NCP1203
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8
Current−Mode Operation
As the UC384X series, the NCP1203 features a
well−known current mode control architecture which
provides superior input audio−susceptibility compared to
traditional voltage−mode controllers. Primary current
pulse−by−pulse checking together with a fast over current
comparator offers greater security in the event of a difficult
fault condition, e.g. a saturating transformer.
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 noise−free operation with cheap transformers.
Skip cycle offers a proven mean to reduce the standby power
in no or light loads situations.
Wide Switching−Frequency Offer
Four different options are available: 40 kHz − 65 kHz –
100 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)
When the auxiliary winding collapses below UVLOlow,
the controller stops switching and reduces its consumption.
It stays in this mode until Vcc reaches 4.9 V typical, where
the startup source is reactivated and a new startup sequence
is attempted. The power supply is thus operated in burst
mode and avoids any lethal thermal runaway. When the
default goes way, the power supply automatically resumes
operation.
Wide Duty−Cycle Operation
Wide mains operation requires a large duty−cycle
excursion. The NCP1203 can go up to 80% typically.
Low Standby Power
If SMPS naturally exhibit a good efficiency at nominal
load, they begin to be less efficient when the output power
demand diminishes. By skipping un−needed switching
cycles, the NCP1203 drastically reduces the power wasted
during light load conditions. In no−load conditions, the
NCP1203 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
NCP1203 waits until the peak current demand falls below a
user−adjustable 1/3
rd
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 cycle−by−cycle
simulations is available but also an averaged version to help
you closing the loop. Ready−to−use templates can be
downloaded in OrCAD’s Pspice and INTUSOFT’s from ON
Semiconductor web site, NCP1203 related section.
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, NCP1203
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 7.8 V. When this happens,
NCP1203 immediately stops the switching pulses and
unbias 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
4.8 V, the startup source turns−on 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 15, on the
following page, portrays the typical operating signals in
short circuit.
