Resonant converter 24 300V. News and analytical portal "electronics time". Examples of tasks for converters with solutions
Recently I got to grips with a resonant half-bridge LLC converter circuit, and I thought that this experience could be used to create a series of articles: start with the basics and gradually delve into the topic. It took me a long time to get familiar with publications, dissertations and manuals before I figured out how this scheme works. It turned out that studying the sources of information given in the bibliography took more time than writing the article itself. Please note that none of the above sources does a complete analysis of the operation of this converter, which has many different modes and operating conditions. I hope you can get a general idea of \u200b\u200bhow the circuit works with my help. This assistance will include filtering information and focusing on the most important key points of the proposed documents.
Figure: 1. DC /AC resonant converter
Figure: 2. DC /AC resonant converter with transformer decoupling
LLC converters are a type of Switched Mode Power Supply (SMPS). Most publications on this topic begin with a description of the basic principles of the LLC. I'll start by explaining how LLC differs from other types of switching converters.
- The operation of a conventional pulse converter consists of two phases. In the first phase, energy is stored in the inductor. In the second phase, the accumulated energy is consumed to maintain the current. You probably remember that, according to the laws of commutation, the current in an inductor cannot change abruptly (in the case of correct commutation), just like the voltage across a capacitor. This principle is the basis for most switching converters.
- The LLC converter works by creating a sinusoidal current that is rectified and stored in a large capacitor. The inductance is not used for simple energy storage, but acts as a resonant element. It acts as a filter that helps convert the square wave to a sinusoidal waveform, while the magnetizing inductance still operates with the traditional triangular waveform. This is one of the features that needs further explanation.
With the operating modes in LLC converters, things are even more complicated, since they have many differences:
- instead of operating at a fixed switching frequency and changing the PWM duty cycle, LLC converters change the frequency and the PWM duty cycle is constant at 50%;
- power transmission in LLC converters is based on the operating point of the magnetizing inductance;
- lLC converters use a variable rate of voltage change depending on the load current;
- they have two resonant frequencies that affect each other;
- continuous current mode (CCM) for LLC converters refers to the rectifier current, not inductance, since there is no traditional inductance in the circuit.
Most of the above may seem complicated and confusing, especially for those who are just getting started with power electronics. The second part of this publication will cover the main sources of information, as well as some key points that I find useful. However, talking about resonant converters requires some basic introductory material.
Switching regulators have revolutionized the field of DC voltage conversion and power conversion in general. Engineers quickly realized that a combination of power switch, rectifier, inductor, and capacitor can perform voltage conversion with high efficiency even with large differences between input and output voltage levels (Figure 1). In addition, transformers can solve the problems of galvanic isolation and matching of large voltage level differences (Fig. 2).
The fact is that the double "L" in the name "LLC-converter" indicates two resonant frequencies in the operating range. We will talk about this in more detail in one of the subsequent articles of this series. For now, just remember that the choice of operating points used in LLC converters provides both ZVS and ZCS switching in the power switches of the MOSFET, as well as ZCS switching in the rectifier diodes. This solves the problems associated with rectifier reverse diode recovery.
Now that the basic features of the operation of pulsed resonant converters are given, we will give a brief description of the information sources used.
Figure: 3.
Figure: 4.
The first reference in the bibliography refers to Bo Young's doctoral dissertation "Topology Investigation for Front End DC / DC Power Conversion for Distributed Power Systems." It contains links to other publications that will help you understand the topic of LLC and the thesis itself. Note that the first link contains links to the fourth part of the thesis as well as to Appendix B for an important stress plot (this link contains Appendices A through D and additional links). Although this graph is found in most sources, it took me hard work and filling in some knowledge gaps to create it (Figure 5).
Figure: five. The dependence of the gain of the converter on the valuefs /fr
References 3 and 4 were crucial for me in plotting the gain of the converter, as they noted the effect of capacitance on gain and explained why negative impedance was causing confusion in the graphs. We will talk about this in more detail in one of the subsequent articles of this series.
Link 5 - guide from Infineon, which contains a detailed description of the most useful design steps. This document compares the switching and rectifying features of bridge and half-bridge circuits and the trade-offs associated with them. I used bridge and half bridge circuits to explain how voltage and current are related. In a bridge circuit, MOSFETs are cascaded to produce the required voltage. Parallel connection of transistors is necessary to increase the load current. A common requirement for switching voltage regulators is to eliminate DC bias to prevent saturation of the transformer. As mentioned earlier, LLC converters differ in that they need a bridge to create positive and negative half-waves of the signal, which, when filtered, takes a sinusoidal shape.
