(12) United States Patent
`Hui et al.
`
`USOO65O1364B1
`(10) Patent No.:
`US 6,501,364 B1
`(45) Date of Patent:
`Dec. 31, 2002
`
`(54) PLANAR PRINTED-CIRCUIT-BOARD
`TRANSFORMERS WITH EFFECTIVE
`ELECTROMAGNETIC INTERFERENCE
`(EMI) SHIELDING
`Inventors: Ron Shu Yuen Hui, Shatin (HK); Sai
`Chun Tang, Yuen Long (HK)
`Assignee: City University of Hong Kong,
`Kowloon (HK)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(75)
`
`(73)
`
`Notice:
`
`(21)
`(22)
`(51)
`(52)
`(58)
`
`(56)
`
`Appl. No.: 09/883,145
`Filed:
`Jun. 15, 2001
`Int. Cl................................................... H01F 5/00
`U.S. Cl. ........................ 336/200; 336/232; 336/223
`Field of Search ................................. 336/200, 232,
`336/223; 29/606, 602.1
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`3.866,086 A 2/1975 Miyoshi et al.
`4,494,100 A 1/1985 Stengel et al.
`4,510,915 A 4/1985 Ishikawa et al.
`4,613.843 A 9/1986 Esper et al.
`4,748,532 A 5/1988 Commander et al.
`4,890.083 A * 12/1989 Trenkler et al. ............ 335/301
`5,039,964 A 8/1991 Ikeda
`5.431,987 A 7/1995 Ikeda
`5,502.430 A * 3/1996 Takahashi et al. .......... 336/232
`5,579.202 A 11/1996 Tolfsen et al.
`5,592,089 A * 1/1997 Danby et al. ............... 324/318
`5,844,451 A 12/1998 Murphy
`6,023,161. A
`2/2000 Dantsker et al. ............ 324/248
`FOREIGN PATENT DOCUMENTS
`
`EP
`JP
`JP
`JP
`JP
`
`7/1985
`O 147 499
`8/1979
`54-110424
`1/1992
`4-1068O
`601.3247 A * 1/1994
`200111651 A * 4/2001
`
`OTHER PUBLICATIONS
`Tang et al., “Coreless planar printed-circuit-board (PCB)
`transformerS-A fundamental concept for Signal and energy
`transfer, IEEE Transactions On Power Electronics, vol. 15,
`No. 5, pp. 93 1941 (Sep. 2000).
`Hui et al., “Coreless printed-circuit board transformers for
`signal and energy transfer.” Electronics Letters, vol. 34, No.
`11, pp. 1052–1054 (May 1998).
`Hui et al., “Some electromagnetic aspects of coreless PCB
`transformers.” IEEE Transactions On Power Electronics,
`vol. 15, No. 4, pp. 805–810 (Jul. 2000).
`Onda et al., “Thin type DC/DC converter using a coreless
`wire transformer," IEEE Power Electronics Specialists Con
`ference, pp. 1330–1334 (Jun. 1994).
`Coombs, C.F., “Printed Circuits Handbook,” 3rd Ed.
`McGraw-Hill, p. 6.32 (1998). No month.
`Tang et al., “Characterization of coreleSS printed circuit
`board (PCB) transformers.” IEEE Transactions on Power
`Electronics, vol. 15, No. 6, pp. 1275–1282 (Nov. 2000).
`Paul, C.R., Introduction to Electromagnetic Compatibility,
`Chapter 11-Shielding, pp. 632-637 (1992), no month.
`Tang et al., “A low-profile power converter using printed
`circuit board (PCB) power transformer with ferrite polymer
`composite," IEEE Transactions on Power Electronics, vol.
`16, No. 4, pp. 493-498 (Jul. 2001).
`Hui et al., “Coreless PCB based transformers for power
`MOSET/IGBT gate drive circuits," IEEE Power Electronics
`Specialists Conference, vol. 2, pp. 1171–1176 (1997). No
`month.
`Bourgeois, J.M., “PCB Based Transformer for Power MOS
`FET Drive," IEEE, pp. 238–244 (1994). No month.
`Goyal, R., "High-Frequency Alalog Integrated Circuit
`Design,” pp. 107-126 (1995). No month.
`* cited by examiner
`Primary Examiner Anh Mai
`(74) Attorney, Agent, or Firm Merchant & Gould P.C.
