基于噪声平衡的抵消共模干扰新技术

1、Novel Techniques to Cancel Common-mode Noise Based on Noise Balance基于噪声平衡的抵消共模干扰新技术Abstract:概要:Role of winding shielding on the parasitic capacitances of transformer and common-mode (CM noise is analyzed in details when considering the effects of the secondary side noise source. Based on the propose

2、d model 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.详细分析线圈屏蔽对变压器寄生电容和共模(CM 噪声的影响。基于共模噪声模型, 俩平衡噪声的新技术用来抵消共模噪声;实验证明共模噪声被显著降低当技术被应用时。 . Introduction一.导言A switching power converter g

3、enerates larger CMnoise as a result of the switching operations in thepresence of parasitic capacitance between windings oftransformer. In order to reduce common-mode EMIemission, a Faraday shielding between the primary andsecondary windings of the transformer is often adopted inpractice to reduce t

4、he 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 considerthe effects of the shielding and were not good enough forEMI analysis in practical design.变压器绕组间存在技术电容

5、因而开关电源运行时会产生大量的共模噪声。 为减少共模电 磁干扰发射, 通常在变压器初次级绕组间加入法拉第屏蔽来减少绕组间的电容耦合。 一些研 究这些杂散电容的报告见参考文献 1,2,3。但是,通常的屏蔽做法在实际设计中对电磁干 扰分析来说并不够好。Typically, CM noise makes up a significant fraction of electromagnetic interference (EMI, so large size of CM choke is needed if we want to suppress EMI noise in the input line

6、. In order to reduce the size of EMI filter and cost, noise cancellation techniques have been introduced to the area of EMI in resent years 45, Those techniques have the disadvantages of complexity and need additional components. In this paper, role of winding shielding on parasitic capacitance of t

7、ransformer and CM noise when taking into account the effects of the secondary side noise source is analyzed in details; based on model of CM noise, two novel techniques to cancel CM noise by balancing noise are given, It is simpler or less cost compared with previous techniques, the techniques can b

8、e applied to isolated converters, such as Fly-back converter, Forward converter, etc. In the last section of the paper, effect of the method on CM noise reduction is verified by experiments.共模噪声是构成电磁干扰的典型的一部分,为抑制电磁干扰必需使用大尺寸共模滤波 器。为缩小电磁干扰滤波器的尺寸和成本,噪声抵消技术方兴未艾 45。那些技术实现复 杂并要额外增加零件。 本文详细分析变压器屏蔽绕组寄生电容对次级

9、共模噪声的影响; 基于 共模噪声模型, 提出共模噪声抵消技术, 相对以前的技术简单且成本低, 该技术可应用于隔 离变换器, 例如反激、 正激等等。 本文结尾讲述经过实验验证该技术对共模干扰的降低程度。 . Principle of CM Noise Balance二.共模噪声抵消原理Takes fly-back converter as an example, Fig.1 showsthe flowing path of CM current when shielding is used inthe transformer; Vp and Vs denote the EMI noise sou

10、rcesby 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

11、the secondary,C psh and Cssh are introduced to represents the equivalentlumped capacitances of primary winding and secondarywinding to the shielding respectively. Cp0 represents thecapacitive coupling of MOSFET to heat sink. Usuallyprimary side voltage is higher than secondary side, soshielding foil

12、 and heat sink is connected to primary minusto reduce the effect of Cps, as in Fig.1. In this case, Cpsh and 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 Fi

13、g.1, then Cpsh has contributions to CM noise but Cssh has not, this case willnot be discussed in the paper for it is only used whensecondary side voltage is higher than primary side.以反激变换器为例,图 1显示了变压器使用屏蔽后的共模噪声传输途径; V p 和 V s 分别表 示初级场效应晶体管开关和次级整流二极管产生的电磁干扰噪声。初级热端为 2、次级热端 为 3, C ps 表示端点 2、 4间的总杂散电容,

14、C psh 和 C ssh 代表初级、次级对屏蔽地的总杂散电容。 C p0代表场效应晶体管和散热器之间的等效电容。通常初级电压高于次级电压,所以屏蔽铜 箔和散热器连接至初级地以减小等效 C ps ,如图 1所示。 C psh 和 C p0因位移电流穿越它们而对共 模噪声抑制不起作用。如果屏蔽层连接至次级低(图 1中的端点 4, C psh 有效抑制共模噪声 但是 C ssh 不会,本文不对次级电压高于初级电压的案例进行论述。 Fig.1. Coupling path of CM noise in Fly-back converter图 1. 反激变换器共模噪声的耦合路径Fig.2 shows

15、the simplified model of CM noise for fly-back converter, icp and issh are the current caused by primary side noise source Vp and secondary side noise source Vs respectively; i cm is the current of CM noise. From the model, we know that,图 2展示反激变换器共模噪声的简化模型, i cp 和 i ssh 分别是初级噪声电压源 V p 和次级噪声电压源 V s 产生

