Power integrations LinkSwitch-4 LNK4*15D Series Application Note page 23

AN-69
D 1-4
IN
RF1
Figure 31. V and Bias Current Loops.
CC
D 1-4
IN
RF1
Figure 32. Input Components and Tracks Susceptible to Noise from BJT Collector Node.
lead loop down to the output capacitor +V
this is not optimal for good thermal cooling of the diode. The
cathode side track can be made large, as it only has a small ac
ripple voltage on it. It can act as a heat sink but the diode heat
has now to flow along the cathode lead before reaching the track.
Testing may be required to determine the best compromise. An
SMT diode would not have this problem.
EMC will be reduced by ensuring a small loop area, for the primary
•
clamp circuit. Keep the components close to the transformer
primary pins.
Noisy paths (shown in red in Figure 32) should be kept away from
•
the input loop (shown in green) to reduce capacitive coupling and
improve EMC. In particular, keep the BJT away from the input
diodes, L and R . BJT collector node is the noisiest.
IN1 IN
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L
IN1
+ +
C C
IN1 IN2
L R
IN2 C2
R
CS
L
IN1
+ +
C C
IN1 IN2
L R
IN2 C2
R
CS
track. Unfortunately
E
C
T1
C1
R
C2
R
R C1
HT
D
C1
Q1
C
C
VCC1 VCC2
C
T1
C1
R
C2
R
R C1
HT
D
C1
Q1
C
C
VCC1 VCC2
A Y capacitor is effective at reducing EMI conducted emissions,
•
particularly in the upper bands of 10 MHz plus. Power levels of 10 W
and above may benefit from the use of a Y capacitor. However,
only low values should be used in wall adapters, about 470 pF
maximum, due to leakage currents presenting an unpleasant mild
shock risk. The Y capacitor should be connected from the +HT pin
of the transformer primary to the –V
shown in Figure 33. The Y capacitor provides a path for the high
frequency current through the transformer, capacitively coupled
from the primary to the secondary windings, to loop back to the
primary side, instead of flowing down the output lead in a loop via
the EMI test equipment (LISN) and back up in a loop via the AC
input leads to the primary side. Currents flowing through the
Y capacitor will not be measured by the LISN so the resulting
measurements will be lower.
Application Note
C R
SSNUB SSNUB
C R V
OUT
OUT OUT
+
+
D
OUT
C R
BS1 BS1
D
BIAS
R
FB1
R
FB2
PI-8164b-110116
C R
SSNUB SSNUB
V
C R
OUT OUT OUT
+
+
D
OUT
C R
BS1 BS1
D
BIAS
R
FB1
R
FB2
PI-8164c-110116
of the secondary output as
E
23
Rev. B 10/17