DAP018x Marking, DAP018DDR2G datasheet(27/28 Pages) ONSEMI

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Figure 54. The Zener Diode can Introduce a Threshold which

Disables OPP at Low Line for Maximum Power Capability

OPP

RoppU

This point will

be adjusted to

reduce the ref

at hi line to the

desired level.

Iopp

RoppL

VDD

Reset

from FB

ref

VCC aux

0.8 V ±5%

ref = 0.8 V + VOPP

(VOPP is negative)

SUM2

Rbias

Swings to:

N1Vout during toff

−N2Vout during ton

Suppose we need a threshold placed at 150 Vdc. In that

case, give the turn ratio, we need to install a zener diode

featuring the following breakdown voltage:

Vz + 150

0.16 + 24 V

(eq. 8)

In high line conditions (Vbulk = 375 Vdc), the voltage on the

zener diode voltage will now swing to:

Vaux + −N2Vin,max ) 24 + −0.16 375 ) 24 + −36 V

(eq. 9)

Applying Equation 6 again, we have:

ROPPU +

36 * 0.16

0.16 1k +

224 kW

(eq. 10)

The new compensation now looks like what Figure 55

shows:

100%

20%

Peak current

setpoint

Vbulk

375

150

100%

20%

Peak current

setpoint

Vbulk

375

150

Figure 55. The Addition of the Zener Diode

Introduces a Threshold at 150 Vdc

The bias resistor Rbias, makes sure that enough current

circulates in the zener diode to make it work far enough from

its knee. The tradeoff includes the power consumption

brought by the addition of this new component. Further

reduction can be obtained by inserting a standard diode in

series with the zener to block the positive excursion.

DAP018x Marking, DAP018DDR2G Datasheet(PDF) 27 Page - ON Semiconductor

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