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The secret story behind the birth of chip ferrite beads (part 2/2)

2013年02月28日　Category: Noise suppression filter Room

This article is a continuation of "The secret story behind the birth of chip ferrite beads (part 1/2)".

[Naming]
  Discussions of "What should we name the product?" and "What should we call it in Japanese?" arose in parallel with product development. We investigated Murata Manufacturing's product naming system and came up with a number of proposals. It had already been decided to use BL (beads), but the third letter and the characteristics notation became subjects of discussion. BL+M (Monolithic) was selected from among the various product name proposals, and the product name was decided as BLM41A01. (A number of years later, as the number of product types increased, the name was revised to BLM41AG700.)
  On the other hand, there were few good proposals for the Japanese name. After a few days, our General Manager came to work one morning and said, "How about 'chip solid inductor' as the Japanese name?" And with that it was decided. Actually, the Japanese name was also changed a number of years later to the "chip bead inductor" currently used in Europe and America...

[BLM series shipment support]
  After the product design examination was completed, our Manager took various materials introducing the new products and commenced PR activities to customers. Even though the products were introduced only via PR materials, about a week later we received a report that a certain customer wanted to use the products in its printers. Well, that's when things really started getting hectic.
  The only equipment we had on hand were an old laminator and external electrode coater from the Yokaichi Plant that we had received from the monolithic multilayer capacitor (GR series) line. We had no characteristic selector, taping machine, or other equipment whatsoever. Well, what were we to do? We checked through the sales representative how many units per week the customer would use, and from when, and began arranging with the Yokaichi Plant to prepare for production. We brought measuring instruments, embossed tape, jigs and tools to a Group company and asked them to handle the measurement and taping by manual operations. I'm fairly ceratin  it was around 7,000 units per week, but that was a small enough quantity that we were somehow able to handle it.
  However, at that point our General Manager gave the order to construct a system capable of producing three million units per month. This was a shocking number to me, and my only thoughts were "Can we really sell that fantastic number?" and "Is it okay to introduce mass-production facilities for that quantity?" We visited the Head Office Production Technology Division (Yasu) together with people from the Yokaichi Plant to discuss which characteristics should be measured and what measurement items were required. We also visited facility manufacturers after that, and then at long last the first BST (BLM measurement and taping machine) was introduced to the Yokaichi Plant.

[BLM complaint and change to external electrodes]
  We received a complaint less than a month after the start of BLM delivery. Our Manager said that there had been a problem at the customer, and told me to go with him to visit the customer the following day. So my first business trip to visit a customer turned out to be a complaint visit.
  We were ushered into a meeting room that appeared to be a corridor divided by partitions, and listened to the details of the complaint. The complaint was that the products had been mounted using flow soldering, which was new at the time, and that electrode leaching had occurred. This marked the start of a brief but decisive battle.
  First, we gained approval for tentative countermeasures to double-coat (form a thick film) the Ag/Pd, and received a grace period of approximately one month. We knew that for the actual countermeasures we would have to change to plated specifications. We visited Group companies continuously for days to ask them to investigate plating. At the time it was said that plating caused the electrode strength to drop, and we were told that we would need to develop a thick-film electrode that could support plating. However, we were dealing with a complaint, so we did not have the time for that. Instead, we collected and evaluated all of the more than 10 different types of thick-film electrodes used within Murata Manufacturing at the time, determined the thick-film electrode with the highest strength, and ultimately used that electrode to deal with the customer's complaint.
  As a result of this complaint, the external electrode specifications were changed to the current Ag thick film + Ni/Sn plating specifications just a few months after commercialization.

[Acknowledgements and thanks]
  I feel that we were able to develop and support mass production of BLM series products in a short time thanks to the cooperation of persons in charge of the previously developed monolithic multilayer capacitors (GR series) and chip coils (LQH series), Group companies that helped with mounting evaluation and plating, and the Plant for mass production.
  Even 27 years later, I am fortunate to have the opportunity to still work together with the people I met and struggled together with at that time. I am extremely grateful to have been able to participate in this enjoyable product development and for the extensive experience gained .
  Since then, the chip ferrite beads series has been developed to include many different product types including array products (8, 6 and 4 elements) and products that support high-speed signal frequencies. But this growth into a major product line is a story for the next time... (Look forward to it!)

Written by: H.T., EMI Division, Murata Manufacturing Co., Ltd.

The secret story behind the birth of chip ferrite beads (part 1/2)

2012年12月14日　Category: Noise suppression filter Room

["What is ferrite?"]
  It was May 1985. I was riding in the Raicho limited express train from Fukui bound for Kyoto on a business trip to the Head Office (Murata Manufacturing Co., Ltd.). That was my first business trip since joining the company a month earlier. On the train, the General Manager of our department explained to me the purpose of the business trip: "It seems that we will be able to make ferrite surface-mount components, so we are going to hear about them." I remember asking what is now quite an embarrassing question: "What is ferrite?" The General Manager replied, "Rusted iron," which I accepted with a simple, "I see."
That may have been my first encounter with ferrite.

[What is monolithic multilayer ferrite with a low Q used for?]
  Development of monolithic multilayer ferrite started by aiming for a Q value equivalent to that of winding types, however ferrite with high-Q characteristics could not be created easily,  so we were left searching for something that we could use. Since low-Q ferrite is advantageous as a noise filter, it was decided to investigate commercialization as noise filters.
Development and prototyp manufacturing were performed at the then Murata Manufacturing Head Office (currently the Nagaoka Plant), and product development, the was performed at Fukui Murata Manufacturing.

