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Military standard requirements for electromagnetic interference and suppression of transient voltage spikes.

1. Background: Challenges from electromagnetic interference and transient voltages

Engineers in the military and aerospace fields face many challenges regarding electromagnetic interference (EMI) and transient voltage compliance. Different military organizations develop different standards that are more stringent than similar standards in commercial applications. These standards sometimes fail to agree on test limits and methods. Electronic noise from one device may affect the normal operation of other devices. The purpose of electromagnetic interference standards is to prevent those possible problems. The lack of necessary electromagnetic interference control can lead to noise interference, such as unwanted noise in communication or computing equipment, as well as false triggering and false readings in sensor circuits.

In addition to generating noise signals that can cause interference, the normal operation of equipment also generates considerable transient voltages. These transient voltages occur at the input terminals of the equipment and are specified in military standards and adjusted for the specific environment in which the different levels of equipment are located. For example, land-based equipment has more different requirements than airborne equipment.

Therefore, the challenge for military system designers is to address the electromagnetic interference and transient voltage performance of devices that will be manufactured to meet the most stringent requirements imposed by the end customer in a timely manner. This operating instructions will explain the basics of electromagnetic interference and transient voltage compatibility as applied to military systems using switched-mode power supplies (SMPS). It will focus on the basic terminology that occurs in electromagnetic interference and transient voltage suppression and Handle various requests from several different military organizations.

2. Electromagnetic interference (EMI)

Electromagnetic interference can be divided into four major categories:

1) Conducted emission

2) Conducted susceptibility

3) Radiated emission

4) Radiated susceptibility

Conducted noise arises from cable transmissions connecting the input power bus and equipment, while radiated interference arises from unintended transmission or reception of noise signals. Electromagnetic interference emission standards (EMI emission standards) are about the “noise” generated by equipment, while electromagnetic interference susceptibility standards (EMI susceptibility standards) describe the “noise environment* that equipment can tolerate—in order to operate normally.

From a design perspective, conducted emissions can be further divided into common-mode noise and differential-mode noise. Differential mode conducted noise arises from the input and output current of the converter (the normal current in the circuit). On the other hand, common-mode current flows in the chassis ground in the same direction as the power line and return line. Differential mode current is generally related to switching currents in power converters, while common mode current is mainly the result of pulsating voltages in the circuit.

Electromagnetic compatibility requires the system to meet all types of standards for conducted interference and radiated interference. However, when considering filter function, it is usually based on how well the filter protects the mains supply from emissions emitted by its equipment. Figure 1 shows a typical switching power supply load with external filter and transient voltage suppression circuitry. The switching current (SMPS switching current) I can be attenuated by built-in and external filters.

The input current I2 of the switching power supply is mainly DC, plus a specific current ripple (ripple current). This current ripple and higher frequency peaks can be further attenuated by the filter, so the current I3 flowing out of the power bus is essentially DC. If there is still a small amount of AC in I3, the filter must be designed to keep the AC content below the level specified in the application standard. The attenuation of the switching current to the DC level is a measure of the differential mode performance of the filter. The common mode effectiveness of the filter can be determined by the reduction in common mode current produced by the common mode current delivered through the common mode current source. An effective electromagnetic interference filter must keep the common mode current and differential mode current in the power circuit within specified standards.

3. Transient suppression

Filters are used to attenuate the electrical noise generated by the normal operation of electronic equipment, while transient voltage suppression works to overcome occasional or intermittent disturbances, which usually occur at the power distribution bus. This type of power disturbance occurs from large generator switches, starting engines, transient loads, etc. They can be roughly divided into three categories:

1) Voltage ripple

2) Voltage surge

3) Voltage spike

Voltage ripple refers to the difference between the actual DC input voltage and the standard DC input voltage. Surges are generated by load transients on the distribution bus and typically last from a few milliseconds to a hundred milliseconds. On the other hand, voltage spikes are generally caused by reactive load switching. The spikes have considerable high-frequency, high-voltage oscillations that last less than 5 milliseconds. The function of the transient suppression circuit is to protect the EMI filter and at the same time protect the downstream circuits from damage caused by transient voltages. Figure 1 shows an input bus voltage and an input surge operating at standard DC levels. The transient voltage suppressor forces the input voltage to a level that ensures the safety of the EMI filter and downstream converters, as shown in Figure 1 for Vbus. In this way, the transient voltage suppression function is similar to the sensitivity function of the EMI filter: they both protect the load equipment from interference originating from the power distribution bus.

In conclusion

The electromagnetic interference filtering and transient voltage suppression required in military systems must ensure:

1. Various electrical parts must not produce additional electrical interference that affects other equipment.

2. All components attached to the power bus can tolerate power bus interference and operate in this typical interference manner.

Several different sets of electromagnetic interference and voltage transient specifications have been established over the years to illustrate many needs. In some cases, the standards of the various organizations are very similar, while in other cases they are quite different, either in the methods used to test equipment compatibility or in the actual levels of interfering signals. Solutions that meet multi-standard specifications enable hardware to be used in various regions around the world. However, designers must be aware that in some applications, a design that meets multiple different specifications is often over-engineered (and thus more expensive, larger, and more cumbersome). Therefore, it is necessary to make trade-offs in the overall performance of the system to meet the requirements of specific applications and schedules.

Packaging electromagnetic interference/transient suppression modules for load converters have important advantages in solving some conflicting requirements: rapid system development, wide range of end uses and efficient packaging.

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