WO2013061063A1

WO2013061063A1 - Magnetron power supply filter

Info

Publication number: WO2013061063A1
Application number: PCT/GB2012/052649
Authority: WO
Inventors: Robert Richardson
Original assignee: E2V Technologies (Uk) Limited
Priority date: 2011-10-27
Filing date: 2012-10-25
Publication date: 2013-05-02
Also published as: GB2495968A; GB2495968B; GB201118589D0

Abstract

A filter (20) for reducing a ripple voltage component of a magnetron power supply comprises a first filter element comprising a first inductor (L1) in series with a power supply circuit for the magnetron (13) and a first capacitor (C1) in parallel with the magnetron; and a second filter element (21) which is resonant substantially at a frequency of the ripple voltage component, the second filter element comprising a second inductor (L2) in series with the first inductor (L1) and a second capacitor (C2) bridging the second inductor (L2).

Description

Magnetron Power Supply Filter

[0001] This invention relates to a filter for a power supply for a magnetron.

BACKGROUND

[0002] In many applications, magnetrons are operated with a DC supply connected across anode and cathode terminals of the magnetron. The DC supply is usually derived from an AC supply which is rectified and filtered to obtain a steady DC supply with a small AC component, known as ripple, inevitably superimposed. An inductance and capacitance (LC) filter is usually included in the supply line to obtain a sufficiently low ripple current of the order of 3% rms current. However, energy stored in such filters can be sufficient that, should the magnetron malfunction, resulting in an internal discharge know as an arc, a discharge into the magnetron from the filter of more than 10J may result that is sufficient to damage the magnetron.

[0003] GB462823 discloses an electrical power supply for a variable load, the power supply comprising a DC source and a filter comprising a resonant circuit to prevent currents of a undesired frequency reaching the load, where the resonant circuit is resonant at the undesired frequency and the power supply, viewed from the load, has a substantially purely resistive impedance over a predetermined range of load current and a predetermined working range of frequency. The power supply is for a variable load comprising a thermionic valve amplifier for a television signal having a DC component in which the current drawn varies with variations in the amplified signal. [0004] Figures 1A-1E illustrate a number of aspects of prior art magnetron power supply circuits.

[0005] Figure 1A shows a typical power supply circuit 10 incorporating a rectifier system 11 with a single stage LC filter 12 comprising an inductor LI in series in the power supply circuit and capacitor CI connected in parallel with the magnetron 13 across the anode and cathode.

[0006] Referring to Figure IB, a typical rectifier ripple frequency waveform 14 from a three phase 50 Hz mains application has a ripple period of 1/Fr where Fr is the ripple frequency of 300 Hz with pulses of negative peak sine form.

[0007] Figure 1C shows a typical switched mode supply waveform 15 usually, but not exclusively, of a pulse width modulated (PWM) format. In this case the ripple period of 1/Fr where Fr is the ripple frequency that typically is between 5kHz and 100kHz or even higher. [0008] The inductance of LI and capacitance of CI are chosen by well-known methods. The inductance of LI must be large enough to ensure continuous current flow into the magnetron 13. The higher the ripple frequency Fr then the lower the inductance of the inductor LI can have. The capacitance of capacitor CI is chosen so that the product L1*C1 is low enough that the ripple attenuation is sufficiently high at the desired frequency while ensuring that the resonant frequency Fc of the filter is well below 0.707*Fr. This is because above the resonant frequency Fc of the filter, the rate of attenuation increases at 40dB/decade.

[0009] Figure ID shows the ripple content 16 of the DC voltage applied to the magnetron after filtering. The supply tends to have a substantially sinusoidal form of period 1/Fr where Fr is the ripple frequency as the harmonics are attenuated more by the LC filter than the fundamental Fr frequency so that the original fundamental frequency continues to dominate.

