GB2475262A - Isolation transformer having monitor winding passing through primary core assemblies of primary winding - Google Patents

Abstract

An isolation transformer, particularly for a cathode heater supply for a microwave source, has a first U-shaped secondary winding 421, comprising two parallel leg portions joined at one end thereof by a bridging portion. A primary winding 422 is located around and concentric with a portion of the bridging portion. A monitor winding passes through primary core assemblies of the primary winding. Electrical insulation 115 insulates the secondary winding from the primary winding. For a DC cathode heater supply a second U-shaped secondary winding is spaced from and aligned with the first U-shaped secondary winding and first and second smoothing chokes comprising respective core assemblies 1127, 1129 are fitted over connection leads arranged for connecting two ends of the first secondary winding to the cathode heater. Synchronised rectifiers are connected between the secondary windings and the first smoothing choke; and control means are provided for the synchronised rectifiers.

Description

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 use of the term "cathode", "cathode heater" or "heater" throughout this document implies this definition where the context does not demand otherwise. With thoriated tungsten or pure tungsten cathodes used in such tubes the heater power required is usually quite high, for example 12V at 120A implying a relatively low load resistance of 0.1 ohm.

Also 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.

Thus, in such embodiments, 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.

It is important that 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.

Moreover, known transformers for supplying the heater current are expensive and very large, occupying a volume of 0.07 m3 and weighing 100 kg in the example given above. Moreover, thyristor controllers for power regulation are problematic in that they have limited control capabilities and poor transient response characteristics.

It is an object of the present invention at least to ameliorate the aforesaid

disadvantages in the prior art.

According to a first aspect of the invention there is provided 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.

Conveniently, the first U-shaped primary winding is tubular.

Advantageously, 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.

Conveniently, the primary windings comprise a plurality of cores with windings connected in series.

Conveniently, the cores are held in place by adhesive.

Advantageously, 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.

Conveniently, the isolation transfonner further comprises two current monitors arranged to monitor current to each synchronous rectifier, respectively.

Conveniently, the isolation transformer further comprises a single turn winding located within one of the tubular secondary windings and arranged to power the control means.

According to a second aspect of the invention, there is provided a method of manufacturing 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.

Conveniently, 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.

Advantageously, 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.

Conveniently, 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.

Conveniently, 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.

Conveniently, the method further comprises providing a single turn winding located within one of the tubular secondary windings and arranged to power the control means.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: 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; Figure 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; and Figure 14 is a block diagram useful in modelling the heater supply for use with the invention for providing digital control thereof.

In the Figures, like reference numbers denote like parts.

AC cathode heating supply 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.

Referring to Figure 1, 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: 1 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.

When operating at higher frequencies 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). As an example, a known magnetron BM75L available from e2v technologies plc, Chelmsford, UK has a cold resistance of around mohms and a hot working resistance of around 100 mohms. The cathode assembly inductance is of the order of 0.5 pH. At normal 50/60 Hz values the reactance of this inductance is only around 0.16 mohm but at, for example, 15 kHz the inductance is 47 mohms; almost half that of the required hot working resistance.

Further additional problems arise in that an interconnection inductance and transformer (Tfmrl) leakage inductance 124, shown in Figure 1 as circuit stray inductance (Ls), can easily approach 1 pH 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. However, 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.

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.

As 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.

In the case of a magnetron, 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. By sensing electrical resistance of the cathode, 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.

If 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 11 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. Usually the inverter will operate from a rectified 3 phase mains supply so the voltage Edc will be of the order of 560V. As indicated above, 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 capacitance Cb of the DC blocking capacitor 142 is selected to produce a resonant circuit such that the resonant frequency w0 of the capacitance (Cb) and total inductance (Ls+ N2Lt) is approximately 2irF/1.15 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.

When dlp/dt = 0 the induced voltage in the inductors Ls and Lt will be zero and so at this time the voltage Vp seen at the transformer primary will be the voltage Vh across the load multiplied by the transformer ratio N2.

In the invention 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.

By monitoring the primary signals of voltage and current a signal proportional to power (or resistance) can be obtained.

As also shown in Figure 1, 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 (Tfmr 1) 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 (Tfrnrl) 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*N2*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.

With the availability of the monitoring signals Vb and Va and because of the low loss in the isolation transformer (Tfmrl) 12 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.

To determine power alone applied to the cathode heater 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 VhlIh.

DC Cathode Heating Supply The basic arrangement of a DC cathode heating supply system is shown in Figure 4 with corresponding waveforms illustrated in Figure 5.

Referring to Figure 4, 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 42 i. The behaviour of the transformer (Tfmri) 42 is now importantly different from the transformer i 2 used in the previously described AC heater supply. Transformer leakage inductances (Lssi and Lss2) have currents with DC components in them while only the primary leakage inductance (Lspi) has an AC component of current flowing in it.

iO It is unavoidable that the secondary leakage inductances (Lssi and Lss2) are closely coupled due to the proximity of the secondary windings 42 i, and relatively large because of the needs of high voltage isolation. Suitable construction methods are described herein in a description of transformer construction and design.

