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WO2011058360A1 - Transformateur d'isolement pour alimenter un filament de cathode de source de micro-ondes - Google Patents

Transformateur d'isolement pour alimenter un filament de cathode de source de micro-ondes Download PDF

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Publication number
WO2011058360A1
WO2011058360A1 PCT/GB2010/051880 GB2010051880W WO2011058360A1 WO 2011058360 A1 WO2011058360 A1 WO 2011058360A1 GB 2010051880 W GB2010051880 W GB 2010051880W WO 2011058360 A1 WO2011058360 A1 WO 2011058360A1
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WO
WIPO (PCT)
Prior art keywords
winding
primary
secondary winding
isolation transformer
shaped
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/GB2010/051880
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English (en)
Inventor
Robert Richardson
Michael Bland
Joe Hutley
Nigel Cox
Colin Bennett
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Teledyne UK Ltd
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e2v Technologies UK Ltd
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Publication of WO2011058360A1 publication Critical patent/WO2011058360A1/fr
Anticipated expiration legal-status Critical
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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/13—Solid thermionic cathodes
    • H01J1/135—Circuit arrangements therefor, e.g. for temperature control
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
    • H02M7/42—Conversion of DC power input into AC power output without possibility of reversal
    • H02M7/44—Conversion of DC power input into AC power output without possibility of reversal by static converters
    • H02M7/48—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • H02M7/539—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters with automatic control of output wave form or frequency
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F27/00—Details of transformers or inductances, in general
    • H01F27/28—Coils; Windings; Conductive connections
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F27/00—Details of transformers or inductances, in general
    • H01F27/40—Structural association with built-in electric component, e.g. fuse
    • H01F27/402—Association of measuring or protective means
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J23/00—Details of transit-time tubes of the types covered by group H01J25/00
    • H01J23/02—Electrodes; Magnetic control means; Screens
    • H01J23/04—Cathodes
    • H01J23/05—Cathodes having a cylindrical emissive surface, e.g. cathodes for magnetrons
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J25/00—Transit-time tubes, e.g. klystrons, travelling-wave tubes, magnetrons
    • H01J25/50—Magnetrons, i.e. tubes with a magnet system producing an H-field crossing the E-field
    • H01J25/52—Magnetrons, i.e. tubes with a magnet system producing an H-field crossing the E-field with an electron space having a shape that does not prevent any electron from moving completely around the cathode or guide electrode
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J25/00—Transit-time tubes, e.g. klystrons, travelling-wave tubes, magnetrons
    • H01J25/50—Magnetrons, i.e. tubes with a magnet system producing an H-field crossing the E-field
    • H01J25/52—Magnetrons, i.e. tubes with a magnet system producing an H-field crossing the E-field with an electron space having a shape that does not prevent any electron from moving completely around the cathode or guide electrode
    • H01J25/58—Magnetrons, i.e. tubes with a magnet system producing an H-field crossing the E-field with an electron space having a shape that does not prevent any electron from moving completely around the cathode or guide electrode having a number of resonators; having a composite resonator, e.g. a helix
    • H01J25/587—Multi-cavity magnetrons

