Power electronic equipment consists of two parts, namely the conversion part and the control part. The former belongs to the category of power flow and strong electricity, and the latter belongs to the category of information flow and weak electricity. In general, the former is the main source of electromagnetic interference, and the latter is the object to be interfered with. In order to make the power electronic equipment operate reliably, in addition to solving the electrical isolation between the conversion part and the control part, the problem of anti-electromagnetic interference of the control part should also be solved, especially when the conversion part is in high voltage, strong current, high frequency conversion situation is particularly important. The anti-interference problem is essentially to solve the electromagnetic compatibility problem of power electronic equipment.
Isolation technology is one of the important technologies in electromagnetic compatibility. The isolation technology in electromagnetic compatibility is divided into several isolation methods such as magnetoelectric, photoelectric, electromechanical, acoustoelectric and floating.
2 Magnetic isolation technology
2.1 The basic principle of using a transformer to achieve magnetoelectric isolation
Transformers are mainly composed of two or more windings wound around a common core. When an alternating voltage is applied to one winding, the alternating voltage is induced on the other winding due to electromagnetic induction. Therefore, several windings of the transformer are connected with each other through the alternating magnetic field, and are isolated from each other on the Circuit. The dielectric strength of its isolation depends on the dielectric strength between several windings and their ground.
2.2 Characteristics of an ideal transformer
The ideal transformer assumes that the resistance of the transformer winding is zero; the leakage flux of the transformer is zero; the loss of the core is zero and the permeability of the core is infinite.
2.2.1 Voltage relationship
In the formula: E1 – the induced potential of the primary side of the transformer;
E2 – the induced potential on the secondary side of the transformer;
U1 – the voltage on the primary side of the transformer;
U2 – the voltage on the secondary side of the transformer;
N1 – the number of turns of the primary winding of the transformer;
N2 – the number of turns of the secondary winding of the transformer;
f – the frequency of the primary voltage of the transformer;
Φm – the peak value of the magnetic flux in the transformer core;
n – the turns ratio of the primary and secondary windings of the transformer.
2.2.2 Current relationship
In the formula: I1 – the current of the primary side of the transformer;
I2 – Current on the secondary side of the transformer.
2.2.3 Power relationship
In the formula: P1 – the input power of the primary side of the transformer;
P2 – output power on the secondary side of the transformer.
2.2.4 Impedance relationship
The impedance of the secondary side is:
The impedance of the primary side is:
In the formula: Z1 – the impedance of the primary side of the transformer;
Z2 – Impedance of transformer secondary.
2.3 Actual Transformer
2.3.1 Magnetic permeability of iron core
Since the magnetic permeability of the actual transformer core is not infinite, there is an excitation current when the transformer is no-load. If the performance of the core material is not good, the ratio of the excitation current to the input current of the primary side of the transformer will increase, and the output current of the secondary side of the transformer will decrease.
Since the magnetic permeability of the actual transformer core is not constant, it will cause distortion of the output waveform. Especially when the iron core is saturated, the magnetic permeability of the iron core is greatly reduced, causing the excitation current to increase rapidly, which may cause the transformer to burn out.
2.3.2 There is loss of iron core
Due to the eddy current loss and hysteresis loss in the actual transformer core, these losses not only reduce the efficiency of the transformer, but also cause the core to heat up, and may even lead to insulation damage. Since the eddy current loss and hysteresis loss of the core are related to voltage and frequency, different core materials should be selected for different voltages and frequencies.
2.3.3 There is resistance in the winding
Since the winding of the actual transformer has resistance, the winding will inevitably generate heat loss when the transformer is working. Especially when the operating frequency is high, the skin effect will lead to an increase in the winding resistance and increase the heat loss.
Due to the poor heat dissipation conditions of the actual transformer winding, attention should be paid to the heat dissipation of the transformer and the selection of the current density of the winding wire.
