EMI (Electromagnetic Interference), also known as RFI (Radio Frequency Interference), is unwanted electromagnetic energy polluting the environment. Its propagation via radiation and power conduction over system signal and power lines can affect the operating of electrical equipment around the source. EMC (Electromagnetic Compatibility) is the ability of a system to function reliably in the presence of significant levels of EMI and at the same time to limit its internally generated EMI to avoid interference with the operation of other systems around it.

EMI effects are not only a nuisance but can also be destructive. Effects can range from annoying picture disturbances on TV sets to an aeroplane crash due to an EMI related flight control failure. EMI can not be completely eliminated but can be attenuated to safe levels recommended by the International Special Committee on Radio Interference (CISPR). Most countries, especially the EU and US, have regulatory bodies that set EMC regulations based on CISPR recommendations. In Europe the national regulatory bodies of member countries operate within the scope of the EMC Directive issued by the European Commission, while in the US, the Federal Communications Commission (FCC) is the guiding body.

The European Commission has issued The EMC Directive, 89/336/EEC, which requires that end user equipment meets prescribed electromagnetic compatibility requirements. National government implement the directive by reference to harmonised standards. In the majority of cases, conformity to the EMC directive can be demonstrated by compliance with EN50081 for emissions and EN50082 for susceptibility. These are known as the “Generic EMC Standards”. These standards, in turn, refer to further standards, as follows:

EN50082 (Susceptibility)

The FCC Rules-Part 15, Subpart J, sets regulations in the US to minimize the interference potential of devices using RF energy incidentally as an intermediate step an end function other than communication. Examples of these devices are digital equipment and switching power supplies. Devices or systems covered by this definition into two classes.

Class A (Consumer)-computing devices for use in commercial, industrial or business areas.
Class b (Consumer)-computing devices for use in residential areas.
The FCC conducted emission limits for these two classes starts at 450Hz as shown in Fig.A

Electromagnetic compatibility (EMC) has become a major challenge for equipment and switching power supply designers. As more and more countries apply EMC regulations on products entering their respective markets, EMC compliance of a product has become a major factor defining its acceptance.

The EMC challenge has two parts:

EMI is usually classified into two types depending on how it is propagated:

Note: The scope of this application note will be limited to the subject of conducted EMI.

Conducted EMI signals are in the frequency range up to 30MHz and are carried mainly in the AC or DC power line from the source to the affected systems. The solution is to fit an input EMI filter.
An EMI filter used on the power line is basically a low pass filter that:

Common mode component
Common mode EMI appears as a voltage on both lines with respect to earth. Common mode current flows through both supply lines to earth (see FigC). If the system has no protective earth connection, common mode current will flow through the capacitance between the case of the system and earth.

Most power converter test are done with the general test set-up showed in Figure I. Some general conditions that apply (except where note) to test methods outlined in this notes are:

Note:
If the converter under test with remote sense pins, connect these pins to their respective output pins. All tests are made in “ Local sensing ” mode.

The minimum and maximum input voltage limits within which a converter will operate to specifications.

An input filter consisting of two capacitors connected in paralled with a serial inductor. Often used in DC/DC converters to reduce input reflected ripple current.

With nominal input voltage and rate output load from the test set-up, the DC output voltage is measured with an accurate, calibrate DC voltmeter. Output voltage accuracy is the difference between the measured output voltage and specified nominal value in percentage. Output accuracy (as a%) is then dervied by the formula:

 
Vnom is the nominal output voltage specified in the converter data sheet.

For a multiple output power converter, the percentage difference in voltage level of two outputs with opposite polarities and equal nominal values.

Make and record the following measurements with rated output load at 25^oC 

Make and record the following measurements at nominal line voltage at 25^oC

The ratio of output load power consumption to input power consumption expressed as a percentage. Normally measured at full rated output power and nominal line conditions.

The rate at which the DC voltage is switched in a DC-DC converter or switching power supply.

Because of the high frequency content of this ripple, special measurements techniques must be employed so that correct measurements are obtained. First a 20MHz bandwidth oscillosocpe is normally used for the measurement so that all significant harmonics of the ripple spike are included.

This noise pickup is eliminated as shown in Figure 3 by using a scope probe with an external ground band or rind and pressing this band directly against the output common terminal of the power converter while the tip contact the voltage output terminal. This makes the shortest possible connection across the output terminals.

Figure3.

Figure4 show a complex ripple voltage waveform that may be presentage on the output of a switching power supply. There are three components of the waveform, first is a 120Hz component that originates at input rectifier and filter, next is the component at the switching frequency of the power supply, finally there are small high frequency spikes imposed on the high frequency ripple.

Figure4 AMPLITUDE

The time required for the power supply output voltage to return to within a specified percentage of rated value following a step change in load current.

Figure 5 Transient Recovery Time

Input current drawn by a power supply with the output short circuit.

A method of protecting a power supply from damage in an overload condition reducing output current as the load approaches short circuit.

Figure 6. Fold Back Current Limiting

The electrical separation between the input and output of a converter given of capacitance and resistance. Normally determined by transformer characteristics and circuit spacing.

The maximum DC of DC voltage which may be applied between the input and output terminal of a power supply without causing damage. Typical break-down voltages for DC-DC converters is 500 VDC minimum.

Figure 7

With the power converter in a temperature test chamber with rated output load, make the following measurements.

The temperature of the still-air immediately surrounding an operating power supply.

The range of ambient or case temperatures within a power supply may be safely operated and meet its specifications.

The range of ambient temperatures within a power supply may be safely stored non-operating with no degradation in its subsequent operation.