Definitions

  • Isolation transformer for a cathode heater supply for a microwave source
  • This invention relates to an isolation transformer for a cathode heater supply for a microwave source.
  • Radio frequency (RF) heating is used for a wide range of industrial processing applications such as metal melting, welding, wood drying and food preparation.
  • the output powers required range from a few kilowatts to values in the megawatt region.
  • the frequency range can be a few hundreds of kilohertz to several tens of megahertz using triodes or tetrodes.
  • For microwave applications of RF in the frequency range above 500 MHz it is usual, but not necessary, to use magnetrons.
  • Thermionic tubes require a heater supply to heat the thermionic cathode and in high power thermionic tubes the cathode is heated directly, i.e. the heater acts as the cathode.
  • the heater acts as the cathode.
  • the heater power required is usually quite high, for example 12V at 120A implying a relatively low load resistance of 0.1 ohm.
  • practical and convenient embodiments of the microwave generator frequently require that the heater circuit is operated not at ground potential but at an eht potential of 20kV or higher.
  • the cathode supply has to provide several kW of power to a low resistance load with a voltage isolation >20kV. It is well-known to provide this power with a large power frequency transformer operating at 50Hz or 60Hz and constructed with large spacing and typically immersed in oil to provide high voltage isolation. Generally the voltage applied to the cathode has to be carefully controlled and adjusted during operation and thyristor regulators are used for this function, typically operating on the primary of a mains transformer.
  • the cathode being one of the most fragile components of a magnetron, operates at its design temperature to prolong the life of the cathode by avoiding overheating while maintaining the required emissivity and preventing arcing by avoiding under heating. It is known in the art to seek to monitor the cathode temperature with a pyrometer, but with use of the magnetron the pyrometer window becomes occluded leading to false temperature readings. Alternatively, a varying schedule of power supplied, developed on a trial and error basis, may be applied during warm-up and operation of the magnetron.
  • transformers for supplying the heater current are expensive and very large, occupying a volume of 0.07 m 3 and weighing 100 kg in the example given above.
  • thyristor controllers for power regulation are problematic in that they have limited control capabilities and poor transient response characteristics.
  • an isolation transformer for a cathode heater supply for a microwave source comprising: a first U-shaped secondary winding, comprising two parallel leg portions joined at one end thereof by a bridging portion; a primary winding concentric with the bridging portion; a monitor winding passing through primary core assemblies of the primary winding; and electrical insulation material insulating the secondary winding from the primary winding.
  • the first U-shaped primary winding is tubular.
  • the isolation transformer further comprises hollow U-shaped former means of greater cross-sectional diameter than, and concentric with, the first U-shaped secondary winding, for containing the electrical insulation material.
  • the primary windings comprise a plurality of cores with windings connected in series.
  • the cores are held in place by adhesive.
  • the isolation transformer for a DC cathode heater supply further comprises: a second U-shaped secondary winding spaced from and aligned with the first U-shaped secondary winding; first and second smoothing chokes comprising respective core assemblies fitted over connection leads arranged for connecting two ends of the first secondary winding to a cathode heater; synchronised rectifiers connected between the secondary windings and the first smoothing choke; and control means for the synchronised rectifiers.
  • the isolation transformer further comprises two current monitors arranged to monitor current to each synchronous rectifier, respectively.
  • the isolation transformer further comprises a single turn winding located within one of the tubular secondary windings and arranged to power the control means.
  • an isolation transformer for a cathode heater supply for a microwave source comprising the steps of: forming a first U-shaped single secondary winding, comprising two parallel leg portions joined at one end thereof by a bridging portion; mounting a primary winding concentric with the bridging portion; passing a monitor winding through primary core assemblies of the primary winding; and providing electrical insulation material to insulate the secondary winding from the primary winding.
  • providing electrical insulation material comprises providing hollow U- shaped former means of greater cross-sectional diameter than, and concentric with, the first U-shaped single secondary winding; and filling a space between the secondary winding and walls of the former means with electrical insulating material.
  • mounting a primary winding concentric with the bridging portion comprises threading a leg portion and a portion of the bridging portion through a core of the primary winding.
  • mounting a primary winding concentric with the bridging portion comprises threading a leg portion and a portion of the bridging portion through a plurality of cores of portions of the primary winding and connecting the portions of primary winding in series.