`(57)
`ABSTRACT
`Novel designs for printed circuit board transformers, and in
`particular for coreleSS printed circuit board transformers
`designed for operation in power transfer applications, are
`disclosed in which shielding is provided by a combination of
`ferrite plates and thin copper sheets.
`5 Claims, 13 Drawing Sheets
`
`
`
`
`
`- Copper Sheet
`
`
`
`-
`
`Primary
`to this to side of
`ie is:
`
`
`
`cyfare-gaec:
`series
`
`Cogger Seei
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 001
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 1 of 13
`
`US 6,501,364 B1
`
`C.
`(Prior Art)
`
`
`
`
`
`
`
`Ferrite pate
`
`8:3:y Ce:dictive
`- is raig aye!
`
`primary wiring
`the tag site of
`- the pcs.
`
`ba
`
`Faiy: 8:38-cated
`stated Coppe wires
`- Dielectric ...arminate
`
`- Secondary widing
`of the botics side
`of the 8:8
`
`X X
`
`- fernaily Conductive
`::$tiatig.8yer
`
`Y
`
`Ferrite pate
`
`i.e. ex: Fire:
`Primary Wiriding
`- a -
`{{c}
`*ickess, :
`
`
`
`tack Separatigii, S
`k-ki
`
`foctor
`fic,
`
`.
`---.... .
`Primary Sicise
`
`::::iic
`hick:"Ess, 2.
`
`
`
`
`
`-
`
`
`
`/
`soider Mask
`
`t
`X s & & f & X t
`8883ty Costic:ve
`...Y.
`Firs
`- /
`88:38; aye!
`k Sexiciary insing
`*::::: *ies
`raisfor 33 raigs,
`*::::::::first 8:3:8 .33::iiate
`G.
`Secondary Side
`(Prior Art)
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 002
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 2 of 13
`
`US 6,501,364 B1
`
`FIG.3(a)
`
`
`
`Friary hiring
`Y.
`
`ciccic:
`Thickness, i.
`
`
`
`
`
`
`^
`Sairier &task
`
`- Copper Sheet
`
`Ferrite Pate
`
`efraiy Codictive
`1 inskiaig i.ayer
`
`Primary sincing
`ior the top side of
`- the P{3}
`
`$oy:8tate-coat:
`its state Cope: ;i&s.
`is Bielectric aminate
`N \ Secordiary Wiriding
`(on the botton sixie
`of 8 °C8
`
`sherina y Conductive
`s: statig layer
`
`^
`
`reise 3:
`
`- Copper sheet
`
`Primary Side
`
`i&is:ctic
`iickress, 2.
`
`
`
`Sexiary isdig
`if
`opper Sheets
`y 838g.
`x asis 38; 38&iss, 8
`FIG.3(b)
`
`\
`
`8888y Cartistie
`c. *.
`Stsiating kaya:
`3: 388
`sitei-Siricit 8:03:33:38
`Seconday Side
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 003
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 3 of 13
`
`US 6,501,364 B1
`
`Free Space
`Ferrite Plates
`
`Free Space
`
`Axis of Symmetry
`
`
`
`
`
`
`
`
`
`FIG.5
`
`, , , ,
`, , , ,
`
`, , , , ,
`!
`| | | | 1 , , , ,
`t t
`
`It
`it
`
`!
`
`,
`
`f
`
`insulating layer
`
`Primary Windings
`
`/
`
`0.07mm
`
`12.5m
`
`a
`
`r u + i r
`
`, s
`
`-
`
`- -
`
`- - -
`
`- - - - - - - - - - - - - - -
`Ferrite Plate . .
`. . . .
`. . . . .
`, , ,
`' ' .
`, ,
`.
`.
`. .
`. . . . . . . . . . .
`..' ... 1.2 . . .
`. .” - - - - -
`, ,
`, , , , , , , , ,
`i
`- 1 - F - - -
`
`a y
`
`, ,
`
`. . i - - - - - - -
`
`insulation layer
`
`
`
`FIG.7
`
`
`
`N
`Cop effrack \
`type tracks...
`2.5
`3
`
`15 ' ' ' ' ',
`
`--------------.
`.
`.
`.
`.
`.
`.
`.
`.
`.
`.
`.
`.
`.
`.
`.
`0.81
`- - - - - - - - - - - - - - - - - - - - - - - -
`- - - - - - - - -
`-
`' ' - - -
`' '
`'
`' ' '
`' ' ' ' ' ' ' ' ' ' '
`- - - - - - - - - - - - - - - - - - - -
`' ' '
`' ' ' '
`' ' ' ' Ferrite Plate
`' ' ' ' ' ' ' ' ' ' ' / / /
`.