[0010] Figure IE shows a typical magnetron l-V characteristic 17. Magnetrons usually have their anodes earthed, forming a resonant structure with a negative supply connected to the cathode and with the anode earthed. The magnetron has a constant voltage characteristic. That is, virtually no current flows until a minimum negative voltage V_diode is reached then current builds up rapidly with increasing voltage until the operating current l_mag is reached at V_mag. Typically, for a magnetron that runs at V_mag and l_mag of 20kV and 5A respectively V_diode may be 19.3kV. The small change of voltage between V_diode and V_mag has a slope resistance, as it is known, of a.

[0011] This behaviour of a magnetron power supply may be represented in a computer or

mathematical model by a large avalanche diode Dz in series with a resistance Ra, as shown in Figure 1A.

[0012] This very non-linear characteristic of a magnetron means that a small change in ripple voltage Vr results in a much larger change of magnetron ripple current. Typically a 1% ripple voltage can result in a 20% change in magnetron current.

[0013] The ripple current can be reduced by using larger filters with much larger LC products. However, the use of larger values for L and C increases the stored energy. As an example, to reduce the ripple current of a magnetron such as a BM100 operating at 20kV and around 5A DC current to < 4% rms would require a 50mH inductor and a 250nF capacitor if the ripple frequency was a PWM waveform at 90% duty cycle and Fr was 16.7kHz. In this situation, the stored energy in the 250nF capacitor would be 50J, which is potentially harmful to the magnetron.

[0014] There is therefore a requirement for a filter that can attenuate the ripple by a similar amount but that will store much less energy.

BRIEF SUMMARY OF THE DISCLOSURE

[0015] In accordance with a first aspect of the present invention there is provided a magnetron power supply comprising a filter for reducing a ripple voltage component the filter comprising: a first filter element comprising a first inductor in series with a power supply circuit for the magnetron and a first capacitor in parallel with the magnetron; and a second filter element which is resonant substantially at a frequency of the ripple voltage component, the second filter element comprising a second inductor in series with the first inductor and a second capacitor bridging the second inductor.

[0016] Conveniently, the magnetron power supply further comprises a first varistor in parallel with the second filter element.

[0017] Conveniently, the magnetron power supply further comprises a second varistor in parallel with the first capacitor.

[0018] Advantageously, the magnetron power supply further comprises a resistor in series with the second capacitor within the second filter element for determining a quality factor of the second filter element. [0019] Conveniently, the quality factor of the second filter element provides predetermined dynamic impedance at resonance with a bandwidth to accommodate tolerances of ±5% of the inductance of the second inductor and ±5% of the capacitance of the second capacitor.

[0020] Advantageously, a value of the resistor is selected to provide a value of the quality factor of substantially 10.

[0021] Advantageously, the quality factor is selected so that the values of the inductance of the second inductor and the capacitance of the second capacitor can be within a tolerance of substantially ±5% of values necessary for the resonant frequency of the second filter element to be equal to the frequency of the ripple component.

[0022] Conveniently, the first filter element comprises a plurality of stages each arranged to be supplied by a separate rectified power supply.

[0023] Conveniently, the second filter element comprises a separate respective resonant filter element for each stage of the first filter element.

[0024] According to a second aspect of the invention, there is provided a method of reducing a ripple voltage component of a magnetron power supply comprising: filtering the power supply with a first filter element comprising a first inductor in series with the power supply circuit and a first capacitor in parallel with the magnetron; and further filtering the power supply with a second filter element which is resonant substantially at a frequency of the ripple voltage component, the second filter element comprising a second inductor in series with the first inductor and a second capacitor bridging the second inductor.

[0025] Conveniently, the method further comprises limiting a voltage applied across the second filter element when the magnetron short circuits by a first varistor bridging the second filter element.