  • the method of manufacturing an isolation transformer for a DC cathode heater supply for a microwave source further comprises: forming a second U- shaped secondary winding spaced from and aligned with the first U-shaped secondary winding; fitting first and second smoothing chokes comprising respective core assemblies over connection leads arranged for connecting two ends of the first secondary winding to the cathode heater; connecting synchronised rectifiers between the secondary windings and the first smoothing choke; and providing control means for the synchronised rectifiers.
  • the method further comprises providing a single turn winding located within one of the tubular secondary windings and arranged to power the control means.
  • Figure 1 is a circuit diagram of an embodiment of an AC heater supply using the isolation transformer of the invention
  • Figure 2 illustrates waveforms generated by the circuit of Figure 1
  • Figure 3 is a circuit diagram showing in more detail the resistance or power monitoring and control circuits of Figure 1 ;
  • Figure 4 is a circuit diagram of a DC heater supply using an embodiment of the isolation transformer of the invention.
  • FIG. 5 illustrates waveforms generated by the circuit of Figure 4.
  • Figure 6 is a circuit diagram showing in more detail the resistance or power monitoring and control circuits of Figure 4.
  • Figure 7 is a circuit diagram of a suitable drive circuit for the synchronous rectifiers of Figure 4.
  • Figure 8 is a perspective view of an embodiment of a transformer according to the invention suitable for the AC heater supply of Figures 1 to 3;
  • Figure 9 is a vertical cross-section of the transformer of Figure 8.
  • Figure 10 is a perspective view of a transformer suitable for the DC heater supply of Figures 4 to 7;
  • Figure 11 is a vertical cross-section of the transformer of Figure 10.
  • Figure 12 is a perspective view of the transformer of Figure 10 with a shielding cover removed;
  • Figure 13 is a perspective view of the transformer of Figure 12 with a PCB removed.
  • Figure 14 is a block diagram useful in modelling the heater supply for use with the invention for providing digital control thereof.
  • FIG. 1 A basic circuit diagram of an AC cathode heating supply according to the invention is shown in Figure 1 and corresponding waveforms are shown in Figure 2.
  • the AC cathode heating supply 10 for heating an electronic tube heater 11 comprises an isolation transformer 12 the secondary windings 121 of which are electrically connected to the heater and the N primary windings 122 of which are electrically connected to and powered by a Switched Mode Power Supply (SMPS) inverter H-bridge 13, so that the ratio of the transformer from the primary is N: l step down.
  • the isolation transformer 12 also comprises a single turn monitor winding 123 which passes through each core assembly of the primary windings 122.
  • the monitor winding is electrically connected to a first input of a module 14 of resistance or power monitor and control circuits.
  • a current monitor 141 arranged to monitor an electrical current in the primary windings is electrically connected to a second input of module 14.
  • An output of the module 14 is electrically connected to one input of an error amplifier or comparator 131, a second input to the error amplifier is provided by a variable reference voltage module 132.
  • An output of the error amplifier is electrically connected to a control input of the SMPS inverter H-bridge 13.
  • a power input of the SMPS inverter H-bridge 13 is connected to mains control inputs and outputs.
  • a capacitor 142 is connected in series between one of the two outputs of the SMPS inverter H-bridge 13 and the primary windings 122.
  • a voltage at terminals of the magnetron comprising the cathode heater 11 may not be a same voltage Vh as presented to the cathode resistance (Rh) 111 of the cathode heater 11. This is because of inevitable inductance 112 of the tube heater connections and of the heater itself which may well provide a significant tube inductance (Lt).
  • a known magnetron BM75L available from e2v technologies pic, Chelmsford, UK has a cold resistance of around lO mohms and a hot working resistance of around 100 mohms.
  • the cathode assembly inductance is of the order of 0.5 ⁇ . At normal 50/60 Hz values the reactance of this inductance is only around 0.16 mo hm but at, for example, 15 kHz the inductance is 47 mohms; almost half that of the required hot working resistance.
  • an interconnection inductance and transformer (Tfmrl) leakage inductance 124 shown in Figure 1 as circuit stray inductance (Ls), can easily approach 1 ⁇ thus adding to the problem caused by the tube inductance (Lt) 112.
  • Electrical resistance (Rh) 111 of the cathode heater 11 may also vary due to skin or proximity effects that occur at higher frequencies in conductors.
  • the relatively poor electrical conductivity of the materials used for typical tube cathodes, such as tungsten, and their high operating temperature >1800°C generally result in minimal resistance variation of the cathode due to frequency-related effects over the frequency range of interest.
  • the inverter 13 During warm up of the cathode the inverter 13 provides power to heat the cathode 11. Once in operation with full anode input power to the tube (that may be several hundreds of kilowatts), however, circuit operation may result in further power being fed to, or removed from, the cathode resulting in a change in temperature of the cathode heater. As emission and cathode life are sensitive to temperature it is very desirable to keep the cathode temperature at its specified optimum value.
  • the cathode 11 is made from a material with a significant temperature coefficient of resistance it is possible to use resistance change of the cathode to monitor changes in cathode temperature.
  • back bombardment power when anode current starts to flow can contribute approximately 70% of the required heating power to the cathode and if no adjustment is made the cathode would overheat.