6 link from Fairchild is the only one I have found where the gain equation also includes secondary leakage inductance. Note that the secondary leakage inductance as well as the load resistance are reflected through the transformer and thus can be adjusted by changing the ratio of the winding turns. This guide contains a number of key tips to help you design a real circuit.
The documentation from Infineon / Fairchild also details the design of the transformer. Since LLC resonant tuning is based on both leakage inductance and transformer magnetizing inductance, this information is useless in our case.
Our college friends in Colorado have shared some insights into power conversion. In particular, the Colorado State ECE 562 Electrical Engineering course contains many examples of simulations performed in MATLAB.
In terms of modeling, it is worth noting that many sources provide references to SPICE models. I do not give preference to any particular link and I believe that by studying them, you can be convinced of the existence of different modes of operation of the LLC converter. But it should be noted again that LLC has many differences from traditional switching converters.
The prototype I'm working with was created by Texas Instruments... Thanks to the power factor corrector, this system provides stable operation with 400 VDC input voltage. The study of the sample showed the tolerance of large fluctuations in the load current and demonstrated the effect of the current on the operating point and resonant frequency.
In conclusion, I would like to note that if you think that you can find the same equations for determining the gain in different articles, then you are wrong. Using the M variable allows you to take into account factors that differ in each specific article, manual, dissertation, training course. If I have time, I will put together a comparison chart to show how they differ.
I decided to devote a separate article to the manufacture of a DC AC step-up voltage converter for 220V. This, of course, is remotely related to the topic of LED spotlights and lamps, but such a mobile power source is widely used at home and in the car.
- 1. Build options
- 2. The design of the voltage converter
- 3. Sinusoid
- 4. An example of the filling of the converter
- 5. Assembly from UPS
- 6. Assembling from ready-made blocks
- 7. Radio constructors
- 8. Circuits of powerful converters
Build options
There are 3 optimal ways to make a 12 to 220 inverter with your own hands:
- assembly from ready-made blocks or radio constructors;
- manufacturing from an uninterruptible power supply;
- use of amateur radio schemes.
The Chinese have good radio constructors and ready-made blocks for assembling DC-to-AC 220V converters. For the price, this method will be the most expensive, but it takes the least time.
The second way is to upgrade an uninterruptible power supply (UPS), which without a battery are sold in large quantities on Avito and cost from 100 to 300 rubles.
The most difficult option is assembling from scratch, you can't do without amateur radio experience. We'll have to make printed circuit boards, select components, a lot of work.
Voltage converter design
Consider the design of a conventional step-up voltage converter from 12 to 220. The principle of operation for all modern inverters will be the same. The high-frequency PWM controller sets the operating mode, frequency and amplitude. The power section is made on powerful transistors, the heat from which is transferred to the device case.
A fuse is installed at the input to protect the car battery against short circuits. A thermal sensor is attached next to the transistors, which monitors their heating. In case of overheating of the 12v 220v inverter, an active cooling system is turned on, consisting of one or more fans. In budget models, the fan can run continuously, and not only under high load.
Power transistors at the output
Sinusoid
The signal shape at the output of the car inverter is formed by a high-frequency generator. A sinusoid can be of two types:
- modified sinusoid;
- pure sine wave, pure sine.
Not every electrical device can work with a modified sine wave, which is rectangular in shape. For some components, the operating mode changes, they may heat up and begin to scrape. A similar thing can be obtained by dimming an LED lamp whose brightness is not adjustable. Crackling and flashing starts.
Expensive DC AC boost converters 12v 220v have a pure sine output. They cost much more, but electrical appliances work great with it.
Example of converter filling
Assembling from a UPS
In order not to invent anything and not buy ready-made modules, you can try a computer uninterruptible power supply, abbreviated as IPB. They are designed for 300-600W. I have an Ippon for 6 sockets, 2 monitors, 1 system unit, 1 TV set, 3 surveillance cameras, a video surveillance control system are connected. Periodically I put it into working mode by disconnecting from the 220 network so that the battery is discharged, otherwise the service life will be greatly reduced.
Colleagues electricians connected an ordinary car acid battery to an uninterruptible power supply, worked perfectly continuously for 6 hours, watched football in the country. The UPS usually has a built-in gel battery diagnostic system that detects low battery capacity. How she will react to the automobile is unknown, although the main difference is the gel instead of the acid.
UPS filling
The only problem is that the uninterruptible power supply may not like the jumps in the car network with the engine running. For a real radio amateur, this problem is solved. Can only be used with the engine stopped.
