BG2571U1 - A radiofrequency heating apparatus with direct digital operation of the radiofrequency power and control of the power’s fine tuning - Google Patents

Abstract

The current utility model relates to a radiofrequency heating apparatus with direct digital operation of the radiofrequency power and control of the power’s fine tuning, which can be used in radiofrequency industrial energy systems, specifically in radiofrequency heating energy systems, which are used for radiofrequency induction heating and melting, as well as for generating plasma, corona discharge and radiofrequency heating of hydrocarbons. The created radiofrequency heating apparatus, according to the utility model, includes an alternating current source (12), connected to a central direct current rectifier (14), the outlet of which is connected to a multitude of radiofrequency devices (16.1… 16.N and 34). Each of the multitude of devices (16.1… 16.N and 34) is in turn connected to a transformer (18.1… 18.N and 40) whose secondary coils are successively connected to generate exit voltage V3. The exit voltage Vs is fed to resonance circle (22), which includes working component of the type bobbin for induction heating (24) or condenser of a corona discharge (26). The resonance circle (22) through backwards connection (28) is connected to the operations block (30) which is in turn connected to also each radiofrequency device (16.1… 16.N and 34), for securing and feeding operation radiofrequency signals as to activate the devices (16.1… 16.N and 34). To the operation block (30) is connected to adjustable charge (36), which ensure adjustable controlled direct current voltage for the last radiofrequency device (34.)

Description

Field of technology

This utility model relates to a radio frequency heating apparatus with direct digital control of the radio frequency power and fine tuning of power, which will find application in the radio frequency industrial energy systems and in particular in the radio frequency heating energy systems used for radio frequency induction heating and melting, as well as for generating plasma, corona discharge and radiofrequency heating of hydrocarbons.

BACKGROUND OF THE INVENTION

Induction heating is a method for non-contact heating of materials for the implementation of production operations welding, hardening, annealing, melting of metals and others, which uses electromagnetic induction created by the flow of high-frequency electric current. This method is applied in production operations related to the processing of electrically conductive materials.

Induction heating is performed by technological devices with an inductive element, which is a source of alternating magnetic field. It is built as a coil with one or more windings, through which a strong high-frequency electric current flows from a powerful generator. In such an embodiment, the inductive element is a primary winding of a transformer without an iron core.

The main units that make up the induction heating devices are directly dependent on the current state of technology in the production of power semiconductor devices. The classic block diagram includes a three-phase thyristor rectifier; low frequency, mains L-C filter and bridge thyristor inverter. The coordination of the parameters of the inductor with those of the inverter in most cases is done with the help of a transformer. The main advantage of the classic circuit is its simplicity, but on the other hand the low-frequency L-C filter is too bulky and expensive. The regulation of the output power by means of a thyristor rectifier leads to a deterioration of the energy indicators of the device in relation to the power supply network (low efficiency). The reaction of the thyristor rectifier, with rapid changes in the parameters of the induction load, is too slow (the minimum reaction time is 3.3 milliseconds). The thyristor inverter is not able to operate at high frequencies (tens or hundreds of kilohertz).

The advent of powerful MOSFET and IGBT transistors leads to a change in the approach to the construction of induction devices. Initially, the thyristor inverter unit was replaced with a transistor, which increases its operating frequency to hundreds of kilohertz, but retains the thyristor rectifier, and thus its associated shortcomings.

The block diagram on the basis of which the induction devices have recently been built includes a three-phase diode rectifier; low frequency, mains L-C filter; step-down DC transistor regulator; high frequency L-C filter and bridge transistor inverter.

The main disadvantage of the induction device realized in this way is the triple conversion of the electric energy: rectifier, DC regulator, inverter. This leads to a decrease in the overall efficiency.

Technical essence of the utility model

The object of the present utility model is to create a radio frequency heating apparatus with direct digital control of the radio frequency power of the induction heating, combined with one or more techniques for fine-tuning the power control, allowing control of the output power in an industrial radio frequency energy system. led to an increase in the overall efficiency.

The problem was solved by creating a radio frequency heating device with direct digital control of the radio frequency power and control of the fine tuning of the power, including an alternating current source connected to a central rectifier of direct current.

2571 UI current, the output of which is connected to a plurality of radio frequency devices. Each of the plurality of devices in turn is connected to a transformer whose secondary windings are connected in series to generate an output voltage. The resulting output voltage is applied to a resonant circuit comprising an operating component of the induction heating coil or corona discharge capacitor. The resonant circuit is connected via feedback to a control unit, which in turn is connected to each of the radio frequency devices for providing and transmitting control and radio frequency signals for actuation of the devices. An adjustable power supply is also connected to the control unit, providing alternating controlled DC voltage for the last radio frequency device.