  • the input power from the main power source can be reduced to compensate for this additional heating and thus if adjustments are made to the power supplied to keep the temperature constant, then a measured resistance of the cathode will be constant.
  • the optimum resistance is dependent on the anode input power to the device. That is, the required resistance, and thus the cathode temperature, vary with anode power.
  • the resistance can be set to any required value to optimise the performance of the system.
  • the cathode temperature may be varied to suite a particular operating scenario.
  • the temperature relates to the resistance and the resistance control may thus not be set to a fixed value but a pre-programmed series of values. So, for example, if a user requires high power a higher resistance may be set implying a higher temperature thus more emission. Conversely if a user wants an extended run at low power, a lower resistance, and thus temperature and emission may be appropriate.
  • a digital implementation permits a wide variety of options to be readily programmed into the control system.
  • the electronic tube is of a type that does not have a cathode the power input of which is affected by the anode input power, then satisfactory control can be implemented by applying constant power to the tube cathode 1 1 via the inverter 13.
  • a drive voltage waveform 21 of the Switched Mode Power Supply (SMPS) inverter 13 is shown in Figure 2. It is convenient to generate a voltage waveform 22 that provides peak output primary voltage Vp of the form shown in Figure 2 with a corresponding primary current Ip.
  • This waveform is of a well-understood form providing an output cycling through +Edk, zero and -Edk and the output impedance must be low in any of these states when either sinking or sourcing current.
  • the inverter will operate from a rectified 3 phase mains supply so the voltage
  • the inverter 13 incorporates an error amplifier 131 , one input of which is connected to a reference voltage supply 132 via a control VR1.
  • the reference voltage supply 132 can be used to set an output power or the resistance setting of the load. Power (or resistance) control is effected by using the error amplifier 131 to compare a signal proportional to power (or resistance) of the load with the know reference 132. The output of the error amplifier provides a signal that allows a duty cycle (a ratio of T1/T2 as shown in Figure 2) to be varied to maintain the power or the resistance at a set value in a known manner.
  • a duty cycle a ratio of T1/T2 as shown in Figure 2
  • a capacitance Cb of the DC blocking capacitor 142 is selected to produce a resonant circuit such that the resonant frequency ⁇ 0 of the capacitance (Cb) and total inductance (Ls+ N 2 Lt) is approximately 2 ⁇ / ⁇ ⁇ 5 where F is the operating frequency of the SMPS 13.
  • This results in the primary current (Ip) being of rounded (quasi-sine) form so that it is relatively easy to detect and sample the peak value Ipk of the current Ip where the rate of change of current is zero (i.e. dlp/dt 0), that is a stationary point in the waveform.
  • the sensing of the signals to provide the power (or resistance) feedback is implemented on the primary side of the isolation transformer (Tfmrl) 12. This requires a transformer with very low losses and reasonably well-controlled residual values. Using the method of the present invention, complex monitoring circuits are not required at the secondary side of the transformer.
  • a known current monitor 141 in the form of a current transformer arranged around the primary feed from the inverter H-bridge 13 monitors the primary current Ip. Because the isolation transformer (Tfmrl) 12 is designed to have very low loss and a high value of shunt inductance, the current Ip is a faithful reproduction of heater current Ih, but scaled down in amplitude by ratio N of the isolation transformer (Tfmrl) 12. The output from this monitor 141 forms the basis of a current monitoring signal Va.
  • a voltage monitoring signal Vb is obtained by a single turn pickup winding 123 close to the primary winding 122 of the transformer (Tfmrl) 12. If the monitor winding 123 is close to the primary cores and if it is lightly loaded (Rload >500*N 2 *Rb) the monitor winding will give a faithful representation of the voltage Vp applied to the transformer. The applied voltage Vp will be stepped down by the transformer ratio N to provide the voltage monitoring signal Vb for a power (or resistance) calculation.
  • the resistance of the heater can be calculated by taking the ratio of Vb/Va with a divider circuit for use by the inverter module 13 in order to regulate the power applied to the cathode heater to maintain the resistance and thus the temperature constant.
  • a multiplier is required to calculate the product Va*Vb to determine Ip*Vp and hence Ih*Vh while to determine resistance of the heater a division function is required to calculate Vb/Va to determine Vp/Ip and hence Vh/Ih.
  • the DC cathode heating supply 40 for heating an electronic tube heater 41 comprises isolation transformer 42 the secondary windings 421 of which are electrically connected via synchronised rectifiers TR1 and TR2 to the cathode heater 41 and the primary windings 422 of which are electrically connected to and powered by a Switched Mode Power Supply (SMPS) inverter H-bridge 43.
  • the isolation transformer 42 also comprises a monitor winding 423 which passes through each core assembly of the primary windings 422.
  • the monitor winding is electrically connected to a first input of a module 44 of resistance or power control and monitor circuits.
  • a current monitor 441 arranged to monitor an electrical current in the primary windings 422 is electrically connected to a second input of module 44.
