DE29905137U1 - Circuit arrangement for feeding a dielectric barrier discharge - Google Patents

Description

Description Circuit arrangement for feeding a dielectrically impeded discharge

The invention relates to a circuit arrangement for feeding a dielectrically impeded discharge. Dielectrically impeded discharge is usually understood to mean in particular barrier discharges with solid insulation. In the following and in particular in the claims, however, dielectrically impeded discharges are also to be understood to mean those in which, as a result of the presence of gaseous and similar dielectrics, volumes with at least temporarily dielectric properties occur. Furthermore, the electrical conductivity of this adjacent layer or this adjacent volume is sufficiently low to prevent the discharge from being focused on a single small volume element.

In the majority of such discharges, energy adjustment is usually carried out by means of a transformer due to the necessary voltage level for the electrical supply of the barrier arrangement; the current density within the plasma discharge is limited by an adjacent layer or an adjacent volume with at least temporarily dielectric properties.

The invention is applicable for the purposes of plasma heating, plasma coating, plasma chemical material conversion, light source technology, plasma environmental technology, disinfection or similar applications, changing biological properties of cell structures and plasma physics, in which changes are achieved through the effect of a surface or volumetric plasma.

It has long been known from power supply technology for direct current gas discharges that discharges which have a negative differential resistance in the discharge characteristic i.e. where the discharge voltage drops as the current increases can be stabilized particularly easily if the supply technology has a current control. This is advantageously achieved with the help of current-controlled power electronic circuits with an inductively smoothed output. It is also known for the same reason that such discharges have an unstable character if they are fed directly from voltage sources or capacitors.

Therefore, for discharges that are fed with alternating voltage, for example at a higher frequency, current-limiting components such as inductors or resonance circuits are often connected to the alternating current circuit [EP 0824300A1, EP 0808 086Al, EP 0828 407 Al]. In some cases, additional smoothing of the direct current is also carried out in an intermediate circuit, which, however, does not have to be regulated if reactive components with a current-limiting effect are present in the alternating current circuit of the discharge [EP 0788 298 Al; DE 44 25679Al]. These arrangements have the disadvantage that they are usually designed for a fixed operating point due to the integrated reactive components, that these reactive components are functionally necessary in the alternating current circuit and cannot be variably controlled, that in addition, due to the inductive or resonance-determining components, they only cause a sinusoidal current curve with the period determined by the resonance of these components, and that they are designed for arc or glow discharges, which usually have lower stability requirements due to their lower negative differential resistances.

It is also known that a plasma discharge path can be supplied with a pulsed alternating current by means of two power sources connected in opposition to one another, at the outputs of which there is an inductance and a switching element as a short-circuiter, if this discharge is arranged between the power sources connected in opposition to one another and one or both short-circuiters are operated alternately or simultaneously (Scholl, R. A.; Asymmetry pulsed power: a new power technology. Surface and Coatings Technology 98 (1998) 823-827.) This technology has the advantage over the methods and arrangements described that the discharge current is regulated in each half-period, thus ensuring greater stability of the discharge and a period of plasma supply lasting almost the entire half-period. However, this arrangement is not suitable for plasma arrangements that require transformer adaptation; because when switching from one polarity to the other, overvoltages arise which are derived from the remagnetization process. In addition, the principle has the disadvantage that it is not possible to interrupt the current within a discharge pulse without foregoing the advantages resulting from the smoothing function of the choke inductances.

Dielectrically impeded discharge arrangements have the special property that they only allow alternating current due to an insulating barrier, that they are extremely

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steeply falling negative differential resistance, they have increased requirements for stabilization and that, due to the high voltage level of the discharge voltage, they have to be adapted to the power electronic circuit arrangement almost exclusively by means of a transformer arrangement. Since the naturally existing barrier capacitance limits the energy input into the plasma per half-wave both locally and in relation to the half-period, the energy supply of dielectrically impeded discharges from alternating voltage sources with different or variable frequencies has become common. However, since this capacitance has no influence on instantaneous value changes in the plasma, the influence of the supplying circuit arrangement is significant. Arrangements are known in which a direct voltage is first generated, this is smoothed many times using a capacitance and then converted into an alternating voltage using an inverter circuit. This configuration is usually described with the term voltage-controlled inverter. This creates bipolar rectangular voltage blocks, which can be influenced by a pulse width control for the purpose of energy control. Such voltage blocks lead to unstable behavior in the case of discharges with the properties of a negative differential resistance described above. When the level is adjusted to the dielectrically impeded discharge using transformers, this usually leads to a pulse discharge being generated for a short time, the current amplitude of which is initially only limited by the transformer leakage reactance and parasitic components. The rectangular voltage in conjunction with the parasitic reactances mean that the discharge duration and amplitude are determined by the oscillation process from the capacitance of the barrier arrangement and the transformer leakage reactance. Such circuits tend to fail due to the considerable shock load on the semiconductor components and lead to poor circuit utilization, poor overall efficiency and low effective power. The special technological effects hoped for in individual cases from such short pulses through the generation of a non-equilibrium plasma often remain doubtful, since this requires rise times in the ns range, which are usually not achievable due to existing parasitic reactances [EP 0809275 Al].