`. .
`.
`.
`.
`.
`.
`. . . . . . --- - - - - - - - - - - - - - - . . . .
`0.6 !
`| | | | | | 1
`!
`14, 21. 21.2-l.
`- - - - - - - - - -
`' ' ' ' ' ' '
`' ' ' ' ' ' ' ' ' '
`. . . . . .
`.
`. . .
`. . . . . .
`. . . . . . . .
`. - - - - - - - - -
`.
`III.
`insulation Layer E. i
`0.2 HHHHHHHH.
`||
`.."
`PCBDielectric
`1
`
`f f
`
`
`
`
`
`0.4 H
`
`Ot.
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 004
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 4 of 13
`
`US 6,501,364 B1
`
`Magnetic Field intensity, H (A/m in dB)
`uly
`Boundary
`
`Ferrite Plate
`
`Free Space
`
`FIG.6
`
`35
`
`30
`
`25
`
`12 5O
`
`1 O
`
`O.8
`
`0.85
`
`0.9
`
`0.95
`z (mm)
`
`1
`
`1.05
`
`11
`
`25
`
`
`
`20
`
`15
`
`an
`O 10
`.
`l l VY
`l
`l
`f l
`E 5
`s: 5
`Boundary
`0 | Ferrite-Copper
`
`Free Space
`
`LL---- O
`
`Boundary
`
`I
`M Copper-Air
`III
`N
`it!"
`
`0.8
`
`0.85
`
`0.95
`z (mm)
`
`1
`
`1.05
`
`1.1
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 005
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 5 of 13
`
`US 6,501,364 B1
`
`
`
`& 3:33
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 006
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 6 of 13
`
`US 6,501,364 B1
`
`
`
`88:
`
`
`
`
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 007
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 7 of 13
`
`US 6,501,364 B1
`
`FG.
`
`
`
`ass s
`
`--
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 008
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 8 of 13
`
`US 6,501,364 B1
`
`G. 2
`
`28
`
`43
`
`8
`
`8
`X (3:3:
`
`}{}
`
`{}
`
`:3
`
`{}
`
`
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 009
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 9 of 13
`
`US 6,501,364 B1
`
`FG.
`
`3
`
`
`
`?
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 010
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 10 of 13
`
`US 6,501,364 B1
`
`G.
`
`4.
`
`
`
`
`
`83
`
`83
`
`
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 011
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 11 of 13
`
`US 6,501,364 B1
`
`G.
`
`
`
`3
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 012
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 12 of 13
`
`US 6,501,364 B1
`
`F.G. (s
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 013
`
`

`

`U.S. Patent
`
`Dec. 31, 2002
`
`Sheet 13 of 13
`
`US 6,501,364 B1
`
`No Shielding
`Ferrite Plates Only
`Copper Sheets Only
`Ferrite Plates + Copper Sheets
`
`- Ferrite Plates Only
`||
`- Copper Sheets Only
`- Ferrite Plates + Copper Sheets
`
`
`
`FIG.18
`100%
`90%
`80%
`
`70%
`60%
`50%
`
`40%
`
`30%
`20%
`
`
`
`I/ | 1 ||
`/-1
`
`10%
`O%
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 014
`
`

`

`US 6,501,364 B1
`
`1
`PLANAR PRINTED-CIRCUIT-BOARD
`TRANSFORMERS WITH EFFECTIVE
`ELECTROMAGNETIC INTERFERENCE
`(EMI) SHIELDING
`
`FIELD OF THE INVENTION
`This invention relates to a novel planar printed-circuit
`board (PCB) transformer structure with effective (EMI)
`Shielding effects.
`
`1O
`
`15
`
`BACKGROUND OF THE INVENTION
`Planar magnetic components are attractive in portable
`electronic equipment applications Such as the power Sup
`plies and distributed power modules for notebook and
`handheld computers. AS the Switching frequency of power
`converter increases, the size of magnetic core can be
`reduced. When the Switching frequency is high enough (e.g.
`a few Megahertz), the magnetic core can be eliminated.
`Low-cost coreless PCB transformers for signal and low
`power (a few Watts) applications have been proposed by the
`present inventors in U.S. patent applications Ser. No.