Mostly UPSs are designed for short-term operation, when 220V is lost in the outlet. With long-term continuous operation, it is highly desirable to supply active cooling. Ventilation is useful for the stationary version and for the car inverter.
Like all devices, it will behave unpredictably when starting the engine with a connected load. The starter of the car drains the Volts a lot, at best it will go into defense as if the battery fails. In the worst case, there will be jumps at the 220V output, the sine wave will be distorted.
Assembling from ready-made blocks
To assemble a stationary or automotive inverter 12v 220v with your own hands, you can use ready-made blocks that are sold on Ebey or from the Chinese. This saves time on board fabrication, soldering and final setup. It is enough to add a body and wires with crocodiles to them.
You can also purchase a radio designer, which is equipped with all radio components, it remains only to solder.
Estimated price for autumn 2016:
- 300W - 400 rubles;
- 500W - 700 rubles;
- 1000W - 1500 rubles;
- 2000W - 1700 rubles;
- 3000W - 2500 RUB
To search on Aliexpress, enter your query in the search box "inverter 220 diy". The abbreviation “DIY” stands for “DIY assembly”.
Board for 500W, output for 160, 220, 380 volts
Radio constructors
A radio constructor is cheaper than a finished board. The most complex elements may already be on the board. After assembly, virtually no setup is required, which requires an oscilloscope. The range of parameters of radio components and ratings are well matched. Sometimes they put spare parts in a bag, suddenly, due to inexperience, you will tear off the leg.
Power converter circuits
A powerful inverter is mainly used to connect construction power tools when building a summer house or hacienda. A low-power voltage converter for 500 watts differs from a powerful one for 5000 - 10000 watts in the number of transformers and power transistors at the output. Therefore, the complexity of manufacture and the price are almost the same, transistors are inexpensive. The optimal power is 3000W, you can connect a drill, grinder and other tool.
I will show several schemes of inverters from 12, 24, 36 to 220V. It is not recommended to put such in a car, you can accidentally spoil the electrician. The circuitry of DC AC converters 12 to 220 is simple, the master oscillator and the power section. The generator is made on the popular TL494 or analogs.
A large number of booster circuits from 12v to 220v for DIY manufacturing can be found at the link
http://cxema.my1.ru/publ/istochniki_pitanija/preobrazovateli_naprjazhenija/101-4
There are about 140 circuits in total, half of them are boost converters from 12, 24 to 220V. Capacities from 50 to 5000W.
After assembly, you will need to adjust the entire circuit using an oscilloscope, it is advisable to have experience with high-voltage circuits.
To build a powerful 2500 watt inverter, you need 16 transistors and 4 suitable transformers. The cost of the product will be considerable, comparable to the cost of a similar radio designer. The advantage of such costs will be a pure sine at the output.
"Specializing in the field of power supplies, we managed to create a method for constructing resonant converters with deep adjustment of output parameters, which differs from the known ones. An international patent has been obtained for this method. The advantages of the method are most fully manifested in the construction of powerful - from 500 to tens of kilowatts - The converter does not require fast protection circuits against short-circuit at the output, since it practically does not have a mode of breaking the current of the switches in any mode. Also, the possibility of the occurrence of through currents is eliminated. Since physically (without feedback) the converter is a current source, it became possible to transfer the capacitor of the filter of the supply network rectifier to the output of the converter, which made it possible to obtain a power factor of 0.92-0.96 depending on the load.The frequency of the resonant circuit does not change, and this makes it possible to effectively filter the radiation of the converter in all directions. The implementation was carried out in the form of current sources for electrochemical protection - cathodic protection stations of the "Elkon" brand. Power 600, 1500, 3000 and 5000 watts. The efficiency in the nominal mode is at the level of 0.93-095. SKZ passed certification tests at NPO VZLET. There is a slow, stringy implementation. All this confirms the vitality of the idea. However, it seems to me that in order to achieve commercial success, it is necessary to popularize an idea in order to attract attention to it. "
Well, it is always a pleasure to help colleagues, especially since the idea behind Elkon products is novel.
Currently, devices and power electronics devices developed for professional use are actively optimized according to criteria such as weight, dimensions, efficiency, reliability, and cost. These requirements are steadily becoming more stringent, that is, the customer wants to have a device with minimal dimensions and weight, and at the same time - with high efficiency, high reliability and low cost.
In order to improve the consumer properties of products, it is necessary to resort to well-known measures: to increase the operating frequency of conversion, reduce power losses in the power elements, reduce or eliminate dynamic overloads in the power section of the circuit. Often these measures contradict each other, and in order to achieve certain results, the developer makes some, sometimes very difficult, compromise. Therefore, further optimization of the parameters of the converting technology is possible only through the transition to new principles of building these devices.
