Reducing the level of interference from switching power supplies. Competent manufacturers should have foreseen everything! EDP ​​Series Filters

To prevent interference from electrical and radio devices, it is necessary to equip them with a filter to suppress interference from the mains, located inside the equipment, which allows you to combat interference at their source.

If you cannot find a ready-made filter, you can make it yourself. The noise suppression filter circuit is shown in the figure below:

The filter is two-stage. The first stage is made on the basis of a longitudinal transformer (two-winding choke) T1, the second is a high-frequency choke L1 and L2. The windings of the transformer T1 are connected in series with the line wires of the supply network. For this reason, low-frequency fields with a frequency of 50 Hz in each winding have opposite directions and mutually cancel each other. Under the influence of interference on the power wires, the transformer windings are connected in series, and their inductive resistance XL increases with an increase in the interference frequency: XL = ωL = 2πfL, f is the frequency of the interference, L is the inductance of the transformer windings connected in series.

The resistance of the capacitors C1, C2, on the contrary, decreases with increasing frequency (Xc = 1 / ωC = 1 / 2πfC), therefore, noise and sudden jumps are "short-circuited" at the input and output of the filter. The same function is performed by capacitors C3 and C4.

Chokes LI, L2 represent one more series additional resistance for high-frequency interference, ensuring their further attenuation. Resistors R2, R3 reduce the Q-factor of L1, L2 to eliminate resonance phenomena.

Resistor R1 provides a quick discharge of capacitors C1-C4 when the power cord is disconnected from the mains and is necessary for safe handling of the device.

The parts of the surge protector are located on the printed circuit board shown in the figure below:

The printed circuit board is designed for the installation of an industrial longitudinal transformer from personal computer units. You can make a transformer yourself by making it on a ferrite ring with a permeability of 1000NN ... 3000NN with a diameter of 20 ... 30 mm. The edges of the ring are treated with fine-grained sandpaper, after which the ring is wrapped with fluoroplastic tape. Both windings are wound in the same direction with a PEV-2 wire with a diameter of 0.7 mm and have 10 ... 20 turns each. The windings are placed strictly symmetrically on each half of the ring, the gap between the terminals must be at least 3 ... 4 mm. Chokes L2 and L3 are also of industrial production, wound on ferrite cores with a diameter of 3 mm and a length of 15 mm. Each choke contains three layers of PEV-2 wire with a diameter of 0.6 mm, the length of the winding is 10 mm. To prevent the turns from slipping, the choke is impregnated with epoxy glue. The parameters of the winding products are selected from the condition of the maximum filter power up to 500 W. For higher power, the dimensions of the filter cores and the diameter of the wires must be increased. The dimensions of the printed circuit board will also have to be changed, but you should always strive for a compact arrangement of the filter elements.

Switching power supplies, thyristor controllers, switches, powerful radio transmitters, electric motors, substations, any electrical discharges near power lines (lightning, welding machines, etc.) generate narrow-band and broad-band interference of various nature and spectral composition. This complicates the operation of low-current sensitive equipment, introduces distortions in the measurement results, causes malfunctions and even failure of both instrument assemblies and entire complexes of equipment.

In symmetrical electrical circuits (ungrounded circuits and circuits with a grounded midpoint) antiphase interference appears in the form of balanced voltages (across the load) and is called symmetric, in foreign literature it is called differential mode interference. Common mode interference in a balanced circuit is called asymmetric or common mode interference.

Symmetrical line noise usually predominates at frequencies up to several hundred kHz. At frequencies above 1 MHz, asymmetric interference predominates.

A rather simple case is narrow-band interference, the elimination of which is reduced to filtering the fundamental (carrier) frequency of the interference and its harmonics. A much more complicated case is high-frequency impulse noise, the spectrum of which covers a range of up to tens of MHz. Dealing with such interference is quite challenging.

Only a systematic approach will help to eliminate strong complex interference, which includes a list of measures to suppress unwanted components of the supply voltage and signal circuits: shielding, grounding, correct installation of supply and signal lines and, of course, filtering. A huge number of filtering devices of various designs, quality factors, applications, etc. produced and used all over the world.

Filter designs differ depending on the type of interference and the area of ​​application. But, as a rule, the device is a combination of LC-circuits, forming filter stages and P-type filters.

