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NCL30001
Output Overload
The Feedback Voltage, V FB , is directly proportional to the
output power of the converter. An internal 6.7 k W resistor
pulls ? up the FB voltage to the internal 6.5 V reference. An
external optocoupler pulls down the FB voltage to regulate
the output voltage of the system. The optocoupler is off
during power up and output overload conditions allowing
the FB voltage to reach its maximum level.
The NCL30001 monitors the FB voltage to detect an
overload condition. A typical startup time of a single PFC
stage converter is around 100 ms. If the converter is out of
regulation (FB voltage exceeds 5.0 V) for more that 160 ms
(typical) the drivers are disabled and the controller enters the
double hiccup mode to reduce the average power
dissipation. A new startup sequence is initiated after the
double hiccup is complete. This protection feature is critical
to reduce power during an output short condition.
260 mV. A new Soft ? Skip period starts once the voltage on
the AC ? IN pin increases to 260 mV.
An increase in output load current terminates a Soft ? Skip
event. A transient load detector terminates a Soft ? Skip
period once V FB voltage exceeds V SSKIP (1.75 V nominal).
This ensures the required output power is delivered during
a load transient and the output voltage does not fall out of
regulation. Figure 57 shows the relationship between
Soft ? Skip and the transient load detector.
Soft ? Skip ? Cycle Mode
The FB voltage reduces as the output power demand of the
converter reduces. Once V FB drops below the skip
threshold, V SSKIP , 410 mV (typical) the driver is disabled.
The skip comparator hysteresis is typically 90 mV.
The converter output voltage starts to decay because no
additional output power is delivered. As the output voltage
decreases the feedback voltage increases to maintain the
output voltage in regulation. This mode of operation is
V FB
V SSKIP
known as skip mode. The skip mode frequency is dependent
of load loop gain and output capacitance and can create
audible noise due to mechanical resonance in the
transformer and snubber capacitor. A proprietary Soft ? Skip
mode reduces audible noise by slowly increasing the
primary peak current until it reaches its maximum value.
The minimum skip ramp period, t SSKIP , is 2.5 ms. Figure 56
shows the relationship between V FB , V SSKIP and the
primary current.
Figure 56. Soft ? Skip Operation
Skip mode operation is synchronized to the ac line
voltage. The NCL30001 disables Soft ? Skip when the
rectified ac line voltage drops to its valley level. This ensures
the primary current always ramp up reducing audible noise.
A skip event occurring as the ac line voltage is decreasing,
causes the primary peak current to ramp down instead of
ramp up. Once the skip period is over the primary current is
only determined by the ac line voltage. A Soft ? Skip event
terminates once the AC ? IN pin voltage decreases below
Figure 57. Load transient during Soft ? Skip
The output of the Soft ? Skip Comparator is or ? ed with the
PWM Comparator output to control the duty ratio. The
Soft ? Skip Comparator controls the duty ratio in skip mode
and the PWM Comparator controls the duty cycle during
normal operation. In skip mode, the non ? inverting input of
the Soft ? Skip Comparator exceeds 4 V, disabling the
drivers. As the FB voltage increases, the voltage at the
non ? inverting input is ramp down from 4 V to 0.2 V to
enable the drivers.
Multiplier and Reference Generator
The NCL30001 uses a multiplier to regulate the average
output power of the converter. This controller uses a
proprietary concept for the multiplier used within the
reference generator. This innovative design allows greatly
improved accuracy compared to a conventional linear
analog multiplier. The multiplier uses a PWM switching
circuit to create a scalable output signal, with a very well
defined gain.
The output of the multiplier is the ac-reference signal. The
ac-reference signal is used to shape the input current. The
multiplier has three inputs, the error signal from an external
error amplifier (V FB ), the full wave rectified ac input
(AC_IN) and the feedforward input (V FF ).
The FB signal from an external error amplifier circuit is
applied to the V FB pin via an optocoupler or other isolation
circuit. The FB voltage is converted to a current with a V-I
V CC
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