Absorption of interference in switching power supplies. High-frequency pulse electromagnetic interference suppression filter, radiation

Impulse interference refers to various types of interference created by surges of direct or alternating voltage or current occurring in any circuits and devices. Pulse pickups include:

direct aiming of video impulses;

shock excitation of high-frequency devices with video impulses or passing through them of a spectrum of frequencies of video impulses obtained in special generators, auxiliary circuits of various devices and televisions;

shock excitation of high-frequency devices arising from the operation of collector motors, relays, switches, telephones and other contact equipment;

shock excitation of high-frequency devices by video pulses resulting from the detection of high-frequency pulses

frequencies in overloaded amplifier stages and in other nonlinear resistances.

The sources and paths of such interference were discussed in § 1-7, 1-8, 1-9, 1-10, 1-11, 1-12.

The first stage of work on suppression of impulse pickups is to find out their specific sources and ways of communication with the pickup receiver.

This requires:

a) Turn off all possible circuits and parts of devices one by one until the interference disappears completely or decreases.

b) Reduce the steepness of the jumps by connecting smoothing filters to various points at which jumps are observed, thereby reducing the pickup and changing the shape of the induced pulse.

c) Increase the duration of the pulses in various circuits, observing how they are distorted at the output of the pickup receiver in order to find out whether they are differentiated or integrated (if they go directly to the video amplifier) ​​or split into two (if they pass through a high or intermediate frequency and de-

tector), Fig. 1-18 and 1-29.

d) Turn off the pickup in the receiver in sequence, starting from the input (antenna), various cascades and other circuits, achieving the disappearance of pickup.

e) Shunt with a high-capacity capacitor with short leads various circuits along which interference can be transmitted, and achieve it

decrease.

As a result of the first stage of work, a clear diagram of at least one communication channel, through which the interference passes, should be drawn up. In this case, the source of the pickup, its output, communication circuits, receiver input, circuits and methods of passing the impulse to the receiver and the guidance must be known.

The second stage of work is making changes to the device necessary to suppress interference. It should be borne in mind that, depending on the nature of the impulse pickups, they are suppressed in the following ways.

To suppress pickup from video pulses and other DC voltage surges that go directly to video amplifiers, low-frequency amplifiers and other devices without high-frequency resonant amplifiers, according to one of the circuits in Fig. 1-28, it is necessary to introduce additional details that weaken the connection between the source and the receiver

2. Guidance from gating video pulses supplied to high-frequency amplifiers for gain control is obtained due to sharp surges in the anode current of the controlled lamps, leading to shock excitation of the amplifier circuits. To suppress such interference, it is necessary to reduce the steepness of the edges of the gate pulses. If such a smoothing of the control pulse is unacceptable, then the only way to suppress the pickup would be to use push-pull circuits in the controlled stages of the high-frequency amplifier with a strobe pulse to the middle point of the grid winding of the transformer.

3. All other types of shock excitation of high-frequency amplifiers (radio receivers) by video pulses and any DC voltage surges arise mostly through the penetration of interference into the input circuits of the amplifier (antenna) along with useful signals. The suppression of such interference is performed at the source, first of all, by switching on filters in the supply circuit of the interference source and shielding in

power supply, as discussed in the previous paragraph.

In rare cases of close proximity of a source of such interference with its receiver (at distances of 1 m or less), in addition to filters, it may be necessary to completely shield the source by placing it in a metal casing (for example, shielding a relay located at the antenna input of a radio receiver) or partial shielding of internal elements source (for example, the shielding of the graphite coating of the cathode ray tube in televisions, recommended in the literature

round.

4. When suppressing the pickup of high-frequency pulses arriving at a high-frequency amplifier that is not tuned to the carrier frequency of the pulses, it is necessary that the pickup receiver elements do not detect interfering pulses, that is, the pickup receiver does not overload and operate in a linear mode. To do this, it is necessary to reduce the noise voltage in the circuit located in front of the first non-linear element of the receiver (lamp or semiconductor detector). The selectivity of a preselector consisting of one or two circuits turns out to be insufficient when high-frequency high-power pulses are applied to it.

If a radio receiver is redesigned to work together with powerful high-frequency pulse generators, then it must be equipped with a special multi-circuit preselector that provides a large attenuation of signals of any frequencies, except those included in the receiver's passband. If you need to adapt a ready-made radio receiver for a specified purpose, then you can get a good result if you add a one or two-cell filter to the antenna water, designed to attenuate the carrier frequency of interfering pulses.