An important characteristic of a line filter is the maximum leakage current. In power applications, this current can reach a value that is dangerous to humans. Based on the leakage current values, the filters are classified according to the safety levels: applications that allow human contact with the device housing and applications where contact with the housing is undesirable. It is important to remember that the filter housing requires mandatory grounding.

TE-Connectivity, building on Corcom's more than 50 years of experience in the design and development of electromagnetic and RF filters, offers the broadest range of devices for use in a variety of industries and hardware assemblies. A number of popular series from this manufacturer are presented on the Russian market.

B Series General Purpose Filters

Series B filters (Figure 1) are reliable and compact filters at an affordable price. A wide range of operating currents, good quality factor and a wide selection of connection types provide a wide range of applications for these devices.

Rice. 1.

Series B includes two modifications - VB and EB, the technical characteristics of which are shown in table 1.

Table 1. Main technical characteristics of B series line filters

The electrical circuit of the filter is shown in Figure 2.

Rice. 2.

The attenuation of the interference signal in dB is shown in Fig. 3.

Rice. 3.

T Series Filters

Filters of this series (Figure 4) are high-performance RF filters for power circuits of switching power supplies. The advantages of the series are excellent suppression of antiphase and common mode noise, compact size. Low leakage currents allow the T-series to be used in devices with low power consumption.

Rice. 4.

The series includes two modifications - ET and VT, the technical characteristics of which are shown in table 2.

Table 2. Main technical characteristics of T series surge protectors

The electrical diagram of the T series filter is shown in Figure 5.

Rice. 5.

The attenuation of the noise signal in dB when the line is loaded onto a 50 Ohm terminating resistor is shown in Fig. 6.

Rice. 6.

K series filters

K series filters (Figure 7) are general purpose RF power filters. They are designed for use in high-impedance power circuits. Ideal for applications where pulsed, continuous and / or pulsed RF interference is induced on the line. Models with the EK index meet the requirements of the standards for use in portable devices, medical equipment.

Rice. 7.

Filters with index C are equipped with a choke between the frame and the grounding conductor. The main electrical parameters of K series line filters are shown in Table 3.

Table 3. Basic electrical parameters of K series surge protectors

The electrical circuit of the K series filter is shown in Figure 8.

Rice. eight.

The attenuation of the noise signal in dB when the line is loaded onto a 50 Ohm terminating resistor is shown in Fig. 9.

Rice. nine.

EMC Series Filters

Filters in this series (Figure 10) are compact and efficient two-stage RF power filters. They have a number of advantages: high coefficient of common-mode noise reduction in the low-frequency region, high coefficient of antiphase noise reduction, compact size. The EMC series is focused on applications with switching power supplies.

Rice. ten.

The main technical characteristics are shown in table 4.

Table 4. Main electrical parameters of EMC series surge protectors

The EMC filter wiring diagram is shown in Figure 11.

Rice. eleven.

The attenuation of the noise signal in dB when the line is loaded onto a 50 Ohm terminating resistor is shown in Fig. 12.

Rice. 12.

EDP ​​Series Filters

2. Corcom Product Guide, General purpose RFI filters for high impedance loads at low current B Series, TE Connectivity, 1654001, 06/2011, p. 15

3. Corcom Product Guide, PC board mountable general purpose RFI filters EBP, EDP & EOP series, TE Connectivity, 1654001, 06/2011, p. 21

4. Corcom Product Guide, Compact and cost-effective dual stage RFI power line filters EMC Series, TE Connectivity, 1654001, 06/2011, p. 24

5. Corcom Product Guide, Single phase power line filter for frequency converters FC Series, 1654001, 06/2011, p. thirty

6. Corcom Product Guide, General purpose RFI power line filters - ideal for high-impedance loads K Series, 1654001, 06/2011, p. 49

7. Corcom Product Guide, High performance RFI power line filters for switching power supplies T Series, 1654001, 06/2011, p. 80

8. Corcom Product Guide, Compact low-current 3-phase WYE RFI filters AYO Series, 1654001, 06/2011, p. 111.

Obtaining technical information, ordering samples, delivery - e-mail:

Network and signal EMI / RFI filters from TE Connectivity. From board to industrial installation

Company TE Connectivity is a world leader in the design and manufacture of surge protectors for effective suppression of electromagnetic and radio frequency interference in electronics and industry. The product line includes more than 70 series of devices for filtering both power supply circuits from external and internal sources, and signal circuits in the widest scope of applications.