[Product development by trial and error]
  At the time, the EMI Group was an organization comprising less than 20 people that had just split off as an independent section from the MLCC Division. I had been assigned there as a new employee, and was engaged in product development. This was the first chip component product to be developed by the EMI Group, and at the time, Murata Manufacturing had only two types of surface-mount components: monolithic multilayer capacitors (GR series) and chip coils (LQH series).
  There were so many things that I didn't know: "What characteristics are required?" "What should the outer dimensions be?" "What about the internal structure?" "How should reliability be evaluated?" "How should surface mounting be evaluated?" "What needs to be investigated for surface mounting in the first place?"
The situation was such that I could not be concerned with how it looked to be asking even such elementary questions as "What is a noise filter?" I remember going around fearlessly asking questions of various departments and people. Looking back, I am keenly aware that it was "newbie" privilege.
  In terms of characteristics, we aimed for characteristics equivalent to those of ferrite beads with lead wires (BL01/02 series). We wanted a size that would be easy to use mounted in the serial interface lines of the target applications. As a result, it was decided to use 4516-size outer dimensions, which are long with a narrow width.
  In terms of design, monolithic multilayer ferrite at that time had a higher firing temperature than current products, and when 100% Ag internal electrodes were tested, the Ag dispersed or vaporized during firing, resulting in frequent open defects. Therefore, Ag/Pd prototypes were created in an attempt to suppress Ag dispersion and vaporization. We wanted to go with 100% Pd to stabilize conduction, but with the addition of some structural countermeasures we were able to achieve prospects for mass production of Ag/Pd products to reduce costs. There were some concerns about the reliability of Ni/Sn plating for the external electrodes, so the commercialized products used a thick Ag/Pd film. However, this later led to a complaint.
  The evaluation samples were finished around this time, but the issue of "what to do about the characteristics guarantee?" remained. At the time, only the outer dimensions of ferrite beads with lead wires were guaranteed, so this was of no use as a reference. The developed products were noise filters, so it was quickly decided that impedance should be guaranteed, but the question was at what frequency? There were no official standards, and also no preceding manufacturers, so there was nothing we could use as a reference. Ultimately, we decided on our own to guarantee the impedance (Z) and DC resistance (RDC) at 100 MHz, which is within the 30 to 300 MHz band for which noise was an issue at the time, and commenced various reliability evaluations.
  Design of the embossed tape also finished around the end of commercialization, and a mounter was borrowed from Komatsu Murata Manufacturing to perform the mounting evaluation. However, it was found that the mechanical centering method* used as the positioning method at the time had problems with the narrow 4516-size dimensions. This issue was cleared with the cooperation of various parties, and the products were provided to customers together with the mounting method.
  In this manner, product development also finished, in this case merely meaning the end of prototype manufacturing on a prototype line at the Head Office, with the cooperation of many different people, and the first BLM41/31 series products underwent in-house design examination in autumn of 1986.

* Mechanical centering method: A positioning method that mechanically restricts the component center based on the package outline.


~ To be continued (2013 February 28) ~


Written by: H.T., EMI Division, Murata Manufacturing Co., Ltd.

What is inrush current?

2012年10月29日　Category: Noise suppression filter Room

  When electrical equipment is first turned on,a large current flows that exceeds the steady-state current value. This current is called an inrush current.
  Why does this inrush current occur? There are many factors that cause it, and the following are some examples:
  - In equipment with large-capacity smooth capacitors or decoupling capacitors,
    when the power is first turned on, a large current flows through to charge those
    capacitors - a necessity when first powering up the equipment.
  - Immediately after the power is turned on, the filament and other parts have low
    resistance, and a large current flows.
    (As they begin to generate heat and warm up, their resistance increases and the
     current drops to the steady-state current.)

  To provide a more easy-to-understand image of inrush current, Figure 1 shows the current waveform when the power is turned on. When the power is turned on, current begins to flow, and the initial current flow reaches the peak current value that is larger than the steady-state current value. Following this, the current value gradually decreases until it stabilizes at the steady-state current. The part during which a large current flows before reaching the steady-state current is the inrush current. If the size of the inrush current exceeds that allowed by the part in use, depending on the magnitude of the inrush current (difference between the peak current value and the steady-state current value) and length of its duration (the length of time until the peak current value converges with the steady-state current value, hereafter called the pulse width), the part used in the circuit may overheat, potentially causing the electrical device to malfunction or break down.


  Next, I will give an example of common malfunctions in parts used for noise suppression in a power supply line. I will describe the case for chip ferrite beads (Murata's BLM series), which are relatively lower cost and user-friendly.
  If the inrush current exceeds the rated current value in chip ferrite beads that are used for noise suppression, they will overheat. In the worst-case scenario, an open circuit fault will occur. Figure 2 shows different waveforms obtained when changing the conditions for peak current value and pulse width of the inrush current waveform. Waveform (1) has a large peak current, waveform (2) has a large pulse width, and waveform (3) shows a case where malfunction does not occur.
In waveform (1), an excessive current flows instantaneously through the chip ferrite bead and the internal electrode melts, causing an open circuit fault. When the electrode melts, the chip itself may crack and break. In waveform (2), the chip ferrite bead continues to heat up, eventually causing the internal electrode to melt, similar to the case with waveform (1). The chip ferrite bead then becomes a source of heat and may cause the circuit board it is mounted on to burn out.

Basics of Noise Countermeasures [Lesson 9] Microwave Absorber Sheets

2012年09月14日　Category: Noise suppression filter Room

In this lesson we will discuss microwave absorber sheets. Various types of microwave absorber sheets have been developed by different companies, and this article introduces microwave absorber sheets supplied by Murata Manufacturing.

===SplitPage===
<Methods of using microwave absorber sheets>
Like ferrite cores, microwave absorber sheets of the type that absorb by magnetic loss are used as EMI countermeasures. These types are generally shaped like sheets, so while they are occasionally wrapped around cables, they are more often attached to boards or inside set cabinets (Figure 4). Users can prepare their own adhesive materials when attaching these sheets, but these sheets are generally purchased with seal-type adhesive pre-applied to the rear of the sheet. In addition, some sheets use materials with high thermal conductivity, so effects as heat radiating materials can also be expected.

