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**Dr. Franki Poon, Dr. Joe Liu**

Explanation of Electromagnetic Interference (EMI) in Switching Power Supply Speakers Dr. Franki Poon, Dr. Joe Liu Power eLab The PowerELab Limited © copyright 2006

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**EMI solutions: Passive filter**

Conventional EMI solutions depend on passive filter using inductors and capacitors: Inductors: CMC, DMC Capacitors: X-cap, Y-cap There are limitations when using passive filters: Inductors: Large size and high conduction loss Capacitors: leakage current specifications

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**Active EMI cancellation IC WT6001: An effective EMI solution – Y-cap booster**

A patented technology developed in PowerELab. An SO-8 IC WT6001 developed with W2. Equivalent to a Y-cap with very large value within the EMI concerned frequency range only. No boosting effect in the leakage current test concerned frequency range (50 – 800Hz). Greatly reduce the common mode inductor size and requirements. Reduce converter size and improve conversion efficiency. Provide effective and efficient EMI solution. Built-in electrical surge protection which can easily pass the EN and EN immunity standard.

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**Applications: Replace passive Y-cap**

Replaced by Y-cap booster LISN

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**Effect measurable by oscilloscope when using Y-cap booster**

Noise voltage across Y-cap in switching converter Noise voltage across Y-cap booster in switching converter More application examples can be found in the datasheet of WT6001

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**Practical application examples**

The original EMI filter design cannot pass the EN55022 class B limit. Filter component: 2 x 20mm high mu toroid for common mode filters 2 x 0.15uF X – cap 1 x 1n Y1-cap connected between primary and secondary

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**Practical application examples: Original filter schematic**

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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**Practical application examples: EMI measurement**

Y-cap: From 1n to 3n3 DM noise Improved but not enough CM noise Failure was identified to be caused by: Insufficient leakage inductance of the common mode choke for DM noise attenuation. The two common mode choke cannot effectively block out the common mode current. Further increase of Y-cap can reduce CM noise but fail to meet leakage current specifications.

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**Practical application examples: Solution using Y-cap booster**

Y-cap booster is used to replace the primary to secondary Y-cap After using the Y-cap booster, L2B is replaced by a small differential mode filter and L1B is reduced to a 9mm toroid with only a few turns to tackle the high frequency common node noise. The test results pass the required limit lines

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**Practical application examples: Filter comparison: Before and After...**

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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**Another practical application examples: Filter comparison: Before and After...**

Original EMI solution using passive filter in ATX converter New EMI solution using Y-cap booster in ATX converter

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Conclusion Y-cap booster breaks the relationship between the Y-cap values and leakage current requirement. Greatly reduce product design period and resources. It can be applied to any position with conventional Y-cap. Significantly reduce 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 low leakage current like medical equipment.

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**Active Diode – An easy to use and high efficiency rectifier suitable for all converters**

Stringent requirements of nowadays converters: Compact size Low heat generation and high conversion efficiency High output power and output current Low cost ………!!! Conventional technologies cannot meet the requirements!!

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**Synchronous rectifier**

Use MOSFETs to replace diode rectifiers. State of the Art 30V SCK Average 30V SO8 MOSFET 9 m 0.24 V A K A K 7.8 m

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**Synchronous rectifier**

Provides low conduction loss. Can operate at higher high current without heatsink. SCK 2.8W 10A MOSFET 0.7W 10A

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**Synchronous rectifier**

Usage not limited to converters with high profit margin. Price of nowadays low RDSon MOSFETs comparable to schottky diodes using the state of the art technology. Provide even lower converter cost because of reduced heatsink, more output power, higher conversion efficiency….. Emerge in low cost converter like adaptors, standard open frame converters, ATX …… .

