Novel Techniques to Cancel Common-mode Noise
Based on Noise Balance

reduction is verified by experiments.

Abstract:
Role of winding shielding on the parasitic capacitancesof transformer and common-mode (CM) noise isanalyzed in details when considering the effects of thesecondary side noise source. Based on the proposedmodel of CM noise, two novel techniques to cancel CM
noise by balancing noise is given; experiment results
show CM noise is greatly reduced when the techniques
are adopted.

Ⅰ. Introduction
A switching power converter generates larger CM
noise as a result of the switching operations in thepresence of parasitic capacitance between windings oftransformer. In order to reduce common-mode EMI
emission, a Faraday shielding between the primary andsecondary windings of the transformer is often adopted inpractice to reduce the effective coupling capacitance
between the windings. Some researches on the modelingof the stray capacitive effects in the transformer werereported [1, 2, 3]. However, they usually did not consider
the effects of the shielding and were not good enough for
EMI analysis in practical design.
Typically, CM noise makes up a significant fraction ofelectromagnetic interference (EMI), so large size of CM
choke is needed if we want to suppress EMI noise in theinput line. In order to reduce the size of EMI filter andcost, noise cancellation techniques have been introduced
to the area of EMI in resent years [4][5], Thosetechniques have the disadvantages of complexity andneed additional components. In this paper, role of winding
shielding on parasitic capacitance of transformer and CM
noise when taking into account the effects of thesecondary side noise source is analyzed in details; basedon model of CM noise, two novel techniques to cancel
CM noise by balancing noise are given, It is simpler orless cost compared with previous techniques, thetechniques can be applied to isolated converters, such as
Fly-back converter, Forward converter, etc. In the lastsection of the paper, effect of the method on CM noise

Ⅱ. Principle of CM Noise BalanceTakes fly-back converter as an example, Fig.1 shows
the flowing path of CM current when shielding is used in
the transformer; Vp and Vs denote the EMI noise sourcesby the operations of primary MOSFET switch andsecondary rectifier diode respectively. The hot-voltage
point in primary is 2 and the hot-voltage point insecondary is 3, Cps denotes the equivalent lumped
capacitance between terminal 2 and 4, representing thecapacitive effect of primary winding to the secondary,
Cpsh and Cssh are introduced to represents the equivalent
lumped capacitances of primary winding and secondary
winding to the shielding respectively. Cp0 represents thecapacitive coupling of MOSFET to heat sink. Usually
primary side voltage is higher than secondary side, soshielding foil and heat sink is connected to primary minus
to reduce the effect of Cps, as in Fig.1. In this case, Cpshand Cp0 have no contribution to the CM noise because
displacement current flowing through it is circulating tonoise source. If shielding foil is connected to secondaryside minus (terminal 4 in Fig.1), then Cpsh has
contributions to CM noise but Cssh has not, this case will
not be discussed in the paper for it is only used whensecondary side voltage is higher than primary side.

Fig.1. Coupling path of CM noise in Fly-back converter

Fig.2 shows the simplified model of CM noise forfly-back converter, icp and issh are the current caused byprimary side noise source Vp and secondary side noise
source Vs respectively; icm is the current of CM noise.
From the model, we know that,icm = icp − issh(1)

Vp and Vs has the same frequency but opposite phase,as the waveforms shown in Fig.3, therefore icp and issh has
the effect of counteraction with each other. Ideally, whenequation (2) is met, then CM noise icm will be reduced tominimum.

V p ⋅ C ps = Vs ⋅ Cssh

Fig.4. Sectional view of transformer and the art of(2)modulating shielding length

Capacitive coupling effect of the open area between theprimary winding and secondary winding can be equatedto Cps; Capacitive coupling effect of the area between
secondary winding to shielding can be equated to Cssh.
Though Cps and Cssh are equivalent lumped
capacitances, it is actually a distributed capacitance sincevoltage is distributed along the windings of thetransformer when switch is operating, as shown inFig5.and Fig.6. Therefore charge will distribute alongwinding surfaces of these two parts of area,

Fig.2. Model of CM noiseFig.3. Waveforms of Vp, Vs and icm

Ⅲ. Methods to Cancel CM Noise
Cssh is greatly larger than Cps when Faraday shielding isused between primary winding and secondary winding.
Therefore, issh is usually larger than icp though Vp is higher
than Vs in practical applications, and CM noise will bedominated by secondary noise issh. In such cases, we canreduce CM noise by decrease Cssh or increase Cps, asfollowing:

1. Optimal Design of shielding

Fig.6. Voltage distribution and Capacitive coupling between

For simplification, assume both primary winding andsecondary winding of transformer are single-layer and ashielding is added between primary winding andsecondary winding of the transformer.
A. Modulate the length of winding shielding
The art of modulating shielding length is shown in
Fig.4. W is the window width of bobbin,θis the central
angle of the open area of the shielding, The length of theopen area is:

1x = ⋅ d ⋅θ mm
2and the length of shielding is:
1l = ⋅ d ⋅ (2π − θ ) mm
2while x = π ⋅ d − l mm

Fig.5. Voltage distribution and capacitive coupling in the open area

(3)

(4)
(5)

Secondary winding and shielding

In the Fig.5 and Fig.6, Vp and Vs is supposed tolinearly distribute along primary winding Np andsecondary winding Ns respectively, surface of shielding
can be considered as zero voltage potential. Cpsw is thecapacitance per unit area of winding surface of the openarea; Csshw is the capacitance per unit area of winding
surface of the area between secondary winding andshielding. Both Cpsw and Csshw are ‘static, volumetric’
capacitances and can be calculate by analytical method[6]. According to the CM model in Fig.2 and thedefinition of Cps and Cssh, The total charge in the surfaceof open area isC ⋅ W ⋅ (V p − VS )(6)
V p ⋅ C ps = psw⋅x2andC ps =C psw ⋅ W ⋅ (V p − VS )2 ⋅V p⋅x(7)

Cssh will be very sensitive to the change of X, as shownin Fig.9, it indicates that uniformity of canceling CM
noise is not good in such case.

The total charge in the surface of the area between
secondary winding and shielding isVs ⋅ Cssh =Csshw ⋅W ⋅Vs⋅l2(8)

andCssh =Csshw ⋅W⋅l2(9)

To cancel CM noise, the optimal length of shielding
Can be calculated whenV p ⋅ C ps = Vs ⋅ Cssh
The position of the open area of shielding is not criticalto the modulation effect because voltage of per turnwinding is almost uniform. Fig.7 shows the rate of Cps
and Cssh change along with x linearly. It indicates that themethod will have good uniformity of canceling CMnoise.

(a)

(b)

Fig.9. (a) Position of the open area of shielding at high voltage sidewinding; (b) effect of modulating shielding width

2. Adding a capacitance to balance noise
Another simple method to balance issh and icp is to add aproper capacitance between terminal 2 and terminal 4 inFig.1. The additional capacitance increase the effect of Cpsand issh, so CM noise icm will be reduced to minimum if
equation (2) is met.
In some applications, icp still is larger than issh eventhough Faraday shielding is used, so the art of modulating
shielding length or width is not effective, in such case,additional capacitance can be added between terminal 3
and terminal 1 in the Fig.1 to make issh and icp balance.

Ⅳ. Application Example and Validation
Fig.7. Effect of modulating shielding length

B. Modulate the width of winding shielding
Fig.8 represents the art of modulating shielding width.
X represents the width of open area between the primaryand secondary winding or the reduced width of shielding.
With the increase of X, Cps will increase and Cssh will
decrease. The optimal width of shielding can be obtainedwhenV p ⋅ C ps = Vs ⋅ Cssh

A 65 Watts flyback power supply with 65 kHz
operation frequency was used for experiment. Fig10
shows the winding structure and winding arrangement of
the transformer. If shielding1 and shielding2 are
traditional Faraday shielding, the shorted length ofshielding1 and shielding2 will be 45mm and 56mm
respectively. The optimal length of shielding waspredicted by Calculation, result showed when the lengthof shielding2 was reduced to 26mm while shieldling1
uses Faraday shielding, Vp*Cps will be equal to Vs*Cssh,
the CM noise will be reduced to its minimum.

Fig.8. Sectional view of transformer and the art ofmodulating shielding widthFig.10. winding structure and winding arrangement

Due to the voltage distribution along winding, different
position of open area of the shielding makes different
modulation effect. If position of open area is at the highvoltage side of primary and secondary winding, Cps andof the transformer

Two transformers with different shielding were designed,
The first transformer is designed with traditional Faradayshielding, shielding of the second transformer use

predicted optimal length. When both transformers aretested in the same prototype without any filter, The CM
noise of the second transformer is about 23dBuV lower incomparison with the first one. Fig.12 shows the test result.

Fig.12. Tested CM noise of the two transformers

Ⅴ. Conclusions
An accurate model of CM noise and two novel
techniques to cancel CM noise are introduced in the paper.
Experiment results verified that:1). The secondary side noise source has contribution toCM noise, particularly when output voltage is high. Its
mechanism and effect on CM noise need to be consideredwhen modeling CM noise.
2). Different connection of shielding makes different
contribution of the secondary side noise source to CMnoise. The proposed model of CM noise shows that theprimary side and secondary side noise source have
opposite effect on CM noise.
3). The art of modulating winding shielding oftransformer or adding a compensate capacitance are thesimple but effective methods to cancel CM noise, it will
help to reduce the size of EMI filter.

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