`08/018,871 and U.S. Ser. No. 09/316,735 the contents of
`which are incorporated herein by reference.
`It has been shown that the use of colorless PCB trans
`former in Signal and low-power applications does not cause
`a Serious EMC problem. In power transfer applications,
`however, the PCB transformers have to be shielded to
`comply with EMC regulations. Investigations of planar
`transformer shielded with ferrite sheets have been reported
`and the energy efficiency of a PCB transformer shielded with
`ferrite sheets can be higher than 90% in Megahertz operating
`frequency range. However, as will be discussed below, the
`present invention have found that using only thin ferrite
`35
`materials for EMI shielding is not effective and the EM
`fields can penetrate the thin ferrite sheets easily.
`
`25
`
`PRIOR ART
`FIGS. 1 and 2 show respectively an exploded perspective
`and cross-sectional view of a PCB transformer shielded with
`ferrite plates in accordance with the prior art. The dimen
`Sions of the PCB transformer under test are detailed in Table
`I. The primary and Secondary windings are printed on the
`opposite sides of a PCB. The PCB laminate is made of FR4
`material. The dielectric breakdown voltage of typical FR4
`laminates range from 15 kV to 40 kV. Insulating layers
`between the copper windings and the ferrite plates should
`have high thermal conductivity in order to facilitate heat
`transfer from the transformer windings to the ferrite plates
`and the ambient. The insulating layer should also be a good
`electrical insulator to isolate the ferrite plates from the
`printed transformer windings. A thermally conductive sili
`con rubber compound coated onto a layer of woven glass
`fibre, which has breakdown voltage of 4.5 kV and thermal
`conductivity of 0.79 Wm'K', is used to provide high
`dielectric strength and facilitate heat transfer. The ferrite
`plates placed on the insulating layers are made of 4F1
`material from Philips. The relative permeability, u, and
`resistivity, p, of the 4F1 ferrite material are about 80 and
`102m, respectively.
`SUMMARY OF THE INVENTION
`According to the present invention there is provided a
`planar printed circuit board transformer comprising at least
`one copper sheet for electromagnetic shielding.
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`Viewed from another aspect of the invention provides a
`planar printed circuit board transformer comprising,
`(a) a printed circuit board,
`(b) primary and Secondary windings formed by coils
`deposited on opposed Sides of Said printed circuit
`board,
`(c) first and Second ferrite plates located over said primary
`and Secondary windings respectively, and
`(d) first and Second copper sheets located over said first
`and Second ferrite plates respectively.
`BRIEF DESCRIPTION OF THE DRAWINGS
`An embodiment of the invention will now be described by
`way of example and with reference to the accompanying
`drawings, in which:
`FIG. 1 is an exploded perspective view of a PCB trans
`former in accordance with the prior art,
`FIG. 2 is a cross-sectional view of the prior art trans
`former of FIG. 1,
`FIGS. 3(a) and (b) are exploded perspective and cross
`sectional views respectively of a PCB transformer in accor
`dance with an embodiment of the present invention,
`FIG. 4 shows the R-Z plane of a prior art PCB
`transformer,
`FIG. 5 is a plot of the field intensity vector of a conven
`tional PCB transformer,
`FIG. 6 plots the tangential and normal components of
`magnetic field intensity near the boundary between the
`ferrite plate and free space in a PCB transformer of the prior
`art,
`FIG. 7 is a plot of the field intensity vector of a PCB
`transformer according to the embodiment of FIGS. 3(a) and
`(b),
`FIG. 8 plots the tangential and normal components of
`magnetic field intensity near the copper sheet in a PCB
`transformer according to the embodiment of FIGS. 3(a) and
`(b),
`FIG. 9 is shows the simulated field intensity of a PCB
`transformer without shielding and in no load condition,
`FIG. 10 shows measured magnetic field intensity of a
`PCB transformer without shielding and in no load condition,
`FIG. 11 shows simulated magnetic field intensity of a
`PCB transformer with ferrite shielding in accordance with
`the prior art and in no load condition,
`FIG. 12 shows measured magnetic field intensity of a
`PCB transformer with ferrite shielding and in no load
`condition,
`FIG. 13 shows simulated magnetic filed intensity of a
`PCB transformer in accordance with an embodiment of the
`invention and in no load condition,
`FIG. 14 shows measured magnetic field intensity of a
`PCB transformer in accordance with an embodiment of the
`present invention and in no load condition,
`FIG. 15 shows simulated magnetic field intensity of a
`PCB transformer in accordance with an embodiment of the
`present invention and in 20 S2 load condition,
`FIG. 16 shows measured magnetic field intensity of a
`PCB transformer in accordance with an embodiment of the
`present invention and in 20 S2 load condition,
`FIG. 17 plots the energy efficiency of various PCB
`transformers in 100 S2 load condition, and
`FIG. 18 plots the energy efficiency of various PCB
`transformers in 100 S2/1000 pF load condition.
