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Very often an engineer resolves a stubborn EMI problem by just 'playing' with the transformer. With its windings carrying high-frequency current, it becomes an effective H-field antenna.
Since parts of the windings have a swinging voltage across them, they can also become effective E-field antennas.
The parasitic capacitance between the Primary and Secondary carries noise across the isolation boundary. In a safety-approved transformer, there are three layers of safety-approved polyester ('Mylar?'') tape between the Primary and Secondary windings, for example the popular #1298 from the 3M Company.
There is usually also a circumferential copper shield (or 'band') around the entire transformer.
To reiterate, even from the point of view of EMI, a flyback transformer should be preferably center-gapped.
There is usually an auxiliary winding present on the Primary side which provides a low voltage rail for the controller and related circuitry.
Figure 1 shows also shows a low noise construction principle as applied to a flyback transformer. Since the Drain of the Fet is swinging, it is a good idea to keep this end of the Primary winding buried as deep as possible, i.e.
Comparing the diagram on the left with its schematic on the right, we see that the start and finish ends of any winding have been indicated. Which means that for the flyback transformer of Figure 1, the diode end of the Secondary winding will necessarily fall adjacent to the safety barrier. The transformer in Figure 1 has the advantage that the quiet end (ground) of the Secondary is now the outermost layer. When we go through the same reasoning for a forward converter transformer, we will find that with the described winding sequence, we will automatically have the quiet ends of both Primary and Secondary overlooking each other across the safety barrier. A way out of this forward converter outer surface radiation problem is to ask our production to reverse the direction of the Secondary winding (only). When the transistor is mounted on the chassis, there is a technique that is used to actually try and cancel the current injected through the heatsink capacitance. Note: The above technique does nothing to cancel the noise injected through the capacitance between the Primary to Secondary windings. Input bridges which use ultrafast diodes are available, and their vendors claim significant reduction in EMI. For medium to high power converters, snubbers are usually placed across the ultrafast catch diode. As compared to the diode turn-off (recovery) speed, the Fet turn-on should be 2 to 3 times slower.
Ferrite beads (of nickel-zinc type) are often placed in series with catch diodes, such as the output diode of a flyback. Backtrack: In one industry documented case, the output Schottky diode of an off-line flyback had a small but unacceptably mysterious ppm (parts per million) failure rate in production testing. We need to re-examine each topology carefully to figure out which PCB trace segments are 'critical'. We need to be careful about inadvertently making traces that have a swinging voltage on them too wide, as then they become good E-field antennas. The ground plane is a very effective method of bringing down the overall level of the EMI emissions. It is helpful to separate the CM and DM components to be able to study them and debug a bad spectrum. Tip: When viewing pulse transition times below 100 nanoseconds, or emission noise frequencies above a few megahertz, it is advisable to keep the cable length small.
Tip: A quick diagnostic test in understanding a high frequency conducted EMI problem at the inputs of a power supply, is to twist the output cables tightly together (along with their respective returns). Backtrack: This procedure was actually implemented in full production on a particular high volume commercial design.
In Figure 5, we show a practical technique to separate that part of the conducted emissions spectrum that is due to radiation from within the converter.
Most of the small companies cannot afford a pre-compliance setup for radiated emission tests. To my wife Disha and daughter Aartika for tremendous patience while I frittered away what was to be quality family time on this. Isolation transformers are designed to provide superior line noise reduction and spike suppression, eliminating the need for a dedicated AC line.
Tripp Lite Isolator series isolation transformers offer complete line isolation, continuous noise filtering and enhanced common mode surge suppression.
Welcome to Jensen Transformers.  As you go through these factory tour images, you will quickly come to the realization that building a Jensen Transformer requires an incredible degree of precision and hand assembly during every stage of production. Calibration performs a number of tests on the instrument and its cell cable in order to give the best accuracy and stability. The coexistence of equipment of different technologies and the inadequacy of the installations favors the emission of electro-magnetic energy and often causes problems of electro-magnetic compatibility. EMI is the energy that causes undesirable response to any equipment and may be generated by sparking on the motor brushes, tension circuits switching, activation of inductive and resistive loads, activation of switches, circuit breakers, fluorescent bulbs, heaters, automotive ignitions, atmospheric discharges and even the electrostatic discharge between persons and equipment, microwaves devices, mobile communication equipment etc.
The biggest problem caused by EMI is the occasional situations that slowly degrade the equipment and its components. The topology and the distribution of the wiring, types of cables, proctection techniques are factors that must be considered to minimize the EMI effects.
