400EADAB1 datasheet(16/47 Pages) ONSEMI

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NCP12400

www.onsemi.com

APPLICATION INFORMATION

Functional Description

The NCP12400 includes all necessary features to build a

safe and efficient power supply based on a fixed−frequency

flyback converter. The NCP12400 is a multimode controller

as illustrated in Figure 43. The mode of operation depends

upon line and load condition. Under all modes of operation,

the NCP12400 terminates the DRV signal based on the

switch current. Thus, the NCP12400 always operates in

current mode control so that the power MOSFET current is

always limited.

Under normal operating conditions, the FB pin commands

the operating mode of the NCP12400 at the voltage

thresholds shown in Figure 43. At normal rated operating

loads (from 100% to approximately 33% full rated power)

the NCP12400 controls the converter in a fixed−frequency

PWM mode. It can operate in the continuous conduction

mode (CCM) or discontinuous conduction mode (DCM)

depending upon the input voltage and loading conditions. If

the controller is used in CCM with a wide input voltage

range, the duty−ratio may increase up to 50%. The build−in

slope compensation prevents the appearance of

sub−harmonic oscillations in this operating area.

For loads that are between approximately 32% and 10%

of full rated power, the converter operates in frequency

foldback mode (FFM). If the feedback pin voltage is lower

than 1.4 V the peak switch current is kept constant and the

output voltage is regulated by modulating the switching

frequency for a given and fixed input voltage VHV.

Effectively, operation in FFM results in the application of

constant volt−seconds to the flyback transformer each

switching cycle. Voltage regulation in FFM is achieved by

varying the switching frequency in the range from 65 kHz

to 28 kHz. For extremely light loads (below approximately

6% full rated power), the converter is controlled using bursts

of 28 kHz pulses. This mode is known as skip mode. The

FFM, keeping constant peak current and skip mode allows

design of the power supplies with increased efficiency under

the light loading conditions. Keep in mind that the

aforementioned boundaries of steady−state operation are

approximate because they are subject to converter design

parameters.

Figure 43. Mode Control with FB Pin Voltage

V FB

VFBilim

VON

VFB(foldS)

VFB(foldE)

Vskip(out)

Vskip(in)

VOFF

FFM

PWM at f OSC

Skip mode

0 V

Low consumption off mode

OFF

There was implemented the low consumption off mode

allowing to reach extremely low no load input power. This

mode is controlled by the FB pin and allows the remote

control (or secondary side control) of the power supply

shut−down. Most of the device internal circuitry is unbiased

in the low consumption off mode. Only the FB pin control

circuitry and X2 cap discharging circuitry is operating in the

low consumption off mode. If the voltage at feedback pin

decreases below the 0.6 V the controller will enter the low

consumption off mode. The controller can start if the FB pin

voltage increases above the 2.2 V level.

See the detailed status diagrams for the both versions fully

latched A and the autorecovery B on the following figures.

The basic status of the device after wake–up by the VCC is

the off mode and mode is used for the overheating protection

mode if the thermal shutdown protection is activated.

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