NCP1207AADAPGEVB Datasheet, PDF (7/17 Pages) ON Semiconductor – PWM Current-Mode Controller for Free Running Quasi-Resonant Operation

NCP1207A, NCP1207B
APPLICATION INFORMATION
Introduction
The NCP1207A/B implements a standard current mode
architecture where the switch−off time is dictated by the
peak current setpoint whereas the core reset detection
triggers the turn−on event. This component represents the
ideal candidate where low part−count is the key parameter,
particularly in low−cost AC/DC adapters, consumer
electronics, auxiliary supplies, etc. Due to its
high−performance High−Voltage technology, the
NCP1207A/B incorporates all the necessary components /
features needed to build a rugged and reliable Switch−Mode
Power Supply (SMPS):
• Transformer core reset detection: borderline / critical
operation is ensured whatever the operating conditions
are. As a result, there are virtually no primary switch
turn−on losses and no secondary diode recovery losses.
The converter also stays a first−order system and
accordingly eases the feedback loop design.
• Quasi−resonant operation: by delaying the turn−on
event, it is possible to re−start the MOSFET in the
minimum of the drain−source wave, ensuring reduced
EMI / video noise perturbations. In nominal power
conditions, the NCP1207A/B operates in Borderline
Conduction Mode (BCM) also called Critical
Conduction Mode.
• Dynamic Self−Supply (DSS): due to its Very High
Voltage Integrated Circuit (VHVIC) technology,
ON Semiconductor’s NCP1207A/B allows for a direct
pin connection to the high−voltage DC rail. A dynamic
current source charges up a capacitor and thus provides
a fully independent V_CC level to the NCP1207A/B. As
a result, there is no need for an auxiliary winding whose
management is always a problem in variable output
voltage designs (e.g. battery chargers).
• Overvoltage Protection (OVP): by sampling the plateau
voltage on the demagnetization winding, the
NCP1207A/B goes into latched fault condition
whenever an over−voltage condition is detected. The
controller stays fully latched in this position until the
V_CC is cycled down 4.0 V, e.g. when the user un−plugs
the power supply from the mains outlet and re−plugs it.
• External latch trip point: by externally forcing a level
on the OVP greater than the internal setpoint, it is
possible to latchoff the IC, e.g. with a signal coming
from a temperature sensor.
• 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 transformer. This
option also offers the ability to fix the maximum
switching frequency when entering light load
conditions.
• Overcurrent Protection (OCP): by continuously
monitoring the FB line activity, NCP1207A/B enters
burst mode as soon as the power supply undergoes an
overload. The device enters a safe low power operation
which prevents from any lethal thermal runaway. As
soon as the default disappears, the power supply
resumes operation. Unlike other controllers, overload
detection is performed independently of any auxiliary
winding level. In presence of a bad coupling between
both power and auxiliary windings, the short circuit
detection can be severely affected. The DSS naturally
shields you against these troubles.
Dynamic Self−Supply
The DSS principle is based on the charge/discharge of the
V_CC bulk capacitor from a low level up to a higher level. We
can easily describe the current source operation with some
simple logical equations:
POWER−ON: IF V_CC < VCC_OFF THEN Current Source
is ON, no output pulses
IF V_CC decreasing > VCC_ON THEN Current Source is
OFF, output is pulsing
IF V_CC increasing < VCC_OFF THEN Current Source is
ON, output is pulsing
Typical values are: VCC_OFF = 12 V, VCC_ON = 10 V
To better understand the operational principle, Figure 12’s
sketch offers the necessary light.
V_RIPPLE = 2 V
VCC_OFF = 12 V
VCC_ON = 10 V
ON
OFF
Output Pulses
Figure 12. The Charge/Discharge Cycle Over a 10 mF
V_CC Capacitor
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