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NCP1254

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

ON Semiconductor 

NCP1254

Figure 42 shows the voltage evolution from almost 0 V to

the open−loop level, around 4.5 V. At low power levels or in

no−load operation, the feedback voltage stays in the vicinity

of 400 mV and ensures skip−cycle operation. In this mode,

the peak current is frozen to 31% of its maximum value. This

freeze lasts as long as VFB stays below 1 V. Beyond 1 V, the

peak current is authorized to follow VFB through a ratio of

4. When the power demand goes up, the switching frequency

linearly increases from 26 kHz up to 65 kHz, a value reached

when the feedback voltage exceeds 1.5 V. Beyond 1.9 V, the

frequency no longer changes. As VFB still increases, we are

in a fixed−frequency variable peak current mode control

type of operation until the feedback voltage hits 3.2 V. At this

point, the maximum current is limited to 0.8 V/Rsense. If VFB

further increases, it means the converter undergoes an

overload and requires more power from the source. As the

peak current excursion is stopped, the only way to deliver

more power is to increase the switching frequency. From

3.2 V up to 4 V, the frequency linearly increases from

65 kHz to 130 kHz. The maximum power delivered by the

converter depends whether it operates in Discontinuous

Conduction Mode (DCM) or in Continuous Conduction

Mode (CCM):

Pmax,DCM

+

1

2

LpFsw,maxIpeak,max 2

h

(eq. 23)

ǒ Ǔ Pmax,CCM

+

1

2

LpFsw,max

Ipeak,max 2 * Ivalley 2

h

(eq. 24)

VFB (V)

Where Ipeak,max is the maximum peak current authorized by

the controller and Ivalley the valley current reached just

before a new switching cycle begins. This current is

expressed by the following formula:

Ivalley

+

Ipeak

*

Vout )

NLp

Vf

toff

(eq. 25)

In DCM, the valley current is equal to 0.

Two Levels of Protection

Once the feedback voltage asks for the maximum peak

current, the controller knows that an overload condition has

started. An internal timer is operated as soon as the

maximum peak current setpoint is reached. Its duration is

internally set to 200 ms. If the feedback voltage continues its

rise, it means that the converter output voltage is going down

further, close to a short−circuit situation. When the feedback

voltage approaches the open−loop level (above 4.0 V

typically), the original timer duration is divided by 4. For

instance, at start−up, even if the overload timer is

programmed to 200 ms, when the feedback voltage jumps

above 4.0 V, the controller will wait 50 ms before fault

detection occurs. Of course, if the feedback does not stay

that long in the region of concern, the timer is reset when

returning to a normal level. Figure 43 shows the timer values

versus the feedback voltage.

max

4.5

Open loop voltage

4.0

25% timer − short−circuit

3.2

1.9

1.5

Frozen current

1

0.4

100% timer − overload

Fixed−frequency variable peak current

Frequency foldback variable peak current

Fixed frequency variable peak current

Frozen peak current

Skip cycle

Max Ipeak

31% max Ipeak

Figure 43. Depending on the feedback level, the timer will take two different values:

it will authorize a transient overload, but will reduce a short−circuit duration.

130 kHz

65 kHz

65 kHz

26 kHz

t

http://onsemi.com

23

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