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NCP1217 Datasheet(PDF) 9 Page - ON Semiconductor

Part # NCP1217
Description  Enhanced PWM Current--Mode Controller for High--Power Universal Off--Line Supplies
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

NCP1217 Datasheet(HTML) 9 Page - ON Semiconductor

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NCP1217, NCP1217A
http://onsemi.com
9
Startup Sequence
When the power supply is first powered from the mains
outlet, the internal current source (typically 7.0 mA) is
biased and charges up the VCC capacitor. When the voltage
on this VCC capacitor reaches the VCCON level (typically
12.8 V), the current source turns off and no longer wastes
any power. At this time, the VCC capacitor only supplies the
controller and the auxiliary supply is supposed to take over
before VCC collapses below VCCmin. Figure 16 shows the
internal arrangement of this structure.
Figure 16. The Current Source Brings VCC Above 12.8 V and then Turns Off
--
+
8
6
4
6mAor0
CVCC
Aux
HV
12.8 V/5.6 V
Once the power supply has started, the VCC shall be
constrained below 16 V, which is the maximum rating on
Pin 6. Figure 17 portrays a typical startup sequence with a
VCC regulated at 12.5 V.
Figure 17. A Typical Startup Sequence for
the NCP1217
t, TIME (sec)
3.00 M
8.00 M
13.0 M
18.0 M
23.0 M
13.5
12.5
11.5
10.5
9.5
REGULATION
12.8 V
Overload Operation
In applications where the output current is purposely not
controlled (e.g. wall adapters delivering raw DC level), it is
interesting to implement a true short--circuit protection. A
short--circuit actually forces the output voltage to be at a low
level, preventing a bias current to circulate in the
optocoupler LED. As a result, the auxiliary voltage also
decreases because it also operates in Flyback and thus
duplicates the output voltage, providing the leakage
inductance between windings is kept low. To account for this
situation and properly protect the power supply, NCP1217
hosts a dedicated overload detection circuitry. Once
activated, this circuitry imposes to deliver pulses in a burst
manner with a low duty--cycle. The system auto--recovers
when the fault condition disappears.
During the startup phase, the peak current is pushed to the
maximum until the output voltage reaches its target and the
feedback loop takes over. The auxiliary voltage takes place
after a few switching cycles and self--supplies the IC. In
presence of a short circuit on the output, the auxiliary
voltage will go down until it crosses the undervoltage
lockout level of typically 7.6 V. When this happens,
NCP1217 immediately stops the switching pulses and
unbiases all unnecessary logical blocks. The overall
consumption drops, while keeping the gate grounded, and
the VCC slowly falls down. Assoon asVCC reachestypically
5.6 V, the startup source turns--on again and a new startup
sequence occurs, bringing VCC toward 12.8 V as an attempt
to restart. If the default has gone, then the power supply
normally restarts. If not, a new protective burst is initiated,
shielding the SMPS from any runaway. Figure 18 portrays
the typical operating signals in short circuit.


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