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 015
`
`

`

`3
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`In accordance with the present invention, the ferrite
`shielded transformer of the prior art shown in FIGS. 1 and
`2 can be modified to improve the magnetic field Shielding
`effectiveness by coating a layer of copper sheet on the
`surface of each ferrite plate as shown in FIGS.3(a) and (b).
`AS an example, the modified transformer and the ferrite
`Shielded transformer are of the same dimensions as shown in
`Table I. The area and thickness of the copper sheets in the
`example are 25 mmx25 mm and 70 um, respectively.
`The magnetic field intensity generated from the Shielded
`PCB transformers is simulated with a 2D field simulator
`using a finite-element-method (FEM). A cylindrical coordi
`nates System is chosen in the magnetic field simulation. The
`drawing model, in R-Z plane, of the PCB transformer shown
`in FIG. 4 is applied in the field simulator. The Z-axis is the
`axis of Symmetry, which passes through the centre of the
`transformer windings. In the 2D simulation, the Spiral cir
`cular copper tracks are approximated as concentric circular
`track connected in Series. The ferrite plates and the insulat
`ing layerS adopted in the Simulation model are in a circular
`shape, instead of in a Square shape in the transformer
`prototype. The ferrite plates and the insulating layerS may be
`made of any conventional materials.
`A. Transformer Shielded with Ferrite Plates
`The use of the ferrite plates helps to confine the magnetic
`field generated from the transformer windings. The high
`relative permeability, u, of the ferrite material guides the
`magnetic field along the inside the ferrite plates. In the
`transformer prototype, 4F1 ferrite material is used though
`any other conventional ferrite material cold also be used.
`The relative permeability of the 4F1 material is about 80.
`Based on the integral form of the Maxwell equation,
`(1)
`fSB-ds=0
`the normal component of the magnetic flux density is
`continuous acroSS the boundary between the ferrite plate and
`free Space. Thus, at the boundary,
`(2)
`B=B2,
`where B, and B, are the normal component (in
`Z-direction) of the magnetic flux density in the ferrite plate
`and free Space, respectively.
`From (2), uttoH=lloH2
`
`From (3), at the boundary between the ferrite plate and free
`Space, the normal component of the magnetic field intensity
`in free Space can be much higher than that in the ferrite plate
`when the relatively permeability of the ferrite material is
`very high. Therefore, when the normal component of the
`H-field inside the ferrite plate is not sufficiently suppressed
`(e.g. when the ferrite plate is not thick enough), the H-field
`emitted from the Surface of the ferrite plates can be enor
`mous. FIG. 5 shows the magnetic field intensity vector plot
`of the transformer shielded with ferrite plates. The primary
`is excited with a 3 A3 MHz current source and the secondary
`is left open. The size of the arrows indicates the magnitude
`of the magnetic field intensity in dBA/m. FIG. 5 shows that
`the normal component of the H-field inside the ferrite plate
`is not Suppressed adequately and So the H-field emitted from
`the ferrite plate to the free Space is very high.
`The tangential (H) and normal (H) components of
`magnetic field intensity near the boundary between the
`
`15
`
`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`US 6,501,364 B1
`
`4
`ferrite plate and free Space, at R=1 mm, are plotted in FIG.
`6. The tangential H-field (H) is about 23.2 dB and is
`continuous at the boundary. The normal component of the
`H-field (H) in the free space is about 31.5 dB and that inside
`the ferrite plate is about 12.5 dB at the boundary. The normal
`component of the H-field is, therefore, about 8% of the
`resultant H-field inside the ferrite plate at the boundary.
`Thus, the ferrite plate alone cannot completely guide the
`H-field in the tangential direction. As described in (3), the
`normal component of the H-field in the free space is 80 times
`larger than that in the ferrite plate at the boundary. From the
`simulated results in FIG. 6, the normal component of the
`magnetic field intensity in the free Space is about 19 dB, i.e.
