Considering EMI at an early design stage yields a shorter time to market
The ongoing trend toward smaller, lighter and more compact and higher throughput power supplies leads to a higher power density and switching frequencies which as its by-product, increased the chances of EMI (Electromagnetic Immunity) related problems that further contributes to the already crowded electromagnetic spectrum available.
As a result, government agencies around the world are pushing for more and much stricter rules and regulations to bring electromagnetic disturbances to an acceptable level. The European Economic Community (EEC) with several member countries like Italy, Germany, Luxembourg, France to name a few came up with its own set of rules and directives. One of which known as the Electromagnetic Directive covers the specific requirements necessary for the manufacturer to import their products in the European market. The consolidation of several European countries into one big market gives ease to manufacturers in the sense that they don't need to comply individual government agency requirements to import their products. Instead, complying the appropriate directives to their products allows them to introduce it to any of the member country without separate approvals.
The EMC Directive which is enforced several years ago, is by itself broad and not specific but with provisions that are binding to all member countries. Its primary intent is to see to it that the electromagnetic interference being generated and emitted by an electronic equipment should be reasonably low and at the same time it is reasonably immune to the threat from its environment. In this way products are said to be sufficiently well designed and built that they will fit for the purpose to which they are sold and that considerations are taken to protect the users against injury while the product is being used. EMI issues usually creates the bottleneck for most Power Supply manufacturers race to introduce their product to the market first. Owing to the fact that whoever comes first always gets a larger portion of the market pie, attacking EMI is always the engineers biggest challenge.
The directive also ensures that devices are to a certain degree, immune to electromagnetic disturbances. Testing for immunity includes Radiated and Conducted Susceptibility to check the device under test if it can withstand typical level of electrical noise either through air or input and output cables. The most stressful tests to demonstrate immunity are the ESD (Electro-Static Discharge) Surge and EFT (Electrical Fast Transient) tests. ESD simulates the electrostatic discharge a person can induce to a device especially when walking in a carpeted floor during a dry day. Both contact and air discharge are applied with magnitude dependent on the class of the device. Surge test simulates the effect of lightning and destructive line transients to electronic devices and is perceived to be the most stressful test among the three. Application of surge voltage up to several kilovolts to the device under test is not uncommon. EFT, which simulates line switching, switch bouncing and inductive line transients, is another stressful test applied in a very fast rate.
Several approaches can be taken in solving for electromagnetic compatibility. Attacking straight to the source itself is the most effective and economical one. Using filters and shielding techniques together with proper grounding also works equally well.
Electromagnetic interference takes into two forms, Conducted and Radiated. The former is transmitted through the input and output cables and the latter one through space. Conducted noise is further divided into Differential Noise and Common Mode Noise. Differential noise are those that flows in both directions along the line and neutral wires and common mode noise are characterize as those that flows between one line and the ground. Typically, conducted emission problems below 2 to 3Mhz are differential mode and above 3Mhz, they are common mode noise. Common mode noise is not as dominant as the differential mode at low frequency for several reasons. The parasitic inductance of the track, filters it and the common mode noise are in general capacitively coupled which reduces the low frequency components.
The switching device either FET or BJT is one of the biggest contributors of electrical noise. High dI/dT makes it easy for noise to couple to surrounding circuitry. Slowing down the switching yields improvement at the expense of higher switching losses. Sometimes, ferrite beads placed at the drain of the FET do the trick. Improper use of heat-sink to cool down the FET could sometimes increase the noise by acting as antenna. Connecting it to bulk negative or earth help minimize its effect. Minimizing the loop area covered between the bulk capacitor, primary winding and the FET improves the noise margin by several dB. Loop area covered by the gate drive circuitry is as critical and should be minimized as well.
Proper Power Transformer construction makes a great difference in EMI performance. Winding techniques such as interleaving primary and secondary windings improves coupling, yielding better result. Low inter-winding capacitance gives lower EMI as well. Sometimes faraday shield and belly band with proper termination spells the difference. The latter one is necessary in fly back topology owing to the considerable flux leakage caused by the gap introduced in the core to avoid saturation.
Secondary rectifiers also gives its share in noise interference problem. Considering that secondary side carry more current than the primary side, secondary rectifiers are potential generators of EMI. Minimizing the loop area covered by the secondary winding, the rectifier and either another rectifier serving as flywheel or a capacitor, depending on the topology should be minimized. RC snubbers across the diodes or sometime across the secondary winding helps in minimizing the noise generation. Such snubbers should be located as close as possible to the device it is suppose to snub to be more effective. A non-inductive type resistor should be used and optimize in value to limit the power dissipation . Sometimes if snubber is not sufficient, ferrite beads with square hysteresis loop are added to the leads of the diode. Also proper selection of the type of rectifier used is as important. It is advisable to use the type with very fast recovery time.
Filtering the EMI should be the last resort in attacking the noise problem. If all potential sources are minimized, the number of stages and size of the filter can be significantly reduced. Components used in filtering EMI are usually passive thus bulky. Chokes takes the greatest space together with the X and Y capacitors. Ideally, the filter should be located near the AC inlet and if application permits, the inlet should be directly soldered to the board to avoid the flying wires that connects them which could possibly pick up noise and bypass the filter making it ineffective. Winding technique for the EMI choke helps in increasing the effectivity of the filter. They should be tightly wound to minimize winding capacitance and to avoid spraying the magnetic energy to its surroundings. If cost allows, toroidal choke are preferred, they are better also in terms of construction and thermal effect.
EMI chokes should also be checked for possible saturation especially at low line input of 100 volts. Saturated choke acts as shorted wire giving no impedance of any kind to the noise. Occasionally choke orientation helps in improving the response. Differential choke placed perpendicular to each other yields better effect than in any other positions. X and Y capacitors is as important as well. X capacitors blocks the differential noise from flowing outside the power supply going to the AC mains. Y capacitors diverts the common mode noise to earth. Sometimes an X capacitor connected directly across the inlet gives a significant improvement. In cases where it is not as effective, properly terminated shield is used to prevent it from picking up noise.
Ground or earth wire should have a low impedance connection to the chassis. For secondary side, a ceramic capacitor connected between the secondary ground to earth may improve radiated EMI. Wire harness should also be routed away from the output chokes especially those that are made from ferrite rods and secondary rectifiers. The magnetic flux of such devices could couple to the wire and increased the conducted and radiated EMI. If allowed, place a ferrite sleeve around the output wires just before it exits the enclosure. Such ferrite sleeve forms a common mode choke and may also help in output ripple reduction.
Heat-sinks, either primary or secondary should be terminated properly. Usually primary heat-sinks are tied to bulk negative and secondary heat-sinks to secondary ground.The complete line of GlobTek power supplies with output wattage ranging from 10 watts to several hundred watts has an optimized EMI filter design giving a better response and profile that allows conducted and radiated emissions to be reduced to a lower level more than enough to pass FCC and CISPR specified limits. See Globtek's web page at www.globtek.com or contact sales at (201) 784-1000.