Difficulties in the development of such a filter are that it must simultaneously satisfy two requirements: not to degrade the performance of the receiver and to provide a sufficiently large attenuation of the interference. If the interfering pulses have a very high carrier frequency, then a slight capacitive coupling inside the receiver between any wires entering the receiver from the outside and the parts of the high-frequency part of the receiver is sufficient for the interfering pulse to arrive in addition to the preselector or antenna.

heating filter. Therefore, in receivers operating in such conditions, it is necessary to have filter cells in the places where any wires enter, including the telephone cord in the radio communication receiver.

5. The level of shock excitation by high-frequency pulses is very low (§ 1-10 and 1-11). Therefore, such interference arrives at the pickup receiver only through the antenna input at the same frequencies as the useful signals. The only way to suppress this interference is to limit the frequency spectrum emitted by the high frequency pulse generator.

4-9. APPLICATION OF DOUBLE LAMPS

Among the double lamps assembled in one cylinder, there are a large number of triodes (the letter H is in the second place of the symbol) and several types of triode-pentodes (the letter F is in the second place of the symbol). The designs of the individual types of double lamps are made differently. In some types of lamps there is a screen with a separate terminal between the parts of the lamp, in other designs the screen is connected to one of the cathodes and

v thirdly, the screen is completely absent.

V The specifications for double lamps mostly specify the capacity between the anodes or between the anode of one half and the grid of the other half. The value of these capacities ranges from 0.02 to 0.5 pf depending on the type of lamp. They are the link that connects the chains that include the different halves of the same lamp. In the technical conditions for some types of double lamps, the values ​​of the connecting capacities are not specified at all. However, they can be quite large and can vary from specimen to specimen within wide limits.

In addition to capacitive coupling, there can be communication between the individual parts of the double lamp due to the electron flux penetrating through the slots and holes in the lamp structure from one half to the electrodes of the other half. This type of communication is not provided by the technical specifications, although sometimes it may turn out to be unacceptable.

As a result of analyzing the influence of both types of communication, the following recommendations can be made for the use of double lamps. Such lamps work best in circuits with a strong connection of both parts to each other: multivibrators, kippers, triggers, blocking generators with a starting lamp, two-phase and push-pull amplifiers, frequency converters consisting of a mixer and a local oscillator, etc. Double tubes in two adjacent amplifier stages work well at not very high frequencies. When using

The use of double lamps in two different channels of a radio device is in principle undesirable and should be resorted to only in cases of extreme necessity. In this case, one should compare the levels of alternating voltages and powers in both combined elements. The less these levels differ from each other, the more likely it is that the use of a double lamp will be painless.

ny wires is also a microwave resonant circuit tuned by the grid-cathode capacitance.

Both circuits are connected through a capacitance grid - a screening grid Cg1,2, which plays here the role of a through passage capacity.

Thus, the diagram of the cathode circuits, ec- Fig. 4-23. Amplifier generation the wounding and control grids of the equi-cascade on the microwave.

valence circuit of a triode generator with coupling through an intra-lamp through passage capacity. If favorable (with

generation occurs.

Having arisen in intermediate stages, this generation may not manifest itself clearly, but affect such usually rarely controlled parameters as the anode current of individual lamps, the linearity of the amplitude characteristic, etc. Sometimes the same generation, changing the operating mode of the amplifier, can cause feedback on fundamental frequency. With the destruction of such generation, the distortion of the frequency characteristics of the amplifier will disappear at the same time.

the mouth of the waves.

The same circuit of a push-pull microwave generator is easy to see in a cathode follower circuit with parallel switching off of lamps, if we take into account the inductance and capacitance of the connecting wires between the anodes and between the grids.

It is somewhat easier to detect microwave generation in powerful low-frequency amplifying stages by the glow of a neon lamp. To carry out such an experiment, a small light bulb is attached to

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 pulsed 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.

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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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.

The German company Epcos (formerly Siemens' Passive Components Division) has a wide range of products for solving electromagnetic compatibility (EMC) issues of electrical or electronic devices.

A significant subgroup of EMC components of Epcos is made up of filters designed to protect devices from high-frequency electromagnetic interference (radio interference).

Electromagnetic interference (EMI) occurs as a result of the operation of devices designed to generate or convert electricity. They represent electromagnetic fields in the space surrounding such technical equipment (TS).