A typical example of how the EMI may affect the work of an electronic component is a capacitor exposed to a voltage peak higher than its specified nominal voltage. Do not be neglectful, imprudent, irresponsibly inexpert or incompetent on technical problems.
Whenever possible refer to the physical regulations, as well as the safety practices for each area.
Act safely on measurements, avoiding contact between terminals and wiring, as high voltage may cause electric shock. In order to minimize the risk of potential problems related to safety, comply with the safety standards and those of the local classified areas regulating the equipment installation and operation.
The inadequate installation or use of equipment in non-recommended applications may damage the system performance and consequently the process, as well as be a source of danger and accidents.
It is extremely important that every person involved is aware and conscious and moreover committed with the plant operational reliability and personal safety.
The control of noises in automation systems is vital, as it may become a serious problem even with the best devices and hardware to collect data and work.
Any industrial environment has electric noises in sources, including AC power lines, radio signals, machines and stations etc. Fortunately, simple devices and techniques as the use of adequate grounding methods, shielding, twisted wires, the average signal method, filters and differential amplifiers may control noise on most measurements. Frequency inverters have commuting systems that may generate electromagnetic interference (EMI).
One of the main goals of a project is to keep all common signal return points on the same potential. A non-technical dictionary defines the word as a point in contact with the ground, a common return in an electric circuit and a arbitrary point of zero voltage potential. To ground or to connect some part of an electric circuit guarantees personal safety and generally improves the circuit work. Unfortunately, a safe and robust environment in terms of grounding quite often does not occur simultaneously. Grounding systems must execute multiple simultaneous functions: provide personal safety and protect the equipment.
Regardless of its protective or functional aim, grounding must be a single one on each installation spot. Each building must have a principal equipotentialization and the installation masses located in the same edification must be connected to the main one, thereby creating the same and only grounding electrode. The functional equipotentialization equalizes the grounding and guarantees that the signal circuits and the electromagnetic compatibility work well. Figure 5 shows a generating source for high voltage and high frequency noises, besides a system for temperature measuring 23 m distant from the control room. In distributed systems like industrial process control, with distant physical areas and power supplied by different power sous, it is recommended grounding on each location and to apply EMI control techniques on each signal routing, as shown on figure 2. The result is that equipment with metal housing is subject to noise on the power source and lightning grounding loops. In order to meet the safety standards and protection against lightning and EMI the grounding system should be a zero impedance plan, whose mixture of different current levels would turn these systems free from interference. The grounding system at a single point can be seen on figure 6, whose striking feature is a single grounding point evenly distributed to the entire installation.
This configuration is best suitable for low-frequency spectrum and satisfies perfectly high frequency electronic systems installed in reduced areas. Moreover, this system must be insulated not to work as a return path for signal currents circulating through signal conductors with balanced pair, for example. This type of parallel grounding eliminates the common impedance problem, although detrimental to the use of a pile of wiring. For high frequencies, the multipoint system is the most adequate and simplifies the installation, as shown on figure 7.
Many low impedance connections between the PE conductors and the grounding electrodes combined with multiple-impedance paths between the electrodes and the impedance on conductors create a complex grounding system with an impedance network (see figure 7 b), and the currents that flow through it cause different grounding potentials on the the network interconnections.
The multipoint grounding systems that use balanced circuits normally do not have noise problems due to filtering, where the noise field is limited between the cable and the grounding plan. Figure 9 shows adequate grounding whose individual currents are conducted to a single grounding point.
A grounding loop occurs when there is more than a grounding path, which generates undesirable currents between these points.
These paths form the equivalent to an antenna loop that captures the interference currents with high efficience. Conduction through common impedance (grounding): It occurs when the current on two different areas pass by the same impedance. The capacitive coupling is represented by the interaction of electric fields between conductors. The electric field effect is proportional to the frequency and inversely proportional to the distance.
The distance between the disturber and the victim cables and the length of the cable that runs alongside them.
The impedance of the victim circuit input (circuits of high input impedance are more vulnerable).
The victim cable insulation (cable insulation εr), mainly for strongly coupled cable pairs. Figure 13 shows the coupling and its voltage and current sources in common and differential modes. Use of conduits and grounded metal boxesFigure 16 shows the capacitance on the coupling between two conductors separated by a D distance. In high frequencies ground the shield on two points (caution!) and in low frequencies on a single point. The use of twisted pair cables is very efficient as long as the induction in each tortion area is approximately equal to the adjacent induction.
