rsense and asserts an error flag
indicating that a maximum current condition is being
observed. In this mode, the controller must determine if it is
a normal startup period (or transient load) or is the controller
is facing a fault condition. To determine the difference
between a normal startup sequence, and a fault condition, the
error flag is asserted, and the 125 ms timer starts to count
down. If the error flag drops prior to the 125 ms time−out
period, the controller resets the timer and determines that it
was a normal startup sequence and enables the low
impedance switch (SW1), enabling the PFC_Vcc output.
If at the end of the 125 ms period the error flag is still
asserted, then the controller assumes that it is a fault
condition and the PWM controller enters the skip mode and
does not enable the PFC_Vcc output.
Figure 38.
−
+
8
6
4
3.2 mA or 0
CVcc Aux
HV
12.6 V/
5.6 V
ON Semiconductor recommends that the Vcc capacitor be at
least 47 F to be sure that the Vcc supply voltage does not drop
below Vccmin (7.7 V typical) during standby power mode and
unusual fault conditions.
Soft−Start
The NCP1230 features an internal 2.5 ms soft−start
circuit. As soon as Vcc reaches a nominal 12.6 V, the
soft−start circuit is activated. The soft−start circuit output
controls a reference on the minus (−) input to an amplifier
(refer to Figure 39), the positive (+) input to the amplifier is
the feedback input (divided by 3). The output of the
amplifier drives a FET which clamps the feedback signal. As
the soft−start circuit output ramps up, it allow the feedback
pin input to the PWM comparator to gradually increased
from near zero up to the maximum clamping level of 1.0
V/Rsense. This occurs over the entire 2.5 ms soft−start
period until the supply enters regulation. The soft−start is
also activated every time a restart is attempted. Figure 40
shows a typical soft−start up sequence.
