P1012AP13 datasheet(10/25 Pages) ONSEMI

10 / 25 page

NCP1010, NCP1011, NCP1012, NCP1013, NCP1014

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Figure 18. NCP101X Facing a Fault Condition (Vin = 150 Vdc)

Tstart

Tsw

TLatch

1 V Ripple

Latch−off

Level

The rising slope from the latch−off level up to 8.5 V

is expressed by: Tstart

DV1 · C

IC1

. The time during which

the IC actually pulses is given by tsw

DV2 · C

ICC1

Finally,

the

latch−off

time

can

derived

using the same formula topology: TLatch

DV3 · C

ICC2

From these three definitions, the burst duty−cycle

can be computed: dc

Tsw

Tstart

) Tsw ) TLatch

(eq. 2) .

DV2

ICC1 · D

ICC1 )

DV1

IC1 )

DV3

ICC2

(eq. 3) .

Feeding

the

equation with values extracted from the parameter section

gives a typical duty−cycle of 13%, precluding any lethal

thermal runaway while in a fault condition.

DSS Internal Dissipation

The Dynamic Self−Supplied pulls energy out from the

drain pin. In Flyback−based converters, this drain level can

easily go above 600 V peak and thus increase the stress on the

DSS startup source. However, the drain voltage evolves with

time and its period is small compared to that of the DSS. As

a result, the averaged dissipation, excluding capacitive losses,

can be derived by: PDSS + ICC1 · t Vds(t) u . (eq. 4) .

Figure 19 portrays a typical drain−ground waveshape where

leakage effects have been removed.

Figure 19. A typical drain−ground waveshape

where leakage effects are not accounted for.

Vds(t)

Vin

toff

ton

Tsw

By looking at Figure 19, the average result can easily be

derived by additive square area calculation:

t Vds(t) u+ Vin · (1 * d) ) Vr · toff

Tsw

(eq. 5)

By developing Equation 5, we obtain:

t Vds(t) u+ Vin * Vin · ton

Tsw

) Vr · toff

Tsw

(eq. 6)

toff can be expressed by: toff

+ Ip ·

(eq. 7) where ton

can be evaluated by: ton

+ Ip ·

Vin

(eq. 8) .

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