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**Synchronous rectifier**

AD AD Problematic for some conventional topologies Special and sometimes complicated driving circuits SR are needed for different topologies Performance sensitive to transformer leakage inductance and operating conditions Converter cannot be paralleled – Reverse current Poor efficiency at low load Limited input voltage range Simple circuit Discontinuous mode is allowed Good low load efficiency Converter can be paralleled High conversion efficiency Works just like a diode

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**Active diode – Operating principle**

K A A1 N1 N2 N3 N4 N1 is the current sense winding N2 provides MOSFET driving signals A1 driver circuit (IC WT6002) N1 N3 & D1 form energy recovery circuit N4 & D2 form reset circuit D1 D2

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**Active diode – Operating principle**

Voltage across winding N2 or gate drive voltage Von of SR depends on ratio of N2 to N3 and voltage Vo Toff Ii A K N1 N2 N3 N4 D1 D2 A1 Von VN2 Voltage source Vo can be any voltage source in a converter, e.g. output voltage Vo VN3 Vo VN4

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**Application of Active Diode in different topologies**

+ + f + C o V + o Magnetic Freewheel Vin Reset SR C o V o - S Vin - - Flyback SR S Forward SR - Flyback Forward

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**Application of Active Diode in different topologies**

+ S + + 1 C SR1 S C 1 1 C V 1 SR1 C o o o Vin - Vin + SR 2 - S 2 C S C 2 SR 2 V - 2 2 L - o f2 Half Bridge centre tap Current Doubler

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**Application of Active Diode in different topologies**

SR1 + C V o o - I SIN and many others…. SR 2 Resonant converter

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**Successful application of Active Diode in converter products**

AD on 150W ACDC AD on 120W ACDC AD on 1.5 V 200 A ACDC AD on 50W ACDC AD on 300W ACDC AD on 60W ACDC AD on 100W DCDC

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**Conclusion A new “Active Diode” technology is presented.**

A kind of current driven synchronous rectifier technology that provides high conversion efficiency and eliminates many conventional synchronous rectifier application problems. Patented technologies. An Active Diode driver IC WT6002 for easy implementation of the technology. Well proven by many converter product design.

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References Liu, J.C.P.; Poon, F.N.K.; Xuefei Xie; Pong, M.H.; current driven synchronous rectifier with energy recovery sensor Power Electronics and Motion Control Conference, Proceedings. PIEMC The Third International , Volume: 1 , 2000, page(s): vol.1 Xuefei Xie; Liu, J.C.P.L.; Poon, F.N.K.; Man Hay Pong; Current-driven synchronous rectification technique for flyback topology, Power Electronics Specialists Conference, PESC IEEE 32nd Annual , Volume: 1 , 2001, Page(s): vol. 1 Xuefei Xie; Liu, J.C.P.; Poon, F.N.K.; Man Hay Pong; A novel high frequency current-driven synchronous rectifier for low voltage high current applications, Applied Power Electronics Conference and Exposition, APEC Sixteenth Annual IEEE , Volume: 1 , 2001, Page(s): vol.1 Liu, J.C.P.; Xuefei Xie; Poon, F.N.K.; Pong, B.M.H.; Practical solutions to the design of current-driven synchronous rectifier with energy recovery from current sensing, Applied Power Electronics Conference and Exposition, APEC Seventeenth Annual IEEE , Volume: 2 , 2002, Page(s): vol.2 Xuefei Xie; Joe Chui Pong Liu; Poon, F.N.K.; Man Hay Pong; A novel high frequency current-driven synchronous rectifier applicable to most switching topologies, Power Electronics, IEEE Transactions on , Volume: 16 Issue: 5 , Sep 2001, Page(s): Xie Xuefei; Liu, J.C.P.; Poon, F.N.K.; Pong, B.M.H.; Two methods to drive synchronous rectifiers during dead time in forward topologies, Applied Power Electronics Conference and Exposition, APEC Fifteenth Annual IEEE , Volume: 2 , 2000, Page(s): vol.2 US patent "Current driven synchronous rectifier with energy recovery" patent number 6,134,131 US patent “Self-driven synchronous rectifier by retention of gate charge” patent number 6,377,477 US patent “Current driven synchronous rectifier with energy recovery using hysterisis driver”, patent number 6,597,587

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