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NCP1379

QUASI−RESONANT PWM CONTROLLER FOR HIGH POWER AC−DC WALL ADAPTERS

ON Semiconductor 

NCP1379

10.4

10.2

10.0

9.8

9.6

9.4

9.2

−40 −20 0

20 40 60 80 100 120

TJ, JUNCTION TEMPERATURE (°C)

Figure 21. IBO vs. Junction Temperature

APPLICATION INFORMATION

NCP1379 implements a standard current−mode

architecture operating in quasi−resonant mode. Thanks to a

proprietary circuitry, the controller prevents

valley−jumping instability and steadily locks out in selected

valley as the power demand goes down. Once the fourth

valley is reached, the controller continues to reduce the

frequency further down, offering excellent efficiency over

a wide operating range. Due to a fault timer combined to an

OPP circuitry, the controller is able to efficiently limit the

output power at high−line.

• Quasi−Resonance Current−mode operation:

implementing quasi−resonance operation in peak

current−mode control, the NCP1379 optimizes the

efficiency by switching in the valley of the MOSFET

drain−source voltage. Due to a proprietary circuitry, the

controller locks−out in a selected valley and remains

locked until the output loading significantly changes.

This behavior is obtained by monitoring the feedback

voltage. When the load becomes lighter, the feedback

setpoint changes and the controller jumps into the next

valley. It can go down to the 4th valley if necessary.

Beyond this point, the controller reduces its switching

frequency by freezing the peak current setpoint. During

quasi−resonance operation, in case of very damped

valleys, a 5.9 ms timer adds the missing valleys.

• Frequency reduction in light−load conditions: when the

4th valley is left, the controller reduces the switching

frequency which naturally improves the standby power

by a reduction of all switching losses.

• Overpower protection (OPP): When the voltage on

ZCD pin swings in flyback polarity, a direct image of

the input voltage is applied on ZCD pin. We can thus

reduce the peak current depending of the ZCD pin

voltage level during the on−time.

• Internal soft−start: a soft−start precludes the main

power switch from being stressed upon start−up. Its

duration is fixed and equal to 3.8 ms.

• Fault input: the NCP1379 and D versions include a

brown−out circuit which safely stops the controller in

case the input voltage is too low. Restart occurs via a

complete startup sequence (latch reset and soft−start).

During normal operation, the voltage on this pin is

clamped to 1.2 V to give enough room for OVP

detection. If the voltage on this pin increases above

2.5 V, the part latches−off.

• Short−circuit protection: short−circuit and especially

over−load protections are difficult to implement when a

strong leakage inductance between auxiliary and power

windings affects the transformer (where the auxiliary

winding level does not properly collapse in presence of

an output short). Here, when the internal 0.8 V

maximum peak current limit is activated, the timer

starts counting up. If the fault disappears, the timer

counts down. If the timer reaches completion while the

error flag is still present, the controller stops the pulses

and goes into auto−recovery mode.

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