16、的电流; i cm 是共模噪声电流。从模型可知:i cm = i cp i ssh (1共模噪声 =初级噪声电流 -次级噪声电流 (1V p 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, when equation (2 is met, then CM noise icm will be reduced to min

17、imum.V p 和 V s 频率相同相位相反,波形如图 3所示,因此 i cp 和 i ssh 相互抵消。当等式(2出现, 共模噪声 i cm 将最小化。V C =V C (2电压×电容 =电压×电容 (2 Fig.2. Model of CM noise图 2. 共模噪声模型 Fig.3. Waveforms of Vp, Vs and icm图 3. Vp 、 V s 和 i cm 波形 . Methods to Cancel CM Noise三.抵消共模噪声的方法C ssh is greatly larger than Cps when Faraday shield

18、ing isused between primary winding and secondary winding. Therefore, issh is usually larger than icp though Vp is higherthan Vs in practical applications, and CM noise will bedominated by secondary noise issh . In such cases, we can reduce CM noise by decrease Cssh or increase Cps , asfollowing:C ss

19、h 远大于 C ps 当初次级间使用法拉第屏蔽的时候。 因此, 虽然通常应用中 V p 高于 V s 但是 i ssh 通常还是比 i cp 大, 共模噪声主要受控于 i ssh 。 这种情况我们可通过减小 C ssh 或增大 C ps 来抑制共模噪声,如下:1. Optimal Design of shielding1. 优化屏蔽设计For simplification, assume both primary winding and secondary winding of transformer are single-layer and a shielding is added betw

20、een primary winding and secondary winding of the transformer.假设初级绕组和次级绕组都是单层这样屏蔽层可尽量覆盖变压器初次级绕组。A. Modulate the length of winding shieldingA. 调整屏蔽层长度The art of modulating shielding length is shown in Fig.4. W is the window width of bobbin,is the central angle of the open area of the shielding, The le

21、ngth of the open area is:屏蔽层长度示意图如图 4所示。 W 是骨架窗口宽度, 是屏蔽层开口区域的中心角,开 口区域的长度如下:x = d mm (3and the length of shielding is:屏蔽层长度:l = d mm (4while x = d l mm (5Fig.4. Sectional view of transformer and the art of modulating shielding length图 4. 变压器剖面图和屏蔽层长度调整Capacitive coupling effect of the open area betw

22、een the primary winding and secondary winding can be equated to 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 since voltage is distributed alo

23、ng the windings of the transformer when switch is operating, as shown in Fig5.and Fig.6. Therefore charge will distribute along winding surfaces of these two parts of area,开口区域等效初次级电容换算为 Cps ;次级绕组与屏蔽层间等效电容换算为 Cssh 。虽然 Cps 和 Cssh 是等效的集总电容,实际上开关运行期间变压器绕组在分布电容上产生分布电动 势,如图 5和图 6所示。因此电荷在绕组表面有不同的分布。 Fig.5

24、. Voltage distribution and capacitive coupling in the open area图 5. 开口区域电压分布和电容耦合 Fig.6. Voltage distribution and Capacitive coupling between Secondary winding and shielding 图 6. 次级绕组与屏蔽层间的电压分布和电容耦合In the Fig.5 and Fig.6, Vp and Vs is supposed to linearly distribute along primary winding Np and seco

25、ndary winding Ns respectively, surface of shielding can be considered as zero voltage potential. Cpsw is the capacitance per unit area of winding surface of the open area; Csshw is the capacitance per unit area of winding surface of the area between secondary winding and shielding. Both Cpsw and Css

26、hw are static, volumetric capacitances and can be calculate by analytical method 6. According to the CM model in Fig.2 and the definition of Cps and Cssh, The total charge in the surface of open area is在图 5、图 6中, V p 和 V s 假定为分别沿初级绕组 N p 和次级绕组 N s 线性分布,屏蔽层表面 为等势面。 C psw 为开口区域初次级绕组间单线表面间的分布电容; C sshw

27、 为次级绕组与屏蔽层 间单线表面间的分布电容;所有的 C psw 和 C sshw 是式(6可以计算的集总电容。依图 2的共模 模型和 C ps 、 C ssh 的定义,开口区域的总表面电荷为 and和 The total charge in the surface of the area between secondary winding and shielding is 次级绕组与屏蔽层间总表面电荷为 and和 To cancel CM noise, the optimal length of shielding Can be calculated when为抵消共模噪声,当下式成立可计算

28、出最佳屏蔽层长度 The position of the open area of shielding is not critical to the modulation effect because voltage of per turn winding is almost uniform. Fig.7 shows the rate of Cps and Cssh change along with x linearly. It indicates that the method will have good uniformity of canceling CM noise. 因每匝绕组的电