In order to understand the fundamental difference between the voltage regulation method offered by "Elkon", what novelty it contains, let's first talk about the traditional construction of regulators. DC-to-DC converters (DC / DC converters), which are a significant class of devices from the field of power electronics, are traditionally built according to the following scheme: the primary link converts DC voltage into a variable high frequency; the secondary link converts alternating voltage to direct voltage. The converter usually contains a regulator that controls the value of the output DC voltage or maintains it at the required level.
High-frequency conversion can be carried out using various schemes, but if we talk about push-pull circuits, then here we can name two types: circuits with a rectangular shape of the current of the power switches and resonant with a sinusoidal (or quasi-sinusoidal) shape of the current of the keys.
The efficiency of the converters is largely determined by the dynamic switching losses on the power elements when switching the operating current values. The experience of developing converters with a power of more than 100 W shows that it is possible to reduce these losses mainly due to the use of switching elements (transistors) with low switching times and due to the formation of the correct trajectory of their switching. The current element base, of course, has rather high dynamic characteristics, but, nevertheless, they are still far from ideal. Therefore, technological limitations often lead to significant overvoltages on the elements of the power circuit, which means that the overall reliability of the converter decreases.
Forming the correct switching path is an important task, which can also greatly reduce switching overvoltages. This method provides the so-called "soft" switching by redistributing energy between the actual power part of the switching element (transistor switch) and the forming element. Reduction of losses occurs due to the return of the energy accumulated by them. Recall that the well-known representatives of the forming elements are all kinds of RCD circuits, damping resistors, snubbers, etc.
The practice of developing real converters shows that when creating a device with a rated power of hundreds to thousands of watts, one has to literally "give" for each watt of effective power, to minimize heat losses, which reduce the overall efficiency of the converter.
Another problem relates to the need for high-speed short-circuit (SC) protection in the load. The problem is mainly that overly fast protection becomes too susceptible to false trips, tripping the inverter even when no danger arises for it. A protection that is too slow is immune to false positives, but will hardly protect the device. You have to spend a lot of effort to design optimal protection.
In connection with the above, the classic high-frequency converter does not quite meet the modern requirements for power converting equipment. There is a need to search for new ways of constructing these devices.
Recently, engineers have been looking at resonant converters as devices with great potential. In resonant converters, there are fundamentally less dynamic losses, they create much less interference, since switching occurs not with straight edges rich in harmonics, but with a smooth waveform close to sinusoidal Resonant converters are more reliable, they do not need fast short-circuit protection (SC) in the load, because the short-circuit current limitation occurs naturally. True, due to the sinusoidal shape of the current, the static losses in the power elements slightly increase, but since the resonant converters are not so demanding on the switching dynamics of the power elements, standard-class IGBT transistors can be used, whose saturation voltage is lower than that of the warp-speed IGBT -transistors. One can also recall SIT transistors and even bipolar ones, although, in the opinion of the author of the site, it is better not to remember the latter in this context.
From the point of view of building a power circuit, resonant converters are simple and reliable. However, until now, they have not been able to displace conventional half-bridge and bridge converters due to fundamental problems with the regulation of the output voltage. Conventional converters use the principle of control based on pulse width modulation (PWM), and there is no difficulty here. In resonant converters, the use of PWM and other special methods (for example, frequency regulation by changing the switching frequency) leads to an increase in dynamic losses, which in some cases become comparable to or even exceed the losses in classical converters. The use of shaping circuits justifies itself in a limited frequency range and with a very small depth of regulation. There is a somewhat more effective method based on a significant decrease in the switching frequency, which leads to a decrease in the average load current, and hence the output power. But this method of frequency regulation can also be called a compromise, which means that it does not sufficiently meet modern requirements.
Still, resonant converters were so tempting that several more ways were invented to increase their efficiency and control depth. Alas, these ideas also proved to be insufficiently effective. The use of an additional pulse regulator installed at the output leads to the need to use one more conversion link, which means that it reduces the efficiency. The switch-over design of the transformer again greatly complicates the converter, increases its cost, and makes it unfeasible for consumer applications.
From the foregoing, we can conclude that the main problem preventing the widespread use of resonant converters lies in the creation of an effective method for deep regulation of the output voltage. If this problem is solved, it will be possible to significantly improve the characteristics of power electronics devices, their further spread to the already mastered and new areas of application of converter technology.