Filters have the following design options: miniature for installation on a printed circuit board; housings of various sizes and types of connection of supply lines and load lines; in the form of ready-made power connectors and communication connectors for network and telephone equipment; industrial, made in the form of ready-made industrial cabinets.

Line filters are manufactured for AC and DC applications, single and three-phase networks, cover the range of operating currents 1… 1200 A and voltages 120/250/480 VAC, 48… 130 VDC. All devices are characterized by a low voltage drop - no more than 1% of the operating voltage. Leakage current, depending on the power and filter design, is 0.2 ... 8.0 mA. The averaged frequency range over the series is 10 kHz… 30 MHz. Series AQ designed for a wider frequency range: 10 kHz… 1 GHz. Expanding the applications of its devices, TE Connectivity produces filters for low and high impedance load circuits. For example, high impedance filters of the series EP, H, Q, R and V for low impedance loads and low impedance series B, EC, ED, EF, G, K, N, Q, S, SK, T, W, X, Y and Z for high impedance loads.

Communication connectors with integrated signal filters are available in shielded, dual, and low profile designs.

Each filter from TE Connectivity is double tested: at the assembly stage and already as a finished product. All products comply with international quality and safety standards.

EMI suppression filter (10+)

High frequency electromagnetic interference filter

The reason for the occurrence of high-frequency impulse noise is trivial. The speed of light is not infinite, and the electromagnetic field travels at the speed of light. When we have a device that somehow converts the mains voltage by frequent switching, we expect that ripple currents directed towards each other will appear in the power wires going to the mains. Through one wire, current flows into the device, and through the other, it flows out. But it’s not at all like that. Due to the finiteness of the field propagation velocity, the impulse of the incoming current is phase-shifted relative to the outgoing one. Thus, at a certain frequency, high-frequency currents in the network wires flow in the same direction, in phase.

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If something is not clear, be sure to ask!

Shevkoplyas B.V. “Microprocessor structures. Engineering solutions. " Moscow, publishing house "Radio", 1990. Chapter 4

4.1. Primary Line Noise Suppression

The waveform of the alternating voltage of an industrial power supply network (~ "220 V, 50 Hz) for short periods of time can differ greatly from the sinusoidal one - surges or" insertions "are possible, a decrease in the amplitude of one or several half-waves, etc. The reasons for the occurrence of such distortions are related usually with a sharp change in the mains load, for example, when a powerful electric motor, furnace, welding machine is turned on. Therefore, it is necessary, if possible, to isolate from such sources of interference via the mains (Fig. 4.1).

Rice. 4.1 Variants of connecting a digital device to a primary supply network

In addition to this measure, it may be necessary to introduce a surge protector at the power input of the device in order to suppress short-term interference. The resonant frequency of the filter can be in the range of 0.1.5-300 MHz; broadband filters provide interference suppression over the entire specified range.

Figure 4.2 shows an example of a mains filter circuit. This filter has dimensions of 30 XZOX20 mm and is mounted directly on the mains input block into the device. Filters should use high frequency capacitors and inductors, either coreless or high frequency cores.

In some cases, it is obligatory to introduce an electrostatic shield (an ordinary water pipe connected to a grounded power switchboard housing) to lay the wires of the primary supply network inside it. As noted in, the shortwave transmitter of the taxi fleet, located on the opposite side of the street, is capable, with a certain relative orientation, to induce signals with an amplitude of several hundred volts on a piece of wire. The same wire, placed in an electrostatic screen, will be reliably protected from this kind of interference.

Rice. 4.2. Example of a network filter circuit

Consider methods for suppressing network noise directly in the device's power supply. If the primary and secondary windings of the power transformer are located on the same coil (Fig.4.3, a), then due to the capacitive coupling between the windings, impulse noise can pass from the primary circuit to the secondary. According to the recommended four methods of suppression of such interference (in order of increasing efficiency).