The main sources of high-frequency interference are pulsed power supplies (household electronics, industrial and medical devices, etc.), nonlinear circuits

To combat interference in the circuits of neighboring vehicles, as well as nodes and blocks within individual vehicles, EMI filters are used. In general, EMI filters are usually low-pass filters and can be installed both directly at the source of interference and in front of the receiver of interference (receptor). EMI filters Epcos (mains filters) are designed to suppress interference coming through the wires of a two- or three-phase network to the input of the protected device, that is, these are filters of the "receiving side". This article is devoted to Epcos line filters, each of which is a separate complete node installed in front of the receiver. All considered filters pass the voltage of the mains frequency 50/60 Hz unhindered.

Common-mode interference voltage occurs as a potential difference between the phase (signal) wire, the return wire (the so-called ground or neutral wire) and ground (device case, heat sink, etc.). The common mode disturbance current has the same direction in the forward and return conductors of the network.

In symmetrical electrical circuits (ungrounded circuits and circuits with a grounded midpoint) antiphase interference appears in the form of symmetrical 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 kilohertz. At frequencies above 1 MHz, asymmetric interference predominates.

Interference arising in unbalanced circuits is called unbalanced. For antiphase interference, an unbalanced circuit is a circuit with a divided (balanced with respect to earth) load.

For power circuits, asymmetric loading is more typical, but, for example, the sources of high-frequency interference (converters on IGBT transistors, etc.) themselves can generate asymmetric (common-mode) interference. On the other hand, common mode noise under certain conditions is converted to antiphase.

EMI filters are characterized by a set of parameters. Let us dwell on the parameters characterizing the Epcos EMI filters:

1. Number of wires in the network: 2, 3 (4).
2. Rated (mains) voltage: 250 (220), 440 (380) V, etc.
3. interference suppression range (barrage frequency band);
4. interference suppression level (standard; with enhanced suppression, etc.);
5. rated current, A;
6. type of interference suppressed by the filter:
   • general type;
   • differential type;
   • asymmetrical interference;
7. connector type;
8. type of shell;
9. climatic category (temperature range in which the filter meets the requirements (standards) for other technical characteristics).

Filter designs differ depending on the type of interference. So, to compensate for symmetrical interference, when voltage distortions occur between the phase conductors of the network, a so-called du / dt low-frequency filter is used, containing interference suppression X-capacitors. Note that X-capacitors are those capacitors that shunt the line wires together at a high frequency.

Due to the fact that with a low internal resistance of the source of interference, its elimination would require excessively large capacities necessary to provide a given voltage division, in practice, inductors are connected in series with the capacitor, which increases the resistance in a series circuit. As a result, a so-called T-shaped (or U-shaped) low-pass filter is formed.

At high frequencies, in order to limit its own capacity, the choke is often performed in the form of a set of separate inductances (sections or so-called "beads", the English name is beads) connected in series. At high frequencies, ferrite chokes can be used, for example, for frequencies of 30, 50 and 100 MHz, Epcos serially produces chokes / beads of the B8248x series in chip sizes 0603 ... 1806, designed for a current of 0.05 ... 4 A. chokes in the output version. At higher frequencies, low inductance can provide sufficient reactance. In this case, to obtain a choke, it is enough to pass the power cable through a group of ferrite rings.

In fig. 1 shows the equivalent circuit of an EMI du / dt filter. It performs the procedure for subtracting the differentiated signal from the original. As a result, the filter smooths out peaks and eliminates voltage surges caused by symmetrical noise. However, it has almost no effect on the disturbance voltage between the mains conductors and earth ground, as well as on the leakage current.

Rice. one

Along with X-capacitors and conventional chokes, Epcos EMI filters use two types of inductors connected (with a common core).

Epcos current compensated EMI suppression chokes are usually made on a ring ferrite core. They use two coils (two wires) for a two-wire network, three for a three-wire network, etc. In this case, the opposite winding of wires can be geometrically implemented by co-winding them on two halves of a ferrite ring.

A Z-shaped choke from Epcos is made by winding two wires on a ring core made of metal powder and having a high saturation threshold, which linearizes the I - V characteristics of the coils and reduces the risk of distortions associated with their nonlinearity.

Below is a number of specific examples of Epcos EMI filters with schematic diagrams and explanation of features.

Example A1: Epcos B84110-B series du / dt EMI filter with common mode rejection (no Y-capacitors).

This filter is used to protect switching power supplies, TVs, computers, industrial and portable equipment. The use of asymmetric noise filters, in particular, significantly removes the restrictions on the length of the cable supplied to the motor from the inverter in industrial applications.