[0026] Conveniently, the method further comprises limiting an overvoltage when the magnetron stops conducting by a second varistor bridging the magnetron in parallel with the first capacitor. [0027] Conveniently, the method further comprises selecting a quality factor for the second filter element which permits a tolerance of ±5% in the selection of values of the inductance of the second inductor and the capacitance of the second capacitor substantially to match a resonant frequency of the second filter element to a frequency of the ripple voltage component.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1A is a circuit diagram of a known magnetron power supply circuit;

Figure IB is a typical ripple waveform from an AC supply for the circuit of Figure 1A; Figure 1C is a typical ripple waveform from a switched mode power supply for the circuit of

Figure 1A; Figure ID is a typical filtered ripple waveform for the circuit of Figure 1A;

Figure IE is a graph of the l-V characteristics of the magnetron of Figure 1A;

Figure 2A is a circuit diagram of a first embodiment of a magnetron power supply circuit according to the invention;

Figure 2B is the circuit diagram of Figure 2A with the LC resonant filter element of Figure 2A replaced by an equivalent circuit;

Figure 3 is a model circuit diagram of the magnetron power supply circuit of Figure 2;

Figure 4 shows in the upper graph ripple performance with a filter according to the invention using the model circuit of Figure 3 and in the lower graph ripple performance with a prior art filter; and

Figure 5 is a circuit diagram of a second embodiment of a magnetron power supply circuit according to the invention.

DETAILED DESCRIPTION

[0029] Referring to Figure 2A, there is illustrated a circuit diagram of a power supply circuit 20 according to the invention. This is basically the same as the circuit of Figure 1A except, in particular, there is provided a parallel tuned circuit 21 comprising a second inductor L2 in parallel with a second capacitor C2 and resistor Rl in series, in which the second inductor L2 is in series with the first inductor LI in a feed to the magnetron 13. This resonant LCR filter element 21 has a resonance frequency F_R. If the resonance frequency F_R is equal to the ripple frequency Fr, the LCR filter element 21 effectively blocks the fundamental ripple frequency Fr, which is the major component of the ripple. The employment of this filter allows the use of a substantially lower capacitance capacitor CI in parallel with the magnetron equivalent circuit 13 than in the prior art circuit of Figure 1A, so that it is also effective in filtering any of the harmonic content in the ripple waveform. This much smaller capacitance capacitor CI thus stores less energy than in the prior art. Under some circumstances the capacitor CI may not be required at all.

[0030] Figure 2A illustrates a practical embodiment of the circuit 20 and includes a first protection varistor VR1 in parallel with the resonant LCR filter element 21 and a second protection varistor VR2 in parallel with the first capacitor CI to minimise the effects of magnetron malfunctions. In the event of a magnetron short circuiting due to an internal breakdown, the first protection varistor VR1 prevents the full voltage being applied across the LCR filter element 21. When the capacitance of capacitor CI is large, as in the prior art, the resultant over-voltage, when the magnetron stops conducting, is not usually too high. The second protection varistor VR2 limits the much higher overvoltage when the magnetron stops conducting which results from the much reduced value of the capacitance of capacitor CI in the present invention. [0031] Figure 2B shows an equivalent circuit 20A, used in the analysis that follows, of the circuit 20 of Figure 2A.

[0032] It is necessary to ensure that the LCR filter element 21 is tuned to the ripple frequency Fr. Generally the ripple frequency Fr for the ripple source will be well defined either because the mains frequency is well defined or in the case of a switched mode supply the basic frequency is usually defined from a quartz crystal source of more than adequate accuracy (typically F ± 50 ppm).

[0033] It is most desirable in practice that the values of L2 and C2 do not have to be tuned precisely. To achieve this, a quality factor Q of the filter 21 is deliberately controlled by providing the resistor Rl in series with the second capacitor C2. By suitable selection, a Q factor can be selected so that the impedance of the filter will be adequately high over a bandwidth that will permit the selection of values for L2 and C2 with reasonable tolerance of say ±5%. The interaction of the various parts will now be outlined mathematically.

Determination of Component Values

[0034] The resonant frequency Fr of the L2 C2 filter element 21 is given by equation 1:

1

Fr _.

2πϋ2 · C2

[0035] The surge impedance of the filter is given by equation 2:

[0036] The quality factor Q of the filter circuit, which is typically set to 10 to provide adequate dynamic impedance at resonance with adequate bandwidth B to cope with component tolerances, is given by the equation:

"7 P I

^ ^{= =} T.