The addition of the rectifiers TRi and TR2 could, if avoiding steps were not taken, iS introduce significant loss. With a supply of 12V at i2OA for the known BM7SL magnetron from e2v technologies plc, for example, a drop of up to iV or more in the diodes TRi and TR2 would represent a significant loss of power and render power or resistance measurement at the transformer primary winding 422 less effective.

To overcome the rectifier loss problem, synchronous rectification with MOSFETs is used. This implementation optimises the drive to the FETs to take into account the unusually high leakage inductances in the secondary side of the isolation transformer (Tfrnri) 42.

Referring to Figure 5, the inverter waveform S i is shown in (a). The transformer drive waveform 52 via the blocking capacitor Cb i42 is shown as (b). The droop A\T 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. During a time T4 the current will fall in one rectifier while it rises in the other, because of the leakage inductances and the coupling between them. Thus both rectifiers TR1 and TR2 conduct for the period T4 so that each rectifier must be triggered on during the period T4. Thus an overlap in the conduction of the rectifier TRi and TR2 is required. The overlap time T4 is determined by the value of the voltage droop AV on Cb and the inductance Lssl and Lss2 and their degree of coupling. The resultant primary current waveform 53 is shown in (c) and the required drives 54, 55 for the synchronous rectifiers are shown in (d) and (e). The rectification action produces a voltage 56 at the choke input (f). 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. By careful selection 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 TR1, TR2 is shown in Figure 7. Referring to Figure 4, power to operate the drive circuit is provided by a further secondary winding (Tfrnr3) 424. Referring again to Figure 7, the further secondary winding 424 feeds a rectifier BR1 in parallel with a filter capacitor Cl 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 TR1a and TR1b 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 1C2 and 1C3, such as MAX4422 that provide a gate drive to the FETs via D4, Ri, R2 and D6, R5, R6. An AND gate ICla and ICib such as a 78HC08 controls the driver circuits and prevents a signal being applied to the driver chips 1C2 and 1C3 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 CT1 and CT2 monitor a current to each synchronous rectifier TR1 and TR2. Rectifying burdens D3, RiO and D5, Ru are used on each current monitor so that the current monitors output signals to an AND Gate (IC 1 a or b) only when current is flowing in a given rectifier TR1 and TR2.

During start up, the synchronous rectifiers TR1 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 TR1 and TR2 from occurring. Once the LT supply rails (+12V and +5V) are established, the output resistances of the driver chips 1C2 and 1C3 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 1V during conduction in TR1 and TR2. When 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.

Transformer Construction AC Heating Supply When operating any SMPS at higher frequencies, volts/turn of the transformer increase compared with operation at lower frequencies. Eventually by suitable design selection a low voltage winding may be reduced to a single turn and this characteristic is exploited in the design of a transformer of the present invention.

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. At high frequencies due to skin and proximity effects any current tends to flow at a surface of a conductor with a circular cross-section. Thus a tubular conductor with a wall thickness of approximately the skin depth utilises the area of copper very effectively. At 15 kHz 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 121 and primary winding 122 in very high. However, it is also desirable that the transformer be compact. As an example for the BM75L magnetron available from e2v tecimologies plc, a working voltage of up to 25 kV is desirable. For high voltage design 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. Thus a circular cross-section single conductor constitutes an ideal form of winding for a system involving high voltage insulation requirements.

Referring to Figures 8 and 9, 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. By suitable selection of straight pipe 87 and 90° elbows 89 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. After moulding, the plastic pipe and elbows used for the tool can be left in place and form an additional part of the electrical insulation circuit.

Rather than use a single core, M narrow cores are used, where M=2 in the embodiment illustrated in Figures 1 and 8 to 13. These pass around the 90° elbows 89 more readily than would a single longer core and their primary windings 122 are then connected in series. They can be held in place by the use of, for example, hot melt adhesive 85.

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. Furthermore the core windings 122 can be a single layer winding of suitably sized wire. For an example, with the BM75L, cores of magnetic area 162 mm2 and magnetic length 225 mm prove a suitable choice. As can be seen 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. For example, 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.

DC Heating Supply Rectifier and Transformer Construction.

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:1 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: 1:1 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.

The overall assembly of the synchronous rectifiers system TR1, TR2 is in the screened metal box 109. 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. The connection leads 1123, 1125 can be solid rods as with DC the full conductor cross-section will be utilised. The core assembles 1021, provided they are a sufficient length to obtain the desired inductance, can be longer if wished to reach to the magnetron terminals. This expedient is most useful in finalising a particular design.

A lid 1333 of the screened metal box 109 forms one of the connections between the transformer (Tfmrisi and Trfmis2) 42 and the second smoothing choke L2. Connections 1335, 1237 between TRin drains and Tfmrlsi and TR2n drains and Ttfrmis2 respectively are made with flat copper strips. A further copper strip 1339 makes a connection between Li and Trin, Tr2n sources and Li. Connections for high current are made on the Tfmri 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 CT1 and CT2 1243, 1245 are mounted around the main tubes that feed sources of Trin 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.

