EP1624546A1 - Arrangment and method for generating a corona discharge - Google Patents

Abstract

The DC source (1) for the corona discharge has an AC supply with a step-up transformer (2) followed by a rectifier bridge (3). The AC supply may be at 400 V 50 Hertz. The rectifier terminals (G1,G2) are connected via a capacitor (C3). Two electronic switches (S1,S2) in series are connected across the rectifier terminals and are connected to a control circuit (4). Connecting lines (A,C) are connected to the rectifier terminals and a third line (B) is connected to the midpoint (V) between the switches. A first corona discharge system (5) has electrodes (E1,E2) connected to the second and third lines. The electrode arrangement (E) incorporates a resistor (RE) and a capacitor (CE).

Description

The invention relates to an arrangement for generating a corona discharge according to the preamble of claim 1 and a method directed thereto according to the preamble of claim 8.

An arrangement for generating a corona discharge is shown in DE 26 52 283. There, an electrode arrangement of two electrodes is shown, which is laid and charged to a high voltage via switches or rotary switches configured as spherical spark gaps. The ignition of the discharge takes place via an ignition electrode or by approaching the electrodes themselves. The disadvantage of this arrangement is that the local switches charge the ozonizer until reaching the breakdown voltage and then a voltage breakdown occurs.

An apparatus for the indirect corona treatment of conductive and non-conductive materials is shown in EP 0 497 996. In the apparatus shown there, the treatment of a film takes place via two electrodes spaced on one side from the film to be treated, to which a sinusoidal high voltage is applied the charging always takes place with alternating polarity. The discharge is controlled there by the application of compressed gas to the discharge arc via a blowing channel provided for this purpose. The disadvantage here is that in addition to the electrode assembly, a further blowing device with a corresponding channel and a complicated control of the blowing process necessary for this purpose is necessary. In addition, this device is limited to the treatment of one side of the surface to be treated.

A corona generator, with which a corona discharge between two electrodes as a special form of atmospheric plasma can be generated, is the subject of DE-GM 88 07 090. Here, a roller is connected to one end of the secondary coil of a transformer whose other end connected to an elongated electrode is, which is arranged parallel and spaced from the roller. Parallel to the DC voltage source, a capacitor is connected, to which a first pair of switches is connected in parallel, between whose switches the primary coil of the transformer is connected in series. Further, a second pair of switches is connected in parallel with the capacitor between which Switches the primary coil of the transformer is also connected in series. The electronic switches are each connected in the same forward direction. The two switch pairs are controlled in phase opposition by a frequency generator. In this way, between the electrode and the roller in each case a high voltage is built up, which leads to a corona discharge, over the roller, for example, a plastic film runs whose surface is pretreated by the corona discharge in a manner that reduces the surface tension of the plastic film and this makes printable.

The transformer described in said utility model is designed as a matching transformer, in which the number of turns of the primary winding is variable to on the one hand to generate the required high voltage and on the other hand by suitable choice of the gear ratio to match the generator resistance and the load impedance, the change of Load impedance is caused by z. B. various pretreated materials, various dielectric materials with which the electrode is coated, and various mechanical dimensions.

A development of the circuit described is the subject of DE 39 23 694 C1, which has a corona generator to the object, which is characterized in that the frequency generator is variable in frequency and having a phase measuring circuit, wherein the phase angle between the current flowing through the primary winding and the voltage applied to the primary winding is determined and the output signal proportional to the phase angle between current and voltage drives the frequency generator and changes its frequency to a value at which the phase angle assumes a predetermined value.

Similar arrangements and methods are described in IEEE Transactions on Industry Applications, vol. la-11, no. 3, May / June 1975, 328-335, and in U.S. patents US 5,401,368 and US 4,145,386.

In the described publications, the voltage applied between the electrodes is substantially sinusoidal, which is already by sectarian measures already due to the high inductance of the transformer, which is directly coupled to the electrodes and forms an electrical resonant circuit with these, can not be avoided.

It has now been found that the effectiveness of surface treatment, e.g. As a plastic film, is not determined by the transmitted power, so not even integral of the applied waveform, but essentially by the rate of voltage increase (12th International Symposium on Plasma Chemistry, August 21-25, 1995, Proceedings, Vol. II, pp. 735-740). WO 2004/016052 A1 therefore describes a method for generating a gas plasma, wherein the waveform, in contrast to the sinusoidal curve, is cut off in the region of the maximum, so that the integral decreases, but the region of the voltage increase over time is preserved substantially unchanged remains. Such a method offers the advantage that the same surface treatment effect is achieved with lower power transmission and thus less heating of the electrodes.