Radio frequency devices are built as silicon steerable rectifiers (SCR), or bipolar transistors with isolated gate (IGBT) or field metal oxide transistors (MOSFET), configured as a circuit of a full bridge amplifier, half-bridge amplifier, class A amplifier AB, Class C amplifier and / or Class D amplifier.

In a preferred embodiment of the apparatus, the radio frequency devices are constructed as a full class D bridge amplifier.

In a preferred embodiment, the latter RF device may include a plurality of special low power RF devices, each of which may have a transformer with its own conversion factor.

In another preferred embodiment, the latter radio frequency device is a full bridge inverter comprising two field-effect transistors connected in series between a positive voltage V + and ground, and two other field-effect transistors also connected in series between a positive voltage V + and ground. The primary winding of the last transformer connected to this last radio frequency device is connected in series at the point between the first field effect transistors and the point between the other field effect transistors.

In another preferred embodiment of the apparatus, the last transformer connected to the last radio frequency device has a different transformation coefficient than that of the other transformers.

The radio frequency heating apparatus, according to the utility model, also includes an alternative rectifier connected respectively to said radio frequency devices.

The advantages of the radio frequency heating device thus created are mainly due to the fact that it is built on a modular principle. The power output to the output from each module is equal to the output power divided by the number of modules. The individual modules are of the same type and are of a lower class. This leads to a lower cost of the induction device compared to those devices in which all the output power is given by one module.

In addition to the simplified configuration of the individual modules, a very good efficiency has been achieved. This is also due to the failure of the DC regulator and the high-frequency L-C filter with their associated losses, which has improved the overall performance of the induction device as a whole.

The created radio frequency heating device is also distinguished by its reliability. Each damaged module is easily bypassed and the technological process can be completed by the other modules. In addition, the price of one module is one ηth of the price of the whole induction device. This allows the maintenance of one module, as a backup, in stock. The modular structure allows quick replacement of any damaged module.

Explanation of the attached figures

This utility model is illustrated in the accompanying figures, where:

Figure 1 is a block diagram of a radio frequency heating apparatus using direct digital radio frequency control and fine tuning of the power;

Figure 2 is a graph of the output voltage as a function of the number of activated RF devices;

Figure 3 - graph of the output power as a function of the number of activated radio frequency devices;

Figure 4a is a diagram of a phase shift circuit

2571 UI, providing control of fine-tuning the power;

Figure 46 shows the phase control signals and the output power signal associated with the phase shift circuit shown in Figure 4a;

Figure 5a shows the last radio frequency device 34 and the transformer 40;

Figure 56 shows an embodiment of the radio frequency device 34 and transformer 40 shown in Figure 5a;

Figure 5b shows a second embodiment of the radio frequency device 34 and transformer 40 shown in Figure 5a by means of binary weighing devices.

Examples of implementation of the utility model

Figure 1 shows a schematic diagram of a radio frequency induction heating apparatus 10 that uses direct digital control of the radio frequency power. The radio frequency induction heating apparatus 10 includes an alternating current (AC) source 12 connected to a central direct current (DC) rectifier 14, which results in a constant voltage V + applied to a plurality of radio frequency devices 16.1 ... 16.N and 34. As an alternative to the central rectifier 14 for all radio frequency devices 16.1 ... 16.N and 34, a special rectifier 20 can be used, respectively connected to each of the radio frequency devices 16.1 ... 16.Nh34, as shown by the dotted line of figure 1.

In turn, each of the many radio frequency devices 16.1 ... 16.N and 34 is connected to a transformer 18.1 ... 18.N and 40. The secondary windings of each of the transformers 18.1 ... 18.N and 40 are connected in series for output voltage generation Vs. The output voltage Vs is used to power a resonant circuit 22, which includes an induction heating coil 24 or a corona capacitor 26. A feedback 28 is used to monitor the output voltage Vs, current and / or frequency of the input voltage. the resonant circuit 22 by a signal to the control unit 30. The control unit 30 transmits control signals for on / off to each radio frequency device 16.1 ... 16.N and 34, as well as radio frequency signals for driving these devices. The radio frequency signals for driving the devices 16.1 ... 16.N and 34 include two phases that are offset from each other. The frequency of the radio frequency signals is determined by the resonant frequency of the resonant circuit 22. The control unit 30 provides closed loop power control based on the power input signal 32 and the feedback signal 28. The input signal 32 may be an analog or digital signal. which is fed to the control unit 30 by a user interface. Adjustable power supply 36 provides alternating controlled DC voltage for the last radio frequency device 34. In some embodiments, the adjustable power supply 36 is not used, wherein the voltage is provided by the output voltage Vs to each of the radio frequency devices 16.1 ... 16.N and 34, as shown by bypass compound 38.

