Power integrations LinkSwitch-4 LNK4*15D Series Application Note page 18

Application Note
Step 13 - Further Component Selection
D
IN1-4
R
F1
AC IN
Figure 24. Input Stage.
The recommended input stage is shown in Figure 24. It consists of a
fusible element (R ), input rectification (D
F1
(C C L and L ).
IN1 IN2 IN1 IN2
The fusible element can be either a fusible resistor or a fuse. If a
fusible resistor is selected to limit switch on inrush current, use a
flameproof type.
Depending on the differential line input surge requirements, a
wire-wound type may be required. Avoid using metal or carbon film
types as these can fail due to the inrush current when VAC applied
repeatedly to the supply. A value of 10 W, 2 W is a typical value.
C and C are approximately equal values, to provide the input pi
IN1 IN2
filter. It is advisable to make at least C
L should be between 220 mH to 2.2 mH, and have a current rating
IN1
of approximately:
^
IL = 40 10
#
IN 1
Although inductors have a current rating for a given temperature rise,
that current is often the level at which the inductance has fallen to
90% of its low current value. So given that it is not desirable that the
inductor saturates and reduces in inductance, hence reducing its
filtering capability, it is a reasonable guide, though the inductor will
be operating at a much lower average current than its rating.
The best EMI results are not always achieved by putting in the
highest available inductor value. The inductor will have a self
resonant frequency (SRF) which tends to decrease with inductor
value. A high value inductor may well have a SRF that coincides with
one of the switching generated frequencies and it will have minimal
attenuating effect upon it. Decreasing the inductor value may move
the SRF out of a sensitive frequency band and give better EMI
results. Alternatively, a resistor can be placed in parallel with the
inductor to damp the SRF, about 4.7 kW would suffice.
L is optional and helps with higher frequency EMI emissions, >20
IN2
Mhz. It is usually a low value SMT ferrite bead inductor of the order
of 150 W to 1000 W impedance at 100 MHz.
Primary Clamp Components
D : 1N4007 / FR107, 1 A, 1000 V
C1
R : 100 W – 300 W
C1
C : 220 pF – 1000 pF 500 V
C1
R : 330 KW - 680 KW
C1
The clamp arrangement, shown in Figure 25, is suitable for LinkSwitch-4
designs. Minimize the value of C
ing the peak Collector voltage to <(V
18
Rev. B 10/17
Input Stage
+
L
IN1
C C
IN1 IN2
L
IN2
PI-5118-042308
), and line filter network
IN1-4
a low ESR type.
IN2
P
h
OUT
3 C +
#
V
IN 2
HT(MIN)
and maximize R while maintain-
C1 C2
× derating factor). Larger
CBO
R
C2
Black Trace: D is a FR107 (fast type, trr = 500 ns)
C1
Gray Trace: D is a 1N4007G (standard recovery, trr = 2 us)
C1
Figure 25. Primary Clamp Components and Effect of Diode Recovery Time of
FEEDBACK Pin Voltage.
values of C may cause higher output ripple voltages due to the
C1
longer settling time of the clamp voltage impacting the sampled
voltage on the feedback winding, particularly at low loads. A value of
470 KW for R with 470 pF for C
C2
ed starting point for the RCD design. Verify that the peak collector
voltage is less than (V × derating factor) under all line and load
CBO
conditions including start-up.
The clamp diode choice is governed by what weight is given to
various performance factors and cost. Customers usually favor
lowest cost at the power level and application type covered by
LinkSwitch-4 devices. This indicates the use of a low-cost 1N4007
slow recovery type diode. Note from Figure 25, that a slow recovery
diode has a lower frequency ring, and faster settling time, this can be
useful in reducing EMI.
The choice of snubber components affects no-load power, no-load
frequency, no-load output voltage stability, peak collector voltage,
efficiency and EMI.
Reducing C will reduce energy lost in the clamp, reduce the no-load
C1
switching frequency and hence reduce the no-load power. The
compromise is that the peak collector voltage is increased and EMI
emissions may increase, but as long they are within the set limits it is
an option to control no-load power.
AN-69
C
C1
R
C1
D
C1
PI-5107-012615
and 100 W for R is a recommend-
C1 C1
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