An alternative solution for generating the highest possible voltage rise values is shown in DE 196 16 187 B4, wherein the superimposed sinusoidal signal curve is superimposed on a voltage pulse having a significantly shorter rise time in order to ignite the gas discharge.

Both documents, however, assume a substantially unchanged sinusoidal waveform which can only be influenced slightly in WO 2004/016052 A1, with the disadvantage that relatively large amounts of energy are transmitted to the electrodes, resulting in a high loss and high generation of wells at the electrodes ,

It is the object of developing a generic arrangement and method for generating a corona discharge so that for generating the corona discharge a much lower energy transfer occurs.

In addition, it would be desirable to be able to dispense with the expensive and sensitive dielectric coating of the electrode in a corona Vorbehandluagsanlage that has hitherto been necessary to avoid uncontrolled penetration of the high voltage and thus damage to vorxubeband the film.

These objects are achieved by an arrangement with the characterizing features of claim 1 and a method with the characterizing Merlamalen of claim 8. Advantageous embodiments are the dependent claims.

Fig. 1

zeigt ein schematisches Schaltbild einer erfindungsgemäßen Anordnung;

Fig. 2

zeigt ein Spannungszeitdiagramm eines Steuerverfahrens für ein erstes Ausführungsbeispiel der Anordnung nach Figur 1;

Fig.3

zeigt ein Spannungszeitdiagramm eines Steuerverfahrens für ein zweites Ausführungsbeispiel der Anordnung nach Figur 1;

Two embodiments of the invention will be explained in more detail below with reference to the accompanying drawings.

Fig. 1

shows a schematic diagram of an arrangement according to the invention;

Fig. 2

shows a voltage-time diagram of a control method for a first embodiment of the arrangement of Figure 1;

Figure 3

shows a voltage time diagram of a control method for a second embodiment of the arrangement of Figure 1;

The circuit shown in Figure 1 for generating a corona discharge has on the input side a DC voltage source 1. The DC voltage source 1 consists of a power transformer 2, which is connected on the input side with an AC voltage / Stroznnetz, for example, a 400 V / 50 Hz voltage network. On the output side, the power transformer 2 is connected to a rectifier 3.

The rectifier 3 has two outputs G1 and G2, wherein the output G1 relative to the output G2 has higher potential. Between the outputs G1 and G2 and parallel to the rectifier 3, a capacitor C3 is connected.

Further, between the outputs G1 and G2 of the rectifier 3 in parallel with the capacitor C3, a pair of series connected electronic switches S1 and S2 are connected. The two electronic switches S1 and S2 are connected to one another at a node V, so that the output of the switch S1 is connected to the input of the switch S2. The switches S 1 and S2 are driven by a drive unit 4 which has a frequency generator, not shown here.

From the connected to the terminal G1 of the rectifier 3 input of the switch S1 leads a connecting line A, from the node V, a connecting line B and of the connected to the output G2 of the rectifier 3 output of the switch S2 a connecting line C to the electrodes described in more detail below.

Block 5 in FIG. 1 shows a first exemplary embodiment of the arrangement according to the invention. In this embodiment, an electrode assembly E for generating a corona discharge by their Ersatzscbaltbild, consisting of the series connection of a capacitor CE and a resistor RE, is shown. The electrode arrangement E corresponds essentially to the electrode arrangement described in DE-GM 88 07 090, however, in the present invention, none of the electrodes has a dielectric coating. A rotatably arranged roller of the electrode arrangement E has an electrically conductive surface which forms a first electrode E1. An elongate electrode with an electrically conductive surface which is arranged parallel to and at a distance from the roller forms a second electrode E2 of the electrode arrangement E. The electrode E1 is connected in this embodiment via the connecting line B to the node V and the second electrode E2 via the connecting line C to the output G2 of the rectifier. The electrode arrangement E is thus connected in parallel to the electronic switch S2.

In order to generate a corona discharge between the electrodes E1 and E2, the input voltage of the mains transformer 2 is transformed to a high voltage alternating voltage, which is then rectified in the rectifier 3 to a high voltage DC voltage applied to the outputs G1 and G2 of the rectifier. This DC voltage has a value of 30 kV. By the capacitor C3, the DC voltage is smoothed and sieved.