2.3.4 There is magnetic leakage in the transformer
The magnetic leakage of the transformer is easy to interfere with the components and wires near the transformer. Therefore, when selecting a transformer for isolation, a transformer with a small magnetic leakage should be selected. Otherwise, the transformer should be strengthened to shield the magnetic field.
2.3.5 There is parasitic capacitance between the primary and secondary sides of the transformer
Due to the parasitic capacitance between the primary and secondary sides of the power transformer, the high-frequency interference entering the primary side of the power transformer can be coupled to the secondary side through the parasitic capacitance. After adding electrostatic shielding between the primary and secondary sides of the power transformer, a new distributed capacitance is formed between the shielding and the winding. Anti-electromagnetic interference effect.
2.3.6 Dielectric strength between several windings and to ground
The dielectric strength between windings and to ground depends on the withstand voltage level that needs to be isolated. This withstand voltage level includes operating voltage, voltage fluctuations, possible transient overvoltages, and a margin for reliable operation.
2.3.7 Operating frequency
The operating frequency not only affects the core loss of the transformer, but also the impedance of the transformer is closely related to the frequency. For example: the impedance of the Inductor L is proportional to the frequency, and the impedance of the capacitor C is inversely proportional to the frequency.
Since the magnetic isolation is achieved by the transformer, when the parasitic capacitance between the transformer windings is large, it should be matched with the shielding and grounding technology.
2.4 Types and applications of transformers
2.4.1 Ordinary Transformer
Ordinary transformers are only used as general power transformers in power frequency occasions, converting a certain level of voltage and current into another level of voltage and current. Since no special measures are adopted, the isolation effect on high-frequency circuits is poor.
2.4.2 Isolation Transformer
Due to the large parasitic capacitance between the windings of ordinary transformers (nF level without shielding, pF level with shielding), in order to improve the isolation effect against high-frequency interference, a layer of shielding can be added between the windings of ordinary transformers, and the The layer shield is grounded (the length of the ground wire should be as short as possible, otherwise the attenuation of the interference will become worse due to the impedance division of the ground wire) and become an isolation transformer. Figure 1 shows the attenuation of interference for a typical single-shielded isolation transformer.
Figure 1 Typical attenuation of interference for single-shielded isolation transformers
On the basis of the above, if a layer of shielding is added to each winding of the transformer, and the shielding of each winding is connected to the low potential of each winding, the isolation effect will be better.
2.4.3 Pulse transformer
In power electronic equipment, pulse transformers are mostly used for interstage coupling of thyristor trigger circuits, squib oscillators and pulse amplifiers. The main parameters of the pulse transformer are effective pulse permeability, initial permeability, leakage inductance, distributed capacitance and turns ratio.
2.4.4 Measuring Transformer
Transformers for general measurement refer to voltage transformers and current transformers. The voltage transformer or current transformer isolates and converts the voltage or current of the strong current into the voltage or current of the weak current. The main parameters of measuring transformer are insulation voltage, voltage (or current) conversion ratio and its accuracy.
2.5 Hall sensor
The Hall sensor is a device that uses the Hall effect for electromagnetic measurement, and achieves electrical isolation due to the intervention of the magnetic field. Hall sensors have the advantages of high precision, good linearity, good dynamic performance, wide frequency response and long life.
3 Optical isolation technology
Photoelectric isolation is realized by optocouplers, that is, through the light emission of semiconductor light emitting diodes (LEDs) and the light reception of photosensitive semiconductors (photoresistors, photodiodes, phototransistors, photothyristors, etc.), to achieve signal transmission. Since the light-emitting diode and the photosensitive semiconductor are insulated from each other, the isolation of the circuit is realized.
When a forward voltage is applied to the light-emitting diode, due to the decrease of the potential barrier of the space charge region, the holes in the P region are injected into the N region, resulting in the recombination of electrons and holes, and most of the energy in the form of light is released during the recombination. The higher the forward voltage applied to the light-emitting diode, the greater the luminous flux released during recombination. Of course, the forward voltage applied to an LED is limited by its maximum allowable current.
