Introduction to an effective EMI solution (PowerPoint)

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

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PowereLab        EMI in switching power supplies

                   All products must comply with various EMI
                   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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PowereLab       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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PowereLab       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
  PowereLab      current in SMPS & the IC



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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
PowereLab           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
                      low line                                     high 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

          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            Passed design using Y-cap
          cost, loss and bigger size for the      booster with much smaller filter
          filter                                  size that saves cost, power and

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                    Filter comparison

                                                    Y-cap booster
                                                    demo board

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              Y-cap booster allow small Y-caps and meets leakage current
              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
                    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

              A very useful way to observe the effectiveness
               of cancellation is by probing the waveform
               across the primary & secondary


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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         EMI IC circuit across
                     primary & secondary         primary & secondary
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                    Waveform shows cancellation

                     Class II – no ground connection, Y-cap only

                      Waveform across             High noise level
                      primary & secondary

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                    Waveform shows cancellation

                     Class II – no ground connection, EMI IC applied

                      Upper Ch : Waveform across
                      primary & secondary                Low noise level
                      Lower Ch : IC output

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                    Waveform shows cancellation

                     Class I –ground connection, Y-cap only

                      Upper Ch : Waveform across
                      primary & secondary                High noise level

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                    Waveform shows cancellation
PowereLab           effect

                     Class I – ground connection, EMI IC applied

                      Upper Ch : Waveform across
                      primary & secondary                Low noise level
                      Lower Ch : IC output

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                Procedures to implement the IC to a
                switching power supply

           1.            Make sure that there is enough differential mode
           2.            Differentiate between Class I & Class II products to
                         see whether the IC can apply
                    a.     Class I product – Put a big capacitor between the primary
                           & secondary
                    b.     Class II product – Short circuit the primary & secondary
           3.            If there is significant reduction in common mode
                         noise on the spectrum, apply the Class I or Class II
                         circuit accordingly.
           4.            Put in small common mode choke for high
                         frequency noise suppression. The IC works up to 7

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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
              The power supply should be properly
               laid out for best effect

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                    Question & Answers

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