TEA1751T All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 3 — 10 January 2013 18 of 31
NXP Semiconductors
TEA1751T
HV start-up flyback controller with integrated PFC controller
The charging current I
start(soft)(FB)
flows as long as V
FBSENSE
is less than approximately
0.5 V. If V
FBSENSE
exceeds 0.5 V, the soft-start current source starts limiting the current.
After the flyback converter has started, the soft-start current source is switched off.
7.3.7 Maximum on-time
The flyback controller limits the on-time of the external MOSFET to 40 s. When the
on-time is longer than 40 s, 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. In the
TEA1751T, the auxiliary voltage is sensed using the current flowing into the FBAUX pin
during the secondary stroke. The auxiliary winding voltage is a well-defined replica of the
output voltage. An internal filter averages voltage spikes.
An internal up-down counter prevents false OVP detection which can occur during ESD or
lightning events. The internal counter counts up by one when the output voltage exceeds
the OVP trip level within one switching cycle. The internal counter counts down by two
when the output voltage has not exceeded the OVP trip level within one switching cycle.
When the counter has reached eight, the IC assumes a true OVP, sets the latched
protection and switches off both converters.
The converter only restarts after the OVP latch is reset. In a typical application, the
internal latch is reset when the VINSENSE voltage drops below 750 mV and is then raised
to 870 mV.
The latched protection is also reset by removing both the voltage on the V
CC and HV pins.
The demagnetization resistor, R
FBAUX
sets the output voltage V
o(OVP)
at which the OVP
function trips:
(7)
where N
s
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.
Fig 13. Soft-start up of flyback.
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V
oOVP
N
s
N
aux
-----------
I
ovp FBAUX
R
FBAUX
V
clamp FBAUX
+=