In order for a corona discharge to take place, a voltage must then be briefly applied between the electrodes E1 and E2 which has the fastest possible voltage increase. The applied voltage can be significantly higher than the breakdown voltage with constant application of a constant voltage, if the duration of the voltage pulses is only short enough. As a result, discharge channels are formed on the electrode arrangement E, so-called streamer, in which no voltage breakdown occurs. If the voltage is applied too long to the electrode arrangement E, then the streamer discharge channels change into so-called leader discharge channels, which lead directly to a voltage breakdown at the electrode arrangement E. Such a Spa Durchungsblase must necessarily be avoided to prevent damage to the film to be treated and the electrode assembly E.

The voltage increase and the duration of the voltage pulses 7 must therefore be such that on the one hand a corona discharge can form, and on the other hand no leader discharge channels are formed which would lead to the voltage deviation. In order to avoid voltage breakdown during the subsequent voltage pulse, the area between the two electrodes E1 and E2 must be as free of charge carriers as possible. It must pass between two voltage pulses a sufficiently long time in which in this area remaining charge carriers disappear, for example, by recombination. This time depends on a variety of boundary conditions, such as the humidity, the temperature, the air flow velocity of the charge carriers, etc.

A possible control method for the switches S1 and S2 in the first embodiment shows the Spaltungszeitdiagramm in Figure 2. In the two upper diagrams, the on states of the switches S1 and S2 are plotted against time, while in the lower diagram, the curve of the voltage UE with the amplitude Û between the electrode E1 and E2 is shown, where Ü can also be significantly higher than the breakdown voltage with constantly applied constant voltage. The representation of the times in Figure 2 takes place here only qualitatively and not to scale due to the very different duration of voltage pulses and switching periods.

Before the corona discharge, the two switches S 1 and S2 are both switched off, that is to say they are not conductive, so that no voltage is applied to the electrode arrangement E and no energy is transmitted to the electrode arrangement E.

At the beginning of the switching period 8 (100 μs), the switch S1 is activated and switched on for a short duration 9 (1 μs), so that the high-voltage DC voltage is applied to the electrode arrangement E. Due to the high switching speed of the switch S1, the voltage increase at the electrode arrangement E takes place very rapidly, for example within 100 ns. The DC voltage at the electrode assembly E may only be present so long that only streamer form and can not train to leaders, the one Voltage breakdown at the electrode assembly E would result. As a result, therefore, the duty cycle of the switch S1 is determined.

To turn off the DC voltage to the electrode assembly E then the switch S 1 is turned off, while the switch S2 is driven and turned on. As a result, the electrode assembly E is disconnected from the DC voltage and short-circuited via the switch S2, so that the stored in the capacitance CE of the electrode assembly E electric charge can discharge through the resistor RE of the electrode assembly and the switch S2. The discharge must take place so long that no streamer discharge channels are no longer present, so that at the next voltage pulse does not immediately form a Leader discharge channel, which would lead to the immediate voltage breakdown at the electrode assembly E.

For the rest of the period 8, the switch S1 remains open, the switch S2 is closed, but can also be opened again. Advantageously, the switch S2 remains closed until the electrode assembly E has completely discharged. The switching period 8 is now completed. To generate the corona discharge of the next period, the switch S 1 is then closed again, while S2 is simultaneously opened in order to avoid a short circuit of the DC voltage source 3.

If the circuit block 6 instead of 5 is used in FIG. 1, a second exemplary embodiment of the arrangement according to the invention results. Circuit block 6 again contains the already explained in circuit block 5 electrode assembly E. The first electrode E1 is again connected via the connecting line B to the node V, while the second electrode E2 via a capacitor C1 and the connecting line A to the first output G1 and a capacitor C2 and the connecting line C is connected to the second output G2 of the rectifier 3

By the two capacitors C1 and C2, which should have identical values as possible, the high-voltage DC voltage at the outputs G1 and G2 of the rectifier 3 is halved, so that between the electrodes E1 and E2 of the electrode assembly E in each case a maximum of half the high voltage DC voltage is applied In this case, the amplitude of UE is thus Û / 2 = ± 15 kV.

The voltage pulses 7 for generating the corona discharge are generated in this embodiment by short-term antiphase driving of the switches S1 and S2, as shown in the voltage-time diagram of Figure 3.

If the switch S1 is conductive, then capacitor C1 is connected to the electrode arrangement E, so that a positive DC voltage is applied. After the short. Period 11, the switch S1 is then turned off, so that no voltage is applied to the electrode assembly E. After half of the period 8, measured from the beginning of the positive voltage pulse 7, then the switch S2 is turned on for a short period of time 11, so that the capacitor C2 is connected in parallel to the electrode assembly E and thereby an opposite, ie negative voltage pulse to the electrode assembly E is present.

