SCY99194 Datasheet, PDF (15/40 Pages) ON Semiconductor – Fixed Frequency Current Mode Controller

SCY99194
APPLICATION INFORMATION
Functional Description
The SCY99194 includes all necessary features to build a
safe and efficient power supply based on a fixed−frequency
flyback converter. The SCY99194 is a multimode controller
as illustrated in Figure 40. The mode of operation depends
upon line and load condition. Under all modes of operation,
the SCY99194 terminates the DRV signal based on the
switch current. Thus, the SCY99194 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 SCY99194 at the voltage
thresholds shown in Figure 40. At normal rated operating
loads (from 100% to approximately 33% full rated power)
the SCY99194 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 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 1.5 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_HV.
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 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 steady−state operation are
approximate because they are subject to converter design
parameters.
Low consumption off mode
ON
OFF
Skip mode
Fixed I_peak
FFM
PWM at f_OSC
0 V _{0.4 V 0.7 V 1.1 V}
1.5V
2.0 V 2.2 V
0.8 V
Figure 40. Mode Control with FB pin voltage
3.5 V VFB
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 0.4 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.
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 V_CC is
the off mode and mode is used for the overheating protection
mode if the thermal shutdown protection is activated.
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