This is the effect of metal plates and screens on the incidence of electromagnetic waves; they generate their own fields that minimize or even nullify the field through them, therefore working as true shields against electromagnetic waves. Although the cables are shielded, the shielding against magnetic field is not as efficient as against electrical cables.
Most makers of field equipment as pressure, temperature transmitters, positioners, converters etc, recommend the local grounding of theirs products. When installing the equipment, normally the housing is in contact with the structural part or piping and consequently is grounded. Some factories recommend their equipment to float insulated from the structure and not grounded, to avoid current loops. For microprocessers and digital communication equipment, some industries incorporate or make surges of transient protectors.available. Use RFI filter and always connect it the closest possible to the noise source (between the RFI filter and the drive). All motors activated by inverters must be powered preferably by shielded cables grounded on both ends.
To meet EMI protection requirements all external cables must be shielded, except power source cables.
On control cables install a small (100 nF a 220 nF) capacitor between the shield and the ground to avoid the return of AC circuit to the ground wire. Use isolated and shielded cables (4 paths) between the inverter and the motor and between the inverter power supply along the inverter. Inverters generate escape currents and in this case a line reactor can be introduced on the inverter output.
Line reactors are a simple and inexpensive way to increase the impedance of an isolated load source (as a variable frequency command, in the case of inverters). The reactors are connected in series to the load that generates harmonics and, by increasing the source impedance, the magnitude of the harmonic distortion may be reduced to the load where the reactor is added.
The main function of the RFI input filter is to reduce the emissions conducted by readiofrequency to the principal lines of distribution and the ground. Reflected waves: if the cable impedance does not match that of the motor, there will be reflections. Special cables: another important detail to help minimize the effects of the electromagnetic noises generated in installations with inverters and AC motors is the use of special cables to avoid the corona effect of discharges that may deteriorate the dielectric rigidity of the insulation and allow the presence of stationary waves and noises on the ground loops.
In terms of digital networks, move them away from the inverter whose signals will go the motors and install repeaters isolating the areas. Ground connectors must be classified according to the maximum capacity of the inverter current.
For applications of AC variable frequency inverters that must comply with EMC standards use the same type as the shielded cable specified for the AC motors to be used between the inverter and the transformer. Keep the motor cable length within the limits established by the inverter user manual to prevent several problems, including the cable load current and the effort of the reflected wave tension. When having TE and PE terminals, ground them separately at the closest panel point using a twisted loop. Motor and input cable shields must be connected on both ends to provide a continuous way to the common-mode noise current. The cable shielding between two cabinets must be connected to the cabinet containing the signal source. The cable shielding between a cabinet and an external device must be connected to the end of the cabinet unless specified otherwise by the external device maker. Never connect a shielding on the common side of a logic circuit, as this will cause noise on the circuit.


Make the termination of the inverter power source, motor and terminal block control connections. In low DC frequencies up to 1 MHz, the cable shielding may be grounded on a single end of the cable and provide good response concerning the electromagnetic interference.
Grounding and shielding are mandatory requirements to guarantee the integrity of a plant data. Noise effects can be minimized with adequate techniques of projects, installation, cable distribution, grounding and shielding. The shield must be connected to the signal reference potential of what is being protected (see figure 25).
When there are multiple segments keep them connected, ensuring the same reference potential, according to figure 26. In this case the current will not circulate through the loop and will not cancel the magnetic fields. A distribution of currents occurs here, in function of the frequencies, since the current tends to follow the course of lower impedance. Up to a few kHz: the inductive reactance is neglectable and the current will circulate by the way of less resistance. Above kHz: the inductive reactance predominates and this will make the current circulate by the way of less inductance.
The way of less impedance is that whose course of return is close to the course of departure for presenting distributed capacitance and lower distributed inductance. The length of the conductor extending outside the conductor should be minimized and guarantee good connection between the the shield and the ground.
Significant damages may be caused to active equipment when the ground potential difference between both ends goes beyond 1 V (rms). The grounding electrical resistance should be the lowest possible on both segment ends to minimize the ground loops, mainly in low frequencies. The cable grounding is used to eliminate interferences by capacitive coupling due to electrical fields. The grounding loop must be connected to the ground reference potential that is bening shielded. Figure 29 – The length of the ground-reference connection should be minimized as it works as a coil.
