SSL8516T All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 1 — 5 May 2014 17 of 33
NXP Semiconductors
SSL8516T
Greenchip PFC and flyback controller
7.3.7 Maximum on-time
The flyback controller limits the on-time of the external MOSFET to 38.5 s. When the
on-time is longer than 38.5 s, the IC stops switching and enters the safe restart state.
7.3.8 Overvoltage protection (FBAUX pin)
An output OVP is implemented in the GreenChip series. In the SSL8516T, 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 in one switching cycle.
When the counter has reached six, the IC assumes a true overvoltage and triggers the
safe start protection
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 windings and N
aux
is the number of auxiliary
windings of the transformer. Current I
ovp(FBAUX)
is internally trimmed.
Accurate OVP detection is made possible by adjusting the value of R
FBAUX
to the turns
ratio of the transformer.
7.3.9 Overcurrent protection (FBSENSE pin)
The primary peak current in the transformer is measured accurately cycle-by-cycle using
the external sense resistor R
SENSE2
. The OCP circuit limits V
FBSENSE
to a level set by
V
FBCTRL
(see also Section 7.3.3). The OCP detection is suppressed during the
leading-edge blanking period, t
leb
(equals t
on(fb)min
t
d(FBDRIVER)
), to prevent false
triggering due to switch-on spikes.
Fig 13. Flyback soft-start
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V
oOVP
N
s
N
aux
-----------
I
ovp FBAUX
R
FBAUX
V
clamp FBAUX
+=
