开关电源布线要素

Transformer noise Use a Y-cap or increase Y-cap size Use transformer E-shield or copper shield
Usually Caused by Output Diode Ringing. Add snubber to output diode Add bead to output diode (power dissipative) Add drain snubber
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EMI and PCB Layout Tips (Common-Mode Noise Reduction)
Shield the drain with copper planes and connect to to source to reduce the drain coupling to GND
EMI and PCB Layout Tips (Overview)
EMI and PCB Layout Tips
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EMI and PCB Layout Tips (Flyback Power Supply Topology)
Common-Mode Choke
Common-Mode Choke
GND
Inductor in GND path reduces common-mode noise flow Needs to pass UL safety requirements
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EMI and PCB Layout Tips (Flyback Critical Loops)
Primary Loop
T1
Snubber Loop
Secondary Loop
1
8
4
5
R
C
1
T1
8
1
T1
8
D
+
+
D C S
L
CONTROL
C
U
R D
4
5
4
5
C
X
F
TOPSwitch
Add by-pass cap if necessary to further shrink loop area
Keep Parts Close No Parts Inside Loops Keep Parts Close No Parts Inside Loops
● Propagation of Noise
– Conduction thru mains lines – Electromagnetic Radiation – Inductive Coupling – Capacitive Coupling
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EMI and PCB Layout Tips (Flyback Power Supply Topology)
● Key EMI Sources
– High di/dt current waveforms in the primary and secondary loops – High dv/dt voltage waveforms from switching transistor – Quick charging and discharging of parasitic capacitances in the transformer windings – Coupling between transistor and heatsink – Noise generated from diodes
Heatsink
N
Heatsink
Common-Mode Noise
G百度文库D
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EMI and PCB Layout Tips (Flyback Power Supply Topology)
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EMI and PCB Layout Tips (Common-Mode Noise Reduction)
Primary shield will reduce coupling between primary and secondary windings Flux band reduces coupling between core and GND
23MHz ringing
Output rectifier voltage
Drain voltage
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EMI and PCB Layout Tips (Output Diode Snubber)
No ringing
Output rectifier voltage
Drain voltage
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EMI and PCB Layout Tips (Output Diode Snubber)
250 Vac 470 pF
250 Vac 470 pF
EMI and PCB Layout Tips (Y-Cap Connection)
Y-cap should have a dedicated trace to transformer pin
`
Incorrect
Correct
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EMI and PCB Layout Tips (Secondary Side Loops)
Keep Output diode loop very small Keep snubber outside to make loop small Use adjacent pins on transformer
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EMI and PCB Layout Tips (Capacitor Traces)
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EMI and PCB Layout Tips (Capacitor Traces)
EMI and PCB Layout Tips (Common-Mode Noise Reduction)
Reduce loop size Reduce current amplitude Reduce di/dt speed Add snubbers to reduce dv/dt speed
Add electrostatic screen to heatsink assembly Or, connect heatisnk to source PI parts do not require this measure because the tab is connected to the source
Noise Current will bypass the filter components
`
`
Incorrect
Correct
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EMI and PCB Layout Tips (Output Diode Ringing)
Noise Current will bypass the filter components
`
`
Incorrect
Correct
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EMI and PCB Layout Tips (Capacitor Traces)
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EMI and PCB Layout Tips (What are “Noisy” and “Quiet” Nodes?)
● Noisy nodes are those nodes which see high voltage switching signals. Potential on these nodes is rapidly changing over a switching period
Quiet Noisy Quiet Noisy
Quiet
• Quiet nodes are low impedance nodes which generally see little switching activity. Potential on these nodes is relatively constant over the switching period
EMI and PCB Layout Tips (Y-Cap Connection and Bypass Capacitor)
Add 10 nF cap to reduce loop further
Before
After
Connect Y cap directly to transformer pin
C O R E
Y-Cap Stirs current into Source to avoid leakage into GND
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EMI and PCB Layout Tips (Common-Mode Noise Reduction)
C2
1 2 3 4 5 6 12 11 10 9 8 7 D4 MBR1645
C2
1 2 3 4 12 11 10 9 8 7 47 680 pF 100 V D4 MBR1645
C9
2200 uF 10 V
5 6
C9
2200 uF 10 V
R5
C8
T1
EER28L
T1
EER28L
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EMI and PCB Layout Tips (Overview)
Differential
Common-Mode Primary-Side Generated Common-Mode Secondary-Side Generated
Differential Filtering Increase CMC choke size Increase X-cap size
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EMI and PCB Layout Tips (Primary and Snubber Loops)
Large drain copper area
`
Incorrect
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The longer the input cable, the greater the coupling The longer the input cable, the greater the coupling
L
C O R E
di/dt H-Field Radiation
Capacitive Coupling of dv/dt E-Field Radiation
Correct
EMI and PCB Layout Tips (Y-Cap Connection)
Y-cap should have a dedicated trace to transformer pin
`
Incorrect
Correct
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