Electric fields are much easier to shield than magnetic fields and the use of shielding on one or more points work against electric fields. To prevent radiator from a conductor, a grounded shield on both sides is generally used above the cut frequency, but some precautions must be taken. Always check the NBR 5418 standard or local regulation for shielding and connection with intrinsically safe equipotential systems.
An intrinsically safe circuit must float or be connected to the equipotantial system associated with the classified area on only one point. Shielding must be connected to the ground or the structure compliant to ABNT NBR IEC 60079-14 standard.
Never install a device that has been previously used without an intrinsically safe barrier on an intrinsically safe system, as the protection zener may be burned and will not work in instrinsically safe areas. According to IEC 61158-2, to ground means to be permanently connected to the ground by a sufficiently low impedance and with enough conductive capacity to prevent any voltage from causing damages to equipment or persons.
Preferably, the shield must be grounded on two points, a the beginning and the end of the bus, provided there is no difference potential between these points and allows the existence and paths to loop currents. The shield must cover entirely the electric circuits through the connectors, couplings, splices and junction and distribution boxes. In classified areas, if the equalization potential between the safe and the hazardous areas is not possible, the shield must be connected directly to ground (Equipotential Bonding System) only at the hazardous area side. Another additional manner is to ground the equipment junction boxes and housings on a ground equipotential line on the non-safe side. Multiple grounding is also usual and provides more effective protection on situations of high frequency and electromagnetic noises. The shield (the loop and the aluminum blade) must be connected to the system functional ground on all stations through the connector and DP cable) to provide a large connection area with the grounded conductive surface.
Maximum protection is provided with all points grounded through a low impedance path to the high frequency signals. In cases with voltage diferential between the grounding points, pass close to the wiring an equalization potential line (the metal duct or an AWG 10-12 cable can be used).
When referring to shield and grounding, in practice there are other ways of handling this subject, one with much controversy, as for example, the shield grounding can be made in each station with the sub D 9-pin connector (figure 34), where the connector housing  makes contact with the shield and is grounded when connecting with the station.
In hazardous areas always use the recommendations from the certifying bodies and the installation techniques demanded by the area classification. For grounding, group circuits and equipment with similar noise characteristics in serial distribution and connect these points on a parallel reference and also ground ducts and boxes. An isolation transformer is designed to specifically address the problems associated with referencing its internal shields to ground.
There is wide range of ISOLATION TRANSFORMERS which reduces the effects of Power line problems like noise spikes, surges transients and neutral to a minimum. The stringent requirement of power of the sophisticated equipment being used today underlines the necessity of clean power. With Sun Electrical “s Isolation Transformer all types of common electrical noises, particularly common mode noise, spikes, surges, and transients can be eliminated to a greater extent. These fields can impinge upon nearby traces and cables, and enlist their help in getting transported out of the enclosure via conduction or radiation.
Since the Secondary side ground is usually connected to the chassis, this noise returns via the Earth plane, in the form of CM noise. In addition to these layers, a copper 'Faraday shield' may be inserted to 'collect' the noise currents arriving at the isolation boundary, and diverting them (usually to the Primary ground). In the discussion below, we note that transformers with split windings are not being discussed, though the principles described here can be easily extended and applied to them too. This is good from the viewpoint of conducted EMI since very little noise will be injected through the parasitic capacitance. So for example, if till that point the machine was spinning clockwise, for the Secondary we specify an anticlockwise direction.
This done by placing another winding, equivalent to the main winding and opposite in phase (though it can be of much thinner wire).
Despite this limitation, a 10dB?V reduction in conducted EMI is still possible (at various points along the spectrum).
But in practice they don't seem to provide much advantage, at least not to justify their cost. But this can create a lot of dissipation in the Fet, since every cycle the capacitor energy is dumped into the Fet. So momentarily the diode forward voltage may be 5 to 10V rather than the expected 1V or so. In that case, the turn-on can be slowed by inserting a resistor of about 10 ohms to 50 ohms in series with the bootstrap capacitor. These beads must be very small, as they can have a great effect on the power supply efficiency. In general the primary winding may be broken up into two windings which are then positioned on either side of the Secondary winding so as to reduce leakage inductance. By careful analysis, this was traced to a very slight wiggle (ringing) somewhere in the middle of the turn-off waveform of the diode. Those traces in which current is forced to either start flowing or stop flowing (suddenly) at the instant of turn-on or turn-off, will produce inductive spikes. On a multi-layer board, if the very next plane to the side containing the power components and their associated traces is a ground plane, the EMI can drop by 10-20 dB. The LISN reading only provides a certain weighted sum of the total conducted noise, and therefore unless special accessories are available (including a modified LISN), we can only guess which part of the EMI scan is mainly DM and which is CM. Then we must terminate the cable at the oscilloscope or measuring instrument with a 50 ohm resistor. This induces field cancellation (also called flux containment), reducing the radiation from the output cables.