Basics of noise countermeasures [Lesson 8] Ferrite Cores

2012年07月13日　Category: Noise suppression filter Room

<Noise countermeasure parts that can be used without changing the board>
Previously, we introduced noise countermeasure parts mounted on a board as part of the electronic circuit. This time we introduce noise countermeasure parts that do not require mounting on a board. (Although they are sometimes fixed to the board...)
As previously introduced, when commercializing an electronic device, it is necessary to check that the noise emitted from the device satisfies the EMI regulations. However, the final check cannot be performed until the device design is complete. Recently, experience concerning designs that do not emit noise is increasing, and various measures are being implemented beforehand to prevent the generation of noise, but of course the effects cannot be known until the final test. There is no problem as long as the noise level is within the regulation value as expected at this point, but the test results sometimes exceed the regulation value. When the delivery date is near, there is no time to change the board, so parts such as ferrite cores that enable countermeasures without changing the board come in handy.

<Ferrite cores are lumps of ferrite>
Ferrite cores are ceramic magnetic bodies consisting of ferrites (soft ferrites) processed into various shapes. Formerly, coils were often made by winding conducting wires around a ring-shaped ferrite core, so ferrite used for noise countermeasures is likewise called a ferrite core.
Ferrites include Mn-Zn ferrite and Ni-Zn ferrite according to the composition. Mn-Zn ferrite is conductive, so it requires insulation work, and Ni-Zn ferrite has better high-frequency characteristics. For these and other reasons, Ni-Zn ferrite is often used for noise countermeasures.

<Principle by which ferrite cores remove noise>
Ferrite cores come in various shapes, but most are ring-shaped. By passing conducting wires through the hole of the ring, the conducting wires and the ferrite core form a coil (inductor). This coil (inductor) is based on the same principle as that of an electronic part inductor, so the impedance increases together with the frequency as shown in Figure 1. Therefore, the coil functions as a low-pass filter that blocks high-frequency current, enabling attenuation of high-frequency noise. Furthermore, the use of a ferrite core also provides an additional effect. When current flows to an inductor comprising a ferrite core, magnetic flux is generated in the ferrite core, and the current energy is converted into magnetic energy. However, when the current changes, this magnetic flux is converted back into current by electromagnetic induction. At this time, not all of the magnetic flux energy is returned to current energy, and some is lost as magnetic loss. (This is called "hysteresis loss.") As a result, part of the noise current passing through the conducting wires is lost as magnetic loss, reducing the energy. The right side of Figure 1 shows the impedance characteristics of a coil with conducting wires passed through a ferrite core. The impedance of a normal coil consists mostly of the reactance (X) component, but when a ferrite core is used, the resistance (R) component becomes extremely large. This is a result of selecting ferrite materials suitable for noise countermeasures, and causes the noise energy consumption effect due to magnetic loss to account for a larger portion of the noise removal effect of the ferrite core than the current limiting effect of high impedance.


===SplitPage===
<Ferrite core specifications and performance>
The noise removal performance of ferrite cores varies according to the ferrite materials and shape.The magnetic permeability changes according to the ferrite materials, so the impedance also differs. In addition, the ratio between the resistance component and reactance component of the impedance also differs according to the materials. However, the materials of ferrite cores sold as noise countermeasures are blended specially for noise countermeasures, so there is no great difference in the characteristics regardless of the material selected.



The inductance increases (proportionally to the square of the number of windings) together with the number of times the conducting wire is passed through the ring core (number of windings). However, when the conducting wire is wrapped around the core, the winding start (entrance) and winding end (exit) come close to each other and have floating capacitance between them. High-frequency noise is conveyed via this floating capacitance area, which is a factor lowering the high-frequency performance. Therefore, in consideration of the target frequencies for noise reduction, the number of windings must either be increased to target the low-frequency range, or reduced to target the high-frequency range.




In addition, the ferrite core dimensions affect the performance as shown in Figure 4. Therefore, a ring core with the smallest inner diameter and widest cross-sectional area possible should be selected.


===SplitPage===
<Using a ferrite core as a common mode choke coil>
Due to their convenience, ferrite cores are often used on cables. However, these cables include interface cables, power cables, and other multiple conducting wires that run in parallel, so common mode noise is often an issue. Common mode choke coils are an effective noise countermeasure in these cases, and common mode choke coil functions can be achieved by passing the cables together through a single ferrite core. For example, in case of an interface cable that has multiple signal lines wired to a single end, it is difficult to wire common mode choke coils. However, common mode choke coil functions can be easily achieved by passing all of the interface cable wires together through a ferrite core.

Case Study of AVN Car Navigation System Noise Countermeasure with common mode choke coil "PLT10HH"series

2012年05月28日　Category: Noise suppression filter Room

Previously, we introduced a case study of an onboard motor noise countermeasure using the PLT10HH. This time we follow up with a case study of an example using the PLT10HH in the power supply block of an AVN car navigation system.
The earlier article introducing the case study of an onboard motor noise countermeasure can be found at the following URL:
http://www.murata.com/products/emicon_fun/2012/02/emc_en14.html

An earlier article introducing the PLT10HH can be found at the following URL:
http://www.murata.com/products/emicon_fun/2011/12/emc_en14.html

AVN car navigation system noise countermeasure
Car navigation systems have diverged into enhanced function AVN (Audio Video Navigation) integrated types and low-cost PND (Portable Navigation Device) types. However, AVN types are not limited to just AVN functions, and are increasingly incorporating diverse functions such as external communication for the purpose of gathering information, and confirmation of safety around the vehicle using multiple cameras. Recently, models have appeared that display various information superposed onto the real-time image on the navigation system screen, like in a science fiction movie. With this enhanced functionality, the speeds and functions of the internal processing elements are also increasing, with the result that much noise is generated. On the other hand, automobiles also use digital devices other than navigation systems, and there is the issue that noise generated by these devices may flow into the navigation system. AVN navigation systems receive radio waves over a wide frequency range from AM radio frequencies to GPS and communications frequencies, so the inflow of noise must also be prevented over a wide frequency range. Most AVN navigation systems use a metal cabinet, so little noise exits or enters the set directly. However, the power cable can easily become an issue as an inflow and outflow path for noise.


