Common-mode

Main system parameters are shown in table 1. The fine field resolution is important. When operating in an unshielded space, however, at least such important is the high common mode rejection and the gradient rejection in order to suppress parasitic magnetic fields. 

Another problem with this approach is common mode failures. A common mode failure is a single event which could lead to the simultaneous failure of several components at the same time. An excellent example of this is a power failure, which could lead to many simultaneous failures. Frequendy, the common mode failure has a higher probabiUty than the failure of the iadividual components, and can drastically decrease the resulting reUabiUty. 

Cell Division Inhibitors. The most common mode of action of soil-appHed herbicides is growth inhibition, primarily through dkect or indkect interference with cell division (163). Such growth inhibitory activity is the basis for most pre- or post-emergent herbicides intended to control germinating weed seeds. In germinating seeds, cell division occurs in the meristems of the root and the shoot. Meristematic cells go through a cycle... 

The most common modes of operation for ms/ms systems include daughter scan, parent ion scan, neutral loss scan, and selected reaction monitoring. The mode chosen depends on the information required. Stmctural identification is generally obtained using daughter or parent ion scan. The mass analyzers commonly used in tandem systems include quadmpole, magnetic-sector, electric-sector, time-of-flight, and ion cyclotron resonance. Some instmments add a third analyzer such as the triple quadmpole ms (27). 

Exotic Decays. In addition to the common modes of nuclear decay, two exotic modes have been observed. These decay modes are of theoretical interest because theh long half-Hves place strict constraints on the details of any theory used to calculate them. 

Common Mode Failure An event having a single cause with multiple failure effects, which are not consequences of each other. 

Naturally, in most cases, we cannot neglect 8L/ , and must derive more general relationships. Let us first consider a cracked plate of material loaded so that the displacements at the boundary of the plate are fixed. This is a common mode of loading a material - it occurs frequently in welds between large pieces of steel, for example -and is one which allows us to calculate 8Lf quite easily. 

Figure 3-54 Typical ac and dc input filter circuits (a) ac input filter circuit for a single or universal input power supply (common-mode EMI filter shown) (b) a voltage doubling ac input circuit for 110V and 220V ac inputs (c) single dc bus input filter.

In ae off-line applications the common-mode choke is typically used and its design procedure can be found in Appendix E. The Alters resemble pi Alters, but are actually bi-directional L-C Alters. The important function is to Alter the noise generated by the switcher before it exits via the input power lines. 

A good starting point is to assume the need for 24 dB of attenuation at 50 kHz. That make the corner frequency of the common-mode filter... 

Assume that a damping factor of 0.707 or greater is good and provides a -3dB attenuation at the corner frequency and does not produce noise due to ringing. Also assume that the input line impedance is 50 ohms since the regulatory agencies use an TISN test which make the line impedance equal this value. Calculate the values needed in the common-mode inductor and Y capacitors ... 

Figure E-3 Common-mode and differential noise models (a) common mode (b) differential mode.

Another major source of noise is the loop consisting of the output rectifiers, the output filter capacitor, and the transformer secondary windings. Once again, high-peak valued trapezoidal current waveforms flow between these components. The output Alter capacitor and rectifier also want to be located as physically close to the transformer as possible to minimize the radiated noise. This source also generates common-mode conducted noise mainly on the output lines of the power supply. 

There are two types of input power buses. DC power buses are single-wire power connections such as found in automobiles and aircraft. The ground connection forms the other leg of the power system. The other form of input connection is the ac, or two or three-wire feed systems as found in ac power systems. The design of the EMI filter for dc systems is covered in Section 3.12 and takes the form of a simple L-C filter. All the noise is common-mode between the single power wire and the ground return. The dc filter is much more complicated, because of the parasitic behavior of the components involved. 

The common-mode filter essentially filters out noise that is generated between the two power lines (Hot and Neutral or HI and H2). The common-mode filter schematic is shown as part of Figure E-4. 

Figure E-4 A complete third-order, input EMI filter (common-mode and differential-mode).

In the common-mode filter the windings of the transformer are in phase, but the ac currents flowing through the windings are out of phase. The result is that the common-mode ac flux within the core for those signals that are equal and opposing phases on the two power lines cancel out. 

Some major transformer manufacturers build standard off-the-shelf components used in the design of common-mode filter transformers such as Coilcraft (Cary, IL). These transformers have various inductance values, and current ratings and also provide the needed creepage dimensions. This can make the designer s job a lot easier. 

The initial common-mode filter component values can be determined in a step-by-step process (like everything else in this book). To begin this process, either a baseline measurement of the unfiltered conducted noise spectrum is... 

Figure E-5 Frequency response of a second-order common-mode filter L = 1 mhl).

Once the component values have been calculated, the physical construction of the transformer and the PCB layout become critical for the effectiveness of the filter stage. Magnetic coupling due to stray inductive pick-up of high-frequency noise by the traces and components can circumvent the filter all together. Added to this is the fact that the common-mode filter choke looks more and more capacitive above its self-resonance frequency. The net result is the designer needs to be concerned about the high-frequency behavior of the filter typically above 20 to 40 MHz. 

Sometimes the high-frequency attenuation is insufficient to meet the specifications and a third pole needs to be added to the EMI filter. This filter is typically a differential-mode filter and will share the Y capacitors from the common-mode filter. Its corner frequency is typically the same as the commonmode filter. This filter is made up of a separate choke on each power line, and is placed between the input rectifiers and the common-mode filter. 

The differential-mode filter should have a lower damping factor than the common-mode because the combined damping response of the entire filter section would be too sluggish if higher damping factors were used. A damping factor of a minimum of 0.5 is acceptable. 

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