A few tie-wraps were used to hold the bunch of wires tightly together in the twisted position.
However a few have a very good idea if they are going to be successful in that test or not, based on some finer aspects of the conducted emissions plot. Rugged all-metal unit housing ships ready for upright tower or wallmount placement in industrial, medical, office or network environment. You will observe the care and attention that each member of our team brings to their craft, whether they are forming Faraday shields or operating high-speed Meteor winders. All this may provoke alterations with the resulting overload, sub-voltage, peaks, voltage transients etc., which may cause high impact on a communication network. Keep in mind that in high frequencies the cables work as a transmission system with crossed and confused lines, reflect and scatter energy from one circuit to another. This may deteriorate the dielectric, whose width is limited by the capacitor operation voltage, which may produce a gradient of potential inferior to the dielectric rigidity of the material, causing malfunctioning and even the capacitor burning. Therefore, only use trained and qualified professionals on installation, operation and maintenance jobs. Commonly, users tolerate them but a close look reveals problems involving cables, their courses and packing, shielding and grounding. This article provides information and tips on grounding but in case of doubt the local regulations always prevail. As high frequency inverters (up to 300MHz) generate harmonics through the commuting amplifiers on these frequency levels, the grounding system resembles more a series of inductors and capacitors rather than a low resistance channel. Depending on how the signals are accommodated there might be up to 2.3KV at the measuring terminals.
The main effects of inadequate grounding are electric shocks to users through contact, low or intermittent response from the protection systems, such as fuses, circuit breakers etc.).
Furthermore, the impedance on each wire may be too high and the ground lines may become a source of noise in the system.
A conductor passes near a noise source (the disturber), captures this noise and sends it to another part of the circuit (the victim). The ideal is to cover 100% of the protected part and to ground this shield so that the parasite capacitance between the conductor and the shield does not function as a repowering or crosstalk element. The one-pair wire is wound in spiral and through the cancellation effect reduce  the noise and keep the medium electric properties constant through its whole length. Its use is efficient in differential mode, balanced circuits and has low efficiency in low frequencies on unbalanced circuits. In addition, this discontinuity may help ease the difference of potential between each duct segment, and, in the event of current surge generated by an atmosphere discharge or a short circuit, the lack of continuity will not allow the current to circulate along the aluminum duct and consequently will not protect the Profibus cable. In low frequencies, the twisted pair absorbs most parts of the effect from electromagnetic interference. In cases where the housing is insulated from any structure point, the manufacturers recommend local grounding with the shortest possible connection with an AWG 12 wire.
Inside the panel create distinct zones and even use separating plates that served as shield. The RFI input filter is connected between the input power source AC line and the inverter input terminals. Remember that the cable between the inverter and the motor presents impedance to the inverter output pulse (called surge impedance). The cable shield is recommended to help reduce the twisted coupling noise on the power source cables. In these cases, it is very important that the differences in ground potential in both connection points are the minimum possible. In practice, it is very common to watch intermittent work and gross errors in measurements due to the bad installations. Inadequate grounding may be the source of undesirable and dangerous potentials that may endanger the effective operation of an equipment or the work system itself. The length of the conductor extending outside the shield should be minimized and guarantee good connection between the ground and the shield. More than one ground connection is allowed on the circuit, provided the circuit is divided in galvanically insulated subcircuits, and each one is grounded at only a single point. Voltage lines with 0 Volts must be connected to ground and galvanically insulated from the fieldbus bus. Always verify the shield continuity until the last PA segment in the segment, with due analysis of the connection and finishing, as it should not be grounded on the equipment housing. In this case, though, annalyze and verify punctually the ground graduation potential and, if necessary, equalize it.
It is constructed with two isolated Faraday shields between the primary and secondary windings. Note that this shield should be a very thin piece of copper to avoid eddy current losses and also to keep leakage inductance down. It is often left floating in low cost designs, though it may be connected to the Secondary ground if desired. Therefore, it can actually double over as a crude Faraday shield if we a) wind it spread out over the available bobbin width, and b) we help it collect and divert more noise by AC coupling its opposite end (i.e. We should also continually compare the left-hand side diagram of Figure 1 with its corresponding schematic on the right-hand side.
