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NCP1205P
Single Ended PWM Controller
AND8043/D
Implementing the SOFA in
AC/DC Power Supplies
Prepared by: Christophe Basso
ON Semiconductor
e–mail: christophe.basso@
INTRODUCTIONThe novel ON Semiconductor NCP1205 represents
a real breakthrough in Switch Mode Power Supply
(SMPS) controllers. By combining free–running operation
and a Voltage Controlled Oscillator (VCO), the
Soft
Oscillating Free–running chArger or SOFA, allows
true Quasi–Resonant (QR) or valley switching operation
and soft frequency foldback when the output power
demand diminishes. Implementing ON Semiconductor
state–of–the–art Very High–Voltage Integrated Circuitry
(VHVIC), the SOFA will naturally find a place in designs
where soft–switching operation and ease of implementation
are premium, as the following features emphasize:
Full Quasi–Resonant Operation: By detecting the endof the transformer core demagnetization to initiate a new
cycle, the SOFA ensures drain–source valley switching or
QR operation. Furthermore, thanks a to comprehensive
logic circuitry, the device jumps between the valleys as
the built–in VCO starts to decrease the switching
frequency. As a result, Electromagnetic–Interference
(EMI) are reduced and turn–on losses are virtually null.
Voltage–Controlled Oscillator: An internal VCO takesover as soon as the free–running frequency hits a
maximum user adjustable value. As the output power
demand further diminishes, the switching frequency is
naturally reduced to ensure a better efficiency at light
loads.
Low Standby–Power: If SMPS naturally exhibit a goodefficiency at nominal load, they begin to be less efficient
when the output power demand vanishes. By smoothly
reducing the number of switching cycles per second, the
SOFA drastically reduces the power wasted during light
load conditions. In no–load conditions, the SOFA allows
the total standby power to easily reach and exceed next
International Energy Agency (IEA) recommendations.
Short–Circuit Protection: By permanently monitoringthe feedback line activity, the IC is able to detect the
presence of a short–circuit, immediately reducing the
output power for a total system protection. Once the short
has disappeared, the controller resumes and goes back to
normal operation. For given applications (e.g. constant
output power supplies), you can easily disconnect this
protective feature.
Over–Voltage Protection: By continuously checking itsown Vcc rail, the SOFA can safely go into permanent
latch–off phase when the operating voltage exceeds 36 V
In Forward winding applications, this options lets you
also protect the design against transient mains over
voltages. For application where an adjustment is
necessary, the SO–16 versions pins out the dedicated
comparator input to let you select the protection level of
your choice.
Large Supply Range: Battery charger require that the controller can still control the output
current when the output voltage is close to zero (e.g. a
discharged battery). This is called Constant–Current/
Constant–V
controller self–supply when the output voltage
disappears, one needs to wire the auxiliary winding in the
Forward mode. However, most of today’s primary side
controllers have difficulty to cope with a Forward
auxiliary winding operated on a universal mains because
of the large voltage dynamics it implies. Fortunately, by
authorizing 7.0 V through 36 V operation, the SOFA eases
the designer task on the self–supply side.
Low Output Ripple in Standby: Some loads aresensitive to the ripple present on the output. This is the
case for Li–Ion batteries where a clean voltage is required
to ensure the longest service. Standard hysteretic
controllers produce unacceptable output ripple. By
smoothly reducing the operating frequency, the SOFA
generates a lower ripple when entering the standby mode.
No Acoustic Noise While Operating: Instead ofreducing the switching frequency at high peak currents,
the SOFA waits until the peak current demand falls below
a fixed 1/3
frequency reduction takes place without having a singing
transformer
components free of noise problems.