Example A2: Epcos EMI filter SIFI-D series (part number B84114-D) with common mode rejection and Y-capacitors6 (in addition to X-capacitors filter B84110-B). The resistor at the input (Fig. 3), installed in parallel with the X-capacitor, is designed to discharge it (large capacitor).

To compensate for several types of interference, a combination of chokes (serial, etc.) is installed.

Example A3: Epcos EMI filter of the SIFI-E series (part number B84115-E). It differs from the previous one additionally connected Z-shaped choke for additional attenuation of symmetrical interference (Fig. 4).

In fig. 5 shows the comparative characteristics of the insertion loss (in terms of symmetric interference) for two series of filters. It can be seen from it that the first filter has a significantly lower level of frequency suppression in the band up to several hundred kilohertz.

Rice. 5

In addition to coupled coils, Epcos EMI filters often include a multi-tier (loop-through) capacitor. The intrinsic inductance of such a capacitor is very small. At the same time, it can compensate for both antiphase and common-mode interference.

Epcos offers EMI filters designed to suppress interference in a wide range of high and ultra-high frequencies, ranging from about 10 kHz up to 40 GHz and above. In this case, the average suppression bandwidth of all filters is about 1 MHz. Among the various models of Epcos EMI filters, one can single out, in particular, special ones, with a given leakage current.

The filter parameters leave an imprint on the possible areas of its application. The scope of application of a specific Epcos filter can be more precisely determined from the corporate catalog and on the website www.epcos.com on the Internet. A number of areas (but not all possible) are listed below where the use of Epcos EMI filters is advisable.

1. Modular systems for automated (soft) starting of electric motor drives ("Active terminal" / AFE) using powerful semiconductor switches (IGBT transistors) controlled by constant voltage. The switches are commutated with constant voltage from the output of voltage converters (AC / DC). For instance:

• CNC machines;
• elevators, etc.

2. Voltage converters of electric generators (wind power plants, etc.).

3. Transport, for example:

• converter drives of modern urban rail vehicles, in particular, trams;
• metro, electric trains, etc .;
• vehicles requiring a low leakage current (with a complex grounding procedure), in particular trolleybuses, etc.
• high-speed trains (long-distance).

4. Drives of steel rolling mills (interference with powerful commutation, as well as regulation of the rotation speed of the sheet feed drives).

5. Conveyor (tape) lines.

6. Filters for switching power supplies and UPS.

7. Pumps.

8. Heating, ventilation and air conditioning systems (HVAC systems).

9. Filters for suppression of interference signals in installations / cabinets with a high concentration of electronic equipment units (with a small volume of space).

10. When using power cables as conductors for communication communications (home Internet, as well as security systems with a limited number of wires in the input cable).

11. Filters for data transmission and telephone lines (ISDN, etc.).

EMI Filters Application Examples

Home Internet: data transmission within the house and between the house and the power substation (Fig. 6). Suppression of interference when using power cables as conductors of communication communications. In the absence of an EMI filter, the subscriber's radio-electronic equipment is noisy with pickups from the mains voltage.

Rice. 6

Shown in Fig. 7 the circuit is used for voltage converters of electric generators. The converter itself is necessary due to the fact that the parameters of the signal, for example, the amplitude of the voltage generated at the output of the generator, usually do not correspond to the parameters of the network. EMI filters protect a generator (for example, a wind farm) from the penetration of high-frequency noise from the voltage converter.

Rice. 7

Modular systems of automated soft start of electric motor drives "Active terminal" / AFE (Fig. 8).

Rice. eight

IGBT transistors, activated by a simple DC voltage from the output of the inverter, enable fast connection or disconnection of high power motor drives. At the input of the converter there is a mains three-phase sinusoidal voltage, and at the output there is a constant voltage. However, the fast switching of the power circuit is a source of high frequency interference. As a result of the penetration of noise into the input, the voltage between the phases of the network is distorted (there is a symmetrical type of noise). The level of asymmetrical interference can also be significant due to the lengthy cable from the voltage converter to the external network. EMI filter 8 Epcos, installed at the input of the converter, compensates practically without a trace both disturbances, "decoupling" the converter and the external network.

Municipal rail transport (trams). The EMI filter is installed between the voltage converter of the electric motor and the supply (contact) line (Fig. 9).

Rice. 9

In conclusion, we can state the wide and varied capabilities of Epcos EMI filters for solving EMC problems of power equipment.