In a typical production sequence, the coil winding machine always spins the bobbin in the same direction for every winding placed, therefore a) all the start ends (dotted ends) are magnetically equivalent (so if one dotted end goes high, the other dots also go high at the same moment, as compared to their opposite ends). With this, the reasoning given above for the flyback will apply to the forward converter transformer too.
We could just wrap a few spaced-out turns of thin wire, with one end connected to Primary ground, and the other end with the small 22pF cap to ground. This could certainly be worth trying out, if there is a last-minute problem, and a major redesign of the board is not an option. But they are still used because of their low cost and the low real estate they need, and so some tricks are possible to control their side effects. Note that a Schottky diode has no reverse recovery time in principle, but its body capacitance is relatively large and can end up resonating with trace inductances. Correctly chosen, the good results obtained with regards to EMI and output ripple are well worth the 1-2% drop in efficiency.
This is always acceptable if the two winding sections are connected in series to form the Primary winding. During crossover transitions, the Source pin develops spikes, and since the gate is referenced to the Source not the Drain, this can lead to spurious turn-ons and consequent reliability issues.
For very high currents (up to thousands of amperes if necessary), a possible choice are probes is based on the Rogowski principle. It could lead to achieving compliance at the input in those cases where the output cables were proven to be the main culprit.
For this, we need to cut the PCB traces just before the input bridge, and then route the AC power from a canned filter outside the enclosure.


This is very common in industries and factories, where EMI is fairly frequent in function of the larger use of machines such as welding instruments, motors (MCCs) and in digital networks and computers in the vicinity of these areas. It is very common the occurrence of noises in power source lines due to bad grounding and shielding or even error in the project.
The user is responsible to determine which rules to follow in his applications and guarantee that each device is installed in compliance with them. The use of loops and twists instead of wires (short wires are better for high frequencies) interconnecting the grounding points is more efficient in this case (see figure 4).
As the conditions for shielding, grounding and equalization improve, we reach the ideal measuring conditions.
When the circuits share the same ground wire, the circuit currents that flow through the finite impedance of the common base line may cause ground potential variations on the other circuits. This capacitance effect between two bodies with electric loads separated by a dielectric is called mutual capacitance effect. Figure 14 shows the interference between cables whose capacitive coupling induces voltage transients, such as electrostatic pickups. In high frequency circuits with grounded multipoints, the efficiency is high, since the return current tends to flow through the adjacent return.
In this case, watch the difference of potential between the grounded point and the panel where the controller (PLC) is located. Also note that commutation frequencies vary from 1k to 34KHz, usually 30khz, and may have great influence on the Foundation Fieldbus and the Profibus-PA networks .
The minimum and maximum gauges depend on the nominal inverter current and the physical limitation of the terminal blocks.
Individual standard conductors that meet the general type specifications concerning temperature, gauge and applicable codes are acceptable, provided they are distant from high voltage cables to minimize the coupling noise.
However, high peak load currents in cables may cause inverter overcurrents or ground failures. If possible, avoid passing large lengths of power source cables and motor conductor by the same duct.
Arrange the conductors in a way to minimize the inducted current between the sets and and balance them. The voltage difference between both ends should be a maximum of 1 V (rms) to minimize the ground loop effects. Make sure the continuity of the shielded cable is longer than 90% of the cable total length. In this method, the shield is grounded on the ground point of the power source or the safe side of the intrinsic safety barrier, besides the ground on the equipment junction boxes and housings, and is also grounded on the non-safe side.
When properly installed, the shield, which is closest to the primary winding, is connected to the common power supply ground and the shield closest to the secondary winding is connected to the shield of the circuit to be isolated. In this case, we want to couple the Primary and Secondary very close to each other in order to reduce leakage inductance, but this also increases their mutual capacitance, and thus the CM noise. If so connected, safety issues may need to be considered in regards to the requirement of reinforced insulation between Primary and Secondary, and also the required Primary to Secondary 'creepages' (distance along the insulating surface) and 'clearances' (shortest distance through air) as applicable.
The fringing fields from exposed air gaps become strong sources of radiated EMI besides causing significant eddy current losses in the copper band. The Drain end of this winding should definitely not be adjacent to the 'safety barrier' (the three layers of tape). We can also see that from the point of view of actual physical proximity, every dotted end of a winding is close to the non-dotted end of the next winding. This technique certainly saves production costs associated with the making and placing of a classic Faraday shield, not to mention the improvement in efficiency due to the reduced leakage inductance (as compared to a formal shield). The idea is that if the noise current is being pushed out from the primary winding, in the cancellation winding it gets pulled in. In fact in a front-end position, any component always need to be able to handle a lot of stress (if not abuse), such as the stresses occurring during power-up at high line.