`79.4 times, higher than that inside the ferrite plate. Thus,
`both simulated results and theory described in (3) show that
`the using ferrite plates only is not an effective way to Shield
`the magnetic field generated from the planar transformer.
`
`TABLE I
`
`Geometric Parameters of the PCB Transformer
`
`Geometric Parameter
`
`Dimension
`
`Copper Track Width
`Copper Track Separation
`Copper Track Thickness
`Number of Primary Turns
`Number of Secondary
`Turns
`Dimensions of Ferrite
`Plates
`PCB Laminate Thickness
`Insulating Layer Thickness
`Transformer Radius
`
`0.25 mm.
`1 mm
`70 um (2 Oz/ft)
`1O
`1O
`
`25 mm x 25 mm x
`0.4 mm
`0.4 mm
`O.228 mm
`23.5 mm.
`
`B. Transformer Shielded with Ferrite Plates and Copper
`Sheets
`A PCB transformer using ferrite plates coated with copper
`sheets as a shielding (FIG.3(a) and (b)) has been fabricated.
`The size of the copper sheets is the same as that of the ferrite
`plate but its thickness is merely 70 um. Thin copper sheets
`are required to minimize the eddy current flowing in the
`Z-direction, which may diminish the tangential component
`of the H-field.
`Based on the integral form of the Maxwell equation,
`
`H. di = 1 + f2P. as
`f dt = f + at
`S
`
`(4)
`
`and assuming that the displacement current is Zero and the
`current on the ferrite-copper boundary is very Small and
`negligible, the tangential component of the magnetic field
`intensity is continuous acroSS the boundary between the
`ferrite plate and free Space. Thus, at the boundary,
`
`H=H2,
`(5)
`where H and H2 are the tangential component (in
`r-direction) of the magnetic field intensity in the ferrite plate
`and copper, respectively. Because the tangential H-field on
`the Surfaces of the copper sheet and the ferrite plates are the
`Same at the boundary, thin copper sheets have to be adopted
`to minimize eddy current loSS.
`Consider the differential form of the Maxwell equation at the
`ferrite-copper boundary,
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 016
`
`

`

`(6)
`
`(7)
`
`where (), u and O are the angular frequency, permeability
`and conductivity of the medium, respectively. Because cop
`per is a good conductor (O=5.80x107 S/m) and the operating
`frequency of the PCB transformer is very high (a few
`magahertz), from (7), the magnetic field intensity, H, inside
`the copper sheet is extremely Small. Accordingly, the normal
`component of the H-field inside the copper sheet is also
`Small. Furthermore, from (3), at the ferrite-copper boundary,
`the normal component of the H-field inside the ferrite plate
`is 80 times less than that inside the copper sheet. As a result,
`the normal component of the H-field inside the ferrite plate
`can be Suppressed drastically.
`By using ferrite element methods, the magnetic field
`intensity vector plot of the PCB transformer shielded with
`ferrite plates and copper sheets has been Simulated and is
`shown in FIG. 7. The tangential (H) and normal (H)
`components of magnetic field intensity near the copper
`sheet, at R=1 mm, are plotted in FIG. 8. From FIG. 8, the
`tangential H-field (H,) is about 23 dB and approximately
`continuous at the boundary. The normal component of the
`H-field (H) in copper sheet is suppressed to about 8 dB and
`that inside the ferrite plate is about -7.5 dB at the boundary.
`Therefore, the normal component of the H-field is, merely
`about 0.09% of the resultant H-field inside the ferrite plate
`at the boundary. Accordingly, at the ferrite-copper boundary,
`the H-field is nearly tangential and confined inside in the
`ferrite plate. Besides, the normal component of the H-field
`emitted into the copper sheet and the free Space can be
`neglected in practical terms. Since the normal component of
`the H-field emitted into the copper is very small, the eddy
`current loss due to the H-field is also very small. This
`phenomenon is verified by the energy efficiency measure
`ments of the ferrite-shielded PCB transformers with and
`without copper sheets described below. As a result, the use
`ferrite plates coated with copper sheets is an effective way
`to shield the magnetic field generated from the transformer
`windings without diminishing the transformer energy effi
`ciency.
`The shielding effectiveness (SE) of a barrier for magnetic
`field is defined as
`
`15
`
`25
`
`35
`
`40
`
`45
`
`SE = 20 logo
`
`O
`
`H
`
`i
`SE = 2x 10 logo. = 2x (H(in dB) - H(in dB)|)
`
`50
`
`(8)
`
`55
`
`where H. is the incident magnetic field intensity and H. is
`the magnetic field intensity transmits through the barrier.
