Transcript Introduction to an effective EMI solution

PowereLab
An Active EMI reduction IC
WT6001
POWERELAB LIMITED
A Power Converter Technology Provider
PowerELab Limited
www.powerelab.com
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EMI in switching power supplies
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All products must comply with various EMI
regulations
EMI reduction in switching power supply is
mainly carried out by passive filters
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filter chokes are big and dissipative
Y capacitors introduce leakage current
It is desired to cut down filter requirement
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An active EMI IC is introduced
EMI reduction by active method
It cuts down common mode current in SMPS
A small SO8 IC that
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cuts down common mode choke size
reduces Y capacitor requirement
eliminates thermal problem in common mode chokes
increase SMPS overall efficiency
reduces product cost
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Common mode current in SMPS
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Common mode current is a major
contributor of conducted EMI
It is difficult to trace as its path is not
readily seen on a circuit diagram
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Generation of common mode
current in SMPS & the IC
Load
LISN
spectrum
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Safety requirement and EMI limits
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The added Y-cap can effectively reduce the
common mode current due to the parasitic
capacitance of the load to the earth
The maximum allowed value is limited by the
leakage current requirement
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The IC provides an effective
Y-cap booster
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Equivalent to a Y-cap with very large value within the EMI
concerned frequency range only
Capacitance remains low in the leakage current test
frequency range (50 – 800Hz)
Provide effective EMI solution
Greatly reduce the common mode filter requirement
Reduce converter size and improve conversion efficiency
Built-in electrical voltage surge protection which can easily
pass EN61000-4-4 and EN61000-4-5 immunity standard
Independent of converter power level, it can handle high
power converters
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An application example – 120W
power adaptor
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The original EMI filter design cannot pass the
EN55022 class B limit
Filter component:
2 x 20mm high mu toroid
2 x 0.15uF X – cap
1 x 1n Y1-cap
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Original filter circuit
P1
TH1
L1B
F1
L2B
4
3
4
3
1
2
C2
1
2
1
C1
P2
1
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It is a commonly used filter configuration
L2B is wound with many turns which intends to suppress the low to
mid-frequency common mode noise. Its leakage inductance together
with C1 also provides differential mode noise filtering
L1B is a single layer, bi-filer wound common mode choke for high
frequency common mode noise filtering
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EMI measured results –
Original design
high line
low line
dBuV
dBuV
80
80
70
70
230V pri-sec Y1n U shielded original
60
60
EN 55022; Class B Conducted, Quasi-Peak
EN 55022; Class B Conducted, Quasi-Peak
100V pri-sec Y1n U shielded original
50
50
EN 55022; Class B Conducted, Average
40
40
30
30
20
20
10
10
0
0
1
10/3/200511:00:49
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EN 55022; Class B Conducted, Average
10
(Start = 0.15, Stop= 30.00) MHz
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10/3/200510:55:40
10
(Start = 0.15, Stop= 30.00) MHz
At 230Vac input, detailed measurement showed that the circled regions
can only marginally pass or fail to meet the average limit line.