[0037] At resonance

L2 make

Rl = Ra

[0038] Component values can be determined by substituting respectively for C2 and L2 in equation 1 from equation 2:

^{^ ' '} 2-fr

1

2KFT∑₉

[0039] If, due to component tolerance, the resonant frequency F_R of the filter is not quite the same as the fundamental frequency Fr of the ripple waveform, then the choice of Q should still permit the impedance ·¾/ i to be adequately high, where Xf is the reactance as a function of frequency and Rf is the resistance as a function of frequency. [0040] If the difference in frequency between the resonant frequency F_R of the filter and the ripple frequency Fr is dF then the impedance of the filter is still adequately high and is given by:

when

then

= Q.QSFr

so with a worst case tolerance on L2 and C2 of 5% then the lowest possible attenuation of the ripple current would correspond to this, assuming the actual ripple frequency can be held accurately. This is usually the case as the ripple frequency Fr is determined by the mains frequency or derived from a circuit with a very stable and accurate oscillator ώ — ^' >—

[0041] Without the filter, the magnetron current ripple is:

[0042] With the filter, the magnetron current ripple is:

[0043] When the resonance frequency F_R of the tuned circuit and the ripple frequency Fr are the same, then for a Q of 10 with &^a = ^ _the ripple current is significantly reduced:

Vr— V _{: i} Fr— l ^.^^

l&R - R ~ IQ iR

With the worst case tolerance then ripple current is as follows, this is still a significant improvement over the case without the filter:

[0044] This invention thus provides a method of obtaining adequate ripple performance with low stored energy. The invention takes advantage of the very nonlinear nature of the magnetron voltage current characteristic and would not be expected to be effective with a normal or resistive load. The impedance of the power supply seen by the magnetron is much higher than the effective resistance of the magnetron, is not constant and may have inductive or capacitive components.

[0045] A SABER computer analysis is shown in Figure 3 for the example outlined earlier. The total stored energy including that in the L2 C2 filter element 21 is significantly reduced but the ripple performance is still quite satisfactory for an industrial magnetron application.

[0046] Figure 4 shows the current in amps supplied to the magnetron plotted against time in milliseconds. The upper trace 41 shows the ripple performance with the invention applied while the lower trace 42 shows the ripple performance with a traditional filter that stores over 15 times the energy stored in the circuit illustrated in Figure 2A using a LI and CI with values 50mH and lOnF respectively with L2 and C2 having values of 12mH and 7.39nF respectively. The energies stored in CI and C2 are 2J and 0.016J respectively. [0047] The lower trace 42 results from using only the shunt filter as shown in the prior art power supply of Figure 1A with LI and L2 50mH and 250nF respectively. With 25 times the capacitor value for CI, compared with that used in the circuit of Figure 2, the stored energy is 50J.

[0048] In both cases the duty cycle of the source was a square wave frequency of 16.67kHz with a 90% duty cycle providing similar ripple (harmonic) content for both systems. [0049] As can be seen the resultant ripple is not completely sinusoidal; there is some harmonic content. This is responsible for some of the distortion seen on the upper waveform.

[0050] The waveforms show that for a substantial reduction in stored energy very similar peak to peak ripple values result. While the form of the ripple is quite different the peak to peak or rms values are similar. While the magnitude of the ripple provided by the invention is 1.16 times that provided in the prior art, the stored energy is l/25^th of that in the prior art, which is a very significant improvement.

[0051] In the context of industrial heating where raw RF power is the requirement the inequalities associated with the ripple shown would not be a cause of concern.

[0052] Figure 5 shows a preferred arrangement of a second embodiment of a magnetron supply circuit 50 according to the invention in which a rectifier system that uses multiple rectifiers ll_l-n powered from individual windings on a transformer or from individual transformers AC_l-n.

[0053] A separate first element of the filter is supplied for each rectifier system 11. In this case, each inductor of the first filter element has an inductance of Ll/n where n is the number of rectifier systems. Each capacitor of the first filter element has a capacitance of n*Cl. This results in an equivalent first element of the filter with a total inductance of LI and a total capacitance of CI as in the first embodiment of Figure 2A.