`Alternatively, the incident field can be replaced with the
`magnetic field when the barrier is removed.
`Magnetic field intensity generated from the PCB trans
`formers with and without shielding has been simulated with
`FEM 2D simulator and measured with a precision EMC
`Scanner. In the field simulation, the primary Side of the
`transformer is excited with a 3MHz, 3 A current Source.
`However, the output of the magnetic field transducer in the
`EMC scanner will be clipped when the amplitude of the
`
`60
`
`65
`
`US 6,501,364 B1
`
`6
`high-frequency field intensity is too large. Thus, the 3 MHz
`3. A current Source is approximated as a Small signal (0.1 A)
`3 MHz source Superimposed into a 3 ADC source because
`the field transducer cannot sense DC Source. In the mea
`Surement Setup, a magnetic field transducer for detecting
`vertical magnetic field is located at 5 mm below the PCB
`transformer.
`A. PCB Transformer without Shielding
`The magnetic field intensity of the PCB transformer
`without any form of Shielding and loading has been Simu
`lated and its R-Z plane is shown in FIG. 9. From the
`Simulated result, the magnetic field intensity, at R=0 mm and
`Z=5 mm, is about 30 dB A/m. The measured magnetic
`intensity, in z-direction, is shown in FIG. 10. The white
`square and the white parallel lines in FIG. 10 indicate the
`positions of transformer and the current carrying leads of the
`transformer primary terminals, respectively. The output of
`the magnetic field transducer, at 5 mm beneath the centre of
`the transformer, is about 130 dB uV.
`B. PCB Transformer Shielded with Ferrite Plates
`The simulated magnetic field intensity of a PCB trans
`former shielded with ferrite plates alone, under no load
`condition, is shown in FIG. 11. The simulated result shows
`that the magnetic field intensity, at R=0 mm and Z=5 mm,
`is about 28 dBA/m. The measured magnetic intensity, in
`z-direction, is shown in FIG. 12. The output of the magnetic
`field transducer, at 5 mm beneath the centre of the
`transformer, is about 128 uV. Therefore, with the use of 4F1
`ferrite plates, the shielding effectivness (SE), from the
`Simulated result, is
`SE=2x(30-28)=4 dB
`The shielding effectiveness obtained from measurements is
`SE=2x(130-128)=4 dB
`Both Simulation and experimental results shown that the use
`of the 4F1 ferrite plates can reduce the magnetic field
`emitted from the transformer by 4 dB (about 2.5 times).
`C. PCB Transformer Shielded with Ferrite Plates and Cop
`per Sheets
`FIG. 13 shows the simulated magnetic field intensity of a
`PCB transformer in accordance with an embodiment of the
`invention Shielded with ferrite plates and copper sheets
`under no load condition. From the Simulated result, the
`magnetic field intensity, at R=0 mm and Z=5 mm, is about
`13 dBA/m. FIG. 14 shows the measured magnetic intensity
`in Z-direction. The output of the magnetic field transducer, at
`5 mm beneath the centre of the transformer, is about 116 dB
`liV. With the use of 4F1 ferrite plates and copper sheets, the
`shielding effectiveness (SE), from the simulated result, is
`SE=2x(30–13)=34 dB
`The shielding effectiveness obtained from measurements is
`SE=2x(130–116)=28 dB
`AS a result, the use of ferrite plates coated with copper sheets
`is an effective way to Shield magnetic field generated from
`PCB transformer. The reduction of magnetic field is 34 dB
`(2512 times) from simulation result and 28 dB (631 times)
`from measurement. The SE obtained from the measurement
`is less than that obtained from the simulated test. The
`difference mainly comes form the magnetic field emitted
`from the current carrying leads of the transformer. From
`FIG. 14, the magnetic field intensity generated from the
`leads is about 118 dB, which is comparable with the mag
`netic field generated from the transformer. Therefore, the
`magnetic field transducer beneath the centre of the trans
`former also picks up the magnetic field generated from the
`lead wires.