The navigation system power line is of course connected with the power lines of various electronic devices in the automobile, so noise from various ECU (Electronic Control Unit) and motors enters via the power line, and noise flowing out from the navigation system also affects these devices. This makes noise countermeasures for the navigation system power line quite important.

===SplitPage===
Case study of car navigation system noise countermeasure with PLT10HH
As a result of enhanced navigation system functionality, the current flowing through the power line has increased as high as approximately 10 A, so parts requiring noise countermeasures must support large currents.
The PLT10HH series of SMD-type common mode choke coils support large currents of up to 10 A, making them suitable for suppressing the radiation and inflow of common mode noise, which easily becomes an issue for the power cable. This case study measured the externally radiated noise from the set, and investigated the noise countermeasure effects of using the PLH10HH1026R0 (rated current: 6 A) in the power supply connector block.
PLT10HH product information can be found at the following URL:
http://search.murata.co.jp/Ceramy/image/img/PDF/ENG/L0119PLT10H.pdf


The results confirmed noise countermeasure effects over a wide frequency band as shown in Fig. 3. This case study evaluated a countermeasure for externally radiated noise from the car navigation system, but common mode choke coils are similarly effective against external noise entering via cables. This makes them an effective means of protecting navigation systems from noise.


Other countermeasures include the use of ring cores for cables, but in this case the number of windings cannot be increased, so sufficient effects often cannot be achieved for low-frequency noise. In addition, when the cable is wrapped around the ring two or more times, line capacitance occurs between adjacent windings, and high-frequency noise can bypass the ring core via this capacitance, with the result that the high-frequency response worsens. The PLT10HH uses a winding method that also takes into account the high-frequency response, enabling effective countering of noise over a wide range from low to high frequencies.


As described above, the PLT10HH is an ideal common mode choke coil for various noise countermeasures in automobiles, such as for onboard motors and car navigation systems. Furthermore, in addition to automotive applications, it is also suitable as a noise countermeasure part for factory automation equipment, amusement machines, and other applications that use large currents and require noise reduction over a wide frequency range.


Written by: Y.M., Murata Manufacturing Co., Ltd.

Featured products! Case Study of Onboard Motor Noise Countermeasure with PLT10HH

2012年02月14日　Category: Noise suppression filter Room

Previously, we introduced the common mode choke coil for strong electric currents. This time we follow up with a case study of an onboard motor noise countermeasure with the PLT10HH.
*An earlier article introducing the PLT10HH can be found at the following URL:
 http://www.murata.com/products/emicon_fun/2011/12/emc_en14.html


Introduction
To make automobiles more energy-efficient and lightweight, an increasing number of their components have become electrically driven.. For this reason, electric motors are used not only for traditional devices like windshield wipers, but also for power steering, fuel pumps and so on. Some DC motors use brushes to rectify the current, but because such brushes are repeatedly contacting and separating from commutators, sharp fluctuations in electric current cause noise and spark noise (Fig. 2). If such noise flows through the power line to other parts, it could have an adverse effect. While it also depends on motor speed, this noise can range widely in frequency from hundreds of kHz to hundreds of MHz, making it necessary to take adequate noise countermeasures.



Figure 2 shows a model of how noise is generated in a brush motor and illustrates key points of a basic method to counter the noise. Basically, there are two kinds of noise: differential mode noise, which is conducted similarly to a power current, and common mode noise that is conducted parallel to two power lines. Effective noise countermeasures must therefore be taken for each type. The use of a capacitor is a relatively easy and inexpensive countermeasure. A countermeasure for differential mode noise uses an across-the-line capacitor (X capacitor) that connects across two power lines. A countermeasure for common mode noise uses a line-bypass capacitor (Y capacitor) that creates a connection between both lines and a chassis ground. If the capacitor alone is not sufficient to suppress noise, the addition of a common mode choke coil can effectively suppress common mode noise that is otherwise difficult to overcome, and it is possible to take measures against even often problematic high-frequency noise.

<Cause of noise in brush motor and countermeasures>
Spark noise occurring between the brush and commutator is the cause. Noise includes both normal mode and common mode noise and occurs over a wide frequency range.

<Key points to noise countermeasures>
(1) Taking measures inside the motor (close to the noise source) is effective.
(2) Using an X capacitor and Y capacitor together is effective against both normal mode
and common mode noise



Example of motor noise countermeasure experiment
Figure 3 shows the  results from an actual experiment with a measure against power-window motor noise.
The results show that a noise countermeasure using only capacitors (across-the-line and line-bypass capacitors) is adequately effective against frequencies lower than 10 MHz, but the effect is not sufficient against frequencies greater than 30 MHz and this measure would not pass CISPR Part 25 Class 4. When a PLT10HH common mode choke coil is added to capacitors, noises of 30 MHz and above are suppressed and CISPR Part 25 Class 4 is successfully cleared. With power line noise, the differential mode is dominant at low frequencies and the common mode at high frequencies, so a common mode choke coil that suppresses common mode noise is effective and high-frequency noises can be suppressed. Additionally, because there are strong electric currents in power lines, when a strong current is passing through a regular inductor, inductance declines because of magnetic saturation and noise is less effectively suppressed. However, with its internal structure, the common mode choke coil makes magnetic saturation unlikely to occur so the noise suppression measure tends to maintain its effectiveness. In this sense also, it is efficient to use a common mode choke coil for power lines.