Again, it is important that the duration 11 of the voltage pulses 7 is dimensioned so that on the one hand a sufficiently rapid increase in voltage of Eiektrodenanoidnung E is ensured in order to achieve a corona discharge, and that on the other hand no leaders form, which would lead to a breakdown. The discharge duration between two voltage pulses 7 is also measured as described above.

Due to the constantly alternating polarity of the voltage applied to the electrode assembly E DC voltage verhiudert that a film to be processed charges due to a constantly recurring Gleichpoligen charging voltage to a polarity, which would ultimately arise after winding the film, a charged wound capacitor.

In order to switch the voltage pulses 7 with frequencies up to 20 kHz, transistors, IGBTs or MOSFETs are used for the switches S1 and S2. Due to the magnitude of the voltage value of the Hocbspanaungs DC voltage of up to 30 kV, the switches S1 and S2 are formed of a plurality of series-connected, cascading composite switching elements, such as transistors, IGBTs, or MOSFETs, which are then driven by the drive unit 4 according to uniform Need to become.

The drive unit 4 has a frequency generator. The drive unit 4 may be a separate drive or driver circuit, but also a microcomputer or a control computer, which performs further tasks.

The arrangement according to the invention has the particular advantage that only as much energy has to be transferred to the electrode arrangement as is necessary for generating the corona discharge. Thus, the energy loss and the heat generation at the electrode assembly E can be optimized.

Likewise, the arrangement makes it possible to generate voltage pulses of a very short duration with a rapid increase in voltage and at the same time a short switching period, that is to say a high switching frequency.

Furthermore, the arrangement according to the invention ensures that no uncontrolled breakdowns occur between the electrodes, which would damage the film to be pretreated. Therefore, the prior art dielectric coatings required on the electrodes in the inventive arrangement may be omitted. But even with electrode arrangements with dielectric coatings, an improvement of the treatment of the films is achieved by the invention.

In the embodiment of Figure 3 is added as a further advantage that due to the ever-changing polarity of the voltage applied to the electrode assembly E DC voltage to be processed suflädt film to a polarity, so that the film can be easily processed after winding.

Claims (14)

Arrangement for generating a corona discharge between a first (E1) and a second electrode (E2), which are subjected to a high-frequency high voltage, characterized by a DC voltage source (1), between whose outputs (G1, G2) two electronic switches connected in series (E1) S1, S2) are arranged, the first electrode (E1) to the connection (V) of the two switches (S1, S2) and the second electrode (E2) to an output (G1) of the DC voltage source (1) is connected. Arrangement according to claim 1, characterized in that the second electrode (E2) is also connected to the other output (G2) of the DC voltage source (1). Arrangement according to claim 2, characterized in that between the second electrode (E2) and each output (G1, G2) of the DC voltage source (1) in each case a capacitor (C1, C2) is arranged. Arrangement according to one of the preceding claims, characterized in that the DC voltage source (1) has a mains transformer (2) and a downstream rectifier (3) and which is connected in parallel with its outputs (G1, G2), a capacitor (C3). Arrangement according to one of the preceding claims, characterized in that a drive unit (4) for controlling the electronic switches (S1, S2) includes a frequency generator. Arrangement according to one of the preceding claims, characterized in that each electronic switch (S1, S2) has a plurality of series-connected switching elements. Arrangement according to one of the preceding claims, characterized in that both electrodes (E1, E2) have conductive surfaces and no dielectric coating. Method for generating a corona discharge between two electrodes using an arrangement according to one of the preceding claims, characterized in that the electronic switches (S1, S2) are controlled such that at the electrodes (E1, E2) in relation to the period (8) short voltage pulses (7) abut. A method according to claim 8, characterized in that the voltage increase du / dt at the rising edges of the voltage pulses (7) takes place with a value of at least 100 kV / μs. A method according to claim 8 or 9, characterized in that the switches (S1, S2) are driven in phase opposition. Method according to one of Claims 8 to 10, characterized in that the voltage pulses (7) applied to the electrodes (E1, E2) have the same polarity. A method according to claim 11, characterized in that a voltage amplitude (Û) of the voltage pulses (7) has a value of at least 20 kV. Method according to one of Claims 8 to 10, characterized in that the voltage pulses (7) applied to the electrodes (E1, E2) have alternating polarity. A method according to claim 13, characterized in that the voltage amplitude (Û) of the voltage pulses (7) has a value of at least 10 kV.

Images