EMI and layout fundamentals for switched mode circuits
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Parasitic inductances of input loop explicitly shown:
Q1
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i1(t) |
D1 |
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Addition of bypass capacitor confines the pulsating current to a smaller loop:
Q1
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ig(t) |
i1(t) |
D1 |
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high frequency currents are shunted through capacitor instead of input source
ECEN 5797 Power Electronics 1 |
10 |
Department of Electrical and Computer Engineering |
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University of Colorado at Boulder |
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Even better: minimize area of the high frequency loop, thereby minimizing its inductance
Q1
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B fields nearly cancel |
i1 |
loop area A c |
i1
ECEN 5797 Power Electronics 1 |
11 |
Department of Electrical and Computer Engineering |
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University of Colorado at Boulder |
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Forward converter |
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Two critical loops: |
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i |
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i2(t) |
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1 |
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Solution: |
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ECEN 5797 Power Electronics 1 |
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12 |
Department of Electrical and Computer Engineering |
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University of Colorado at Boulder |
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Unwanted coupling of signals
via impedance of ground connections
++
––
Power supplies
+48 volts
+15 volts
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Stage 1 |
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Stage 2 |
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Stage 3 |
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input |
output |
input |
output |
input |
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•All currents must flow in closed paths: determine the entire loop in which large currents flow, including the return connections
•Ground (zero potential) references may not be the same for every portion of the system
ECEN 5797 Power Electronics 1 |
13 |
Department of Electrical and Computer Engineering |
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University of Colorado at Boulder |
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Example: suppose the ground connections are
+48 volts |
i3 |
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+15 volts |
i2 |
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i1(t) |
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+ |
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Stage 1 |
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– |
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v |
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– v |
out |
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2 |
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in |
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Zg |
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i2 + i 3
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Stage 2 |
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Stage 3 |
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–
vin2 = vout1 – Zg (i2 + i 3)
“Noise” from stages 2 and 3 couples into the input to stage 2
This represents conducted EMI, or specifically corruption of the ground reference by system currents
ECEN 5797 Power Electronics 1 |
14 |
Department of Electrical and Computer Engineering |
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University of Colorado at Boulder |
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Example: gate driver |
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line input |
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+15 volt supply |
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converter |
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power |
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stage |
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+ |
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ig(t) |
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– |
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analog |
PWM |
gate |
power |
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control |
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control |
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chip |
driver |
MOSFET |
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chip |
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ig(t) |
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ECEN 5797 Power Electronics 1 |
15 |
Department of Electrical and Computer Engineering |
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University of Colorado at Boulder |
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Solution: bypass capacitor and close coupling of gate and return leads
line input
converter +15 volt supply power
stage
+ |
+ |
– |
– |
analog |
PWM |
gate |
power |
control |
control |
driver |
MOSFET |
chip |
chip |
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High frequency components of gate drive current are confined to a small loop
A dc component of current is still drawn output of 15V supply, and flows past the control chips. Hence, return conductor size must be sufficiently large
ECEN 5797 Power Electronics 1 |
16 |
Department of Electrical and Computer Engineering |
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University of Colorado at Boulder |
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About ground planes
Current i(t) flowing in wire
return current i(t) flows in ground plane directly under wire
wire
ground plane
Inductance of return connections is minimized
Hence ground planes tend to exhibit lower impedance ground connections, and more nearly equipotential ground references
Ground planes are especially effective in the analog control portions of switching regulator circuits
But it is still possible to observe significant coupling of noise in ground, by
•poor layout of ground plane, or
•high resistance of ground plane
ECEN 5797 Power Electronics 1 |
17 |
Department of Electrical and Computer Engineering |
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University of Colorado at Boulder |
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A poor ground plane layout
power supply
power supply return
power supply
power supply return
ground plane
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Return current of noisy |
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circuit runs |
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underneath sensitive |
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circuits, and can still |
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sensitive |
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sensitive |
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Noisy |
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corrupt their ground |
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circuit |
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circuit |
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circuit |
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references |
ground plane
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sensitive |
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sensitive |
v |
Noisy |
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circuit |
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circuit |
circuit |
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A solution is to remove the noisy circuit from the ground plane. One could then run a separate ground wire for the noisy circuit. The only drawback is that noise can be coupled into the input signal v.
ECEN 5797 Power Electronics 1 |
18 |
Department of Electrical and Computer Engineering |
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University of Colorado at Boulder |
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Coupling of signals via magnetic fields |
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i(t) |
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Loop containing ac |
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current i(t) |
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generates B field |
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mutual flux |
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which links another |
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+ |
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di (t) |
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conductor, |
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v ( t ) = L M |
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inducing an |
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dt |
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unwanted voltage |
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— |
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v(t) |
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ECEN 5797 Power Electronics 1 |
19 |
Department of Electrical and Computer Engineering |
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University of Colorado at Boulder |
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