NCP1028 datasheet(22/29 Pages) ONSEMI

Part #	NCP1028

Description	High?뭋oltage Switcher for Medium Power Offline SMPS Featuring Low Standby Power

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NCP1028

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22

The principle consists in selecting the RR resistor,

connected from pin 2 to ground, to impose a current

IRR in

the transistor collector.

Figure 38. Maximum Peak Current Setpoint

Variations versus Ramp Compensation

The equation to get the right compensation level is the

following:

RR +

Vp2.75 k

Sa@TSW

(eq. 9)

where Vp, the total voltage swing, equals 2.75 V.

Application example:

Suppose we have the following flyback specifications:

Vout = 5.0 V

output voltage

Vf = 1.0 V

secondary diode forward drop

@ Iout nominal

Np:Ns = 1:N = 1:0.052 transformer turn ratio

Lp = 3.8 mH

primary inductance

We can calculate the off slope, the one actually needed

to evaluate

Sa, by reflecting the output voltage over the

primary inductance. The slope is projected over a complete

switching period. Here, we use a 65 kHz part.

Soff +

Vout ) Vf

NLp

TSW +

6

15u

0.052

3.8m +

455 mA 15 ms

(eq. 10)

Due to the internal sense arrangement, this current slope

will become a voltage slope having a value of:

S off + 455m

0.375 + 170 mV 15 ms (eq. 11)

If we chose 50% of this downslope, then the final

compensation ramp will present a slope of:

Sa + 170m

2

+ 85 mV 15 ms

(eq. 12)

We then have:

RR +

Vp2.75 k

Sa@TSW

+ 2.75

2.75k

85m

+ 89 kW (eq. 13)

In the above calculations, the internal ESD resistor has

purposely been omitted to avoid bringing in another

variable. In case no ramp compensation is required, pin 2

must be tied to VCC, the adjacent pin.

Soft−Start

The NCP1028 features a 1.0 ms soft−start, which

reduces the power−on stress, but also contributes to lower

the output overshoot. Figure 39 shows a typical operating

waveform. The NCP1028 features a novel patented

structure which offers a better soft−start ramp, almost

ignoring the startup pedestal inherent to traditional

current−mode supplies.

Figure 39. 1.0 ms Soft−Start Sequence

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