Power integrations LinkSwitch-4 LNK4*15D Series Application Note page 33

AN-69
Figure 51. Schematic with Extra Diode for EasyStart Feature.
Thermal Shut Down
The LinkSwitch-4 has a thermal shut down feature. If the junction
temperature of the IC exceeds T
down and cease switching. Once the temperature has fallen by T
the chip will restart, perform a normal start-up cycle and continue.
Constant Power (CP) option
In addition to EasyStart, LNK4214D and LNK4215D devices have a
constant power section to the VI curve, see Figure 52. This feature is
intended to further improve start-up into large capacitive loads
(5000 µF at 12 V). At start-up, the constant current (CC) pull-up set
point of the VI characteristic (bottom right of Figure 52) is set by the
transformer turns ratio, the value of R
parameter V . The value of V
CC(SC)
LNK4214D and LNK4215D compared to LNK4014D and LNK4115D, so
for a given transformer and R
CS
about 75% higher than the standard CV/CC VI curve parts, providing
an extra 75% of start-up current into the output capacitance and
aiding a successful rapid start-up. As the output voltage rises, the
controller regulates in CC mode until it is above approximately 50% of
the design CV level. It then operates in a power limited (CP) mode
until the voltage reaches the design CV set point, at which point it
changes to CV mode regulation. I
greater than 57% of I . The exact trajectory is somewhat
OUT(EXT)
dependent on the HT level (mains input level). This is due to the
controller operating in open loop mode over the CP section of the VI
curve i.e. feedback is not effective during this section, so inaccuracies
due to efficiency variation over the CP section and mains input level
are not corrected. Hence there is no single CP curve for a design, but
minimum and maximum curves between which the actual curve will
lie, see I for Low Mains and High Mains in Figure 52. The
OUTCP(TYP)
constant power value is set by the full load switching frequency
parameter F , the primary inductance, the peak primary current ‒
SW
set by the device parameter V
CS(MAX)
the conversion efficiency. Due to the open-loop mode of operation
on the CP section, tolerances in primary inductance, switching
frequency, R , V and V
CS CS(MAX) CSCC
of the CP section of the VI curve. This must be factored in when
undertaking the design of a CP application.
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L
FILT
D
BRIDGE
C
IN1
R
IN
, about 140 ºC, the chip will shut
SD
and the value of the device
CS
is about 75% higher on
CC(SC)
value the CC level, I , will be
OUT(EXT)
at this point, I , will be
OUT OUT(CP)
divided by the value of R , and
CS
play a significant part in the accuracy
T1
2 × R
HT
Q1
C
IN2
EasyStart
R
Diode
SBD
R
CS2
R
CS
V (V)
OUT
,
SD(H)
100%
V
OUTCV(TYP)
75%
50%
33%
0
Figure 52. Typical LNK4214D and LNK4215D CP Output Characteristic.
Primary inductance tolerance should be ±5% or better, F
tolerance of ±7%, V
CSCC
or better.
Application Note
D
OUT
+
C R
OUT OUT
R
FB1
C
VCC
R
FB2
PI-8180-110916
Low Mains
High Mains
I
OUTCP(MIN)
I
OUTCP(TYP)
I
OUT(EXT)
100% 175% 200%
±2%, V ±5.5% and R should be ±1%
CS(MAX) CS
I (A)
OUT
has a
SW
33
Rev. B 10/17