NCP1212 ON Semiconductor, NCP1212 Datasheet - Page 18

• Manufacturer Part Number: NCP1212
• Description: Current Mode Pwm Controller For Both Forward And Flyback Converters
• Manufacturer: ON Semiconductor

in Figure 42. From the captured waveforms, it can be noted
that the Brownout Input Threshold Voltage is 1.21 V and
Brownout Hysteresis Voltage is 1.36 V at BOK pin.
Experimental results for the Brownout action were shown
4. Improving Light Load and No Load Regulation for
High Power Applications
For high power applications, limited by the dynamic
range of the control circuitry, i.e. the control
feedback is limited by the swing of the optocoupler.
When V
conditions, it no longer has the means to further
reduce that voltage because of the saturation of the
optocoupler. At light load or no load conditions, the
primary current is very small and as the current
FB
reaches about 0.1 V at light load
Channel 1: The MOSFET’s V
Channel 2: Primary Bulk Capacitor Voltage
Channel 3: BOK Pin Voltage
Figure 42. Brownout Detect Waveforms
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NCP1212
18
DS
Switching Waveform
sensing resistor is also small for high power
applications, the current sense feedback voltage will
be much smaller than 0.1 V. Consequently, the
control will force to acquire maximum Duty Cycle
operation and the output will increase without
control.
In order to improve the poor regulation at light load,
a small circuit is added as shown in Figure 43. With
the additional circuitry, when V
Q2 will drive additional offset current to CS pin and
modify the current sensing voltage, V
higher than 0.1 V at normal load operation, Q2 will
be turned off due to limited V
be .
FB
falls below 0.1 V,
CS
. For V
FB