NCP1217, NCP1217A
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7
TYPICAL CHARACTERISTICS (continued)
Figure 13. Skip Mode Level vs. Temperature Figure 14. Int Comp Ramp Max Level vs.
Temperature
50
1.15
50
1.05
0
DSKIP MODE LEVEL (V)
1.00
1.20
TEMPERATURE (°C)
1.10
150100
INT COMP RAMP LEVEL (V)
TEMPERATURE (°C)
50
3.00
50
2.80
0
2.70
3.10
2.75
2.90
150100
3.05
2.85
2.95
TEMPERATURE (°C)
50 50
5.0
0
HV SOURCE CURRENT (mA)
3.0
8.0
4.0
6.0
7.0
150100
Figure 15. High Voltage Current Source
(@ V
CC
= 10V) vs. Temperature
NCP1217, NCP1217A
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8
APPLICATION INFORMATION
Introduction
The NCP1217 implements a standard current mode
architecture where the switchoff event is dictated by the
peak current setpoint. This component represents the ideal
candidate where low partcount is the key parameter,
particularly in lowcost AC/DC adapters, TV power
supplies, etc. Due to its highperformance HighVoltage
technology, the NCP1217 incorporates all the necessary
components normally needed in UC384X based supplies:
timing components, feedback devices, lowpass filter and
startup device but also enhances the original component
by offering: 1) an externally triggerable latchoff
2) ramp compensation and finally, 3) shortcircuit
protection. Due to its highvoltage current source,
ON Semiconductors NCP1217 does not need an external
startup resistance but supplies the startup current directly
from the highvoltage rail. On the other hand, more and
more applications are requiring low noload standby power,
e.g. for AC/DC adapters, VCRs, etc. UC384X series have a
lot of difficulty to reduce the switching losses at low power
levels. NCP1217 elegantly solves this problem by skipping
unwanted switching cycles at a useradjustable power level.
By ensuring that skip cycles take place at low peak current,
the device ensures quiet, noisefree operation:
CurrentMode Operation: As the UC384X series, the
NCP1217 features a wellknown current mode control
architecture which provides superior input audio
susceptibility compared to traditional voltagemode
controllers. Primary current pulsebypulse checking
together with a fast over current comparator offers greater
security in the event of a difficult fault condition, e.g. a
saturating transformer.
Ramp Compensation: By inserting a resistor between the
currentsense (CS) pin and the actual sense resistor, it
becomes possible to inject a given amount of ramp
compensation since the internal saw tooth clock is routed to
the CS pin. Subharmonic oscillations in Continuous
Conduction Mode (CCM) can thus be compensated via a
single resistor.
Adjustable Skip Cycle Level: By offering the ability to
tailor the level at which the skip cycle takes place, the
designer can make sure that the skip operation only occurs
at low peak current. This point guarantees a noisefree
operation with cheap transformers. Skip cycle offers a
proven mean to reduce the standby power in no or light loads
situations.
Wide SwitchingFrequency Offer: Three different options
are available: 65 kHz – 100 kHz–133 kHz. Depending on
the application, the designer can pick up the right device to
help reducing magnetics or improve the EMI signature
before reaching the 150 kHz starting point.
Overcurrent Protection (OCP): By continuously
monitoring the V
CC
auxiliary winding voltage, NCP1217
enters burst mode as soon as the power supply undergoes an
overload: when the V
CC
voltage goes down until it crosses
the undervoltage lockout level (VCC
min
). When the
NCP1217 reaches this level (typically 7.6 V), it stops the
switching pulses until the V
CC
pin voltage reaches VCC
latch
(5.6 V). At VCC
latch
, the NCP1217 attempts to restart. As
soon as the default disappears, the power supply resumes
operation.
Overvoltage Protection (OVP): If pin1 is brought to a level
higher than the internal 3.2 V reference voltage, the
controller is permanently shut down until the user cycles the
VCC OFF and ON again. This allows the building of
efficient and lowcost over voltage protection circuits.
Wide DutyCycle Operation: Wide mains operation
requires a large dutycycle excursion. The NCP1217 can go
up to 74% typically.
Low StandbyPower: If SMPS naturally exhibit a good
efficiency at nominal load, they begin to be less efficient
when the output power demand diminishes. By skipping
unneeded switching cycles, the NCP1217 drastically
reduces the power wasted during light load conditions. In
noload conditions, the NPC1217 allows the total standby
power to easily reach next International Energy Agency
(IEA) recommendations.
No Acoustic Noise While Operating: Instead of skipping
cycles at high peak currents, the NCP1217 waits until the
peak current demand falls below a useradjustable 1/3 of the
maximum limit. As a result, cycle skipping can take place
without having a singing transformer You can thus select
cheap magnetic components free of noise problems.
External MOSFET Connection: By leaving the external
MOSFET external to the IC, you can select avalanche proof
devices, which in certain cases (e.g. low output powers), let
you work without an active clamping network. Also, by
controlling the MOSFET gate signal flow, you have an
option to slow down the device commutation, therefore
reducing the amount of ElectroMagnetic Interference
(EMI).
SPICE Model: A dedicated model to run transient
cyclebycycle simulations is available but also an
averaged version to help you closing the loop. Readytouse
templates can be downloaded in OrCAD’s Pspice and
INTUSOFT’s IsSpice from ON Semiconductor web site,
NCP1217 related section.
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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 V
CC
capacitor. When the voltage
on this V
CC
capacitor reaches the VCC
ON
level (typically
12.8 V), the current source turns off and no longer wastes
any power. At this time, the V
CC
capacitor only supplies the
controller and the auxiliary supply is supposed to take over
before V
CC
collapses below VCC
min
. Figure 16 shows the
internal arrangement of this structure.
Figure 16. The Current Source Brings V
CC
Above 12.8 V and then Turns Off
-
+
8
6
4
6 mA or 0
CV
CC
Aux
HV
12.8 V/5.6 V
Once the power supply has started, the V
CC
shall be
constrained below 16 V, which is the maximum rating on
Pin 6. Figure 17 portrays a typical startup sequence with a
V
CC
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 shortcircuit protection. A
shortcircuit 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 dutycycle. The system autorecovers
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 selfsupplies 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 V
CC
slowly falls down. As soon as V
CC
reaches typically
5.6 V, the startup source turnson again and a new startup
sequence occurs, bringing V
CC
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.

NCP1217P133G

Mfr. #:
Manufacturer:
ON Semiconductor
Description:
Switching Controllers Enhanced Current Mode PWM
Lifecycle:
New from this manufacturer.
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