NCP1212 Datasheet, PDF (13/22 Pages) ON Semiconductor – Current Mode PWM Controller for Both Forward and Flyback Converters

NCP1212
SS/DMAX pin is also used for the selection of maximum
turn on Duty Cycle. The oscillator circuit is designed to
operate in either 82% or 48% charging time that corresponds
to either 82% or 48% maximum PWM turn on Duty Cycle.
As discussed in the Oscillator Section, saw−tooth waveform
at C_T pin is different for 82% and 48% maximum turn on
Duty Cycle and it is shown in Figure 32.
The final voltage at SS/DMAX pin determines the
maximum turn on Duty Cycle. If 82% maximum turn on
duty is desired, simply connect a capacitor from SS/DMAX
pin to ground as shown in Figure 36 and the final voltage on
the capacitor will be 5.0 V minus one diode drop (X4.3 V).
5.0 V
I_SS = 0.8 mA
Final Voltage [ 4.3 V
SS/DMAX
82% Max.
C_SS
Duty Cycle
Figure 36. 82% Maximum Duty Cycle Selection
For 48% maximum Duty Cycle selection, we need to
adjust the final voltage at SS/DMAX to lower than 3.2 V
minus one diode drop (X2.5 V). This can be achieved by
connecting a resistor in parallel with C_SS as shown in
Figure 37. The value of this parallel resistor is given by the
equation in below:
Rduty
+
2.5 V
8 mA
5.0 V
Overload Detection
During output overload or short circuit condition, the
PWM controller will pump as much energy as possible to the
secondary side and the power only limited by the
cycle−by−cycle current limit setting. Components in the
power supply circuit such as the power MOSFET and output
rectifier may be damaged by this continuous stress.
Theoretically, fly−back converter has inherent short circuit
protection provided that the PWM controller is supplied by
a fly−back auxiliary winding and it has UVLO function.
Unluckily, it is quite common that the supply will experience
very high leaky voltage spike that prevents the V_CC voltage
to fall below UVLO level during short circuit.
NCP1212 is equipped with an integrated overload
detection mechanism, which is irrespective of auxiliary
winding voltage level. Overload shutdown is no longer
bothered by leakage spike hence a reliable overload
protection system can be easily constructed by NCP1212 for
both forward and fly−back configuration. Overload
detection block is shown in Figure 38. Overload condition
is signified by current sense voltage hitting the maximum
allowable voltage, 1.0 V. To avoid false trigger that may
happen during transient load changes, C_SS starts to
discharge by 20 mA (I_SD−I_SS). If overload condition
persists, V_SST voltage level drops to 0.5 V and triggers the
overload shutdown. Overload shutdown is only enabled
after the soft−start period.
Due to the overload detection mechanism, it is
mandatory to connect a capacitor at the SS/DMAX pin.
Otherwise overload shutdown may be triggered during
startup phase.
I_SS = 8 mA
Final Voltage = 2.5 V
SS/DMAX
48% Max.
C_SS
Duty Cycle
R_duty
Figure 37. 48% Maximum Duty Cycle Connection
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