The specialists of the "Elkon" enterprise managed to make significant progress in researching the regulation method by reducing the switching frequency. It was this method that was taken as a basis, since it retains the main advantage of the resonant circuit - switching switching at zero current. The study of the processes occurring in a conventional resonant converter made it possible to refine its scheme and find a more efficient regulation mechanism in a wide range of loads and an acceptable frequency range, which formed the basis of an international patent. In addition, it was possible to achieve the same amplitude of power transistor currents both in rated load and short-circuit mode, no through currents through power transistors even at maximum switching frequency, "soft" load characteristics (much better than that of a conventional resonant converter).
The complete circuit of the modernized resonant converter is the subject of "know-how" of the "Elkon" enterprise, however, in order to make it clear to the reader what the improvement is, the following information is given from the patent "Method of controlled resonant conversion of constant voltage".
The invention is intended for the implementation of powerful, cheap and efficient adjustable high-frequency transistor resonant voltage converters for various applications. These can be welding converters, induction heating installations, radio transmitters, and more.
There is a prototype of an adjustable resonant voltage converter published in. In the prototype: an oscillation is created with its own period To and the switching period of the power switches Tk; capacitive and inductive energy storage is used with consumption from a constant voltage source and transfer of part of the energy to the load with a rectifier; voltage regulation is carried out due to the detuning from resonance with the period of natural oscillations To of the switching frequency of the switches Tk, close to To.
As mentioned above, detuning leads to a significant increase in dynamic losses and, in general, reduces the reliability of the converter, since detuning loses the main advantage of the resonant converter - switching at zero currents. All this leads to the fact that it is advisable to use the method only in low-power converters.
There is a closer prototype published in the work. In this prototype, an oscillation is also created with its own period To and the switching period of the keys Tk, but Tk\u003e To; capacitive and inductive energy storage is used with consumption from a constant voltage source and transfer of part of the energy to the load with a rectifier; the output voltage is regulated by changing the switching period Tk. However, here the excess energy of the capacitive storage is returned back to the power source due to the discharge of the capacitive storage through the load, and the limitation of the front of the current pulses of the power switches is carried out using additional inductive storage. This method retains the main advantage of the resonant converter - the ability to switch power switches at zero currents.
Unfortunately, this prototype also has several disadvantages. One of the fundamental disadvantages is an increase in the current of the keys in the event of overloads or short-circuits in the load circuit at the rated or maximum frequency. Since in this case the inductive elements store a large amount of energy, it does not have time to fully return to the power source in a short period (Tk-To) / 2. Another drawback is the forced interruption of the current through the switches, despite the fact that the switching front is set. Here it becomes necessary to have complex protection of key elements, narrows the overall voltage regulation range, which leads to a narrowing of the scope of the converter.
The device with which this method can be implemented is a conventional resonant half-bridge converter with a capacitive voltage divider (capacitive storage) and an inductive storage connected with a load between the half-bridge transistor rack and the middle terminal of the capacitive divider. Additional inductive accumulators are included in the branches or in the circuit of each key element.
The device, proposed by the "Elkon" enterprise, solves the problem of providing a wide range of load voltage regulation and, thus, expands the scope of its application. In the new method, you can find some analogies with prototypes and: oscillations are created with an own period To and a switching period Tk, and Tk\u003e To, capacitive and inductive storage is also used with consumption from a constant voltage source and transfer of part of the energy to the load with a rectifier, also return of excess energy of the capacitive storage back to the source, voltage regulation is carried out by changing Tc. The novelty of the method lies in the fact that simultaneously with the first oscillations, second oscillations are created with a natural period To and a switching period Tk, using the same capacitive storage and a second inductive storage with energy consumption from a capacitive storage and energy transfer to a load with a rectifier.
The main feature of the proposed method is the simultaneous flow of the currents of the first and second oscillations through the key elements in such a way that the total current through them does not break, which allows the energy of inductive storage devices to be returned at maximum frequency even in the event of a short circuit. In this case, the amplitude of the current of the key elements remains at the level of the nominal values. This method "works" in the entire range of switching periods Tc, which successfully solves the problem of the resonant converter.
Device shown on picture 1, contains a controlled master pulse generator (1), the outputs of which are connected to the gates of transistors (2) and (3), forming a half-bridge rack (half-bridge arm). The common point of connection of transistors (2) and (3) through a capacitive storage (resonant capacitor), designated (5), is connected to one of the terminals of the transformer-rectifier load (6). Inductive storage (resonant chokes), designated (7) and (8), are connected in series. Their common connection point is connected to the other load terminal (6). The supply voltage source (9) is connected to the lower terminals of the choke (7) and the emitter of the transistor (2). The upper terminal of the choke (8) is connected to the collector of the transistor (3).