29、压一样所以屏蔽层缺口的位置带来的影响可忽略。 图 7显示 C ps 和 C ssh 随 x 线性变化。这表示此法对抵消共模噪声的一致性很好。 Fig.7. Effect of modulating shielding length图 7. 调整屏蔽层长度的结果B. Modulate the width of winding shieldingB. 调整屏蔽层的宽度Fig.8 represents the art of modulating shielding width. X represents the width of open area between the primary and s

30、econdary 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 obtained when图 8展示屏蔽层宽度调整的奥妙。 X 代表初次级绕组间屏蔽层的空白宽度。增加 X , C ps将增加而 C ssh 将减小。下式成立时可获得最佳的屏蔽层宽度 Fig.8. Sectional view of transformer and the art of

31、modulating shielding width图 8. 变压器屏蔽层宽度调整剖面图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 high voltage side of primary and secondary winding, Cps and Cssh will be very sensitive to

32、 the change of X, as shown in Fig.9, it indicates that uniformity of canceling CM noise is not good in such case.绕组有不同的电压梯度, 屏蔽层的位置决定屏蔽效果。 如果屏蔽层空白区域在初次级绕 组的热端, C ps 和 C ssh 对 X 的改变非常敏感, 如图 9所示, 这表示此方式对抵消共模噪声效果不 太好。 Fig.9. (a Position of the open area of shielding at high voltage side winding;(b effe

33、ct of modulating shielding width图 9. (a屏蔽层空白区域在绕组高电势位置; (b调整屏蔽层宽度效果2. Adding a capacitance to balance noise2. 增加一电容配平噪声Another simple method to balance issh and icp is to add a proper capacitance between terminal 2 and terminal 4 in Fig.1. The additional capacitance increase the effect of Cps and is

34、sh , so CM noise i cm will be reduced to minimum if equation (2 is met.另一种配平 i ssh 和 i cp 的方法是在图 1端点 2和端点 4之间加电容。此附加电容增加 C ps 和 i ssh , 共模噪声 i cm 在等式 2成立时最小化。In some applications, icp still is larger than issh even though Faraday shielding is used, so the art of modulating shielding length or width

35、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.在某些应用中,虽然使用了法拉第屏蔽,但是 i cp 还是比 i ssh 大,因此调整屏蔽层长度或宽 度无效,在这些案例中,可在图 1端点 3和端点 1之间附加电容配平 i ssh 和 i cp 。 . Application Example and Validation . 应用案例和验证A 65 Watts

36、flyback power supply with 65 kHz operation frequency was used for experiment. Fig10shows the winding structure and winding arrangement ofthe transformer. If shielding1 and shielding2 aretraditional Faraday shielding, the shorted length ofshielding1 and shielding2 will be 45mm and 56mmrespectively. T

37、he optimal length of shielding waspredicted by Calculation, result showed when the lengthof shielding2 was reduced to 26mm while shieldling1uses Faraday shielding, Vp*Cps will be equal to Vs*Cssh,the CM noise will be reduced to its minimum. 一开关频率 65kHz 的 65瓦反激电源。图 10展示出变压器原理图和绕组结构图。如果是传 统的法拉第屏蔽,屏蔽层

38、1和 2的长度分别是 45mm 和 56mm 。屏蔽层的最佳长度可通过计算 得出, 当屏蔽层 1使用法拉第屏蔽时屏蔽层 2的长度可减少为 26mm , V p 乘 C ps 将等于 V s 乘 C ssh , 共模噪声将最小化。 Fig.10. winding structure and winding arrangement of the transformer图 10. 变压器原理图和绕组结构图Two transformers with different shielding were designed, The first transformer is designed with tra

39、ditional Faraday shielding, shielding of the second transformer use predicted optimal length. When both transformers are tested in the same prototype without any filter, The CM noise of the second transformer is about 23dBuV lower in comparison with the first one. Fig.12 shows the test result.第一个变压器

40、采用传统法拉第屏蔽,第二个变压器采用计算出来的最佳屏蔽层长度。俩 变压器在同样没有滤波器的电源中使用, 第二个变压器的共模噪声比第一个低 23dB V 。 图 12展示了测试结果。图 12. 俩变压器的共模噪声 . Conclusions . 结论An accurate model of CM noise and two noveltechniques to cancel CM noise are introduced in the paper.Experiment results verified that:本文介绍了共模噪声模型和两种抵消共模噪声的技术。验证结果如下:1. The seco

41、ndary side noise source has contribution toCM noise, particularly when output voltage is high. Itsmechanism and effect on CM noise need to be consideredwhen modeling CM noise.1. 除输出电压高的情况外,次级噪声是共模噪声的主要来源。2. Different connection of shielding makes differentcontribution of the secondary side noise source to CMnoise. The proposed model of CM noise