EMI and layout fundamentals for switched mode circuits
.pdf
This phenomenon can sometimes be a problem when ground loops are present. Circulating ground currents are then induced, which lead to variations in the ground reference potential
|
|
|
|
dc power |
|
|
converter |
|
|
|
|
|
|
|
|
|
|
||
|
|
|
|
supply |
|
|
under test |
|
|
|
|
|
|
|
|
|
|
||
|
|
|
|
|
|
|
|
||
|
|
|
|
|
|
|
|
||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
reference input
network analyzer
Measurement of audiosusceptibility: observed unusual and unexpected results
Fixed by breaking ground loops
Audiosusceptibility then was as expected
ECEN 5797 Power Electronics 1 |
20 |
Department of Electrical and Computer Engineering |
|
University of Colorado at Boulder |
|||
|
|
||
|
|
|
Stray capacitances
Most significant at high voltage points in circuit
Two major sources of EMI:
•Transformer interwinding capacitance
•MOSFET drain-to-heatsink capacitance
Drain-to-heatsink capacitance
v(t) |
|
i(t) |
= C |
dv (t) |
|
||||||||
|
|||||||||||||
|
dt |
||||||||||||
|
|
|
|
|
|||||||||
|
|
|
|
|
|
|
|
|
|
|
Drain-to-heatsink |
||
|
|
|
|
|
|
|
|
|
|
|
capacitance |
||
|
|
|
|
|
|
|
|
|
|
|
|||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
power |
|
|
|
||||||||||
MOSFET |
|
|
|
||||||||||
ECEN 5797 Power Electronics 1
When the switched drain voltage is applied to this capacitance, current spikes must flow.
The currents must flow in a closed path (a loop). What is the loop in your circuit?
To control the effects of these currents,
•provide a short path for them to return to their origin
•add common-mode filters
•slow down switching times
21 |
Department of Electrical and Computer Engineering |
|
University of Colorado at Boulder |
||
|
||
|
|
Common mode noise generation |
|||
by drain-to-heatsink capacitance |
|||
|
|
full bridge converter |
|
B |
|
|
|
W |
|
|
|
G |
|
drain-to- |
|
|
|
heatsink |
|
|
|
capacitance |
|
common mode |
Heatsink/chassis |
||
filter |
|
|
|
ECEN 5797 Power Electronics 1 |
22 |
Department of Electrical and Computer Engineering |
|
University of Colorado at Boulder |
|||
|
|
||
Common mode noise generation
by transformer interwinding capacitance
Flyback converter example
|
|
n:1 D1 |
|
|
+ |
|
|
V |
Vg |
+ |
– |
|
||
|
– |
+ |
|
|
|
|
|
vsg (t) |
|
|
Q1 |
|
|
– |
Transformer interwinding capacitance causes currents to flow between the isolated (primary and secondary) sides of the transformer, and can cause the secondary-side ground voltage to switch at high frequency: vsg(t) contains a high-frequency component.
ECEN 5797 Power Electronics 1 |
23 |
Department of Electrical and Computer Engineering |
|
University of Colorado at Boulder |
|||
|
|
||
|
|
|
Modeling transformer interwinding capacitance
Suppose the transformer is wound as follows:
EE core
insulation 
secondary primary return 
primary
secondary return 
A simple lumped element model, including interwinding capacitance:
n : 1
Cw
|
Cw |
|
|
|
|
|
|
ECEN 5797 Power Electronics 1 |
24 |
Department of Electrical and Computer Engineering |
|
University of Colorado at Boulder |
|||
|
|
||
|
|
|
Flyback converter ground potentials
|
|
Cw |
|
|
|
|
– |
|
|
|
V |
Vg |
+ |
|
+ |
|
|
||
|
– |
|
+ |
|
|
Cw |
D1 |
|
|
vsg (t) |
|
|
|
Q1 |
|
|
|
– |
|
|
|
|
Flyback converter circuit, with interwinding capacitance modeled
One can solve the circuit to find the high-frequency ac component of vsg(t). The result is
vsg (t) 
DTs |
Ts |
|
Vpp |
|
t |
V |
= |
n - 1 |
|
Vg |
+ V |
|
|
||||
pp |
|
n |
|
||
|
2 |
|
|
||
The secondary ground potential switches at high frequency with respect to the primary ground.