On the figure 2 graphs showing the operation of this resonant transducer are shown. The master generator (1) generates the paraphase control pulses shown in fig. 2 a-b, duration To / 2 and adjustable switching period Tk, which in turn open transistors (2) and (3). In the steady-state mode of the converter operation, at time t1, a control pulse is applied to the transistor (2), while a sinusoidal current pulse I1, shown in fig. 2 in, - the so-called "first oscillations". Simultaneously with it, current I2 continues to flow through the antiparallel (opposed) diode (4) of the transistor (3) - the "second oscillation".
 |
| picture 3 The first cycle of the circuit |
On the figure 3 shows the first clock of the circuit, which reflects its behavior in the interval (t1 ... t2). Resonant capacitor (5) with voltage U5, the graph of which is shown in fig. 2 d., is recharged through a transformer-rectifier load (6), including a transformer (6.1), a rectifier (6.2) and the load itself (6.3). The first resonant choke (7) stores energy. At the same time, the resonant capacitor (5) is discharged through the second resonant choke (8) with a voltage U8, the graph of which is shown in fig. 2 d... The choke (8) stores energy in accordance with the polarity indicated on the graph.
 |
| picture 4 The second cycle of the circuit |
On the figure 4 the second step of the circuit is shown, reflecting its behavior in the interval (t2 ... t3). The resonant capacitor (5) continues to recharge through the transformer-rectifier load (6) and the first resonant choke (7). Also, the resonant capacitor (5) is recharged through the second resonant choke (8), which already gives off energy in accordance with the indicated polarity.
 |
| picture 5 The third cycle of the circuit |
On the figure 5 the third cycle of the circuit is shown, which reflects its behavior in the interval (t3 ... t4). The resonant capacitor (5) continues to charge through the transformer-rectifier load (6) and the first resonant choke (7) with the voltage U7 shown in the graph fig. 2 f... At the same time, the resonant capacitor (5) is already being charged from the second resonant choke (8), which continues to deliver energy in accordance with the indicated polarity.
 |
| picture 6 The fourth cycle of the circuit |
On the figure 6 the fourth cycle of the circuit is shown, which reflects its behavior in the interval (t4 ... t5). The resonant capacitor (5) continues to charge through the transformer-rectifier load (6) and the first resonant choke (7), which already gives off energy in accordance with the polarity indicated in the figure. At the same time, the resonant capacitor (5) continues to charge from the second resonant inductor (8).
On the figure 8 the sixth clock of the circuit is shown, reflecting its behavior in the interval (t6 ... t7). The resonant capacitor (5) already transfers energy through the transformer-rectifier load (6) and the first resonant choke (7) to the power supply (9). In this case, the current I1 changes its direction.
 |
| picture 9 The seventh cycle of the circuit |
On the figure 9 the seventh cycle of the circuit is shown, which reflects its behavior in the interval (t7 ... t8). A control pulse is applied to the transistor (3), while a sinusoidal current pulse I2 begins to flow according to fig. 2 inthrough this transistor ("second oscillation"). The current I1 also continues to flow through the antiparallel diode (10) of the transistor (2) - the "first oscillation". The resonant capacitor (5) gives off energy through the transformer-rectifier load (6) and the first resonant choke (7) - into the supply voltage source (9) and into the second resonant choke (8).
On the figure 11 the ninth cycle of the circuit is shown, reflecting its behavior in the interval (t9 ... t10). All storage devices give up their energy.
On the figure 13 shows the final cycle of the circuit, which reflects its behavior in the interval (t11 ... t1). The resonant capacitor (5) is discharged, then the processes are repeated.
Pay attention: in the time interval t6-t7, energy is returned to the source, since the current I1 changes its direction. The negative amplitude of the current I1 is determined by the load of the converter. This fact determines the additional advantages of the method - the amplitude of the current through the switches does not increase up to a short circuit in the load. Also, the problem of through currents is completely absent, which makes it easy and reliable to control transistors. The problem of creating fast protections to prevent the short circuit mode also disappears.
This idea was the basis of prototypes and serial products that Elkon is currently producing. For example, a voltage converter with a capacity of 1.8 kW, designed for a cathodic protection station for underground pipelines, receives power from a single-phase AC 220 V 50 Hz. It uses power IGBT transistors of the IRG4PC30UD type of ultra-fast class with a built-in opposed diode, the capacity of the resonant capacitor (5) is 0.15 μF, the inductance of the resonant chokes (7) and (8) is 25 μH each. The period of natural oscillations To is 12 μs, the transformation ratio of the transformer (6.1) is 0.5, which determines the range of the rated load (0.8 ... 2.0) Ohm. For the minimum value of the switching period Tk, equal to 13 μs (at a switching frequency fk equal to 77 kHz) and a load of 1 Ohm, the amplitudes of currents I1 and I2, respectively, are plus 29 A and minus 7 A. For a load of 0.5 Ohm, the amplitudes of currents I1 and I2 were respectively, plus 29 A and minus 14 A. In the case of a short circuit, these values \u200b\u200bare plus 29 A and minus 21 A, the average current through the load is 50 A, that is, the effect of limiting the short circuit current appears.
