NCP1014AP065G datasheet(17/24 Pages) ONSEMI

Il numero della parte	NCP1014AP065G

Spiegazioni elettronici	Self?뭆upplied Monolithic Switcher for Low Standby?뭁ower Offline SMPS

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17

The Flyback transfer formula dictates that:

Pout

η =

1

2

·Lp·Ip2 ·Fsw (eq. 19)

which, by extracting

Ip and plugging into Equation 19, leads to:

Tsw = Lp

2 · Pout

η ·Fsw ·Lp



·

 1

Vin +

1

N · (Vout + Vf)

(eq. 20)

Extracting Lp from Equation 20 gives:

Lpcritical =

(Vin · Vr)2 · η

2·Fsw·[Pout·(Vr2 + 2· Vr · Vin + Vin2)]

(eq. 21)

,with Vr =N .(Vout +Vf) and η the efficiency.

If Lp critical gives the inductance value above which

DCM operation is lost, there is another expression we can

write to connect Lp, the primary peak current bounded by

the NCP101X and the maximum duty--cycle that needs to

stay below 50%:

Lpmax =

DCmax · Vinmin · Tsw

Ipmax

(eq. 22)

where Vinmin

corresponds to the lowest rectified bulk voltage, hence the

longest ton duration or largest duty--cycle. Ip max is the

available peak current from the considered part, e.g. 350 mA

typical for the NCP1013 (however, the minimum value of

this parameter shall be considered for reliable evaluation).

Combining Equations 21 and 22 gives the maximum

theoretical power you can pass respecting the peak current

capability of the NCP101X, the maximum duty--cycle and

the discontinuous mode operation:

Pmax := Tsw2 ·Vinmin2 ·Vr2 · η ·

(eq. 23)

Fsw

(2 · Lpmax · Vr2 + 4·Lpmax · Vr ·Vinmin

+ 2·Lpmax · Vinmin2)

From Equation 22 we obtain the operating duty--cycle

d =

Ip · Lp

Vin · Tsw

(eq. 24)

which lets us calculate the RMS

current circulating in the MOSFET:

IdRMS = Ip ·

d

3

 (eq.25) . From this equation, we

obtain the average dissipation in the MOSFET:

Pavg = 13 ·Ip2 ·d· RDSon (eq. 26) to which switching

losses shall be added.

If we stick to Equation 23, compute Lp and follow the

above calculations, we will discover that a power supply

built with the NCP101X and operating from a 100 Vac line

minimum will not be able to deliver more than 7.0 W

continuous, regardless of the selected switching frequency

(however the transformer core size will go down as

Fswitching is increased). This number increases

significantly when operated from a single European mains

(18 W). Application note AND8125/D, “Evaluating the

Power Capability of the NCP101X Members” details how

to assess the available power budget from all the NCP101X

series.

Example 1. A 12 V 7.0 W SMPS operating on a large

mains with NCP101X:

Vin = 100 Vac to 250 Vac or 140 Vdc to 350 Vdc once

rectified, assuming a low bulk ripple

Efficiency = 80%

Vout = 12 V, Iout = 580 mA

Fswitching = 65 kHz

Ip max = 350 mA – 10% = 315 mA

Applying the above equations leads to:

Selected maximum reflected voltage = 120 V

with Vout = 12 V, secondary drop = 0.5 V  Np:Ns = 1:0.1

Lp critical = 3.2 mH

Ip = 292 mA

Duty--cycle worse case = 50%

Idrain RMS = 119 mA

PMOSFET = 354 mW at RDSon =24 Ω (TJ > 100C)

PDSS = 1.1 mA x 350 V = 385 mW, if DSS is used

Secondary diode voltage stress = (350 x 0.1) + 12 = 47 V

(e.g. a MBRS360T3, 3.0 A/60 V would fit)

Example 2. A 12 V 16 W SMPS operating on narrow

European mains with NCP101X:

Vin = 230 Vac  15%, 276 Vdc for Vin min to 370 Vdc

once rectified

Efficiency = 80%

Vout = 12 V, Iout = 1.25 A

Fswitching = 65 kHz

Ip max = 350 mA – 10% = 315 mA

Applying the equations leads to:

Selected maximum reflected voltage = 250 V

with Vout = 12 V, secondary drop = 0.5 V  Np:Ns = 1:0.05

Lp = 6.6 mH

Ip = 0.305 mA

Duty--cycle worse case = 0.47

Idrain RMS = 121 mA

PMOSFET = 368 mW at RDSon =24 Ω (TJ > 100C)

PDSS = 1.1 mA x 370 V = 407 mW, if DSS is used below an

ambient of 50C.

Secondary diode voltage stress = (370 x 0.05) + 12 = 30.5 V

(e.g. a MBRS340T3, 3.0 A/40 V)

Please note that these calculations assume a flat DC rail

whereas a 10 ms ripple naturally affects the final voltage

available on the transformer end. Once the Bulk capacitor has

been selected, one should check that the resulting ripple (min

Vbulk?) is still compatible with the above calculations. As an

example, to benefit from the largest operating range, a 7.0 W

board was built with a 47 mF bulk capacitor which ensured

discontinuous operation even in the ripple minimum waves.

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