NCP1654

http://onsemi.com

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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:

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,

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,

monitors the output voltage, the coil current and the die

temperature to protect the system from possible

over−stresses. Integrated protections (over−voltage

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,

− Under−Voltage Protection / 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 (e.g.,

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:

Over−Voltage Protection: NCP1654 turns off the

power switch as soon as Vout 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,

Vout Low Detect: NCP1654 drastically speeds up

the regulation loop by its internal 200

mA

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. When the circuit detects an

excessive power transfer, it resets the PWM latch

and pulls down the regulation block output as long

as the calculated power keeps too high,

− 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: Vcontrol is pulled low as the IC is off,

detection activates, or under−voltage protection

activates, and no drive is provided.

The soft−start function is done by disable the

“200

mA enhanced current source” at start up. So

there is only 20

mA to charge the Ccontrol, and

makes Vcontrol increase slowly. This is to obtain a

slow increasing duty cycle and hence reduce the

voltage and current stress on the MOSFET.

This soft−start function is designed in B version

only. A version doesn’t have this soft−start function,

the resistors connected to input voltage and should

be able to boost the PFC output as soon as possible

up period, Ccontrol will be charged by 220

mA

current source and the PFC output will rise rapidly.

a ±1.5A gate driver to efficiently drive TO220 or

TO247 power MOSFETs.