NCP1654
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Detailed Operating Description
Introduction
The NCP1654 is a PFC driver designed to operate in
fixed frequency, continuous conduction mode. The fixed
frequency operation eases the compliance with EMI
standard and the limitation of the possible radiated noise
that may pollute surrounding systems. In addition,
continuous conduction operation reduces the application
di/dt and their resulting interference. More generally, the
NCP1654 is an ideal candidate in systems where
cost−effectiveness, reliability and high power factor are the
key parameters. It incorporates all the necessary features to
build a compact and rugged PFC stage:
• Compactness and Flexibility: housed in a SO8
package, the NCP1654 requires a minimum of
external components. In particular, the circuit scheme
simplifies the PFC stage design and eliminates the
need for any input voltage sensing. In addition, the
circuit offers some functions like the Brown−Out or
the true power limiting that enable the optimizations
of the PFC design,
• Low Consumption and Shutdown Capability: the
NCP1654 is optimized to exhibit consumption as
small as possible in all operation modes. The
consumed current is particularly reduced during the
start−up phase and in shutdown mode so that the PFC
stage power losses are extremely minimized when the
circuit is disabled. This feature helps meet the more
stringent stand−by low power specifications. Just
ground the Feed−back pin to force the NCP1654 in
shutdown mode,
• Safety Protections: the NCP1654 permanently monitors
the output voltage, the coil current and the die
temperature to protect the system from possible
over−stresses. Integrated protections (Overvoltage
protection, coil current limitation, thermal shutdown...)
make the PFC stage extremely robust and reliable:
− Maximum Current Limit: the circuit permanently
senses the coil current and immediately turns off the
power switch if it is higher than the set current limit.
The NCP1654 also prevents any turn on of the
power switch as long as the coil current is not below
its maximum permissible level. This feature protects
the MOSFET from possible excessive stress that
could result from the switching of a current higher
than the one the power switch is dimensioned for. In
particular, this scheme effectively protects the PFC
stage during the start−up phase when large in−rush
currents charge the output capacitor.
− Undervoltage Protection for Open Loop Protection
or Shut−down: the circuit detects when the
feed−back voltage goes below than about 8% of the
regulation level. In this case, the circuit turns off and
its consumption drops to a very low value. This
feature protects the PFC stage from starting
operation in case of low AC line conditions or in
case of a failure in the feed−back network (i.e. bad
connection).
− Fast Transient Response: given the low bandwidth
of the regulation block, the output voltage of PFC
stages may exhibit excessive over or under−shoots
because of abrupt load or input voltage variations
(e.g. at start up). If the output voltage is too far from
the regulation level:
Overvoltage Protection: NCP1654 turns off the
power switch as soon as V
out
exceeds the OVP
threshold (105% of the regulation level). Hence
a cost & size effective bulk capacitor of lower
voltage rating is suitable for this application,
Dynamic Response Enhancer: NCP1654
drastically speeds up the regulation loop by its
internal 200ĂmA enhanced current source when the
output voltage is below 95% of its regulation level.
− Brown−Out Detection: the circuit detects low AC
line conditions and disables the PFC stage in this
case. This protection mainly protects the power
switch from the excessive stress that could damage it
in such conditions,
− Over−Power Limitation: the NCP1654 computes the
maximum permissible current in dependence of the
average input voltage measured by the brown−out
block. It is the second OCP with a threshold that is
line dependent. When the circuit detects an
excessive power transfer, it resets the driver output
immediately,
− Thermal Shutdown: an internal thermal circuitry
disables the circuit gate drive and then keeps the
power switch off when the junction temperature
exceeds 150°C typically. The circuit resumes
operation once the temperature drops below about
120°C (30°C hysteresis),
− Soft Start: V
control
is pulled low brown−out detection
activates, or Undervoltage protection activates, and
no drive is provided.
At start up, the “200 mA enhanced current source” is
disabled. So there is only 28 mA to charge the
compensation components, and makes V
control
raise
gradually. This is to obtain a slow increasing duty
cycle and hence reduce the voltage and current
stress on the MOSFET. Hence it provides a soft−start
feature.
• Output Stage Totem Pole: the NCP1654 incorporates
a ±1.5A gate driver to efficiently drive TO220 or
TO247 power MOSFETs.