To improve the operational stability of the converter, additional reactive components in the form of additional inductances, transformer leakage reactances or

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Resonance branches are inserted into the alternating current circuit of the inverter containing the discharge path, which means additional technical effort and reduces the possible parameter limits [EP 0831 517 A2; ]. This reduces the use of the inverter circuit to the pure provision of an alternating voltage of a defined frequency, without any claim to influence the dynamic behavior of the discharge. These voltage-controlled inverter circuits therefore have the disadvantage that it is not possible to stabilize the discharge at a defined operating point and the energy input is only limited to a small proportion of the supplying alternating voltage half-period. It also proves to be disadvantageous that power control by means of pulse width control of the conduction time of the power electronic valve components is not very effective in this case. This principle also has the disadvantage that the entire circuit is strongly dependent on the resonant frequency and can therefore only be operated within a narrow parameter range. The energetic supply of the discharge is thus stabilized by reactive components, and the supply duration is limited to a maximum of 50% of each half-period (Schorpp, V. Dielectrically hindered noble gas halogen excimer discharge. Diss. Karlsruhe 1991).

In recent years, attempts have been made to achieve technological effects by generating multiple pulse shapes with partially variable frequencies for plasmas. However, the circuit principles and supply methods explained above are not suitable for this, or only to a very limited extent, for the reasons explained. For example, high voltage and current increases are desired at the start of the discharge in order to quickly reach the breakdown voltage or discharge voltage level and to use the advantages of a non-equilibrium plasma within a short initial phase and at the same time to achieve an accelerated change to the other polarity. The reignition voltage for gases with high discharge dynamics should also be reduced by maintaining the discharge over a longer period of time within a half-wave. A frequency that can be changed within wider limits independently of the discharge current and thus of the power input is also desirable. In such a case, it is necessary to avoid parasitic inductances in the alternating current circuit. It is also desirable to be able to adjust the energy supply over a large part of the half-period, in

a relatively wide frequency range and to keep the instantaneous power constant over time and to influence the duration of this phase.

The shortcomings of the state of the art described are due to the fact that a power supply method and circuit principle in the form of a voltage-controlled inverter is used that is unsuitable for stabilizing a discharge and that the principle of discharge stabilization using reactive components is used as a makeshift measure or that when using a current-controlled bipolar power pulsing that is suitable in itself, the special conditions of energy adaptation and energy flow to the barrier discharge were not technologically taken into account. This leads to restrictions in the possible parameter ranges, in the applicability, in the energy utilization or in the functional reliability.

The known methods and arrangements are therefore limited in their area of application and lead to increased energy and technical expenditure outside of the limited optimal parameters.

The invention is based on the object of creating a circuit arrangement for feeding a dielectrically impeded discharge, which avoids the disadvantages listed and opens up expanded technological parameter ranges with an expanded area of application and thus contributes to improving efficiency and reducing operating costs with comparable or reduced implementation effort.

Special aspects of this task consist in particular in implementing the supply of a dielectrically impeded discharge, which enables steep pulses at the plasma discharge path with high frequency variability, ensures a phase of defined, controllable constant current with the effect of a temporarily quasi-constant power input independent of the frequency, which can be changed within extended limits, and also enables a defined pulse pause. In this way, an expanded working range, better process stability and an expansion of the variety of parameters at the discharge path are to be achieved. Furthermore, the proportionality of the medium-frequency discharge current to the easily controllable direct current is to achieve greater stability of the control system and better process controllability.

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This object is achieved according to the invention by the features of claim 1. The essence of the invention is therefore that the energy supply process for a dielectrically impeded discharge, which has the properties mentioned at the beginning, extends over the majority of each discharge half-period, whereby different phases can be set in the sequence of pulse current phase, current constant phase and current decay phase. Here, the energy of the pulse current phase is largely obtained by a reversal process from the reactive energy of the low-inductance alternating current circuit in which the dielectrically impeded discharge forms.

The current constant phase is determined by a transformer-proportionately transmitted, adjusted, regulated and inductively smoothed direct current. If a regulated direct current is now provided on the output side of an inductively smoothed direct current source and is adjusted to the discharge voltage level by inversion and transformer transmission, this constant current is also formed temporarily in the dielectrically impeded discharge. The inductive smoothing of the direct current ensures that the transition from one half-period to the opposite half-period can be very steep and is only limited by the dynamics of the power switching elements and parasitic residual impedances of the alternating current branch of the discharge arrangement. In addition, these residual impedances are not required for stabilizing the discharge and can therefore be minimized. This principle makes it possible to make the inductance of the alternating current circuit of the discharge arrangement as small as desired without causing any restrictions in stability. It also requires low parasitic inductances, which is reflected in the requirement for small transformer inductances and promotes the formation of short and steep pulse phases.

