emi ic application aug 2

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An Active EMI reduction ICWT6001

POWERELAB LIMITEDA 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

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Generation of common mode current in SMPS & the IC

LISN Load

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 – cap1 x 1n Y1-cap

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Original filter circuit

C2

L2B

13

24

C1

L1B

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24

P1

1

P2

1

TH1 F1

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

100V pri-sec Y1n U shielded originalEN 55022; Class B Conducted, Quasi-Peak

EN 55022; Class B Conducted, Average

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10/3/2005 11:00:49 (Start = 0.15, Stop = 30.00) MHz

230V pri-sec Y1n U shielded original


EN 55022; Class B Conducted, Quasi-Peak

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10/3/2005 10:55:40 (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

low line high line

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230V CB U shield SDMC SCMC .33uFEN 55022; Class B Conducted, Average


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10/3/2005 16:39:55 (Start = 0.15, Stop = 30.00) MHz

EMI solution – A better one using Y-cap booster

100V CB U shield SDMC SCMC .33uF



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10/3/2005 16:31:32 (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 morecost, loss and bigger size for thefilter

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 boosterdemo 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 ICPIN 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

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 acrossprimary & secondary EMI IC circuit across

primary & secondary

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Waveform shows cancellation effect

Class II – no ground connection, Y-cap only

Waveform acrossprimary & secondary High noise level

115Vac final Ycap



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4/19/2006 4:30:16 PM (Start = 0.15, Stop = 30.00) MHz

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Class II – no ground connection, EMI IC applied

Upper Ch : Waveform acrossprimary & secondaryLower Ch : IC output

Low noise level

115Vac Final Test



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4/21/2006 12:16:12 PM (Start = 0.15, Stop = 30.00) MHz

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Class I –ground connection, Y-cap only

Upper Ch : Waveform acrossprimary & secondary High noise level

115Vac L IC Disabled EN 55022; Class B Conducted, Quasi-Peak


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7/24/2006 4:12:12 PM (Start = 0.15, Stop = 30.00) MHz

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Class I – ground connection, EMI IC applied

Upper Ch : Waveform acrossprimary & secondaryLower Ch : IC output

Low noise level

115Vac L int +BulkTx FluxBand BdFB10R



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7/21/2006 11:57:28 AM (Start = 0.15, Stop = 30.00) MHz

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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.

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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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Question & Answers

emi ic application aug 2

Documents

shield sdmc scmc

common mode current

high noise levelwww

low noise levelwww

ic outputpowerelab limited

secondary lower ch

ic works

upper ch