The control unit 30 controls the output power using a combination of direct digital radio frequency power control for each of the radio frequency devices 16.1 ... 16.N and 34 and control of the fine power adjustment, including phase shift control and control, as well as and adjustable power supply to the last device 34.

The embodiments set forth herein directly control one or more radio frequency devices for applying closed loop power control instead of amplifying or modulating an external signal. In addition, the above embodiments use fine-tuning power control with direct digital power control in induction heaters. These features significantly reduce the total number of RF devices used to achieve the desired power, which reduces costs and improves the overall efficiency of the device 10.

The presented embodiments apply a scheme for direct digital control of the radio frequency power, combined with one or more techniques for control of the fine tuning of the power without the need for additional amplification stages. The presented embodiments allow working with a variable

2571 UI a frequency that monitors the resonant frequency of the resonant circuit 22, for example by using a phase-locked automatic frequency tuning.

A current transformer sensor (not shown in the figure) can be used to receive a feedback signal 28. Current transformer sensors are used to measure alternating electric current. These sensors are known as measuring transformers. If the current in a circuit is too high to be applied directly to the measuring instruments, the current transformer sensor provides a reduced current proportional to the current in the circuit, which can be easily applied to the measuring and recording instruments, such as the control unit 30. .

Like any other transformer, the current transformer sensor has a primary winding, a magnetic core and a secondary winding. The alternating current in the primary winding generates an alternating magnetic field in the core, which then causes an alternating current in the secondary winding. The main purpose of the current transformer design is to ensure that the primary and secondary circuits are effectively connected so that the secondary current is directly proportional to the primary current. The current transformer sensor includes a wire wound around a silicon steel ring placed on the measured circuit. The primary winding includes, for example, one winding and the secondary winding multiple windings.

Radio frequency devices 16.1 ... 16.Nh34 are based on a number of semiconductor technologies, such as silicon controllable rectifiers (SCR), insulated gate bipolar transistors (IGBT) and field metal oxide transistors (MOSFET), configured in various topologies, including but not only as a scheme of a full bridge amplifier, a half-bridge amplifier, a class A amplifier, a class AB amplifier, a class C amplifier and / or a class D amplifier. class D, but other topologies may be used. The transistors used can range from field metal oxide transistors (MOSFETs) or insulated gate bipolar transistors (IGBTs) to newer switching semiconductors. The typical device used is the HyperFET powerful MOSFET Q3-Class, IXFB44N100Q3.

Transformers 18.1 ... 18.N and 40 are usually implemented with a ratio M: 1 or M: 0.5 with multiple primary windings and a secondary winding with one or two windings. The ratios can be varied to allow for the correct matching of the impedance of the radio frequency device 16.1 ... 16.N and 34 with the output circuit or the resonant circuit 22. Typical transformation coefficients are from 2: 1 to 40: 1.

The control unit 30 provides control signals and frequency signals, independently of each radio frequency device 16.1 ... 16.N and 34, each of which then generates a radio frequency supply voltage, which is applied to the primary windings of the respective transformer 18.1 ... 18.N and 40. The control unit 30 provides a closed loop power control system by comparing the feedback signal 28 with the power input signal 32 to determine the required power level. The control unit 30 operates by selectively activating the radio frequency devices 16.1 ... 16.N and 34 by means of control signals as required. The frequency signals for the individual devices can be the same or different.

Figure 2 shows the output voltage Vs, which is an alternating voltage, of the radio frequency heating apparatus 10 as a function of the number of radio frequency devices 16.1 ... 16.Nh 34 which are activated. Each of the devices 16.1 ... 16.N provides a substantially equivalent output voltage step. By selectively activating the individual radio frequency devices 16.1 ... 16.N and 34, the output voltage Vs gradually changes. The output power, as shown in Figure 3, varies in proportion to the square of the output voltage Vs. When a power change is required, the control unit 30 activates the required number of radio frequency devices. As shown in Figure 3, when using five radio frequency devices, the energy steps are relatively coarse. However, using a larger number of RF devices can be too expensive for many applications.

The latest radio frequency device 34

2571 UI can be implemented in a different way from the other radio frequency devices 16.1 ... 16.N (shown in figures 5a, 56 and 5c), as the associated transformer 40 has a different transformation coefficient from the other transformers 18.1 ... 18.N.

