Power e Lab PowerELab Limitedwww.powerelab.com 1 An Active EMI reduction IC WT6001 POWERELAB LIMITED A Power Converter Technology Provider

Power e Lab PowerELab Limitedwww.powerelab.com 2 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

Power e Lab PowerELab Limitedwww.powerelab.com 3 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

Power e Lab PowerELab Limitedwww.powerelab.com 4 Power e Lab 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 e Lab PowerELab Limitedwww.powerelab.com 5 Generation of common mode current in SMPS & the IC LISN Load spectrum

Power e Lab PowerELab Limitedwww.powerelab.com 6 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

Power e Lab PowerELab Limitedwww.powerelab.com 7 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 EN and EN immunity standard Independent of converter power level, it can handle high power converters

Power e Lab PowerELab Limitedwww.powerelab.com 8 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

Power e Lab PowerELab Limitedwww.powerelab.com 9 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

Power e Lab PowerELab Limitedwww.powerelab.com 10 EMI measured results – Original design 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 low line high line

Power e Lab PowerELab Limitedwww.powerelab.com 11 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

Power e Lab PowerELab Limitedwww.powerelab.com 12 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

Power e Lab PowerELab Limitedwww.powerelab.com 13 Filter comparison Y-cap booster demo board

Power e Lab PowerELab Limitedwww.powerelab.com 14 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

Power e Lab PowerELab Limitedwww.powerelab.com 15 The IC WT pin SO IC PINFunction 1NC 2GND, ref to Vcc 3NC 4 5Out, to Y cap 6Vcc, max 15V 7Ref, set bias point 8In, feedback pin

Power e Lab PowerELab Limitedwww.powerelab.com 16 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

Power e Lab PowerELab Limitedwww.powerelab.com 17 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

Power e Lab PowerELab Limitedwww.powerelab.com 18 Waveform shows cancellation effect  Class II – no ground connection, Y-cap only Waveform across primary & secondary High noise level

Power e Lab PowerELab Limitedwww.powerelab.com 19 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

Power e Lab PowerELab Limitedwww.powerelab.com 20 Waveform shows cancellation effect  Class I – ground connection, Y-cap only Upper Ch : Waveform across primary & secondary High noise level

Power e Lab PowerELab Limitedwww.powerelab.com 21 Waveform shows cancellation effect  Class I – ground connection, EMI IC applied Upper Ch : Waveform across primary & secondary Lower Ch : IC output Low noise level

Power e Lab PowerELab Limitedwww.powerelab.com 22 Procedures to implement the IC to a switching power supply 1. Make sure that there is enough differential mode choke. 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 MHz.

Power e Lab PowerELab Limitedwww.powerelab.com 23 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

Power e Lab PowerELab Limitedwww.powerelab.com 24 Points to note  The IC works best when Vcc is close to 15V  The power supply should be properly laid out for best effect

Power e Lab PowerELab Limitedwww.powerelab.com 25 Question & Answers