NCP1246 ONSEMI [ON Semiconductor], NCP1246 Datasheet - Page 15

• Manufacturer Part Number: NCP1246
• Description: Fixed Frequency Current Mode Controller for Flyback Converters
• Manufacturer: ONSEMI [ON Semiconductor]

Functional Description
safe and efficient power supply based on a fixedïfrequency
flyback converter. The NCP1246 is a multimode controller
as illustrated in Figure 45. The mode of operation depends
upon line and load condition. Under all modes of operation,
the NCP1246 terminates the DRV signal based on the switch
current. Thus, the NCP1246 always operates in current
mode control so that the power MOSFET current is always
limited.
the operating mode of the NCP1246 at the voltage
thresholds shown in Figure 45. At normal rated operating
loads (from 100% to approximately 33% full rated power)
the NCP1246 controls the converter in 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 dutyïratio may increase up to 50%. The buildïin
slope
subïharmonic oscillations in this operating area.
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
shutïdown. 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
The NCP1246 includes all necessary features to build a
Under normal operating conditions, the FB pin commands
There was implemented the low consumption off mode
0 V
compensation
OFF
0.4 V
Skip mode
Low consumption off mode
0.7 V
prevents
0.8 V
1.1 V
the
Figure 45. Mode Control with FB pin voltage
Fixed I
appearance
APPLICATION INFORMATION
peak
http://onsemi.com
NCP1246
1.5V
of
15
FFM
of full rated power, the converter operates in frequency
foldback mode (FFM). If the feedback pin voltage is lower
than 1.5 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 V
constant voltïseconds to the flyback transformer each
switching cycle. Voltage regulation in FFM is achieved by
varying the switching frequency in the range from 65 kHz
(or 100 kHz) to 27 kHz. For extremely light loads (below
approximately 6% full rated power), the converter is
controlled using bursts of 27 kHz pulses. This mode is called
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
steadyïstate operation are approximate because they are
subject to converter design parameters.
decreases below the 0.4 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.
latched A and the autorecovery B on the following figures.
The basic status of the device after wake–up by the V
the off mode and mode is used for the overheating protection
mode if the thermal shutdown protection is activated.
For loads that are between approximately 32% and 10%
Effectively, operation in FFM results in the application of
See the detailed status diagrams for the both versions fully
2.0 V
2.2 V
ON
PWM at
f
OSC
3.5 V
HV
V
.
FB
CC
is