In turn, in various embodiments of the apparatus, the latter radio frequency device 34 may include a plurality of special low-power radio frequency devices, each of which may have a transformer with its own transformation factor, which increases the effective number of radio frequency devices connected to the apparatus. 10. Figure 5c shows an embodiment in which the transformers have a conversion factor that is binary weighed. Thus, the transformation coefficients are given as 2: 1, 4: 1, 8: 1 and 16: 1 in this variant or in general as 2η: 1, where η is an increasing integer.

A variant for fine-tuning the power adjustment is shown in Figures 4a and 46. By applying phase shift control of the RF devices 16.1 ... 16.Ν and 34, the fine-tuning control of the power frequency can be achieved without requiring additional components. The last radio frequency device 34 can be implemented as a complete bridge inverter, shown in Figure 4a. The bridge inverter includes two field-effect transistors connected in series between a positive voltage V + and ground, and two other field-effect transistors connected in series between a positive voltage V + and ground. The primary winding of the last transformer 40 is connected in series at the point between the first field effect transistors and the point between the other field effect transistors. The voltage Vs is obtained on the secondary winding of the transformer 40. Figure 46 shows time diagrams of the phases A and B of the signals from the control unit 30 of the two pairs of field-effect transistors, as well as the power signal provided through the secondary winding of the transformer 40. The signals with phase A and phase B are modulated in phase with respect to each other, for controlling the energy transmission at the output of a radio frequency device 34.

When combining direct digital radio frequency control with phase shift control, such as performing fine-tuning of power control, the value of the phase shift depends on the number of radio frequency devices that are activated. Power is a function of the square of the output voltage Vs. In a system with five radio frequency devices, when one of them is activated, the total power is approximately 4%. When a radio frequency device is activated, 100% of its output is controlled so that the phase shifts by 0-180 degrees. If a second RF device is added, the total power is increased to 18%, but only the partial power difference, which is approximately 12%, is controlled by phase control. That is, approximately 70% of the total power is controlled by phase control or about 0-120 degrees.

Because additional RF devices are used, the change in power that is controlled decreases as a ratio of the total power to a number of RF devices, and as a result, the phase angle used to control these RF devices also decreases. When five radio frequency devices are activated, 45% of the power range or only 78 degrees of phase shift is controlled by phase control.

Activation of additional radio frequency devices allows for a significant reduction in energy loss by reducing the range of the phase angle at higher power, which is a significant improvement over conventional techniques.

Claims (7)

Radio frequency heating apparatus with direct digital control of the radio frequency power and control of the fine tuning of the power, characterized in that it includes an alternating current source (12) connected to a central direct current rectifier (14), the output of which is connected to a number of radio frequency devices (16.1 ... 16.N and 34), each of which in turn is connected to a transformer (18.1 ... 18.N and 40), whose secondary windings are connected in series to generate of the output voltage Vs, wherein the output voltage Vs is applied to the res 2571 UI nance circuit (22) including an operating component of the induction heating coil type (24) or a corona discharge capacitor (26), the resonant circuit (22) being connected to a control unit (30) by feedback (28) , which in turn is connected to each radio frequency device (16.1 ... 16.N and 34) by providing control and radio frequency signals for activation of the devices (16.1 ... 16.N and 34), whereby to the control unit (30) is also connected to an adjustable power supply (36), providing alternating controlled DC voltage for the last radio frequency device (34). Radio frequency heating apparatus according to Claim 1, characterized in that the radio frequency devices (16.1 ... 16.N and 34) are designed as silicon controllable rectifiers (SCR) or insulated gate bipolar transistors (IGBT) or field metal -oxide transistors (MOSFET) configured as a full bridge amplifier, half-bridge amplifier, class A amplifier, class AB amplifier, class C amplifier and / or class D amplifier. Radio frequency heating apparatus according to Claim 2, characterized in that the radio frequency devices (16.1 ... 16.Nn34) are designed as a full class D bridge amplifier. Radio frequency heating apparatus according to claim 1, characterized in that the latter radio frequency device (34) may comprise a plurality of special low power radio frequency devices, each of which may have a transformer with its own transformation coefficient. Radio frequency heating apparatus according to claim 1, characterized in that the last radio frequency device (34) is a complete bridge inverter comprising two field-effect transistors connected in series between a positive voltage V + and ground, and two other field-effect transistors connected. also in series between a positive voltage V + and ground, wherein the primary winding of the last transformer (40) is connected in series at the point between the first field-effect transistors and the point between the other field-effect transistors. Radio frequency heating apparatus according to claim 1, characterized in that the transformers (18.1 ... 18.N) have a different transformation coefficient than that of the transformer (40). Radio frequency heating apparatus according to Claim 1, characterized in that the apparatus also includes an alternative rectifier (20) connected to the radio frequency devices (16.1 ... 16.Nh34) respectively.