What is EMI Suppression?

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.

What are European electromagnetic requirements?

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) EN50081(Emissions) What are FCC electromagnetic requirements?

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
FCC electromagnetic requirements
Why do we need EMC?

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:
  1. Suppression of interference at the source.
  2. Making devices or systems immune to EMI.
There are three general approaches to suppressing of EMI:
  1. Good screening for radiated EMI.
  2. Proper earthing.
  3. Filtering of power and signal lines to reduce conducted EMI.
What are the types of EMI?

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

5-1
Conducted EMI is noise fed back from a system onto the AC or DC power line or signal lines. This noise is in the frequency range of 150KHz to 30 MHz. It usually has a common mode component and a differential mode component. The common mode component appears as a voltage on both line and neutral leads with respect to ground or earth while the differential mode appears between the line and neutral leads. To suppress conducted EMI, LC networks are usually used.

5-2
Radiated EMI comes in the form of electromagnetic waves radiating directly from the circuitry and leads of a system. A common example is the AC power cord of the system which can act as a transmitting antenna for radiated EMI. Ranging from 30 MHz to 1GHz, this type of noise can be effectively suppressed by metal shielding around the source.

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

How do EMI filters work?

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:
EMI filters

Before an effective EMI filter can be selected or designed, system and equipment designers alike should understand the nature of conducted EMI.

What are the components of conducted EMI?

Differential mode component
Differential mode EMI appears as a voltage between the supply lines of the equipment. The resulting current circulated through the supply lines and the source. None flows through the earth conductor.

Differential mode component

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.

Common mode component
What is the general test setup for DC-to-DC Converters?

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:

DC-to-DC Converters

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.

What is a 'pi filter'?

A Pi Filter is 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.

Pi Filter
How is output voltage accuracy calculated?

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:

How is output voltage accuracy calculated

What is the 'input voltage accuracy'?

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

What is the 'voltage balance'?

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

What is line regulation / how is it calculated?

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

The line regulation is Vout M (the maximum of the two deviations of output) from the value at nominal input in percentage.
line regulation calculation

What is load regulation / how is it calculated?

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

Load regulation is the difference between the two measured output voltage as a percentage of output voltage at rated load.
Load regulation calculation

What is efficiency?

Efficiency is 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.

What is switching frequency?

Switching Frequency is 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.

What is output ripple and noise?

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.

output ripple

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.

output ripple

What is 'Transient Recovery Time'?

Transient Recovery Time is 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.

Transient Recovery Time

What is 'current limiting'?

Current Limiting is the input current drawn by a power supply with the output short circuit.

What is 'foldback current limiting'?

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

foldback current limiting

What is 'isolation'?

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

What is the 'breakdown voltage'?

Breakdown Voltage is 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.

breakdown voltage

What is the 'temperature coefficient' / how is it calculated?

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

The temperature coefficient is the higher one of the two values calculated above, expressed as percent per change centigrade.

What is the 'ambient temperature'?

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

What is the 'operating temperature'?

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

What is the 'storage temperature range'?

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

What is XDSL?

DSL TECHNOLOGY (Digitial Subscriber Line Technology), in its various manifestations (ADSL,SDSL,HDSL,AND VDSL), IS EXPECTED TO HAVE A DRAMATIC SOCIETAL IMPACT ON BOTH OUR PERSONAL AND PROFESSIONAL LIVES..

STATED SIMPLY, DSL PROVIDES HIGH-SPEED MULTIMEDIA SERVICES, SUCH AS VIDEO-ON-DEMAND, SUPER-FAST INTERNET ACCESS, DISTANCE LEARNING AND VIDEO PHONING TO ANYONE WITH A STANDARD, COPPER TELEPHONE LINE. BY PLACING DSL MODEMS AT BOTH A PHONE COMPANY'S CENTRAL EXCHANGE OFFICE AND AT A CUSTOMER LOCATION, A TELCO CAN ENABLE ITS USERS TO RECEIVE MULTIMEDIA CONTENT MORE THAN 400 TIMES FASTER THAN WITH TODAY'S 14,400 bps MODEMS, IT IS THE USE OF THE UBIQUITOUS COPPER NETWOEK THAT HAS CAUSED DSL TO GAIN FAVOR AS A BETTER ALTERNATIVE TO ISDN AND CABLE MODEMS..

COMPARED TO OTHER CURRENTLY AVAILABLE TECHOLOGIES, DSL MODEMS OFFER THE HIGHEST TRANSMISSION RELIABILITY. THE FASTEST SPEEDS AND INTERACTIVE CAPABILITIES, FOR THE LEAST INVESTMENT. DSL MODEMS AVAILABLE TODAY CAN DOWNLOAD THE ENTIRE ENCYCLOPEDIA BRITANNICA IN 31 MINUTES COMPARED TO 54 HOURS WITH A COMMON 14,400bps MODEM.

What types of DSL are there?

GENERALLY, WITH FOUR TYPES INCLUDED IN THE DSL FAMILY : (WE COULD CALL IT ALL "XDSL" )

ADSL - ADSL DELIVERS SUPER-FAST RATE OF 32Kbps TO 8.192 Mbps DOWNSTREAM TO THE CUSTOMER AND 32Kbps TO 1.088Mbps UPSTREAM TO THE NETWORK WHILE SIMULTANEOUSLY PROVIDING PHONE SERVICE ON THE SAME STANDARD PHONE LINE. WITH ADSL, INTERACTIVE MULTIMEDIA SERVICES CAN BE PROVIDED TO ALMOST ANYONE WITH A TELEPHONE LINE IN THEIR HOME OR BUSINESS ..

HDSL - THE HIGH COST AND LENGTHY IMPLEMENTATION TIME OF TRADITIONAL T1/E1 AND FIBER LINES HAVE BUSINESSES AROUND THE WORLD SCRAMBLING FOR MORE BANDWIDTH TO ACCOMMODATE APPLICATIONS SUCH AS LAN INTERNETWORKING , VEDIO CONFERENCING AND PBX INTERCONNECT. AS A RESULT, MORE AND MORE SERVICE PROVIDERS ARE DISCOVERING HIGH-BIT-RATE TRANSPORT A FULL DUPLEX T1 (1.544Mbps) OR E1 (2.048Mbps) ACROSS EXISTING TWISTED PAIR COPPER W/O REPERYERS. THIS HIGH-PERFORMANCE TECHNOLOGY IS THE QUICKEST AND MOST COST-EFFECTIVE OPTION TO T1/E1 LINES ..AND PROVIDES A QUALITY TRANSMISSION THAT IS COMPARABLE TO FIBER…

SDSL - THIS SDSL TECHNOLOGY PROVIDES SYMMETRIC (BI-DIRECTIONAL)HIGH SPEED, VARIABLE RATE COMMUNICATIONS. SDSL ADDRESSES THOSE APPLICATION THAT REQUIRE A SYMMETRIC DATA RATE . SDSL DATA RATE RANGE FROM 160Kbps TO 2.084Mbps..

VDSL - VDSL TECHNOLOGY BOOSTS THE DATA RATES OF THE SUBSCRIBER LINE SIGNIFICANTLY. THE CLOSER THE CUSTOMER IS TO THE CRETRAL EXCHANGE OFFICE, THE HIGHER THE DATA RATE. A SUBSCRUBER AT A DISTANCE OF 5000FT AWAY FROM THE CENTRAL EXCHANGE OFFICE CAN GET A DATA RATE OF 13Mbps AT 3000FT THAT SUBSCRIBER MAY ATTAIN DATA RATES UP TO 1026Mbps AND AT 1000FT, A 51 Mbps DATE RATE IS POSSIBLE…

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