Therefore it is not uncommon to intentionally degrade the Fet switching speed by adding a resistor (say 10 ohms to 100 ohms) in series with the gate (maybe with a diode across the resistor so as to leave the turn-off speed unaffected for efficiency reasons. At Fet turn-off, the diodes become strong E-field sources, whereas at Fet turn-on, the diodes will generate strong H-fields. At turn-on, it is asked to provide the high current spike required to charge up the CISS of the Fet. For the same reason, a loop of wire to accommodate a current probe is also correctly placed on the Drain side, never on the Source.
In Figure 4 we have shown two current probes, wired up in such a way that they are actually performing 'simultaneous equation' math to separate the CM and DM components. This type of probe is available from several manufacturers, for example Power Electronic Measurements Ltd. The ends of the existing filter are kept either open (to receive E-fields) or connected together through a small loop (for seeing H-fields). While most manufacturers take due precaution on their projects to minimize this effect, the complete immunity is not attainable.
The transient surges are minimized by proper line filters and energy suppressors on coils and other inductive loads. Cables with thicker gauge help reduce the ground resistance, while the flexible wire reduces the earth impedance. If the currents are large enough, the potential variations may cause serious disturbances on the operation of all the circuits connected to the common signal ground. This is critical in inverters with nominal power of 200 HP (150 KW) and keep the power source cables and the control cables separated. It is also important to be aware that in high frequencies there is parasite coupling capacitance that tends to complete the loop when the shielding is grounded on a single cable extreme. In this instance, the shield is insulated on all grounds, with exception to the negative ground point of the power source or the intrinsic safety barrier on the safe side.
An additional an complementary situation is one whose grounds would be grounded in a set on an equipotential ground line, connecting the non-safe side to the safe side.
On classified areas, avoid grounding intrinsically safe components, unless it is necessary for functional reasons, when employing galvanic insulation. Common high frequency conductors have the disadvantage of having high impedance and current loops should be avoided. A wire is soldered close to its approximate geometric center and goes to the Primary ground.
When the transformer uses an air gap on its outer limbs, the fringing flux emanating from the gap causes severe eddy current losses in the band.
That is usually more helpful than trying to reduce the amount of noise injection through the barrier, because that is something we can control quite well by various tricks --- like having the auxiliary winding double over as a Faraday shield etc. Note that the very same idea can be applied to any topology (and also all high power DC-DC converters), whenever the switch needs to be mounted on the chassis. Then, if two such rods are being used on a given output, we should wind the two rods identically, but reverse the current flow in one of them compared to the other by suitable modification of the PCB.
Any small R-C snubber present across the diode essentially controls the forward voltage spike too. So a resistor placed in series with this capacitor simply limits the charging current somewhat, and thereby slows the turn-on. Sometimes the Secondary winding is also sometimes broken up into two windings, and the intention is usually to increase its current handling capability. Return currents tend to travel by the shortest straight line path at low frequencies, but the higher harmonics tend to image themselves directly under the forward trace on the opposite layer.
By doing these two measurements at the same time rather than one after another, we have also retained valuable information about the relative phase relationship existing between the CM and DM components.
Note that, with this termination, the measured voltage is approximately half of what it really is because we essentially have a voltage divider formed by the cable and the terminating resistor. At that time, a ferrite sleeve or bead, slipped over the output bunch (as an alternative to twisting), was also working equally well, if not better.
The other end of this EMI filter is then routed as usual to the LISN and spectrum analyzer. Even though they may have achieved compliance with the conducted limits in this region, that is not enough for them.
So, some layout, wiring, grounding and shielding techniques offer a significant contribution to this optimization.
When arranging the boxes with cables for large inverters, check if the box or duct containing the signal wiring stay at 30cm or more from the one containing the motor or the power wiring. The standards establish the minimum insulation of 500V resistance between the ground terminal, while the system ground must be lower than 1Ω. The grounding system should be seen as a circuit that favors the flow of current under the minimum possible inductance. Note that the ends of the copper shield should not be galvanically connected, as that would constitute a shorted turn as far as the transformer is concerned. That would have brought the Drain end of the Primary winding right next to the safety barrier, with the Secondary ground end (which is connected to the chassis) directly across it.
