NCP1605DR2G【PDF 17/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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NCP1605, NCP1605A, NCP1605B

http://onsemi.com

DETAILED OPERATING DESCRIPTION

Introduction

The NCP1605 is a PFC driver designed to operate in fixed

frequency, Discontinuous Conduction Mode (DCM). In the

most stressful conditions, Critical Conduction Mode (CRM)

can be achieved without power factor degradation and the

circuit could be viewed as a CRM controller with a frequency

clamp (given by the oscillator). Finally, the NCP1605 tends

to give the best of both modes without their respective

drawbacks. Furthermore, the circuit incorporates protection

features for a rugged operation together with some special

circuitry to lower the power consumed by the PFC stage in no

load conditions. More generally, the NCP1605 functions

make it the ideal candidate in systems where

costeffectiveness, reliability, low standby power and high

power factor are the key parameters:

" Compactness and Flexibility: the controller requires

few external components while offering a large variety

of functions. Depending on the selected coil and

oscillator frequency you select, the circuit can:

1. Mostly operate in CRM and use the oscillator as a

frequency clamp.

2. Mostly operate in fixed frequency mode and only

run in CRM at high load and low line.

3. Permanently operate in fixed frequency mode

DCM.

In all cases, the circuit provides nearunity power factor.

Skipcycle capability for low power standby: among

other applications, the circuit targets power supply where

the PFC stage must keep alive even in standby. A

continuous flow of pulses is not compatible with noload

standby power requirements. Instead, the controller slices

the switching pattern in bunch of pulses to drastically

reduce the overall losses. The skip cycle operation is

initiated by applying to Pin 1, a signal that goes below

300 mV in standby. Typically, this signal is drawn from the

feedback of the downstream converter.

Startup Current Source and large V

range:

meeting low standby power specifications represents a

difficult exercise when the controller requires an external,

lossy resistor connected to the bulk capacitor. The

controller disables the highvoltage current source after

startup which no longer hampers the consumption in

noload situations. In addition, the large V

range (10 V

to 20 V after startup), highly eases the circuit biasing.

Fast Line / Load Transient Compensation: given the

low bandwidth of the regulation block, the output voltage

of PFC stages may exhibit excessive over and undershoots

because of abrupt load or input voltage variations (e.g. at

startup). If the output voltage is too far from the regulation

level:

The NCP1605 disables the drive to stop

delivering power as long as the output voltage

exceeds the Overvoltage Protection (OVP) level.

The NCP1605 drastically speeds up the

regulation loop when the output voltage is below

95.5% of its regulation level. This function is

allowed only after the PFC stage has started up

not to eliminate the softstart effect.

PFC OK: the circuit detects when the circuit is in normal

situation or if on the contrary, it is in a startup or fault

condition. In the first case, Pin 12 is in high state and low

otherwise. Pin 12 serves to control the downstream

converter operation in response to the PFC state.

Safety Protections: the NCP1605 permanently

monitors the input and output voltages, the coil current and

the die temperature to protect the system from possible

overstresses and make the PFC stage extremely robust and

reliable. In addition to the aforementioned OVP protection,

one can list:

Maximum Current Limit and Zero Current

Detection: the circuit permanently senses the

coil current and immediately turns off the

power switch if it is higher than the set current

limit. It also prevents any turn on of the power

switch as long as some current flows through

the coil, to ensure operation in DCM. This

feature also protects the MOSFET from the

excessive stress that could result from the

large inrush currents that occurs during the

startup phases.

Undervoltage Protection: the circuit turns off

when it detects that the output voltage goes

below 12% of the OVP level (typically). This

feature protects the PFC stage from starting

operation in case of too low ac line conditions

or in case of a failure in the OVP monitoring

network (e.g., bad connection).

BrownOut Detection: the circuit detects too

low ac line conditions and stop operating in

this case. This protection protects the PFC

stage from the excessive stress that could

damage it in such conditions.

Thermal Shutdown: an internal thermal

circuitry disables the circuit gate drive and then

keeps the power switch off when the junction

temperature exceeds 150癈 typically. The circuit

resumes operation once the temperature drops

below about 100癈 (50癈 hysteresis).

Output Stage Totem Pole: the NCP1605 incorporates a

0.5 A / +0.8 A gate driver to efficiently drive most TO220

or TO247 power MOSFETs.

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