The PLT10HH common mode choke coil used here supports rated currents of 6 A to a maximum of 10 A and is designed to support various motor applications in which there is a large electric current during startup. It also has a highly reliable design so it can be used in severe environments such as in automobiles. Of course it can also be used for regular electronic devices, making it suitable for countering noise in a wide range of motor power lines.



Person in charge: Murata Manufacturing Co., Ltd.  Yasuhiro Mitsuya

Pick Up Products! Common Mode Choke Coils for Automotive

2011年12月14日　Category: Noise suppression filter Room

Pick-up products!　
Large-current PLT10H series SMD common mode choke coil for automotive

Up until now, we have been giving basic information about parts in the EMICON-Fun! articles. In this new series of 'Pick-up products!' articles, we will be introducing highlighted products not offered by other companies. The first part in this series is about large-current PLT10H series SMD common mode choke coil for automobiles. This article gives an overview of the product, and succeeding articles will discuss the effects of actually using the product.

The PLT10H is a revised version of the lead-type PLT09H common mode choke coil that has been a firm favorite in the past, and that was made into an SMD-type and exhibits drastic expansion of rated current and enhanced reliability. Rated current was greatly improved from the previous 3A to a maximum of 10A,* enabling its potential use in a wider range of applications. The previous type was only available with one inductance variation, but the number of variations for this type has been increased to four, ranging from 6μH to 20μH. Moreover, the coil's coupling coefficient can be controlled to give it the most appropriate differential mode inductance, making it effective against not only common mode noise, but also differential mode noise. Operating temperature ranges from -55ºC to +125ºC, making these coils perfect for vehicle accessory motor power supplies.

*Derating of rated current may be required depending on operating temperature.
For details on use of the product, please see the product specification for approval.

◆Dimensions (When you click it, the image is enlarged.)







     

Basics of Noise Countermeasures [Lesson 6] Common mode choke coils

2011年10月28日　Category: Noise suppression filter Room

Following our previous discussion of chip-type three-terminal capacitors, in this lesson we will discuss common mode choke coils.


<Common mode choke coils separate noise from signals using conduction modes>
  In our previous discussions of chip ferrite beads and chip-type three-terminal capacitors, we explained how they utilize differences in frequency, as noise frequencies are relatively higher than signal frequencies. As such, they function as low-pass filters that selectively suppress only noise. Common mode choke coils are also a type of noise filter, but rather than using differences in frequency, they separate noise from signals by differences in conduction mode. We must therefore first learn the distinction between common modes and differential modes.


<Common modes and differential modes>
  Normally, in the electrical circuit of a circuit board, the current flowing out from a certain part reaches another circuit through the load, and returns to the origin via a different route on the circuit board. (In many cases, the return route is the ground plane of the circuit board.) This type of flow is called differential mode (or normal mode).



  Another conduction route also exists, though not as a clear-cut wire. A tiny amount of stray capacitance is generated between the wires on the circuit board and the reference ground surface, creating a conduction route where the capacitance flows commonly through all wires on the circuit board and returns in the opposite direction along the reference ground surface. This route is called common mode.





  Although the stray capacitance between the wires and the reference ground surface is quite small, impedance drops as the signal frequency rises even with the tiny amount of stray capacitance, so that the common mode current flows more easily. Normally, the common mode current is not actively sent through the electrical circuit, but if the ground of a power supply circuit or driver IC vibrates, the entire circuit it drives will vibrate, resulting in common mode noise. If a cable is externally connected to the circuit, common mode current will also flow through the cable. As it will have an electrical potential with respect to the ground, the current will be released as noise radio waves.


<Common mode choke coils are noise filters that act only on common mode currents>
  Common mode choke coils are noise filters that discriminate between signals and noise from the above mentioned common modes and differential modes, or conduction modes. Simply put, they are filters that act only on common modes.
Figure 3 shows a principle diagram for common mode choke coils.



  Common mode choke coils are made up of two conducting wires wrapped around a single core (a ferrite core when used in high-frequency applications). They therefore have four terminals. The wires are wrapped around the core in opposite directions. When common mode currents flow through coils with this type of structure, flux is generated by the electromagnetic induction phenomenon that occurs in each coil. However, as the direction of the generated flux is the same, both fluxes become stronger to increase their action as inductors. Conversely, differential mode currents flowing through the coil generate flux in opposing directions that cancel each other out. As a result, it no longer acts as an inductor against the differential mode current. Common mode choke coils are therefore filters that only act as inductors for common modes, and not against differential modes.


===SplitPage===
<Advantages of common mode choke coils>
  Common mode choke coils have two advantages.
(1) Even when the frequencies of signals and noise overlap, their different conduction modes enable suppression of only noise.
(2) Performance does not decrease even with a large differential mode current, as the core does not become saturated.

  The most important feature of common mode choke coils is their ability to distinguish between noise and signals even of the same frequency. Recently, more and more electronic devices are starting to use high-speed differential transmission as their method for transmitting signals. USB, SATA, and HDMI are typical examples of high-speed differential transmission. In high-speed differential transmission lines, extremely high frequency signals are transmitted, and filters like ferrite beads that separate noise from signals by differences in frequency cannot distinguish between the two. Emphasizing the impact of the signal means that it cannot suppress noise very well, but focusing on noise suppression attenuates part of the signal, therefore influencing signal integrity. Common mode choke coils, on the other hand, separate signals from noise using transmission modes, so that high-speed signals that flow through the coils are not affected if they are differential modes. In high-speed differential transmission lines, signals are generally only differential modes. The problematic noise is mostly common mode noise, so common mode choke coils can be used to effectively suppress common mode noise without affecting high-speed signals.