  • An output of the module 44 is electrically connected to one input of an error amplifier or comparator 431 , a second input of the error amplifier is provided by a variable reference voltage module 432.
  • An output of the error amplifier is electrically connected to a control input of the SMPS inverter H-bridge 43.
  • a power input of the SMPS inverter H-bridge 43 is connected to mains control inputs and outputs.
  • a capacitor 442 is connected in series between one of the two outputs of the SMPS inverter H-bridge 43 and the primary windings 422.
  • Full wave push pull synchronised rectifiers TR1 and TR2 with chokes LI and L2 input filtering are used to provide a DC output from the secondary windings 421.
  • the behaviour of the transformer (Tfmrl) 42 is now importantly different from the transformer 12 used in the previously described AC heater supply.
  • Transformer leakage inductances (Lssl and Lss2) have currents with DC components in them while only the primary leakage inductance (Lspl) has an AC component of current flowing in it.
  • the inverter waveform 51 is shown in (a).
  • the transformer drive waveform 52 via the blocking capacitor Cb 142 is shown as (b).
  • the droop AV on the drive is produced by the impedance which the capacitor Cb presents at each inverter pulse off commutation.
  • the voltage on the capacitor Cb at the time Toff+n*T2/2 (where n has any integer values including zero) is designed to ensure rectifier commutation takes place desirably quickly.
  • the advantage of this circuit is that the energy in the leakage inductances Lssl and Lss2 is recovered without loss thus making the power and or resistance monitoring at the primary more effective.
  • the capacitor Cb can desirably have a same value for either AC or DC applications so that a common heater inverter can be used for AC or DC applications.
  • a suitable drive circuit 71 for the synchronous rectifier TRl, TR2 is shown in Figure 7.
  • power to operate the drive circuit is provided by a further secondary winding (Tfmr S 3) 424.
  • the further secondary winding 424 feeds a rectifier BR1 in parallel with a filter capacitor CI and a regulator diode chain of a resistor R7 and two diodes Dl and D2 to power LT rails of +5V and +12V for the drive circuits.
  • Synchronous rectifier FETs TRl a and TRlb and TR2a and TR2b are illustrated connected in parallel for each function but a single or multiple FETs may be used as dictated by requirements of the design output current.
  • the pairs of synchronous rectifier FETs are driven by driver chips IC2 and IC3, such as MAX4422 that provide a gate drive to the FETs via D4, Rl, R2 and D6, R5, R6.
  • An AND gate ICla and IClb such as a 78HC08 controls the driver circuits and prevents a signal being applied to the driver chips IC2 and IC3 until the LT rails voltages are established.
  • a delay circuit 72 as shown in Figure 7, of D7, D8, R8, R9, and C2 provides a requisite delay to permit the +12V and +5V rails to establish.
  • Current monitors CT s i and CT s2 monitor a current to each synchronous rectifier TRl and TR2. Rectifying burdens D3, RIO and D5, Rl l are used on each current monitor so that the current monitors output signals to an AND Gate (ICla or b) only when current is flowing in a given rectifier TRl and TR2.
  • the synchronous rectifiers TRl and TR2 are both subjected to rapid switching voltage rises across their drain sources.
  • the additional circuits TR3, R3 and TR4, R4 in the gate prevent Miller capacitance currents in the FETs that may raise the gate voltage and result in undesirable turn-on of the synchronous rectifier TRl and TR2 from occurring.
  • the output resistances of the driver chips IC2 and IC3 are adequate to prevent this spurious turn-on.
  • the circuit arrangement is such that while the LT is being established the circuit behaves as a normal rectifier with diode drops around IV during conduction in TRl and TR2.
  • the trigger circuit is enabled after LT is established the trigger waveforms take over and lowers the voltage drops in the synchronous rectifiers to around 25mV or less.
  • a suitable isolation transformer (Tfmrl) 12 is shown in Figure 8 and has a single turn secondary winding comprising a loop of copper tubing 121.
  • Tfmrl isolation transformer
  • the skin depth is of the order of 0.5 mm so that standard central heating copper tubing of between 0.5 mm and 1 mm makes an ideal conductor for this application.
  • Fabrication of the tube can use a standard soldered end feed fitment that would be used for central heating fittings or the tube can be preformed to the required U shape required. Another key requirement is that the voltage hold-off between the secondary winding
  • the transformer be compact.
  • a working voltage of up to 25 kV is desirable.
  • the use of a circular cross-section conductor is ideal as the electric stress for a given geometry decreases as the radius of the surface increases.
  • a circular cross-section single conductor constitutes an ideal form of winding for a system involving high voltage insulation requirements.
  • a single U-shaped tube comprising two parallel leg portions joined at one end thereof by a bridging portion, constituting the secondary winding 121 is encapsulated in a suitable epoxy resin 95. Threaded inserts 82 for connection to the heater and cathode are brazed into the free ends of the U-shaped tube 121. A spacing 81 of free ends of the U-shaped tube 121 can be such as to connect directly to RF tube heater and cathode terminals.
  • the resin 95 may be contained by a mould tool made up from standard plastic pipe fittings of the type used for waste water. Such pipe fittings are typically made from high temperature PVC which has most advantageous electrical insulating properties at high voltage.
  • a suitable mould can be built around the single tube 121.