When a photosensitive semiconductor, such as a photodiode, is illuminated by light, photogenerated electron-hole pairs generated near the PN junction form a photocurrent under the action of the internal electric field of the PN junction. The stronger the illuminance of the light, the greater the photocurrent. When the photosensitive semiconductor is not exposed to light, there is only a small dark current.
3.2 Characteristics of optocouplers
The characteristics of the optocoupler are represented by the functional relationship between the input current of the light-emitting diode and the output current of the photosensitive semiconductor, as shown in Figure 2.
Fig. 2 Characteristic curve of optocoupler
It can be seen from the characteristic curve of the optocoupler that the linearity of the optocoupler is poor, which can be corrected by feedback technology.
3.3 Application of optocoupler
Since the input impedance of the optocoupler is smaller than that of the general interference source, the interference voltage divided by the voltage at the input end of the optocoupler is small, the current it can provide is not large, and it is not easy to make the semiconductor diode emit light; due to the photoelectric The shell of the coupler is sealed, and it is not affected by external light; the isolation resistance of the optocoupler is large (about 1012Ω), and the isolation capacitance is small (about several pF), so it can prevent the electromagnetic interference caused by circuit coupling. The isolation impedance of the optocoupler decreases as the frequency increases, and the anti-interference effect will also decrease.
3.4 Infrared Remote Control
Infrared remote control is essentially photoelectric coupling, but its light-emitting device and light-receiving device are not packaged together, so the isolation effect of infrared remote control is better.
3.5 Optical cable
Optical cables are also optoelectronic couplings in nature. The light-emitting devices and light-receiving devices are connected by optical cables. Since it is difficult for external interference to enter the optical cables, the isolation effect of optical cables is the best.
4 Electromechanical isolation technology
4.1 Contact Electromagnetic Relay
Electromechanical isolation is generally realized by a contacted electromagnetic relay, that is, the coil of the electromagnetic relay receives the signal, and the mechanical contact sends the signal. When the mechanical contact is disconnected, the impedance is large and the capacitance is small, which prevents the transmission of electromagnetic interference caused by circuit coupling. However, the coil operating frequency of the relay is low, which is not suitable for occasions with high operating frequency. In addition, there are disadvantages such as bouncing and spark interference when the contacts are on and off, as well as contact resistance.
4.2 Precautions for the application of contact electromagnetic relays
4.2.1 Electromagnetic interference of mechanical contacts
In the process of breaking the signal current by the mechanical contacts, overvoltage will be induced between the contacts due to the existence of the circuit inductance. This overvoltage may cause the contact gap to break down and generate an arc; when the contact gap increases, the The arc extinguishes, the voltage between the contacts rises, and the arc reignites; this repeats until the contacts are sufficiently spaced to interrupt the current.
In the above process, the generated arc and the voltage pulse train with large peak value and high frequency will cause strong interference to other circuits and devices through radiation and conduction.
4.2.2 Spark extinguishing circuit for mechanical contacts
The spark extinguishing circuit of the mechanical contact is composed of a resistor R and a capacitor C in series. The principle is to use a capacitor to convert the energy on the load inductance L when the contact is broken, so as to avoid the electromagnetic interference caused by overvoltage and arc on the contact, and finally absorb this part of the energy by the resistor.
The circuit parameters are calculated as follows:
R>2 (L/C) 1/2 (7)
In the formula: R-resistance (Ω);
L – load inductance (μH);
Im – the maximum current in the load inductance (A);
C takes the larger of C1 and C2.
4.2.3 Freewheeling circuit for inductive loads
The freewheeling circuit of the inductive load of the DC circuit is to use a diode to connect the inductive load in anti-parallel. When the inductive load is cut off, the current on it will continue to flow through the diode, and will not generate overvoltage and endanger other devices on the circuit.