[0054] The resistance of the second varistor VR2 is reduced compared with the value in the first embodiment and selected based upon the proportion of the number of rectifier systems or stages and the voltage. For example, for a 20kV magnetron a second varistor VR2 that passes 6A at 25 kV would be suitable, so that at 20kV its current would be <lmA. For 25 stages a varistor with say lkV at 6A and 800V at 1mA would be selected for each stage.

[0055] Generally the second element of the filter can be kept as a single component as its size and stored energy is small and is selected as described above. For the first varistor V 1, a varistor that has, for example, a 1mA voltage of around 5kV would be suitable.

[0056] It would be possible, although more complex, to use a separate resonant second element of the filter for each stage. In this case, each stage would be based around a design value where the resistance per stage would be Ra/n. The individual filters would have a Q of 10 and the design equations would be as described earlier. The multiple first varistors VR1 would be similarly proportioned to values based upon the number of stages.

[0057] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0058] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0059] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

1. A magnetron power supply comprising a filter for reducing a ripple voltage component, the filter comprising:

a. a first filter element comprising a first inductor in series with a power supply circuit for the magnetron and a first capacitor in parallel with the magnetron; and b. a second filter element which is resonant substantially at a frequency of the ripple

voltage component, the second filter element comprising a second inductor in series with the first inductor and a second capacitor bridging the second inductor.

2. A magnetron power supply as claimed in claim 1, further comprising a first varistor in parallel with the second filter element.

3. A magnetron power supply as claimed in claims 1 or 2, comprising a second varistor in parallel with the first capacitor.

4. A magnetron power supply as claimed in any of the preceding claims further comprising a resistor in series with the second capacitor of the second filter element for determining a quality factor of the second filter element.

5. A magnetron power supply as claimed in claim 4, wherein the quality factor of the second filter element provides predetermined dynamic impedance at resonance with a bandwidth to accommodate tolerances of ±5% of the inductance of the second inductor and ±5% of the capacitance of the second capacitor.

6. A magnetron power supply as claimed in claim 4, wherein a value of the resistor is selected to provide a value of the quality factor of substantially 10.

7. A magnetron power supply as claimed in any of claims 4 to 6, wherein the quality factor is selected so that the values of the inductance of the second inductor and the capacitance of the second capacitor can be within a tolerance of substantially ±5% of values necessary for the resonant frequency of the second filter element to be equal to the frequency of the ripple component.

8. A magnetron power supply as claimed in any of the preceding claims, wherein the first filter element comprises a plurality of stages each arranged to be supplied by a separate rectified power supply.

9. A magnetron power supply as claimed in claim 8, wherein the second filter element comprises a separate respective resonant filter element for each stage of the first filter element.

10. A method of reducing a ripple voltage component of a magnetron power supply comprising:

a. filtering the power supply with a first filter element comprising a first inductor in series with the power supply circuit and a first capacitor in parallel with the magnetron; and b. further filtering the power supply with a second filter element which is resonant

substantially at a frequency of the ripple voltage component, the second filter element comprising a second inductor in series with the first inductor and a second capacitor bridging the second inductor.

11. A method as claimed in claim 8 comprising limiting a voltage applied across the second filter element when the magnetron short circuits by a first varistor bridging the second filter element.

12. A method as claimed in claims 8 or 9 comprising limiting an overvoltage when the magnetron stops conducting by a second varistor bridging the magnetron in parallel with the first capacitor.

13. A method as claimed in any of claims 8 to 10 comprising selecting a quality factor for the second filter element which permits a tolerance of ±5% in the selection of values of the inductance of the second inductor and the capacitance of the second capacitor substantially to match a resonant frequency of the second filter element to a frequency of the ripple voltage component.

14. A method as claimed in any of claims 8 to 13, wherein the first filter element comprises a plurality of stages each arranged to be supplied by a separate rectified power supply.

15. A method as claimed in claim 14, wherein the second filter element comprises a separate respective resonant filter element for each stage of the first filter element.

16. A system comprising a magnetron arranged to be powered by a magnetron power supply as claimed in any of claims 1 to 9.