Figure 4. A comparison of noise suppression by different high-speed differential transmission lines



  Cables are connected to business power lines and secondary AC adapters that receive inputs of power. These cables become antennae, and the noise that is released becomes a problem. When using inductor-type filters that act on differential modes, such as ferrite beads or normal mode choke coils, the core becomes magnetically saturated by large currents that flow through, causing a dramatic reduction in their performance as inductors. Common mode choke coils are very useful in these types of applications. In common mode choke coils, no magnetic saturation occurs, as flux generated by differential mode currents cancels each other out and disappears. Common mode choke coils are therefore used to suppress noise in power lines through which large currents flow.


<Examples of common mode choke coils >
  Figure 5 shows some examples of common mode choke coils.



  AC power lines are subject to high voltage, so careful attention is given to safety when constructing coils for these lines. Conversely, as high-speed signal lines require smaller-sized coils, chip-type coils are used in these applications. Other coils available on the market are a winding wire-type that winds a wire around a ferrite core and a film-type that uses film coils. The winding wire-type features high performance, while the film-type features smaller size. Figure 6 shows an example of the structure of a winding wire chip-type common mode choke coil. Two lines are wound around the coil together, so that the outgoing line and the returning line lie next to each other, and the magnetic coupling between the lines increases to raise selectivity between common modes and differential modes.

Figure 6. An example of the structure of a winding wire chip-type common mode choke coil (bottom view)


===SplitPage===
<Precautions for using common mode choke coils>
  The discussion up to now has stated that common mode choke coils do not affect differential modes, but this is only true for ideal common mode choke coils. In reality, some of the flux produced by the opposing coils leaks is not cancelled out, resulting in a small amount of inductance. This differential mode inductance is very small, but its influence must be considered in applications that use extremely high-frequency signals. Figure 7 shows an example of an actual impedance curve for a chip-type common mode choke coil. We know that the differential mode impedance becomes high at around 1 GHz. Chip-type common mode choke coils have recently been developed that suppress differential mode impedance even more. Appropriate chip-type common mode choke coils should be chosen for Display Ports, USB3.0, and other devices that use extremely high-frequency signals.
Have a look at the guide we have made available for selecting chip-type common mode choke coils for use in high-speed differential lines.

Selection Guide for Common Mode Choke Coils for High-Speed Differential Transmission Lines
http://www.murata.com/products/emc/selection_guide/emc2/highspeed/index.html


Figure 7. Example of impedance properties of common mode choke coils

Basics of Noise Countermeasures [Lesson 5] Chip 3 terminal capacitors

2011年09月28日　Category: Noise suppression filter Room

Following our previous discussion of chip ferrite beads, in this lesson we will talk about chip 3 terminal capacitors.

<Lead-type ceramic capacitors>
  Before discussing chip 3 terminal capacitors, an explanation of lead-type 3 terminal capacitors will make the concepts easier to understand.
  Figure 1 shows the structure of a general lead-type ceramic capacitor (2 terminal).


  In lead-type ceramic capacitors, electrodes on both sides of a single panel dielectric are coated and lead terminals are attached. In this structure, the lead terminal parts have minimal inductance (residual inductance), so when this capacitor is used as a bypass capacitor, there is inductance between it and the ground terminal.



  Figure 2 shows an example of insertion loss characteristics when the capacitor is used as a bypass capacitor. As this graph shows insertion loss, noise level decreases towards the bottom of the graph. Impedance normally increases proportionately to increasing frequency in capacitors, so, even in high-frequency regions, insertion loss should continue to increase along the dashed line in the figure. However, in reality, capacitors have the aforementioned residual inductance, and this minimal inductance interferes, causing a decrease in performance at high frequencies represented by the V-shaped insertion loss curve of the solid line.

<3 terminal capacitors are made by projecting two ends of the lead on one side>
  3 terminal capacitors are ceramic capacitors in which the shape of the lead terminals is altered to improve the high-frequency characteristics of 2 terminal capacitors. As shown in Figure 3, one lead in a 3 terminal capacitor has two projections. With this configuration, the projections of the 2 terminal lead are connected to an input and an output of power sources or signal lines, respectively, and the other lead is connected to the ground terminal to create the connections shown in the equivalent circuit schematic on the right. By connecting it this way, the 2 terminal lead inductance does not enter the ground side, thereby making the ground impedance extremely small. Also, as the inductance of the 2 terminal lead works similar to a T-type filter inductor, it works in the direction of reducing noise.



<Monolithic ceramic chip-type capacitors and chip 3 terminal capacitors>
  Currently the most commonly used capacitors are chip-type monolithic ceramic capacitors. Figure 4 shows the structural concept of 2 terminal chip-type monolithic capacitors. A dielectric sheet is placed between the plates and the internal electrodes are connected to alternate projecting ends of the electrodes in a monolithic or layered pattern. Because it is in the shape of a chip, it has no leads, and there is no longer any residual inductance. However, a minimal amount of inductance remains inside, so that performance drops at higher frequencies.



  Similar to a lead-type 3 terminal capacitor, the electrode structure is altered in chip 3 terminal capacitors to improve performance at high frequencies. Figure 5 shows the structural concept of a 3 terminal chip-type capacitor. A ground terminal is attached to each side of the chip, the dielectric is placed between the plates, and feed through electrodes and ground electrodes are alternately stacked up to create a feed through capacitor-like structure. As you can see in the equivalent circuit schematic, the inductance of the feed through electrodes works like a T-type filter inductor, similar to the conditions in the 3 terminal lead capacitor, so that residual inductance has less influence. The distance to the ground side is shorter, resulting in minimal inductance in this part. Moreover, as the ground side is connected to both ends, they become connected in series, and inductance becomes apparently cut in half.



  Figure 6 compares the insertion loss characteristics of chip 3 terminal capacitors and 2 terminal chip-type monolithic capacitors. The capacitance is the same in each type, so similar characteristics are seen in low-frequency regions. However, the performance of the 2 terminal capacitor begins to drop as it exceeds 10 MHz, while the 3 terminal capacitor maintains its performance until the vicinity of 100 MHz. As the performance in the 3 terminal chip-type capacitor does not decrease until it reaches the high-frequency region, it is useful for applications that require noise suppression until it hits a high frequency.