The parameters are selected as follows:
In the formula: IF – the average forward current of the diode;
URRM – diode reverse repetitive peak voltage;
IN – the rated current of the inductive load;
UN – the rated voltage of the inductive load.
If you replace the diode with a varistor, it will work better. Because the varistor absorbs energy faster, the action response time is reduced.
5 Acoustic and electrical isolation technology
5.1 SAW filter
The surface acoustic wave device uses a solid material with piezoelectric effect as a substrate, and two ends of the substrate are respectively provided with interdigitated metal transducers. When an alternating electrical signal is applied to the transmitting transducer, due to the inverse piezoelectric effect, the surface of the piezoelectric body produces a changing strain, and the surface acoustic wave can be excited. When the surface acoustic wave propagates on the solid surface to the receiving transducer, due to the positive piezoelectric effect, an electrical signal will be obtained on the receiving transducer. Circuit isolation is achieved because the two interdigitated metal transducers are electrically disconnected.
Since the interdigital transducer has a natural center frequency, when the electrical signal is consistent with the center frequency, resonance occurs and the strongest surface acoustic wave is emitted. SAWs of other frequencies are weak and are suppressed. So the isolation effect of the SAW filter is very good.
5.2 Application of SAW filter
SAW filters are currently mainly used in television and communications, as band-pass, band-stop filters, frequency discriminators and oscillators and so on.
6 Floating technology
Floating, that is, there is no conductor connection between the ground of the circuit and the earth. The advantage is that the circuit is not affected by the electrical properties of the earth. The disadvantage is that the circuit is susceptible to parasitic capacitance, which makes the ground potential of the circuit fluctuate and increases the inductive interference to the analog circuit.
The floating ground can make the isolation resistance between the power ground (strong electric ground) and the signal ground (weak electric ground) very large, so it can prevent the electromagnetic interference caused by the circuit coupling of the common ground impedance.
6.2 Application of floating technology
6.6.2 Separate AC power ground from DC power ground
Generally, the neutral wire of the AC power supply is grounded. However, due to the existence of grounding resistance and the current flowing on it, the potential of the zero line of the power supply is not the zero potential of the earth. In addition, there is often a lot of interference on the neutral line of the AC power supply. If the AC power supply ground is not separated from the DC power supply ground, it will affect the normal operation of the DC power supply and subsequent DC circuits. Therefore, using the floating technology that separates the AC power ground from the DC power ground can isolate the interference from the AC power ground wire.
6.6.2 Floating technology for amplifiers
For amplifiers, especially those with small input signals and high gain, any small interference signal at the input may cause abnormal operation. Therefore, using the floating technology of the amplifier can block the entry of interference signals and improve the electromagnetic compatibility of the amplifier.
6.3 Precautions for floating technology
1) Try to improve the ground insulation resistance of the floating system, which is beneficial to reduce the common mode interference current entering the floating system.
2) Pay attention to the parasitic capacitance of the floating system to the ground. High-frequency interference signals may still be coupled into the floating system through the parasitic capacitance.
3) Floating ground technology must be combined with electromagnetic compatibility technologies such as shielding and isolation in order to receive better expected results.
4) When using floating technology, attention should be paid to the hazards of static electricity and voltage counterattack to equipment and people.
The main purpose of using the isolation technology in electromagnetic compatibility is to isolate the interference source part from the sensitive part for the reliable operation of the power electronic equipment. The isolation technology in electromagnetic compatibility can be mainly divided into several isolation methods such as electromechanical, magnetoelectric, photoelectric, acoustic and electric and floating. Among them, several isolation methods such as magnetoelectricity, photoelectricity, and sound and electricity are all accomplished by using the mutual conversion between various physical quantities and electricity. Regardless of the isolation method, the essence of electromagnetic compatibility is to artificially create electrical isolation to prevent electromagnetic interference caused by circuit coupling.