In this way, the stationary discharge current can be regulated as an alternating current in the discharge via the regulated direct current, which means that the power input into the discharge can be influenced in a controllable manner in this phase. This controllability remains even when the control frequency of the alternating direction is changed within extended limits, which gives rise to the advantage of being able to change the operating frequency independently of the power input.

In order for the current in the dielectrically impeded discharge to remain constant within a half-wave, the voltage at the dielectric barrier must increase. The image of this voltage increase at the barrier represents the

unipolar voltage, which is largely proportional to the rectified course of this voltage. If this unipolar fluctuating voltage is now limited within a half-wave of the alternating voltage, the voltage at the dielectric barrier can no longer increase and the current becomes 0. In this way, it is possible to regularly generate a third phase, the current decay phase, and to influence its duration by the level of the limiting voltage of the fluctuating unipolar direct voltage. Since the level of the voltage limitation required to ensure a defined current decay phase changes with each current setpoint and this depends on the capacity of the barrier, it is advantageous to make the value of the voltage limitation dependent on the integral of the current of a half-wave.

The invention will be explained below using an exemplary embodiment with reference to the accompanying drawings.

Show

Fig 1: the circuit arrangement for feeding a dielectrically impeded discharge, Fig 2: the current curve for feeding a dielectrically impeded discharge, Fig 3: the voltage curve at the input of the inverter circuit.

According to Fig. 1-3, to feed a dielectrically impeded discharge 1 using an inverter circuit 2 and a transformer 3 for energy adjustment positive and negative current half-waves A of variably controllable frequency are effective at the discharge arrangement 4, which consist of a pulse current phase B, a constant current phase C and a current decay phase D.

Here, the pulse current phase B is characterized by a steep current increase from the opposite polarity or from zero and a subsequent drop to an amount greater than zero or an initial value of the following phase. It is mainly obtained from the reactive energy of the barrier 5 hindering the discharge, the volume enclosing the discharge 1 and adjacent parasitic capacitive reactive elements and is influenced in amplitude and frequency by further parasitic inductive components.

The amplitude of the current constant phase C with a value greater than zero is determined by a direct current 6 that is proportionally transmitted by means of an inverter, adjusted, regulated and inductively smoothed. The amount of this current constant phase C can therefore also be regulated largely independently of the frequency, which results in a

uniform energy input into the plasma over a long period of time within each current half-wave can be ensured. By controlling this current constant phase C using a direct current 6, the control is simplified and the process stability is improved.

The current decay phase D with a length that can be controlled to the maximum time of the current constant phase can be influenced by various measures, individually or in combination, within the duration of the current constant phase C. Firstly, the current decay phase D is determined by temporarily limiting the voltage of the inverter input voltage F, which is assigned to the smoothed direct current 6 and is essentially unipolar. This voltage is equal to the potential difference between the nodes 7 and 8 and identical to the unipolar output voltage of the direct current source. By limiting this voltage to a value E, it is achieved that no current increase is effective at the discharge arrangement 4, which means that the current 9 through the discharge arrangement 4 decays and becomes zero. This effect can also be achieved with variable frequency if, for example, the voltage limit 10 of the unipolar variable inverter input voltage F is compared in its value with the integral of the current over the corresponding half-wave of the supplying inverter current.

To avoid overvoltages in the course of the inverter input voltage F, it is necessary that the conduction time of the power electronic switching elements controlling the positive and negative half-waves together is greater than or equal to the period of the controllably variable frequency.

An extension of the conduction time of the power electronic switching elements controlling the positive and negative half-waves within a period of the current 9 through the discharge arrangement 4 also leads, to a limited extent, to an extension of the current decay phase D. In order to ensure these properties even in a limited, extended frequency range of the current through the discharge arrangement 4 and to ensure the proportionality between the direct current 6 and the amplitude of the current constant phase C, it is necessary to ensure a low-spread transformer energy transfer. In this way, it can be achieved that the pulse current phase B is small compared to the half-wave duration A.

The properties described are achieved in terms of circuitry in that a regulated direct current source 11, an inverter circuit 2 and a regulated voltage limiting circuit 10 are connected in parallel at common nodes, whereby the regulated direct current source is equipped with at least one current measurement and a current or power control circuit and has an inductance on the output side in the power circuit for current smoothing, the value of which is dimensioned such that when used according to the invention, a gap in the current is excluded. Further features according to the invention are that the inverter circuit contains at least two independently controllable power electronic switching elements 12 that can be switched on and off, and circuit branches in which these valve components are located have a freewheeling function in the blocking direction, the inverter circuit in the alternating current branch 13 is connected in series to the primary winding or primary windings of a transformer 3 for voltage and impedance matching, and the leakage reactance of this transformer is preferably less than 10% compared to the main reactance, and the transformer is secondarily connected to a discharge arrangement 4 consisting of a plasma discharge layer or plasma discharge volume 1 and a dielectrically impeding layer or dielectrically impeding volume 5.