`
`Momentum Dynamics Corporation
`Exhibit 1010
`Page 017
`
`

`

`US 6,501,364 B1
`
`7
`D. PCB Transformer in Loaded Condition
`When a load resistor is connected acroSS the Secondary of
`the PCB transformer, the opposite magnetic field generated
`from Secondary current cancels out part of the magnetic field
`Setup from the primary. As a result, the resultant magnetic
`field emitted from the PCB transformer in loaded condition
`is less than that in no load condition. FIG. 15 shows the
`simulated magnetic field intensity of the PCB transformer
`shielded with ferrite plates and copper sheets in 20 S2 load
`condition. From the Simulated result, the magnetic field
`intensity, at R=0 mm and Z=5 mm, is about 4.8 dBA/m,
`which is much less than that in no load condition (13
`dBA/m). FIG. 16 shows the measured magnetic intensity in
`Z-direction. The output of the magnetic field transducer, at 5
`mm beneath the centre of the transformer, is about 104 dB
`liV and that in no load conditions is 116 dB uV.
`Energy efficiency of PCB transformers shielded with (i)
`ferrite plates only, (ii) copper sheets only and (iii) ferrite
`plates covered with copper sheets may be measured and
`compared with that of a PCB transformer with no shielding.
`FIG. 17 shows the measured energy efficiency of the four
`PCB transformers with 100 S2 resistive load. In the PCB
`transformer Shielded with only copper sheets, a layer of
`insulating sheet of 0.684 mm thickness is used to isolate the
`transformer winding and the copper sheets. From FIG. 17,
`energy efficiency of the transformers increases with increas
`ing frequency. The transformer Shielded with copper sheets
`only has the lowest energy efficiency among the four trans
`formers. The energy loSS in the copper-Shielded transformer
`mainly comes from the eddy current, which is induced from
`the normal component of the H-field generated from the
`transformer windings, circulating in the copper sheets.
`The energy efficiency of the transformer with no Shielding
`is lower than that of the transformers shielded with ferrite
`plates. Without ferrite shielding, the input impedance of
`coreless PCB transformer is relatively low. The energy loss
`of the coreless transformer is mainly due to its relatively
`high iR loss (because of its relatively high input current
`compared with the PCB transformer covered with ferrite
`plates). The inductive parameters of the transformers with
`and without ferrite shields are shown in Table II. However
`this shortcoming of the coreless PCB transformer can be
`overcome by connecting a resonant capacitor across the
`Secondary of the transformer. The energy efficiency of the 4
`PCB transformers with 100 S2/1000 pF capacitive load is
`shown in FIG. 18. The energy efficiency of the coreless PCB
`transformer is comparable to that of the ferrite-shielded
`transformers at the maximum efficiency frequency (MEF) of
`the coreless PCB transformer.
`The ferrite-shielded PCB transformers have the highest
`energy efficiency among the four transformers, especially in
`low frequency range. The high efficiency characteristic of
`the ferrite-shielded transformers is attributed to their high
`input impedance. In the PCB transformer shielded with
`ferrite plates and copper sheets, even though a layer of
`copper sheet is coated on the Surface of each ferrite plate, the
`eddy current loSS in the copper sheets is negligible as
`discussed above. The H-field generated from the transformer
`windings is confined in the ferrite plates. The use of thin
`copper sheets is to direct the magnetic field in parallel to the
`ferrite plates So that the normal component of the magnetic
`field emitting into the copper can be Suppressed signifi
`cantly. The energy efficiency measurements of the ferrite
`Shielded transformers with and without copper sheets con
`firm that the addition of cooper sheets on the ferrite plates
`will not cause Significant eddy current loSS in the copper
`sheets and diminish the transformer efficiency. From FIGS.
`17 and 18, the energy efficiency of both ferrite-shielded
`transformers, with and without copper sheets, can be higher
`than 90% at a few megahertz operating frequency.
`
`15
`
`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`It will thus be seen that the present invention provides a
`Simple and effective technique of magnetic field Shielding
`for PCB transformers. Performance comparison, including
`shielding effectiveness and energy efficiency, of the PCB
`transformerS Shielded in accordance with embodiments of
`the invention, copper sheets and ferrite plates has been
`accomplished. Both simulation and measurement results
`show that the use of ferrite plates coated with copper sheets
`has the greatest shielding effectiveness (SE) of 34 dB (2512
`times) and 28 dB (631 times) respectively, whereas the SE
`of using only ferrite plates is about 4 dB (2.5 times).
`Addition of the copper sheets on the Surfaces the ferrite
`plates does not significantly diminish the transformer energy
`efficiency. Experimental results show that the energy effi
`ciency of both ferrite-shielded transformers can be higher
`t