<Chip 3 terminal capacitors actually have 4 terminals>
  As shown in Figure 5, even though we say that chip 3 terminal capacitors have 3 terminals, they actually have four. 4 terminal types can even further reduce the impedance on the ground side, but even when made into chips, they are still called '3 terminal' capacitors because electrically all terminals have the same potential and because the original lead-type 3 terminal capacitors had 3 terminals.

<3 terminal chip-type capacitor mounting method>
  As chip 3 terminal capacitors have feed through terminals and ground terminals, the mounting method differs from that of a regular 2 terminal capacitor. Figure 7 shows the mounting method.



  When mounting a 3 terminal chip-type capacitor as a bypass capacitor, we cut the signal or power pattern and connect a feed through electrode in between, and prepare and connect a ground pattern at the ground terminal. The ground pattern must be connected with the shortest possible connection to a stable ground plane to maintain minimal impedance. When using a double-sided board or multilayer board, it is preferable to connect it to the ground plane via a through hole.

Basics of Noise Countermeasures [Lesson 4] Chip ferrite beads

2011年06月14日　Category: Noise suppression filter Room

　　In this and subsequent parts, we will be discussing typical noise-suppression components. The first such component is the chip ferrite bead. These beads are ferrite bead inductors fabricated in the shape of chips which support surface mounting (SMD).

<A ferrite bead is a bead of ferrite with a lead passed through it.>
　　An example of the shape of a lead-type ferrite bead inductor is shown in Fig.1.　The structure is a simple one, and with this shape a lead has been passed through the bead which is formed by the ferrite. This inductor does not have the lead wound around it as a regular coil does, but when current flows to its lead, magnetic flux is generated inside the ferrite bead. As a result, the ferrite bead functions as an inductor. Incidentally, the ferrite used here is made with materials which have a high loss at high frequencies, so in the high-frequency range the energy of the current is lost as a loss in the ferrite, enabling noise to be absorbed effectively.

<Chip ferrite beads consist of a layered inductor structure.>
　　Chip ferrite beads are made by fabricating these ferrite bead inductors into chips, and Fig. 2 shows their typical structure. Coil patterns are formed between the layers of the sheets of raw ferrite, and by a process of integration and firing, a 3-dimensional coil structure is produced.

　　By fabricating the ferrite bead inductors as chips and by adopting a coil structure inside them, it is possible to achieve a higher impedance than that of the lead type of ferrite bead inductor that simply has the lead through it. (In reality, some chip ferrite beads have only a lead passing through the bead.) This structure is basically the same as that of a monolithic type of chip inductor, but the difference from an inductor lies in the fact that the ferrite material used is better suited to suppress noise.
　　Fig. 3 shows an example of the impedance frequency characteristics of a chip ferrite bead. The basic principle involved is as follows: The impedance increases in proportion as the frequency rises, as in the case of inductors, so by connecting these beads in series in a circuit, they function as a low-pass filter. With regular inductors, the main characteristic among the impedance (Z) values is the reactance component (X). On the other hand, since chip ferrite beads use ferrite materials with a high loss in the high frequencies, the main characteristic in the high-frequency range is the resistance component (R). The reactance component is not accompanied by loss, but the resistance component is. This means that, compared with regular inductors, chip ferrite beads have better properties for absorbing noise energy, providing a higher noise-suppression effect.


<The impedance curve can be selected to suit the intended application.>
　　Chip ferrite beads have been customarily standardized by impedance value at a frequency of 100 MHz. However, many kinds of products with the same impedance value are available. This is for the purpose of making it possible to select the sharpness of the impedance curve.
　　Fig. 4 shows an example of curve variations. Both the BLM18AG601SN1 and BLM18BD601SN1 are chip ferrite beads that have an impedance value of 600Ω at 100 MHz, but Fig. 4 shows that the BLM18BD601SN1 has a sharper impedance curve, whereas the BLM18AG601SN1 has a curve that rises more gently.

　　With the type whose impedance curve rises gently, the impedance begins to increase at a lower frequency level so noise can be suppressed across a wide frequency band from the very low frequencies to the high frequencies. However, if the signal frequency is relatively high, this frequency may also be attenuated. In contrast, with the type whose impedance curve rises sharply, the impedance increases only in the high-frequency range, so even if signals with a comparatively high frequency are used, noise can be suppressed without affecting the signals. For this reason, it is important to factor in the signal frequency and the frequency of the noise to be suppressed when selecting the chip ferrite beads.

<Impedance at high frequencies is improved by altering the internal structure.>
　　Fig. 3 showed the impedance frequency characteristics of chip ferrite beads, and this figure shows that the 400-500 MHz frequency area forms a boundary where the impedance value starts to drop off. This is due to the effects of the chip ferrite bead structure. As a basic rule, the impedance of an inductor continues to increase as the frequency rises. However, regular chip ferrite beads have areas inside them where the winding start (entrance) is close to the winding end (exit), as shown in Fig. 5. In such an area, electrostatic coupling (a state in which extremely small capacitors appear to be present) occurs so the high-frequency current passes through it, and the impedance of the inductor has less of an effect. In areas of electrostatic coupling, current tends to pass through more easily as the frequency increases, so the higher the frequency, the more the apparent impedance decreases.

　　In order to resolve this problem, the structure where the winding starts and winding ends are positioned close together must be altered. Fig. 6 shows an example of a chip ferrite bead whose internal structure has been changed in order to improve the high-frequency characteristics. Whereas, in regular chip ferrite beads, the axis of the coil pattern runs perpendicularly (so-called "vertical winding"), the axis of the coil pattern for chip ferrite beads with improved high-frequency characteristics runs horizontally. As a result, by placing some distance between the coil winding start and winding end, the frequency at which the impedance starts to drop is significantly increased.