On the figure 14 the family of control characteristics of the converter is shown. It is important to note that over the entire switching frequency range, switching pulses are applied at zero currents. These results were obtained in the OrCAD 9.1 circuit modeling system, then tested on a full-scale model.
For comparison, on figure 15 a family of control characteristics of a classical resonant transducer of similar power is presented. The minimum switching period Tk is increased due to the occurrence of through currents and is 14 μs (with a switching frequency fk equal to 72 kHz). For this rated frequency, the switching mode is at zero current. For a load resistance of 1 Ohm, the amplitude of the load current is 30A, for a resistance of 0.5 Ohm, the amplitude is already 58A. In the case of a short circuit, the amplitude of the current through the transistors becomes more than 100 A, and the switching of the power transistors is no longer at zero currents, but the average load current becomes more than 180 A. Thus, as mentioned earlier, there is a need for fast short circuit protection to eliminate an accident ...
The regulation section "A" (thin lines) characterizes the switching mode not at zero current. Of practical interest is the regulation section "B" when the switching frequency is two or more times less than the nominal one. It can be noted that the depth of regulation in this way for a classic converter is much less than in an Elkon converter, and the need to operate at a lower switching frequency worsens the specific energy performance of a classic converter. The offered Elkon converter has practically acceptable control characteristics and a range of switching frequency.
Given the soft load characteristic, it is possible to control the output voltage at a fixed frequency by phase control of two converters connected in parallel with an AC voltage. This option has been tested on a 1.2 kW model. The output voltage ranges from zero to maximum.
The results obtained suggest that voltage converters using the new method of resonant conversion will find wider application in all areas of using conventional converters with PWM regulation for tens or more kW.
And now - a little about serial production. The Elkon enterprise produces:
- cathodic protection stations with a capacity of 0.6, 1.5, 3.0 and 5.0 kW, with an efficiency in the nominal mode not worse than 93%;
- sources for manual arc welding with a power of 5.0 and 8.0 kW powered from a 220 volt 50 Hz network;
- sources for manual arc welding with a power of 12 kW powered by a three-phase network of 380 volts 50 Hz;
- sources for heating forging blanks with a power of 7.0 kW powered from a 220 volt 50 Hz network;
- converters for a high-voltage solar battery with a power of 5.0 kW with an input voltage from 200 to 650 V and an output voltage of 400 V; when modulating the output voltage of the converter according to a sinusoidal law with a frequency of 100 Hz and the subsequent distribution of half-waves, electricity was transferred from the solar battery to the 220 volt 50 Hz network.
The company's employees hope that this idea will also inspire experienced radio amateurs who are engaged in the construction of welding equipment.
LITERATURE
Meshcheryakov V.M. Power electronics is an effective way to solve the problems of the regional program "Energy Saving" // Electrical Engineering. 1996.12. 1.
High-frequency transistor converters. / E.M. Romash, Y.I.Drabovich, N.N. Yurchenko, P.N. Shevchenko -M .: Radio and communication, 1988.-288s.
Goncharov A.Yu. Serially produced transistor power converters // Electronics: Science, Technology, Business. 1998.2. P. 50.
Kovalev F.I., Florentsev S.N. Power electronics: yesterday, today, tomorrow // Electrical Engineering. 1997.11 p. 2.
Dmitrikov V.F. and others. New highly efficient domestic power supplies with transformerless input // http //: www.add.ru/r/konkurs/st.18.html
Patanov D.A. General problems of reducing switching losses in voltage inverters // http://www.add.ru/r/konkurs/avtst8.html
Zhdankin V.K. Power electronics devices from Zicon Electronics // Modern automation technologies. 2001.N1.p.6.
Belov G.A. High-frequency thyristor-transistor DC voltage converters. -M .: Energoatomizdat, 1987.-120s.
PCT patent, WO94 / 14230, 23.06.94, H02M 3/335.
Patent PCT / MD 03/00001. May 16, 2002, H02M3 / 337 What do they write
Resonant inverters are widely known in converter technology. They provide a harmonic current shape in the power circuit due to the oscillatory circuit. Consider the principle of operation of a resonant inverter, which is illustrated by the diagram and diagrams in Fig.5.13.