1. The primary and secondary windings of the power transformer are performed on different coils (Figure 4.3, b). The throughput capacity C decreases, but the efficiency decreases, since not all of the magnetic flux from the primary winding region enters the secondary winding region due to scattering through the surrounding space.
2. The primary and secondary windings are made on the same coil, but separated by a copper foil shield with a thickness of at least 0.2 mm. The screen should not be a short-circuited loop. It is connected to the frame ground of the device (Fig. 4.3, c)
3. The primary winding is completely enclosed in a screen that is not a short-circuited turn. The screen is grounded (fig. 4.3, G).
4. The primary and secondary windings are enclosed in individual screens, between which a dividing screen is laid. The entire transformer is enclosed in a metal case (Fig.4.3,<Э). Экраны и корпус заземляются. Этот тип трансформатора в силу предельной защищенности от прохождения помех получил название «ультраизолятор».

With all of the above methods of suppression of interference, the wiring of the mains wires inside the device should be carried out with a shielded wire, connecting the shield to the frame ground. Invalid uk
tie into one bundle of network and other (power supply, signal, etc.) wires "even in the case of shielding both.

It is recommended to install a capacitor with a capacity of about 0.1 μF in parallel with the primary winding of the power transformer in the immediate vicinity of the winding terminals and, in series with it, a current-limiting resistor with a resistance of about 100 Ohm. This makes it possible to “close” the energy stored in the core of the power transformer at the moment the mains switch is opened.

Rice. 4.3. Options for protecting a power transformer from the transmission of impulse noise from the network to the secondary circuit (and vice versa):
a - no protection; b - the separation of the primary and secondary windings; v- laying the screen between the windings; G - complete shielding of the primary winding; d - complete shielding of all elements of the transformer

Rice. 4.4. Simplified power supply diagram (a) and diagrams (b, c), explaining the operation of a full-wave rectifier.

The power supply is the greater the source of impulse noise over the network, the greater the capacitance of the capacitor C

Note that with an increase in the capacitance C of the filter (Fig. 4.4, a) of the power supply of our device, the likelihood of failures of neighboring devices increases, since the power consumption from the network by our device increasingly acquires the character of blows. Indeed, the voltage at the output of the rectifier also increases in those time intervals when energy is taken from the network (Fig. 4.4, b). These intervals in Fig. 4.4 are shaded.

With an increase in the capacitance of the capacitor C, the periods of its charge become less and less (Fig. 4.4, c), and the current taken in a pulse from the network becomes more and more large. Thus, an outwardly "harmless" device can create interference in the network that is "not inferior" to interference from a welding machine.

4.2. Grounding Rules Providing Protection Against Ground Interference

In devices made in the form of structurally complete blocks, there are at least two types of "ground" buses - housing and circuit. In accordance with safety requirements, the frame bus is necessarily connected to the grounding bus laid in the room. The circuit bus (relative to which the signal voltage levels are measured) should not be connected to the frame bus inside the block — a separate clamp must be brought out for it, isolated from the frame.

Rice. 4.5. Improper and correct grounding of digital devices. Shown is a ground bus that is typically found indoors

In fig. 4.5 shows options for incorrect and correct grounding of a group of devices, which are interconnected by information lines. (these lines are not shown). Circuit buses "ground" are connected by individual wires at point A, and frame - at point B, as close as possible to point A. Point A may not be connected to the ground bus in rooms, but this creates inconvenience, for example, when working with an oscilloscope, which The probe ground is connected to the case.

With an incorrect grounding (see Fig. 4.5), the impulse voltages generated by the equalizing currents along the ground bus will actually be applied to the inputs of the receiving trunk elements, which can cause their false triggering. It should be noted that the selection of the best earthing option depends on the specific “local” conditions and is often carried out after a series of careful experiments. However, the general rule (see Figure 4.5) always holds true.

4.3. Suppression of interference on secondary power supply circuits

Due to the finite inductance of the power and ground rails, impulse currents cause impulse voltages of both positive and negative polarity to be applied between the power and ground pins of the microcircuits. If the power and ground buses are made with thin printed or other conductors, and high-frequency decoupling capacitors are either completely absent or their number is not enough, then when several TTL microcircuits are simultaneously switched at the "far" end of the printed circuit board, the amplitude of impulse noise on the power supply (voltage surges acting between the power supply and the ground of the microcircuit) can be 2 V or more. Therefore, when designing a printed circuit board, the following guidelines should be followed.

