ncp1282 ON Semiconductor, ncp1282 Datasheet - Page 14

• Manufacturer Part Number: ncp1282
• Description: High-performance Active Clamp/reset Pwm Controller Featuring Hv Start-up
• Manufacturer: ON Semiconductor

ac-dc or dc-dc converters requiring high-efficiency and
low parts count. This controller incorporates two in phase
outputs with an adjustable overlap delay. The main output
is designed for driving a forward converter primary
MOSFET. The secondary output is designed for driving an
active clamp circuit MOSFET, a synchronous rectifier on
the secondary side, or an asymmetric half bridge circuit.
soft-stop and a cycle skip overcurrent detector with a time
threshold. The soft-stop powers down the converter in a
controlled manner after a fault is detected. The cycle skip
timer disables the converter if a continuous overcurrent
condition is present.
by incorporating high accuracy on critical specifications
such as programmable maximum duty cycle, undervoltage
detector and overcurrent threshold. A minimum operating
voltage of 8.5 V ensures enough voltage is available for
driving high voltage MOSFETs. Additional features found
in the NCP1282 include line feedforward, bidirectional
frequency synchronization up to 1.0 MHz, cycle-by-cycle
current limit with leading edge blanking (LEB),
independent under and overvoltage detectors, internal
startup circuit and soft-start.
SOFT-STOP AND SOFT-START
soft-start architecture that combines soft-start and
soft-stop functions on a single pin.
a fault is detected. By slowly reducing the duty cycle during
power down, the active clamp capacitor (C
discharged. This prevents oscillations between the power
transformer and C
in a predictable state.
up allowing the controller to gradually reach steady state
operation. Combined, both features reduce system stress
and power surges.
capacitor voltage (V
Soft-start or soft-stop is implemented by slowly charging
or discharging the capacitor on the SS pin. OUT1 is
disabled once the FF Ramp exceeds V
charge current is 20 mA and the soft-stop discharge current
is 100 mA, guaranteeing a faster turn OFF time.
placing an external resistor between the V
The resistor should be sized such that the total charge
current does not exceed 100 mA. Otherwise, the converter
will not be able to complete a soft-stop sequence.
undervoltage,
(V
The NCP1282 is a voltage mode controller designed for
Two distinctive features of the NCP1282 are the
The NCP1282 reduces component count and system size
The NCP1282 incorporates a novel soft-stop and
Soft-stop reduces the duty cycle until it reaches 0% once
Soft-start slowly increases the duty cycle during power
The duty cycle is controlled by comparing the SS
The preset 1:5 charge:discharge ratio can be reduced by
The converter enters a soft-stop sequence if an
AUX(off1)
) or thermal shutdown condition is detected.
overvoltage, cycle skip low V
clamp
SS
, and ensures the converter turns off
) to the Feedforward (FF) Ramp.
DETAILED OPERATING DESCRIPTION
SS
REF
. The soft-start
and SS pins.
clamp
http://onsemi.com
AUX
) is
NCP1282
14
Once the converter enters the soft-stop mode, it will stay
in soft-stop mode until V
is removed prior to reaching 0.2 V.
is handled differently to ensure the fastest response time
and prevent system malfunction. If a soft-stop sequence
starts before V
the FF Ramp (typ. 3.0 V) and the PWM Comparator (V
is not yet controlling the duty cycle, a controlled discharge
of C
However, if V
discharged until soft-stop sets a duty cycle equal to the duty
cycle set by V
afterwards, as shown in Figure 35. If V
Ramp and the V
forced to the peak voltage of the FF Ramp, before starting
a controlled discharge of C
duty cycle set at the beginning of the soft-stop event never
exceeds the duty cycle prior to the soft-stop event.
Depending on the converter state, a soft-stop sequence
Figure 35. Soft-Stop Behavior when V
SS
V
V
V
commences immediately, as shown in Figure 34.
SS
EA
EA
Figure 34. Soft-Stop Before Soft-Start
V
is Complete and V
SS
(V
SS
EA
V
EA
EA
EA
X
exceeds the maximum voltage clamp of
. A controlled discharge commences
= V
is not controlling the duty cycle)
is controlling the duty cycle, C
is not controlling the duty cycle, V
the Duty Cycle.
EA
- V
SS
FF Ramp
SS
f
reaches 0.2 V even if the fault
, as shown in Figure 36. The
EA
is Open.
SS
exceeds the FF
EA
Controls
SS
SS
EA
is
is
)