　　Chip ferrite beads come in many other variations--some support high currents and others have compact sizes, for example--and this variety enables users to select the ones optimally suited to the applications at hand.



Person in charge: Murata Manufacturing Co., Ltd.  Yasuhiro Mitsuya

Basics of Noise Countermeasures [Lesson 2] How noise may be countered

2010年12月28日　Category: Noise suppression filter Room

Basics of Noise Countermeasures

<How is noise generated?>

In the previous lesson, the problem of electromagnetic noise in electronic products and the need for noise countermeasures were discussed. The first question that springs to mind is why not obviate the need for noise countermeasures by designing the products so that no noise will be generated in the first place? The countermeasures would not be needed if the products were designed in this way, but the reality is such that it is not so easy to implement this kind of design. As a matter of fact, some of the signals in digital circuits become noise themselves.

 

Square wave signals (with waves shaped like squares which go back and forth between two values every fixed period of time) are usually used in digital circuits. However, these square waves have a great number of frequency components.

 

Example of a square wave

The following figure shows an example where the harmonics (waves with frequencies which are integral multiples of the fundamental waves) of sine waves are superimposed on one another. It is clear that the greater the number of harmonics, the closer they approach square waves. Square waves correspond to what is produced by the repeated superimposing of these harmonics.

In other words, it means that a great many frequency components are contained in the square waves used by digital circuits and that many components with high frequencies are contained.

When AC current flows through a conductor, it is radiated as radio waves to a greater or lesser extent, and the higher the frequency of the current, the more easily it is radiated as radio waves. It is for this reason that high-frequency radiation is regulated by noise controls. It is clear from the above that some of the square wave signals flowing through digital circuits are controlled as noise. In other words, insofar as digital circuits operate, noise waves will be emitted. However, a glance at the previous figure shows that these waves come to resemble square waves to some extent even if higher-order harmonics are not included. Therefore, if only the signals with the high frequencies among the digital signals could be removed, it would be possible to remove the harmonic components which pose problems as noise while preserving the waveforms that are close to square waves. This approach is used by EMI suppression filters functioning as low-pass filters. It is difficult to regulate the job of "removing the harmonic components," and if the frequencies which are too low are removed as well, the signal waveforms lose their shape, causing the operation timing of the digital devices to be thrown into disarray or errors in operation to occur. Conversely, if the frequencies are not removed sufficiently, the noise countermeasures will no longer be adequate. As a result, parts with many and varied noise countermeasures have been made available, and it has become important to know which parts to use.

 

<Other sources of noise>

In actuality, the harmonic components of the square waves of digital circuits do not constitute the only source of noise. Also present are the switching noise of switching power supplies, the brush noise of motors and various other kinds of noise caused by different factors. Nevertheless, noise is usually dealt with using low-pass filters just like with the noise countermeasures adopted for digital circuits simply because noise is controlled in the harmonic regions where signals tend to be emitted as radio waves.

The next lesson will examine the principles behind the noise countermeasure parts that function as low-pass filters.

person in charge: Murata Manufacturing Co., Ltd.  Yasuhiro Mitsuya

Basics of Noise Countermeasures [Lesson 1] What is an EMI filter?

2010年10月28日　Category: Noise suppression filter Room

Fundamentals of Noise Countermeasures

<About this column>

This column aims to provide a basic explanation about noise countermeasures, from "What is EMI?" to the functions and uses of various noise countermeasure parts. This first column starts with the question, "What is an EMI filter?"

<Introduction>

"EMI" is an acronym which means Electro Magnetic Interference. Thus, an EMI filter refers to a filter used to eliminate electromagnetic interference. However, this may be a little confusing, so let's start by explaining the background of how EMI filters were created.

We are surrounded by electronic devices, and these electronic devices use digital circuits. High-frequency currents flow through digital circuits, so when these currents flow through substrate patterns, cables and other wiring, these paths become antennae and emit radio waves. When other electronic devices are nearby, these radio waves may enter the other electronic devices and produce adverse effects. For example, when a computer is placed near a radio receiver, noise may enter the radio sound. This is an example of noise generated by the digital circuits of the computer becoming radio waves which then enter the radio receiver antenna and thus, becoming noise. In addition, when strong radio waves enter a digital circuit, the digital signal waveform may change and the digital circuit may experience operation errors. The problem of noise is not only confined to radio waves conveyed through space, as noise can also occur between devices connected by power cables or other means.

Under what conditions does noise interference occur?

<How can noise issues be resolved?>

There are two different methods of resolving these noise issues. One is to prevent devices that typically produce noise from actually producing noise in the first place.. This is called emission countermeasures. The other is to implement countermeasures for the receiving-side device so that noise and operation errors do not occur even when the device receives noise. This is called immunity countermeasures. Immunity here means resistance, and immunity countermeasures consist of providing resistance to noise, like immunity to a disease, so that even when noise enters it does not cause problems. Emission countermeasures and immunity countermeasures together are referred to as EMC (Electromagnetic Compatibility) countermeasures.

Two types of noise countermeasures

In most cases, noise issues can be resolved by taking sufficient emission or immunity countermeasures. Emission countermeasures enable a pinpoint response since it is relatively easy to understand what kind of noise is being emitted and from where. However, it is much harder to predict what kind of noise will enter and from where, so immunity countermeasures are more difficult than emission countermeasures. Therefore, approaches around the world emphasize resolving noise issues by emission countermeasures. Based on this thinking, each country has established standards for resolving noise issues, and these standards state that radio waves or conduction noise of a certain level or more should not be emitted. (Some standards also require immunity performance.)

 

EMI filters are noise countermeasure parts used when implementing emission and immunity countermeasures, to prevent the type of electromagnetic interference (EMI) described above.

The next column will introduce approaches to noise countermeasure parts.

person in charge: Murata Manufacturing Co., Ltd.  Yasuhiro Mitsuya