1. The power and ground rails should have minimum inductance. To do this, they are made in the form of lattice structures that cover the entire area of ​​the printed circuit board. It is unacceptable to connect TTL microcircuits to the bus, which is a "branch", because as it approaches its end, the inductance of the power supply circuits accumulates. The power and ground rails should, if possible, cover the entire free area of ​​the printed circuit board. Special attention should be paid to the design of accumulative dynamic memory matrices on the K565RU5, RU7 microcircuits, etc. The matrix should be a square so that the address and control lines have a minimum length. Each microcircuit must be in an individual cell of the lattice structure formed by the power and ground rails (two independent grids). The power and ground buses of the storage matrix should not be loaded with "foreign" currents flowing from address drivers, control signal amplifiers, etc.
2. Connection of external power and ground buses to the board through the connector should be made through several contacts, evenly spaced along the length of the connector, so that the input into the lattice structures of the power and ground buses is made from several points at once.
3. Power supply interference should be suppressed close to where it occurs. Therefore, a high-frequency capacitor with a capacity of at least 0.02 μF must be located near the power pins of each TTL microcircuit. This also applies especially to the aforementioned heap memory chips. To filter low-frequency interference, it is necessary to use electrolytic capacitors, for example, with a capacity of 100 μF. When using dynamic memory microcircuits, electrolytic capacitors are installed, for example, in the corners of the storage matrix or in another place, but near these microcircuits.

According to, instead of high-frequency capacitors, special power buses BUS-BAR, CAP-BUS are used, which are laid under or between microcircuit lines, without disrupting the usual automated technology of installing elements on a board with subsequent "wave" soldering. These buses are distributed capacitors with a linear capacitance of about 0.02 μF / cm. With the same total capacitance as discrete capacitors, buses provide significantly better noise rejection at higher wiring densities.

Rice. 4.6. Variants of connection of P1-PZ boards to the power supply unit

In fig. 4.6 recommendations are given for connecting devices made on printed circuit boards P1 — PZ to the output of the power supply. A high-current device made on the PZ board creates more noise on the power and ground buses, so it should be physically brought closer to the power supply, and even better, it should be powered using individual buses.

4.4. Rules for working with agreed communication lines

In fig. 4.7 shows the waveform of signals transmitted through the cable, depending on the ratio of the resistance of the load resistor R and the characteristic impedance of the cable p. Signals are transmitted without distortion when R = p. The characteristic impedance of a particular type of coaxial cable is known (for example, 50, 75, 100 ohms). The characteristic impedance of flat cables and twisted pairs is usually close to 110-130 ohms; its exact value can be obtained experimentally by selecting a resistor K, when connected, the distortions are minimal (see Fig. 4.7). When conducting an experiment, do not use variable wire resistances, since they have a large inductance and can distort the waveform.

Communication line of the "open collector" type (Fig. 4.8). For transmission of each trunk signal with a rise time of about 10 ns at distances exceeding 30 cm, a separate twisted pair is used or one pair of conductors is separated in a flat cable. In the passive state, all transmitters are turned off. When any transmitter or group of transmitters is triggered, the line voltage drops from more than 3 V to about 0.4 V.

With a line length of 15 m and with its correct matching, the duration of transient processes in it does not exceed 75 ns. The line implements the Editing OR function with respect to signals represented by low voltage levels.

Rice. 4.7. Signal transmission via cable. О - voltage pulse generator

Communication line of the "open emitter" type (Fig. 4.9 "). This example shows an example of a line using a flat cable. Signal wires alternate with earth wires. Ideally, each signal wire is bordered on both sides by its own earth wires, but this is usually not particularly necessary. In Fig. 4.9, each signal wire is adjacent to "own" and "foreign" ground, which is usually quite acceptable. A flat cable and a set of twisted pairs are essentially the same thing, and yet the latter is preferable in conditions of an increased level of external interference. An open emitter line provides a Wired OR function to signals represented by high voltage levels. The timing characteristics are approximately the same as those of an "open collector" line.

Communication line of the "differential pair" type (Fig. 4.10). The line is used for unidirectional signal transmission and is characterized by increased noise immunity, since the receiver reacts to the difference in signals, and the interference induced from the outside acts on both wires in approximately the same way. The line length is practically limited by the ohmic resistance of the wires and can reach several hundred meters.

Rice, 4.8. Open collector communication line

Rice. 4.9. Communication line of the "open emitter" type

Rice. 4.10. Differential pair communication line

All lines discussed should use receivers with high input impedance, low input capacitance, and preferably with a hysteresis transfer characteristic to increase noise immunity.

