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NCP1631
Figure 12. Zero Current Detection Timing Diagram
(V AUX is the Voltage Provided by the ZCD Winding)
R CS
R OCP COIL
R OCP
R CS ILIM1
Current Sense
The NCP1631 is designed to monitor a negative voltage
proportional to total input current, i.e., the current drawn by
the two interleaved branches (I in ). As portrayed by
Figure 13, a current sense resistor (R CS ) is practically
inserted within the return path to generate a negative
voltage (V CS ) proportional to I in . The circuit uses V CS to
detect when I in exceeds its maximum permissible level. To
do so, the circuit incorporates an operational amplifier that
sources the current necessary to maintain the CS pin
voltage null (refer to Figure 13). By inserting a resistor
R OCP between the CS pin and R CS , we adjust the current
that is sourced by the CS pin (I CS ) as follows:
* [R CS I COIL ] ) [R OCP I CS ] + 0 (eq. 10)
Which leads to:
I CS + I (eq. 11)
In other words, the pin 9 current (I CS ) is proportional to
the coil current.
A negative clamp protects the circuit from the possible
negative voltage that can be applied to the pin. This
protection is permanently active (even if the circuit off).
The clamp is designed to sustain 5 mA. It is recommended
not to sink more than 5 mA from the CS pin for a proper
operation.
Two functions use I CS : the over current protection and
the in ? rush current detection.
Over ? Current Protection (OCP)
If I CS exceeds I ILIM1 (210 m A typical), an over ? current
is detected and the on ? time is decreased proportionally to
the difference between the sensed current I IN and the
210 m A OCP threshold.
The on ? time reduction is done by injecting a current I neg
in the negative input of the “V TON processing circuit”
OPAMP. (See Figure 7)
I neg + 0.5(I CS * 210 m ) (eq. 12)
This current is injected each time the OCP signal is high.
The maximum coil current is:
I COIL(max) + I (eq. 13)
In ? rush Current Detection
When the PFC stage is plugged to the mains, the bulk
capacitor is abruptly charged to the line voltage. The
charge current (named in ? rush current) can be very huge
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