ON Semiconductor Fairchild FAN302HL, Руководство по дизайну

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AN-6094
© 2012 Fairchild Semiconductor Corporation www.fairchildsemi.com
Rev. 1.0.0 • 9/27/12 16
5. PCB Layout Guidelines
Printed Circuit Board (PCB) layout and design are very
important for switching power supplies where the voltage
and current change with high dv/dt and di/dt. Good PCB
layout minimizes excessive EMI and prevents the power
supply from being disrupted during surge / ESD tests. The
following guidelines are recommended for layout designs.

To improve EMI performance and reduce line
frequency ripple, the output of the bridge rectifier
should be connected to capacitors C
DL2
and C
DL1
first,
then to the transformer and MOSFET.
 The primary-side high-frequency current loop is
C
DL2
– Transformer – MOSFET – R
CS
– C
DL2
. The area
enclosed by this current loop should be as small as
possible. The trace for the control signal (FB, CS, and
GATE) should not go across this primary high-
frequency current loop to avoid interference.
 Place R
HV
for protection from the inrush spike on the
HV pin (100 k Ω is recommended).

R
CS
should be connected to the ground of C
DL2
directly. Keep the trace short and wide (Trace
4
→
1
)
and place it close to the CS pin to reduce switching
noise. High-voltage traces related to the drain of the
MOSFET and the RCD snubber should be away from
control circuits to prevent unnecessary interference. If
a heat sink is used for the MOSFET, connect this heat
sink to ground.
 As indicated by
2
, the area enclosed by the
transformer auxiliary winding, D
DD
and C
DD
, should
be small.
 Place
C
DD
,
C
S
,
R
S2
,
C
FB,
and
R
BF
close to the controller
for good decoupling and low switching noise.
 As indicated by
3
, the ground of the control circuits
should be connected at a single point first, then to
other circuitry.
 Connect ground in
3
→
2
→
4
→
1
sequence. This helps
avoid common impedance interference for the sense
signal.
 Regarding the ESD discharge path, use the shortcut
pad between the AC line and the DC output
(recommended). Another method is to discharge the
ESD energy to the AC line through the primary-side
main ground
1
. Because ESD energy is delivered from
the secondary side to the primary side through the
transformer stray capacitor or the Y capacitor, the
controller circuit should not be placed on the discharge
path.
5
shows where the point-discharge route can be
placed to effectively bypass the static electricity energy.
 For the surge path, select a fusible resistor of wire-
wound type to reduce inrush current and surge energy.
Use
π input filter (two bulk capacitor and one
inductance) to share the surge energy.
Figure 24. Recommended Layout