  • the primary cores with their windings 122 can be threaded over one of the leg portions to fit on the bridging portion of the U-shaped mould so formed.
  • the plastic pipe and elbows used for the tool can be left in place and form an additional part of the electrical insulation circuit.
  • Material sizes are chosen so that thickness of the epoxy 85 and a surface tracking distance 83 provide adequate electrical isolation for the required eht voltage. For example, where the isolation is 25 kV and the output is 12 V at 120 A, a 15 mm diameter, 1 mm thick copper tube may be used for the single turn 121 and 32mm PVC water fitments for the mould tool 87 and 89. The resulting epoxy thickness is around 8mm and the creep distance 83 is 120 mm.
  • a resultant size of the transformer together with the choice of operating frequency permits the use of amorphous cores for the M cores of the primary windings 122.
  • the cores work at relatively low peak flux density and so the loss is very low.
  • the core windings 122 can be a single layer winding of suitably sized wire.
  • cores of magnetic area 162 mm 2 and magnetic length 225 mm prove a suitable choice.
  • the whole structure has components that have smooth and/or circular type perimeters.
  • Single layer windings 122 and a circular cross-section secondary conductor 121 provide an AC resistance at 15 kHz close to the DC resistance, thus giving best possible utilisation of the copper.
  • Such shapes also represent optimum methods of achieving the lowest electrical stress in a given volume of material. Consequently, for its power throughput and eht isolation, the transformer is very light and compact.
  • a transformer suitable for the e2v BM75L magnetron weighs only 1 kg and has a total loss of ⁇ 15 W at full output.
  • Figure 1 shows a single turn primary winding 123 used for monitoring purposes. This winding is wound through the M cores of the primary winding 122 after fitting the cores to the moulded assembly and before the final application of the hot melt glue 85 used to secure the cores.
  • a transformer 42 suitable for a DC heating supply is similar to the transformer 12 used for the AC supply.
  • An overall assembly with synchronous rectifiers is shown in perspective in Figure 10 and a vertical cross-sectional view is shown in Figure 11.
  • Figure 12 is a perspective view of the transformer 42 without a screened metal box 109 which in Figures 10 and 11 screens the circuitry, including a PCB 1241.
  • Figure 12 is a perspective view of the amplifier without the screened metal box 109 or the PCB 1241.
  • a main difference between the transformer 12 for the AC supply and the transformer 42 for the DC supply is that the transformer 42 for the DC supply has two secondary winding tubes 421. If a single winding were used, i.e. N: l step down, then a bridge rectifier would be required and the current would flow through two rectifiers in series. For high current low voltage applications a push pull secondary is used where each of the secondary windings has a single associated rectifier. This reduces loss as current only flows through a single rectifier. The required transformer now has windings that are N: l : l step down and the current in each turn is half that of the full current. The two individual secondary windings do not conduct together but conduct on alternate half cycles of the input supply.
  • the two secondary winding tubes 421 are closely spaced, to maximise coupling between them, as there is a peak voltage of approximately only 3 times Vh between them.
  • the two secondary winding tubes 421 can be of reduced diameter compared with the secondary winding of a transformer for an AC supply, as the current in them is reduced to around 0.7 Ih. Their close proximity and the fact that they are also circular in cross-section ensures that an electric field stress in the outer layer of the mould 117 and 119 and in the epoxy filling 115 is still suitably low.
  • First and second smoothing chokes LI and L2 are made up of two core assemblies 1021 that fit over connection leads 1123, 1125 from the secondary winding to the tube heater and cathode.
  • the core assemblies 1021 comprise grouped toroids of suitable materials, such as powder iron cores, with smaller radius cores 1129 inside, and concentric with, larger radius cores 1127. This arrangement raises the inductance as well as giving a certain degree of rigidity to the structure. Although two cores sizes are shown in the diagram more than two sizes can be used if desired or, if available, a single large core could be used.
  • Concentric clamps 1031 hold each core assembly to the screened metal box 109.
  • connection leads 1123, 1125 can be solid rods as with DC the full conductor cross-section will be utilised.
  • a lid 1333 of the screened metal box 109 forms one of the connections between the transformer (Tfmrlsl and Trfmls2) 42 and the second smoothing choke L2.
  • Connections 1335, 1237 between TRln drains and Tfmrlsl and TR2n drains and Ttfrmls2 respectively are made with flat copper strips.
  • a further copper strip 1339 makes a connection between LI and Trln, Tr2n sources and LI .
  • Connections for high current are made on the Tfmrl secondary tubes 421 in a similar manner to that used for the AC application, with soldered or brazed in fixing bushes, as in Figure 8, that are tapped with a suitable size thread to ensure a firm fit for the current involved, for example M6 for 120 A.
  • Control for the synchronous rectifiers TR1 , TR2 is mounted on the control PCB 1241 that is mounted above the copper connection strips.
  • Two current monitors CT s i and CT s2 1243, 1245 are mounted around the main tubes that feed sources of Trln and Tr2n.
  • a fixing block 1247 bridging the free ends of the U-shaped secondary windings is used to ensure that the connection between all the elements of the system are held rigidly.