Physical implementation of the highway (Fig. 4. II), Each device connected to the trunk contains two connectors. A circuit similar to that shown in Fig. 4.11, was considered earlier (see Fig. 3.3), therefore, we will dwell only on the rules that must be observed when designing matching units (SB).

Transmission of trunk signals through connectors. The best options for wiring connectors are shown in Fig. .4.12. In these cases, the front of the pulse running along the line almost does not "feel" the connector, since the inhomogeneity introduced into the cable line is insignificant. This, however, requires taking 50% of the contacts used underground.

If this condition is impracticable for some reason, then it is possible, to the detriment of noise immunity, to accept the second, more economical, but the number of contacts option for wiring the connectors, shown in Fig. 4.13. This option is often used in practice. Twisted pair earths (or flat cable earths) are collected on metal strips of the largest possible cross-section, for example 5 mm2.

The wiring of these lands is carried out evenly along the length of the strip, as the corresponding signal wires are wired. Both strips are connected via a connector using a series of jumpers of minimum length and maximum cross-section, and the jumpers are evenly spaced along the length of the strips. Each earthing jumper must correspond to no more than four signal lines, but the total number of jumpers must not be less than three (one in the center and two at the edges).

Rice. 4.13. Acceptable option for signal transmission through the connector. H- = 5 mm2 — cross-section of the bar, 5 ^ 0.5 mm2 — cross-section of the earth wire

Rice. 4.14. Options for performing branches from the trunk

Execution of branches from the highway. In fig. 4.14 shows options for incorrect and correct execution of a branch from the trunk. The path of one line is traced, the earth wire is shown conditionally. The first option (a typical mistake of novice circuit engineers!) Is characterized by splitting the wave energy into two parts,

Rice. 4.15. Options for connecting receivers to the trunk
coming from line A. One part goes to the charge of line B, the other goes to the charge of line C. After the charge of line C, the "full" wave begins to propagate along line B, trying to catch up with the wave with half the energy that left earlier. The signal front thus has a stepped shape.

With the correct execution of the branch, the segments of the lines A, C and B turn out to be connected in series, therefore the wave is practically not split and the signal fronts are not distorted. The transmitters and receivers located on the board should be as close as possible to its edge to reduce the inhomogeneity introduced at the point where the line segments B and C combine.

One or two-way transceivers can be used to decouple the receiver beams from the backbone (see Fig. 3.18. 3.19). When branching a line into several directions, a separate transmitter should be allocated for each (Fig. 4.15, v).

For line transmission, it is better to use trapezoidal rather than rectangular pulses. Signals with shallow edges, as noted, propagate along the line with less distortion. In principle, in the absence of external interference for any arbitrarily long and even inconsistent line, you can choose such a slow rise rate of the signal that the transmitted and received signals will differ by an arbitrarily small amount.

To obtain trapezoidal pulses, the transmitter is made in the form of a differential amplifier with an integrating feedback loop. At the input of the main receiver, also made in the form of a differential amplifier, an integrating circuit is installed to filter high-frequency interference.

When transmitting signals within the board, when the number of receivers is large, "serial termination" is often used. It consists in the fact that in series with the output of the transmitter, in the immediate vicinity of this output, a resistor with a resistance of 20-50 Ohm is switched on. This allows you to damp the oscillatory processes at the signal fronts. This technique is often used when transmitting control signals (KA5, SAZ, \ UE) from amplifiers to dynamic memory LSIs.

4.5. About the protective properties of cables

In fig. 4.16, a shows the simplest scheme for transmitting signals over a coaxial cable, which in some cases can be considered quite satisfactory. Its main disadvantage is that in the presence of pulsed equalizing currents between the frame earths (potential equalization is the main function of the frame earth system), part of these currents 1 can flow through the cable braid and cause a voltage drop (mainly due to the inductance of the braid), which ultimately acts on the load K.

Moreover, in this sense, the diagram shown in Fig. 4.16, a, turns out to be preferable, and with an increase in the number of points of contact of the cable sheath with the frame ground, the possibilities for the induced charges to drain from the braid are improved. The use of a cable with an additional braid (Fig. 4.16, c) allows you to protect yourself from both capacitive pickups and equalizing currents, which in this case flow through the outer braid and practically do not affect the signal circuit.

