TEA1750T/N1,518【PDF 17/29 页】IC PFC CONTROLLER DCM16SOIC

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Sheet目录46

13页 14页 15页 16页 [17页]18页 19页 20页 21页

TEA1750_2

Product data sheet

Rev. 02 15 December 2008

17 of 29

NXP Semiconductors

TEA1750

GreenChip III SMPS control IC

7.3.7 Maximum on-time

The yback controller limits the on-time of the external MOSFET to 25 祍 (typ). When the

on-time is longer than 25 祍, the IC stops switching and enters the safe restart mode.

7.3.8 Overvoltage protection (FBAUX pin)

An output overvoltage protection is implemented in the GreenChip III series. This works

for the TEA1750 by sensing the auxiliary voltage via the current owing into pin FBAUX

during the secondary stroke. The auxiliary winding voltage is a well-dened replica of the

output voltage. Voltage spikes are averaged by an internal lter.

If the output voltage exceeds the OVP trip level, an internal counter starts counting

subsequent OVP events. The counter has been added to prevent incorrect OVP detection

which might occur during ElectroStatic Discharge (ESD) or lightning events. If the output

voltage exceeds the OVP trip level a few times and not again in a subsequent cycle, the

internal counter counts down at twice the speed it uses when counting up. However, when

typically 8 cycles of subsequent OVP events are detected, the IC assumes a true OVP

and the OVP circuit switches the power MOSFET off. As the protection is latched, the

converter only restarts after the internal latch is reset. In a typical application the mains

should be interrupted to reset the internal latch.

The output voltage V

ovp(FBAUX)

at which the OVP function trips, can be set by the

demagnetization resistor, R

FBAUX

where N

is the number of secondary turns and N

aux

is the number of auxiliary turns of the

transformer. Current I

ovp(FBAUX)

is internally trimmed.

The value of R

FBAUX

can be adjusted to the turns ratio of the transformer, thus making an

accurate OVP detection possible.

Fig 12. Soft start-up of yback

014aaa020

SOFT START

CONTROL

OCP

FBSENSE

0.5 V

start(soft)fb d 60 礎

SS2

SENSE2

o ovp

(

)

aux

----------- - I

ovp FBAUX

(

)

FBAUX

?/DIV>

clamp FBAUX

(

)

(

)

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