The lower frequency region seems to be caused by differential mode
noise
The high frequency region is caused by common mode noise
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EMI solution – A better one
using Y-cap booster
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dBuV
dBuV
80
80
70
70
100V CB U shield SDMC SCMC .33uF
60
60
EN 55022; Class B Conducted, Quasi-Peak
50
50
EN 55022; Class B Conducted, Average
40
40
30
30
20
20
10
10
0
10/3/200516:31:32
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230V CB U shield SDMC SCMC .33uF
EN 55022; Class B Conducted, Average
0
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EN 55022; Class B Conducted, Quasi-Peak
10
(Start = 0.15, Stop= 30.00) MHz
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10/3/200516:39:55
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(Start = 0.15, Stop= 30.00) MHz
Y-cap booster is used to replace the 1n Y-cap
In addition to the removal of L2B, L1B can be further reduced to
a 9mm toroid with only a few turns
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Filter size reduction
Failed design even with more
cost, loss and bigger size for the
filter
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Passed design using Y-cap
booster with much smaller filter
size that saves cost, power and
space
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Filter comparison
Y-cap booster
demo board
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Summary
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Y-cap booster allow small Y-caps and meets leakage current
requirement
Greatly reduce product design period and resources
It can be applied to any position with conventional Y-cap
Significantly reduces the size and loss of common mode choke
implies higher power density and efficiency
EMI less sensitive to transformer winding capacitance implies
more rooms for improving transformer coupling
Very suitable for equipment required lower leakage current
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The IC WT6001
• 8 pin SO IC
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PIN
Function
1
NC
2
GND, ref to Vcc
3
NC
4
NC
5
Out, to Y cap
6
Vcc, max 15V
7
Ref, set bias point
8
In, feedback pin
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Observation of cancellation by
an oscilloscope
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 A very useful way to observe the effectiveness
of cancellation is by probing the waveform
across the primary & secondary
Floating
oscilloscope
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Observation of cancellation by
an oscilloscope
 Waveforms observed before & after
application of the EMI IC
A 1 nF Y-cap across
primary & secondary
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EMI IC circuit across
primary & secondary
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Waveform shows cancellation
effect
 Class II – no ground connection, Y-cap only
dBuV
90
80
70
115Vac final Ycap
60
EN 55022; Class B Conducted, Quasi-Peak
50
EN 55022; Class B Conducted, Average
40
30
20
10
1
10
4/19/20064:30:16PM
Waveform across
primary & secondary
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(Start = 0.15, Stop= 30.00) MHz
High noise level
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Waveform shows cancellation
effect
 Class II – no ground connection, EMI IC applied
dBuV
90
80
70
60
EN 55022; Class B Conducted, Quasi-Peak
50
EN 55022; Class B Conducted, Average
40
30
115Vac Final Test
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10
1
4/21/200612:16:12PM
Upper Ch : Waveform across
primary & secondary
Lower Ch : IC output
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10
(Start = 0.15, Stop= 30.00) MHz
Low noise level
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Waveform shows cancellation
effect
 Class I –ground connection, Y-cap only
dBuV
90
80
70
60
115Vac L IC Disabled
EN 55022; Class B Conducted, Quasi-Peak
EN 55022; Class B Conducted, Average
50
40
30
20
10
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7/24/20064:12:12PM
Upper Ch : Waveform across
primary & secondary
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10
(Start = 0.15, Stop= 30.00) MHz
High noise level
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Waveform shows cancellation
effect
 Class I – ground connection, EMI IC applied
dBuV
90
80
70
60
EN 55022; Class B Conducted, Quasi-Peak
50
EN 55022; Class B Conducted, Average
40
30
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115Vac L int +BulkTx FluxBand BdFB10R
10
1
7/21/200611:57:28AM
Upper Ch : Waveform across
primary & secondary
Lower Ch : IC output
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10
(Start = 0.15, Stop= 30.00) MHz
Low noise level
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Procedures to implement the IC to a
switching power supply
Make sure that there is enough differential mode
choke.
Differentiate between Class I & Class II products to
see whether the IC can apply
1.
2.
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b.
3.
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Class I product – Put a big capacitor between the primary
& secondary
Class II product – Short circuit the primary & secondary
If there is significant reduction in common mode
noise on the spectrum, apply the Class I or Class II
circuit accordingly.
Put in small common mode choke for high
frequency noise suppression. The IC works up to 7
MHz.
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Trimming process
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 To cater for different power supply
environment, the following components
can be trimmed
 Y-cap connected to the IC output (pin 5)
 Feedforward resistors in the circuit for
class I products
 Refer to datasheet for details
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Points to note
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 The IC works best when Vcc is close to
15V
 The power supply should be properly
laid out for best effect
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Question & Answers
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