Connecting a cable with additional braiding according to the diagram shown in Fig. 4.16, d, allows you to improve the frequency properties of the line by reducing its linear capacity. Ideally, the potential of any elementary section of the central core coincides with the potential of the elementary cylinder of the inner braid that surrounds this section.

Lines of this type are used in local computer networks to increase the speed of information transfer. The outer sheath of the cable is part of the signal circuit, and therefore, in terms of immunity from external interference, this circuit is equivalent to the circuit shown in Fig. 4.16.6.

Rice. 4.16. Cable Uses

Neither the copper nor the aluminum braid of a simple coaxial cable protects it from the effects of low frequency magnetic fields. These fields induce EMF both on the braid section and on the corresponding section of the central core.

Although these EMFs are of the same name in sign, they do not compensate each other in magnitude due to the different geometry of the corresponding conductors - the central core and the braid. The differential EMF is ultimately applied to load K. Additional braid (Fig. 4. 16, c, d) also unable to prevent the penetration of a low-frequency magnetic field into its inner region

Protection from low-frequency magnetic fields is provided by a cable containing a twisted pair of wires, enclosed in a braid (Fig. 4.16, e). In this case, the EMF induced by an external magnetic field on the wires that make up the twisted pair completely compensate each other both in sign and in absolute value.

This is all the more true, the smaller the pitch of the twisting of the wires in comparison with the zone of action of the field and the more carefully (symmetrically) the twisting is performed. The disadvantage of such a line is its relatively low frequency "ceiling" - about 15 MHz - due to the large energy losses of the useful signal at higher frequencies.

The diagram shown in Fig. 4.16, e, provides the best protection against all types of interference (capacitive pickup, equalizing currents, low-frequency magnetic fields, high-frequency electromagnetic fields).

It is recommended to connect the inner braid to the “radio engineering” or “true” (literally grounded) ground, and the outer braid to the “system” (circuit or frame) ground. In the absence of a "true" ground, you can use the connection diagram shown in fig. 4. 16, f.

The outer braid connects to the system ground at both ends, while the inner braid only connects to the source side. In cases where there is no need for protection against low-frequency magnetic fields and it is possible to transmit information without using paraphase signals, one of the twisted pair wires can serve as a signal wire, and the other as a shield. In these cases, the circuits shown in Fig. 4.16, c, f, can be thought of as coaxial cables with three shields — a twisted pair ground wire, an inner and outer braided cable.

4.6. Using optocouplers to suppress interference

If the devices of the system are separated by a considerable distance, for example, by 500 m, then it is difficult to count on the fact that their lands always have the same potential. As noted, the equalizing currents along the earth conductors create impulse noise on these conductors due to their inductance. This interference is ultimately applied to the inputs of the receivers and can cause false alarms.

The use of differential-pair lines (see § 4.4) can only suppress common-mode noise and therefore does not always bark positive results. In fig. 4.17 shows the diagrams of optocouplers between two devices remote from each other.

Rice. 4.17. Optocoupler isolation schemes between devices remote from each other:
a - with an active receiver, b- with active transmitter

A circuit with an "active receiver" (Fig. 4.17, a) contains a transmitting optocoupler VI and a receiving optocoupler V2. When pulsed signals are applied to the input X, the LED of the VI optocoupler periodically emits light, as a result, the output transistor of this optocoupler periodically saturates and the resistance between points a and b drops from several hundred kilo-ohms to several tens of ohms.

When the output transistor of the transmitting optocoupler is turned on, the current from the positive pole of the source U2 passes through the LED of the optocoupler V2, line (points a and b) and returns to the negative pole of this source. The U2 source is isolated from the U3 source.

If the output transistor of the transmitting optocoupler is off, no current flows through the source circuit U2. Signal X "at the output of the optocoupler V2 is close to zero if its LED is on, and close to +4 V if this LED is off. Thus, when X == 0, the LEDs of the transmitting and receiving optocouplers are on and, therefore, X" == 0. When X == 1 both LEDs are off and X "== 1.

Optocoupler isolation can significantly increase the noise immunity of the communication channel and ensure the transmission of information over distances of the order of hundreds of meters. Diodes connected to the transmitting and receiving optocouplers are used to protect them from reverse voltage surges. The resistor circuit connected to the source U2 serves to set the current in the line and limit the current through the LED of the receiving optocoupler.