We do note however, that a forward converter transformer has no (or very small) air gap, so it is generally considered 'quieter' to start with. A similar technique may be useful on the Secondary side too, if the catch diode is to be mounted on the chassis. So looking from the top, one should be carrying current clockwise and the other anticlockwise. If they are chosen to be fast, at least the diode should have soft recovery characteristics. The final solution here was a small ferrite bead inserted on the leg of the Schottky diode. Currents, given a chance, therefore automatically try to reduce the area they enclose, as this lowers the self-inductance of the trace containing them, and thereby provides the current the lowest impedance route. Oscilloscopes will usually automatically correct for this, if they 'know' that there is a 50?- termination present.
But it was disqualified simply because it was far more expensive than three tie-wraps put together.
They look at the overall shape of the plot and if they find that that it is gradually rising towards 30MHz, they are confident that they have a radiation problem. In Brazil, the installation on potentially explosive atmospheres is regulated by the NBR-5418. Some designs also use an additional Faraday shield on the Secondary side (after the 3 layers of insulation), and that is connected to the Secondary ground. Note that the ends of this band can, and should be, soldered together, because this is an outer shield, and can never constitute a shorted turn for the transformer. Sometimes it is wound bifilar to the Primary winding, though we must be aware that since the 'dots' of these two windings are mutually reversed, we will have a high voltage differential between the two windings at certain points along their length.
This helps redirect the flux from one inductor straight into the other, so there is much less spilled around. A popular choice is the soft recovery fast diode like the BYV26C (or BYM26C for medium power) from Philips. This smoothened out the wiggle, reduced the EMI dramatically in the bargain, and as a proof of the hypothesis, no Schottky failures were seen thereafter. So if ill-considered cuts are made in the ground plane with the intention of 'conveniently' routing some other trace, the return currents of the power converter stage (which really need this ground plane) will get diverted along the sides of the intervening cuts, and in doing so, will form effective slot antennas. The output from a Rogowski probe depends not on the current enclosed but on its rate of change. Fast-rising pulses can also produce spurious ringing due to high-frequency current crowding on the surface of the cable shield. It is also interesting to note here, that though a ferrite sleeve may look like a radiation shield, and smell like one too, and even produce the same results as twisting the cables, it actually works by reducing the common mode noise currents, not by shielding them.
It will give us an indication if a heatsink for example is causing severe E-fields, or a certain core is causing severe H-fields.
But if the plot starts drooping or remains generally flat as 30MHz approaches, they are likely to immediately submit the prototype to a lab for the formal radiated limit compliance certification. This method has the disadvantage that it does not protect totally the high frequency signals and, depending on the topology and cable length, may generate occasional communication intermittence.
However, most commercial ITE power supply designs don't need these shields, provided adequate thought has gone into the winding and construction, as we will discuss.
So if there are pinholes in the enamel insulation, there is a danger of flashover and resulting failure of the power supply.
We can work out that the boost and flyback topologies don't have a problem, though the buck and forward converters do.
All traces leading to components marked 'Critical' or 'Important' must be kept very short (and neither too thin).
So, instead of just placing several turns around the wire to be sensed as in a typical current transformer, the Rogowski probe effectively takes an air-core solenoid and then bends that in a circle around the sensed wire (a doughnut).
This can be suppressed by threading the measurement cable through one or more ferrite beads or toroids. In our case we clearly had significant common mode noise currents present on the output, which were causing the output cables to radiate. We can wave a small plate of thick copper (connected to Earth) in suspected areas to see which component may be the source.
You can actually 'see' the energy level in the 5-30 MHz region, and you need to be sure that it is quieting down.
In paralleled windings, the two supposedly 'equal' sections are actually magnetically slightly different because of their different positions inside the transformer.
The radiation was then being picked up by the inputs causing the conducted EMI test to fail.
For studying the magnetic near-fields, a slab of ferrite (from a typical EMI suppression kit) works far better than a copper plate, and this can be waved around similarly (no need to Earth it though). If there is an undue amount of conducted noise energy, radiated noise can't be too far off. The usual lab active current probes (which also measure DC, and include a Hall sensor), are just not suited for these high-bandwidth measurements. The tie-wraps did not lower the noise, but merely helped prevent the radiation from the cables from spilling out.
If this paralleling is really needed, it is better to use them as shown in the lower schematic of Figure 3. Here the diode forward drops help to 'ballast' the windings and thus they 'iron out' any inequality between the two halves.



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