To power the control PCB 1241 a single turn winding 424 is fed through the centre of one of the secondary tubes 421 of Tfmri. This turn 424 enters and exits the tube at small (1 mm) central drillings in the fixing bushes 1151 on one of the secondary tubes 421.

Although the 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/GB2009/05 0942. It will be understood that with a 3-phase power supply three transformers may be used, one for each phase.

Power and Resistance Control Whether AC or DC heating is used, the transformer and rectifier are realised in a way that incurs very little loss. As a consequence it is possible to measure voltage and current at the transformer primary 122, 422 and from these measurements calculate the load power and/or the secondary resistance. These calculations may be implemented by either analogue or digital means.

Circuitry for heater power and/or resistance measurement using the AC heater supply of Figure 1 is shown in greater detail in Figure 3.

Referring to Figures 1 and 3, outputs of the current monitor 141 are connected to inputs of a differentiator 142 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 SH1 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 142 is connected to respective control inputs of the first and second sample and hold amplifiers SH1, SH2. Second outputs of the first and second full wave rectifiers 144, 145 respectively and of the first and second sample and hold amplifiers SH1, 5H2 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.

As stated earlier, and as shown in Figure 2, the primary current Ip through the isolation transformer 12 is of quasi-sine waveform 23. A point on the primary waveform 23 where di/dt = zero is detected by using the differentiator circuit 142. An output of the differentiator circuit enables the two sample and hold amplifiers SH 1, 5H2 that acquire voltage monitor output from the monitor winding 123 and current monitor output from the current monitor 141 respectively when dildt = zero. When di/dt is zero the voltage drop in the inductance Ls and Lt will be zero (since inductor voltage = L*di/dt) so current and voltage values will be the values applied to the load Rh of the cathode heater 11 multiplied by the transformer ratio N2.

By using an analogue multiplier chip 143 such as an AD534 a voltage proportional to the power in Rh (i.e. Va*Vb) can be obtained. Conversely, 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. By this expedient only +ve numbers are required to be processed by the multiplier and/or divider 143 thus making implementation simpler.

For the DC heater a different method is implemented and the measurement system of Figure 4 is illustrated in more detail in Figure 6.

Referring to Figures 6 and 4, 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.

As has been stated, 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. Thus by rectifying and smoothing via integrators 446, 447 outputs from the current monitor 441 and single turn voltage monitor 423, the power can be obtained by the product Va *Vb or the resistance by the division Yb/Va.

The main difference between the AC and DC heater systems is that the sample and hold amplifiers SH1 and SH2 of the AC supply circuit need to be reconfigured as integrators 446, 447 in the DC supply circuit.

Digital Controller Implementation For both AC and DC variants of the heater power supply 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. Once the necessary measurements have been digitised the load resistance can be calculated using a method appropriate to a connected load variant. For a DC variant the calculation is simply Rload = VhlIh. For an AC version the voltage could be sampled at di/dt = 0. The calculation of the resistance is the same as in the DC version.

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 P1 controller constants to achieve a required closed loop response.

DSP Digital Controller Implementation The two nested P1 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 CResjstce 122. The output of the resistance controller 122 is a power demand Pdemand, from which the demand current is calculated by demand = Pdemand/Vload. The demand current demand is then used as a demand signal for the second, nested P1 control loop 123 that controls the load current, Current. 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 P1 control loops is well understood and not discussed here.

Claims (16)

1. CLAIMS1. 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.
2. 2. An isolation transformer as claimed in claim 1, wherein the first U-shaped primary winding is tubular.
3. 3. An isolation transformer as claimed in claims 1 or 2, further comprising 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.
4. 4. An isolation transformer as claimed in any of the preceding claims, wherein the primary windings comprise a plurality of cores with windings connected in series.
5. 5. An isolation transformer as claimed in claim 4 wherein the cores are held in place by adhesive.
6. 6. An isolation transformer as claimed in any of the preceding claims for a DC cathode heater supply further comprising: 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.
7. 7. An isolation transformer as claimed in claim 6, further comprising two current monitors arranged to monitor current to each synchronous rectifier respectively.
8. 8. An isolation transformer as claimed in claims 6 or 7, further comprising a single turn winding located within one of the tubular secondary windings and arranged to power the control means.
9. 9. A method of manufacturing 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.
10. 10. A method as claimed in claim 9 wherein 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.
11. 11. A method as claimed in claim 10, wherein 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.
12. 12. A method as claimed in claim 11, comprising 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.
13. 13. A method as claimed in any of claims 9 to 12 of manufacturing an isolation transformer for a DC cathode heater supply for a microwave source further comprising: 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.
14. 14. A method as claimed in claim 13, further comprising providing a single turn winding located within one of the tubular secondary windings and arranged to power the control means.
15. 15. An isolation transformer as described herein with reference to and as shown in the accompanying Figures.
16. 16. A method of manufacturing an isolation transformer as described herein with reference to and as shown in the accompanying Figures