The current in the line according to the IRPS interface can be selected equal to 20 or 40 mA. When choosing the resistor values, the ohmic resistance of the communication line must be taken into account. Scheme with an "active transmitter" (Fig. 4.17, b) differs from the previous one in that the power supply of the U2 line is located on the side of the transmitter. This does not offer any advantages - both circuits are essentially the same and are so-called "current loops".

The recommendations given in this chapter may seem too harsh for a beginner circuit designer. He sees the fight against interference as a "battle with the windmill", and the lack of experience in the design of devices of increased complexity creates the illusion that it is possible to create a workable device without following any of the recommendations given.

Indeed, sometimes this is possible. There are even known cases of serial production of such devices. However, in informal reviews of their work, you can hear many interesting non-technical expressions, such as visit effect and some others, simpler and more understandable.

Switching power supplies (UPS), built on the basis of converters of DC (rectified mains) voltage to AC, generate unwanted noise. On the collectors (drains) of the power switches of the UPS controllers, there is a voltage close to rectangular in shape, with a swing reaching 600 ... 700V. In addition, there are closed circuits in the UPS, through which pulse currents circulate with rather steep edges and slopes (0.1 ... 1 μs) and an amplitude of up to 3 ... 5A and more.

Generally speaking, PWM converters that operate at a constant switching frequency generate noise in a known frequency band, which makes it easier to suppress them and is one of the reasons for their widespread use in pulsed power supply circuits for household appliances.

However, switching power supplies, regardless of the type of PWM converter used, must be equipped with suppression circuits for two main types of interference. These noises are single-ended (differential) input and balanced (common-mode) input noise.

The mechanisms of occurrence, propagation and methods of struggle in switching power supplies with these noises will be considered using the example of the corresponding equivalent circuits of converters.

Fig. 1 Occurrence of unbalanced noise

Single-ended input noise is a noise current flowing due to the voltage difference Vin between the two input conductors (Fig. 1). The key transistor of the converter is shown in the figure in the form of a switch Fs, which is sequentially turned on and off at the frequency of the converter's pseudo-frequency. The load is shown as a variable resistor R L, the resistance of which changes depending on the load current. Passive elements L and C correspond to the input filter built into the converter. In addition, almost all converters are equipped with an input capacitor Cb, and some also have at least a small series inductance (choke) taken into account in the source impedance Zs (Zs also takes into account the intrinsic inductance of the line rectifier smoothing electrolytic capacitor).

Effective suppression of asymmetric interference is achieved through the shunting action of the capacitor Cb, which must be of high quality and be characterized by low equivalent series inductance (ESI) and resistance (ESR) in the corresponding frequency range (usually in the switching frequency range and above). In real circuits, Cb is usually a constant capacitor of 0.1 ... 1.0 μF, shunting the electrolytic capacitor of the mains rectifier. In the rectifier, they simultaneously strive to use high-quality, as a rule, tantalum, electrolytic capacitors with small EPI and ESR.

Symmetrical interference is suppressed using a balun, which is an inductor with two windings having the same number of turns. It has a high impedance for symmetrical current, but practically zero for unbalanced.

The unbalanced current (including the current drawn) flows into the upper winding of the transformer and outflows from the lower one. Since the currents through these windings are equal in magnitude and opposite in direction, and the number of turns in the windings is the same, the resulting magnetic flux in the core due to the unbalanced current turns out to be zero, although the amount of current consumed can be very large. Because of this, a high permeability core with no air gap is usually used in a balun transformer. Moreover, it has a sufficiently high inductance for a symmetrical current when using windings of only a few turns. A much smaller symmetrical interference current flows mainly through the lower winding, as well as through the upper one in the same direction. Consequently, the balun transformer has a high impedance for symmetrical disturbance currents.

As additional measures to suppress interference in pulsed power supplies, the following are applied:

The above measures, as a rule, are sufficient, and therefore, in household equipment, impulse power supplies are usually used without shielding enclosures.

Fig. 3 Typical circuit of a line filter and rectifier

Some of the considered methods of dealing with interference in a UPS are illustrated by the example of a typical circuit of a mains rectifier (Fig. 3) used in the designs of VM and TV. Capacitors C5 ... C8 installed in parallel to diodes D1 ... D4 of the bridge rectifier of the mains voltage serve to suppress asymmetric interference. The same role is played by capacitors C1,2, which symmetrical the potentials of the network wire relative to the chassis of the electronic equipment.