Such a discharge arrangement can also be constructed from several elements with similar properties to the overall discharge arrangement. In addition, it is essential that when a direct current 6 is present on the output side of the direct current source 11, the valve components are controlled in such a way that at least one circuit branch of the inverter circuit 2 is always switched to conducting and the control frequency of the inverter circuit is variable. The freewheeling properties ensure that the capacitively stored energy of the discharge arrangement can become effective in the following pulse current phase of opposite polarity through a reversal process and only leads to losses to a small extent. A frequency that can be varied within limits makes it possible for this parameter to be regulated for technological effects independently of the power input. The lower frequency is determined by the saturation limit of the transformer core material, the upper limit follows the condition that the control frequency should be smaller than the duration of the resonance frequency assigned to the current pulse phase. It proves to be advantageous that even if the lower operating frequency is undershot and thus in the event of saturation

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of the transformer core, no damage to the circuit is to be expected, since the DC source prevents this.

In order to limit the duration of the current constant phase C during one or during each current half-wave A and thus, for example, to achieve targeted deionization phenomena, the duration can now be influenced in various ways. For example, limiting the unipolar variable output voltage of the direct current source with the aid of a voltage control geared to this voltage by the voltage limiter circuit 10 inevitably causes the current in the plasma discharge to decay as explained above. With a variable frequency, the required level of the voltage limitation E must change. In order to implement this influence, it is advantageous to arrange a current sensor 14 in one or each valve branch of the inverter circuit, the output signal of which is integrated by means of an integrator 15 with a reset after each current half-wave over the corresponding current half-wave and can thus be seen as a measure of the required comparison level of the unipolar variable output voltage of the direct current source, from which the voltage limitation should become effective. With sufficiently dynamic behavior of the voltage limiting circuit 10, it is thus possible to achieve different forms of the current half-waves A. To a limited extent, it is also possible to achieve, by extending the conduction phases of the inverter branches, that the unipolar variable output voltage of the direct current source is short-circuited and thus to achieve a decay of the current in the plasma discharge.

Claims (5)

Protection claims 1. Circuit arrangement for feeding a dielectrically impeded discharge, in which - a regulated direct current source (11), an inverter circuit (2) and a regulated voltage limiting circuit (10) are connected in parallel at common nodes (7; 8), the regulated direct current source (11) being equipped with at least a current measurement and a current or power control circuit and on the output side in the power circuit having an inductance for current smoothing, the value of which is such that when used according to the invention, a gap in the current is excluded, - the inverter circuit (2) contains at least two independently controllable power electronic switching elements (12) that can be switched on and off, and circuit branches in which these valve components are located have a freewheeling function in the blocking direction, - when a direct current is present on the output side of the direct current source (11), the valve components are controlled in such a way that at least one circuit branch of the inverter circuit (2) is always switched to conducting, the inverter circuit (2) in the alternating current branch is connected in series to the primary winding or primary windings of a transformer (3) for voltage and impedance matching and the leakage reactance of this transformer is preferably less than 10% compared to the main reactance, - the control frequency of the inverter circuit (2) is variable, - and the transformer (3) is secondarily connected to a discharge arrangement (4) consisting of a plasma discharge layer or plasma discharge volume (1) and a dielectrically impeding layer or dielectrically impeding volume (5). 2. Circuit arrangement according to claim 1, characterized in that an actual value is connected to the voltage limiting circuit (10) and a control circuit for the variable limitation of the unipolar variable output voltage of the direct current source is present. -2- 3. Circuit arrangement according to claim 1, characterized in that the resonance frequency of stray reactance, parasitic inductive and capacitive components in the alternating current circuit of the inverter (2) and the capacitance in the discharge arrangement (4) is greater than the control frequency of the inverter (2). 4. Circuit arrangement according to claim 1 or 2, characterized in that an actual value sensor for the voltage limiting circuit is connected to the output terminals of the direct current source (11) and/or one or per branch an actual value sensor of the current of each half-wave of an inverter branch is connected to an integrator (15) with reset after each current half-wave, which is located in the valve current path of one or more valve components of the inverter circuit (2). 5. Circuit arrangement according to claim 1, characterized in that the conduction period of the branches of the inverter circuit (13) influencing the current half-waves (A) together is greater than the period duration assigned to the control frequency and that the conduction period of the branches of the inverter circuit influencing the current half-waves is variable.