An Active EMI reduction IC WT6001

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An Active EMI reduction ICWT6001
POWERELAB LIMITED
A Power Converter Technology Provider





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EMI in switching power supplies

• All products must comply with various EMI
  regulations
• EMI reduction in switching power supply is mainly
  carried out by passive filters
• 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
• 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

• 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

Power eLab
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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

• 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

• 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

• 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

• 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

• 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

• 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
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

• 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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Observation of cancellation by an oscilloscope

• 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
EMI IC circuit across primary secondary
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Waveform shows cancellation effect

• Class II no ground connection, Y-cap only

Waveform across primary secondary
High noise level
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Waveform shows cancellation effect

• Class II no ground connection, EMI IC applied

Upper Ch Waveform across primary
secondary Lower Ch IC output
Low noise level
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Waveform shows cancellation effect

• Class I ground connection, Y-cap only

Upper Ch Waveform across primary secondary
High noise level
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Waveform shows cancellation effect

• Class I ground connection, EMI IC applied

Upper Ch Waveform across primary
secondary Lower Ch IC output
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
• 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

• 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

• 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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