  • a single turn winding 424 is fed through the centre of one of the secondary tubes 421 of Tfmrl .
  • This turn 424 enters and exits the tube at small (1 mm) central drillings in the fixing bushes 1 151 on one of the secondary tubes 421.
  • cathode heater power supply has been described in use with the transformer of Figures 10 to 13 it will be understood that the cathode heater supply can be used with other transformers such as, for example, the transformer described in PCT/GB2009/050942. It will be understood that with a 3 -phase power supply three transformers may be used, one for each phase.
  • outputs of the current monitor 141 are connected to inputs of a differentiator 146 and to inputs of a first full wave rectifier 144.
  • Outputs of the monitor winding 123 are connected to inputs of a second full wave rectifier 145.
  • a first output of the first full wave rectifier 144 is connected to an input of a first sample and hold amplifier SHI and a first output of the second full wave rectifier 145 is connected to an input of a second sample and hold amplifier SH2.
  • An output of the differentiator 146 is connected to respective control inputs of the first and second sample and hold amplifiers SHI, SH2.
  • Second outputs of the first and second full wave rectifiers 144, 145 respectively and of the first and second sample and hold amplifiers SHI, SH2 respectively are connected to four respective inputs of a multiplier / divider module 143.
  • An output of the multiplier / divider module 143 is to a pulse width modulator of the heater supply.
  • the primary current Ip through the isolation transformer 12 is of quasi- sine waveform 23.
  • SHI, SH2 sample and hold amplifiers
  • an analogue multiplier chip 143 such as an AD534 a voltage proportional to the power in Rh (i.e. Va*Vb) can be obtained.
  • the analogue multiplier chip AD534 143 can be programmed to divide so that a voltage proportional to resistance (i.e. Vb/Va) of the load Rh can be obtained.
  • Figure 3 shows that each signal Va and Vb is rectified by the first and second full wave rectifiers 144, 145, respectively.
  • Figure 4 is illustrated in more detail in Figure 6.
  • outputs of the current monitor 441 are connected to inputs of a first full wave rectifier 444.
  • Outputs of the monitor winding 423 are connected to inputs of a second full wave rectifier 445.
  • a first output of the first full wave rectifier 444 is connected to an input of a first integrator 446 and a first and second outputs of the second full wave rectifier 145 are connected to respective inputs of a second integrator 447.
  • An output of the first integrator 446, a second output of the first full wave rectifiers 444 and first and second outputs of the second integrator 447 respectively are connected to four respective inputs of a multiplier / divider module 443.
  • An output of the multiplier / divider module 443 is to the error amplifier 431 shown in Figure 4.
  • the transformer 42, rectifier 444, 445, and monitors 441, 423 are very efficient and virtually without loss. Consequently, the only power flow in the equipment is dissipated in the load Rh of the cathode heater 41.
  • the power can be obtained by the product Va *Vb or the resistance by the division Vb/V a.
  • sample and hold amplifiers SHI and SH2 of the AC supply circuit need to be reconfigured as integrators 446, 447 in the DC supply circuit.
  • the parameters that need to be measured are load voltage and load current.
  • the load voltage and current are obtained by measurement of primary side parameters as described above. The difference between the AC and DC variants is simply timing of the sampling.
  • a same version of software can be used for both AC and DC versions.
  • a small switch or jumper can be used to indicate to a DSP processor which variant of load is connected.
  • Dynamic Model of Cathode Figure 14 shows a controller block diagram of the cathode heater resistance controller implemented by DSP software and also a simplified model of the thermal dynamics of the magnetron cathode structure.
  • the model is based on the thermal mass 121 of the tungsten cathode and a linear approximation of the thermal resistance about the operating point.
  • the Laplace domain dynamic model of the cathode is the basis of the controller design and is used to find the PI controller constants to achieve a required closed loop response.
  • Transducer/measurement gains for i loa d and Vi oa d are not shown because they are cancelled out by the DSP.
  • a is the temperature coefficient of resistance for the tungsten cathode filtering and sampling of i loa d and Vi oa d are also not shown.
  • the tern T 4 is assumed to be linear about the operating point and the thermal coefficient of resistivity a is assumed to be linear about the operating point.
  • the two nested PI controllers 122, 123 shown in Figure 12 are implemented in DSP software. Both controllers have a sample frequency equal to the switching frequency of the inverter. The dynamics of the system are dominated by the thermal time constant of the cathode. Therefore the closed loop bandwidth of the system will be much lower than the controller sample frequency. This means it is possible to design the controllers in the continuous domain and use the bilinear transform to convert the controller constants for digital implementation.
  • the load resistance error signal is passed into the resistance controller CR es istance 122.
  • the demand current idemand is then used as a demand signal for the second, nested PI control loop 123 that controls the load current, Ccunent-
  • the output of the current controller 123 is a duty demand, duty, that feeds a PWM generator for the inverter 13, 43.
  • the control structure is identical for both AC and DC variants. Digital implementation of PI control loops is well understood and not discussed here.

