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NCP1255

Current-Mode PWM Controller for Off-line Power Supplies featuring Peak Power Excursion

ON Semiconductor 

NCP1255

APPLICATION INFORMATION

Introduction

The NCP1255 implements a standard current mode

architecture where the switch−off event is dictated by the

peak current setpoint. This component represents the ideal

candidate where low part−count and cost effectiveness are

the key parameters, particularly in low−cost ac−dc adapters,

open−frame power supplies etc. The NCP1255 brings all the

necessary components normally needed in today modern

power supply designs, bringing several enhancements such

as a non−dissipative OPP, a brown−out protection or peak

power excursion for loads exhibiting variations over time.

• Current−mode operation with internal slope

compensation: implementing peak current mode

control at a fixed 65−kHz frequency, the NCP1255

offers an internal ramp compensation signal that can

easily by summed up to the sensed current. Sub

harmonic oscillations can thus be compensated via the

inclusion of a simple resistor in series with the

current−sense information.

• Frequency excursion: when the power demand forces

the peak current setpoint to reach the internal limit

(0.8 V/Rsense typically), the frequency is authorized to

increase to let the converter deliver more power. The

frequency excursion stops when 130 kHz are reached at

a level of 4 V. This excursion can only be temporary

and its duration is set by the overload timer.

• Internal OPP: by routing a portion of the negative

voltage present during the on−time on the auxiliary

winding to the dedicated OPP pin (pin 1), the user has a

simple and non−dissipative means to alter the

maximum peak current setpoint as the bulk voltage

increases. If the pin is grounded, no OPP compensation

occurs. If the pin receives a negative voltage down to

–250 mV, then a peak current reduction down to 31.3%

typical can be achieved. For an improved performance,

the maximum voltage excursion on the sense resistor is

limited to 0.8 V.

• Low startup current: reaching a low no−load standby

power always represents a difficult exercise when the

controller draws a significant amount of current during

start−up. Thanks to its proprietary architecture, the

NCP1255 is guaranteed to draw less than 15 mA

maximum, easing the design of low standby power

adapters.

• EMI jittering: an internal low−frequency modulation

signal varies the pace at which the oscillator frequency

is modulated. This helps spreading out energy in

conducted noise analysis. To improve the EMI signature

at low power levels, the jittering will not be disabled in

frequency foldback mode (light load conditions).

• Frequency foldback capability: a continuous flow of

pulses is not compatible with no−load/light−load

standby power requirements. To excel in this domain,

the controller observes the feedback pin and when it

reaches a level of 1.9 V, the oscillator then starts to

reduce its switching frequency as the feedback level

continues to decrease. When the feedback level reaches

1.5−V, the frequency hits its lower stop at 26 kHz.

When the feedback pin goes further down and reaches

1 V, the peak current setpoint is internally frozen.

Below this point, if the power continues to drop, the

controller enters classical skip−cycle mode.

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

power switch from being stressed upon start−up. In this

controller, the soft−start is internally fixed to 4 ms.

Soft−start is activated when a new startup sequence

occurs or during an auto−recovery hiccup.

• OVP input: the NCP1255 includes a latch input (pin 1)

that can be used to sense an overvoltage condition on

the adapter. If this pin is brought higher than the

internal reference voltage Vlatch, then the circuit

permanently latches off. The Vcc pin is pulled down to

a fixed level, keeping the controller latched. Reset

occurs when the latch current goes below ICClatch or

when a brown−out transition is sensed by the controller.

• Vcc OVP: a latched OVP protects the circuit against

Vcc runaways. The fault must be present at least 20 ms

to be validated. Reset occurs when the latch current

goes below ICClatch or when a brown−out transition is

sensed by the controller.

• 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 (the aux winding level

does not properly collapse in presence of an output

short). Here, every time the internal 0.8−V maximum

peak current limit is activated (or less when OPP is

used), an error flag is asserted and a time period starts,

thanks to the programmable timer. The controller can

distinguish between two faulty situations:

♦ There is an extra demand of power, still within the

power supply capabilities. In that case, the feedback

level is in the vicinity of 3.2−4 V. It corresponds to

0.8 V as the maximum peak current setpoint without

OPP. The timer duration is then 100% of its

programmed value via the pull−down resistor on

pin 8. If you put a 22−kW resistor, the typical

duration is around 500 ms. If the fault disappears,

e.g. the peak current setpoint no longer hits the

maximum value (e.g. 0.8 V at no OPP), then the

timer is reset.

♦ The output is frankly shorted. The feedback level is

thus pushed to its upper stop (4.5 V) and the timer is

reduced to 25% of its normal value.

♦ In either mode, when the fault is validated, all pulses

are stopped and the controller enters an

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