The peak-peak voltage Vpp is typically approximately equal to Vg. vsg(t) can also have a dc component, not predicted by the circuit model.
ECEN 5797 Power Electronics 1 |
25 |
Department of Electrical and Computer Engineering |
|
University of Colorado at Boulder |
|||
|
|
||
|
|
|
Secondary-side stray capacitances now lead to common-mode currents
Example: diode case-to-heatsink capacitance
|
iCM |
|
vsg (t) |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
– |
|
DTs |
|
Ts |
|
|
|
|
|
|
|
Vpp |
|
|
D1 |
V |
|
|
|
|
t |
|
|
|
|
|
|
||
Vg |
+ |
|
|
|
|
|
|
+ |
|
|
|
|
|
|
|
|
– |
+ |
|
|
|
|
|
|
iCM |
|
|
|
|
V |
|
|
i |
V |
|
= n - 1 |
|||
|
|
CM vsg (t) |
|
g + V |
|||
|
Q1 |
|
pp |
|
2 |
n |
|
|
– |
iCM(t) |
|
|
|||
|
|
|
|
|
|
||
|
iCM |
|
|
|
|
|
|
|
|
|
|
|
|
|
t |
These currents usually corrupt the ground reference voltage
ECEN 5797 Power Electronics 1 |
26 |
Department of Electrical and Computer Engineering |
|
University of Colorado at Boulder |
|||
|
|
||
|
|
|
Discussion
•Transformers can successfully provide dc and low-frequency ac isolation
•Transformer interwinding capacitances couple the primary and secondary voltages, greatly reducing the high-frequency ac isolation and leading to common-mode currents and conducted EMI
Some possible solutions:
•Redesign the transformer to reduce the interwinding capacitance. This usually leads to increased leakage inductance
•Add common-mode filters:
Capacitors which connect the primaryand secondary-side grounds
Common-mode filter inductors
This greatly reduces conducted EMI, and can also reduce radiated EMI. But the capacitors do not allow the secondary ground potential to switch at high frequency.
ECEN 5797 Power Electronics 1 |
27 |
Department of Electrical and Computer Engineering |
|
University of Colorado at Boulder |
|||
|
|
||
|
|
|
Addition of capacitance between primary and secondary grounds
|
iCM |
|
|
|
|
– |
|
|
D1 |
V |
|
Vg |
+ |
+ |
|
+ |
|
||
|
– |
Csg |
vsg (t) = 0 |
|
iCM |
||
|
0 |
– |
|
|
Q1 |
|
|
|
|
iCM |
|
Capacitor Csg is much larger than the stray capacitances, and so nearly all of the common-mode current flows through Csg. If Csg is sufficiently large, then it will have negligible voltage ripple, and vsg(t) will no longer contain a high-frequency component.
ECEN 5797 Power Electronics 1 |
28 |
Department of Electrical and Computer Engineering |
|
University of Colorado at Boulder |
|||
|
|
||
|
|
|
Measurement of common mode current
current |
iCM |
|
|
interwinding |
|
probe |
|
|
capacitance |
||
|
|
|
|
||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
iDM
iCM
to oscilloscope
The common mode current due to transformer interwinding capacitance can be easily measured using a current probe
The differential-mode current iDM(t) cancels out, and the oscilloscope will display 2iCM(t).
ECEN 5797 Power Electronics 1 |
29 |
Department of Electrical and Computer Engineering |
|
University of Colorado at Boulder |
|||
|
|
||
|
|
|