Landscapes

  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Control Of High-Frequency Heating Circuits (AREA)

Abstract

L'invention comprend un transformateur d'isolement pour alimenter un filament de cathode de source de micro-ondes, comprenant un premier enroulement secondaire en U (421) constitué de deux parties de patte parallèles assemblées sur une extrémité de celui-ci par une partie en pont. Un enroulement primaire (422) est situé autour d'une partie de la partie en pont et concentrique à cette dernière. Un enroulement de moniteur traverse des ensembles noyaux primairex de l'enroulement primaire. Une insolation électrique (115) isole l'enroulement secondaire d'un enroulement primaire. Pour alimenter un filament de cathode en courant continu, un second enroulement secondaire en U est espacé du premier enroulement secondaire en U et aligné avec de dernier, et de première et de seconde inductance de lissage comprenant les ensembles noyaux respectifs (1127, 1129) sont ajustées sur des fils de connexion agencés pour connecter deux extrémités du premier enroulement secondaire au filament de cathode. Des rectificateurs synchronisés sont connectés entre les enroulements secondaires et la première inductance de lissage ; et des moyens de commande sont utilisés pour les rectificateurs synchronisés.
PCT/GB2010/051880 2009-11-11 2010-11-11 Transformateur d'isolement pour alimenter un filament de cathode de source de micro-ondes Ceased WO2011058360A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0919720.3 2009-11-11
GB0919720A GB2475262B (en) 2009-11-11 2009-11-11 Isolation transformer for a cathode heater supply for a microwave source

Publications (1)

Publication Number Publication Date
WO2011058360A1 true WO2011058360A1 (fr) 2011-05-19

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GB (1) GB2475262B (fr)
WO (1) WO2011058360A1 (fr)

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0301805A1 (fr) * 1987-07-27 1989-02-01 Matsushita Electric Industrial Co., Ltd. Appareil de chauffage à hautes fréquences
GB2227134A (en) * 1989-01-06 1990-07-18 Hitachi Ltd Control of microwave heating apparatus to avoid overvoltage on starting
US5001318A (en) * 1989-08-09 1991-03-19 Kabushiki Kaisha Toshiba High frequency heating apparatus with abnormal condition detection

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4999743A (en) * 1989-09-27 1991-03-12 At&T Bell Laboratories Transformer with included current sensing element
EP1783788A3 (fr) * 2005-11-02 2009-03-25 DET International Holding Limited Transformateur avec dispositif de mesure de courant

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0301805A1 (fr) * 1987-07-27 1989-02-01 Matsushita Electric Industrial Co., Ltd. Appareil de chauffage à hautes fréquences
GB2227134A (en) * 1989-01-06 1990-07-18 Hitachi Ltd Control of microwave heating apparatus to avoid overvoltage on starting
US5001318A (en) * 1989-08-09 1991-03-19 Kabushiki Kaisha Toshiba High frequency heating apparatus with abnormal condition detection

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GB0919720D0 (en) 2009-12-30
GB2475262A (en) 2011-05-18

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