Figure 5.13 - Principle of operation of a resonant inverter
In this figure, S 1, S 2 are controlled keys operating in antiphase. When the switch S 1 closes, the current i 1 begins to grow according to the harmonic law. The natural frequency of the loop with losses is
(5.8)
Through the interval T 0/2, the current in the circuit will become equal to zero and the switch opens at a zero value of the switched power. At time t1, switch S2 is closed and a negative half-wave of the current in the load is formed due to the vibrational exchange of energy between the reactive elements. Again, through T 0/2, the current in the circuit becomes zero, S2 opens and the key S1 closes, and so on. Quality factor of the contour
(5.9)
If the switching frequency of the keys corresponds to the resonance frequency of the circuit 
, then the voltage waveform across the load is close to harmonic, and its effective value 
(5.10)
The load can be connected in series (as in Figure 5.13) or in parallel with any of the reactive elements, usually a capacitor.
The advantages of resonant inverters:
a) reduction of power losses for switching. Especially in conditions of a large technological spread of the parameters of the keys. The so-called “soft” switching is provided,
b) reducing the level of high-frequency interference, both radiated (radio interference) and propagated through wires (conductive), into the supply network and into the load,
c) the absence of through currents in push-pull circuits leads to
increased reliability.
Disadvantages of resonant inverters:
a) a significant excess of the voltage on the reactive elements over the supply voltage due to the phenomenon of resonance;
b) an increase in the size of smoothing filters in comparison with rectangular voltage;
c) higher installation power of keys.
An approximate diagram of a transistor converter with a resonant inverter is shown in Figure 5.14. The load R H is connected in parallel with the capacitor C K through a full-wave rectifier VD 1 and VD 2.
Figure 5.14 - Converter with a resonant inverter
The TV transformer provides voltage level matching and galvanic isolation between mains and loads. The output voltage is stabilized by frequency modulation of the clock frequency (f T) of the control circuit. For which f T is chosen slightly less than the resonant frequency of the circuit L K C K. By adjusting the frequency, an instability of about 0.1% can be obtained. The noise level is about 15 dB lower than in non-resonant inverter circuits.
Many specialized and universal controllers have been developed to control the keys of inverters, for example, 1114EU1 ... 1114EU5, UC3846, UC3875, TL494, TL599, etc.
5.5 Examples of tasks for converters with solutions
Example 5.5.1
Initial data:there is a voltage converter with a rectifier and an output smoothing filter, the circuit of which is shown in Figure 5.15. Its parameters: 
,,
,
,
.
Define the magnitude of the voltage across the load of this source (all elements are ideal).
Figure 5.15 - Power supply diagram
Decision. The voltage at the input of the smoothing filter (diode VD3) of the power supply has the form shown in Figure 5.16.
The constant component is
,
where 
- transformation ratio,
- pulse duty cycle.
Figure 5.16 - Rectifier output voltage waveform
Example 5.5.2
Initial data:The voltage waveform at the output of the inverter looks like Figure 5.17.
Defineoptimal value of the duty cycle of the inverter control pulses ( 
) in terms of the minimum content of 3rd and 5th harmonics.
Decision. The harmonic components of the output voltage for a square wave have the following dependence on the duty cycle: 
According to this expression, we construct adjustment curves for three harmonics k \u003d 1, k \u003d 3 and k \u003d 5 (Fig. 5.18).
Figure 5.18 - Harmonic components of the inverter output voltage
From the graphical dependences it can be seen that the minimum content of the 3rd and 5th harmonics occurs at K 3 \u003d 0.73.
Example 5.5.3
Initial data: There is a single-ended converter with reverse connection of a rectifier diode (Fig. 5.19). Scheme parameters:
,
,
,
.
Figure 5.19 - Voltage converter
Definethe minimum value of the fill factor for ideal keys.
Decision.At the output of the transformer in the nominal mode, the maximum voltage is 30V, since 
... The average output voltage is 
... The minimum duty cycle corresponds to the maximum voltage deviation, i.e.
.
Example 5.5.4
Initial data:There is a voltage converter (Fig.5.20) based on a half-bridge inverter with parameters:, 
,
, load current 
.
Figure 5.20 - Voltage converter
Definevoltage at the collector of a closed transistor (VT1 or VT2) and the maximum value of the current in the primary circuit of the transformer I 1.
Decision.The voltage on the collector of the closed transistor does not exceed the supply voltage level, i.e. 
.
The maximum value of the current in the primary circuit of the transformer is:
