GB2173062A

GB2173062A - Magnetron heater power supply

Info

Publication number: GB2173062A
Application number: GB08508328A
Authority: GB
Inventors: Robert Longmuir Sinclair
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1985-03-29
Filing date: 1985-03-29
Publication date: 1986-10-01
Also published as: DE3609470A1; SE8601358L; IT1190245B; GB8508328D0; FR2579822A1; FR2579822B1; IT8647829A0; SE8601358D0

Abstract

The power supply includes a source 23 of alternating voltage connected to the heater 22 of the magnetron 20 and regulating means 24 connected in series with the source 23. The regulating means is arranged to control the number of half-cycles of the alternating voltage of one polarity which are applied to the heater 22 during a predetermined time interval, whilst applying all half- cycles of the opposite polarity. Control means 25 apply a control signal to the regulating means 24 in accordance with the required duty ratio of the magnetron. A voltage correction networks 26, 27 corrects, for the fact that the power in the half-cycle waveform is not half of the power in the full waveform. <IMAGE>

Description

SPECIFICATION Magnetron heater power supply This invention relates to a magnetron heater power supply, and in particular to such a supply that will enable a magnetron to be operated over a continuously variable range of duty ratios.

In common with other thermionic valves a magnetron requires a heater power supply to raise the temperature of the cathode to enable the emisson of electrons to take place. In order to enable the magnetron to operate in a reliable manner it is necessary to maintain the cathode temperature within fairly fine limits. A particular problem with a magnetron is caused by secondary emission of electrons which bombard the cathode and cause its temperature to rise. This effect becomes more pro nounced as the duty ratio of the magnetron increases, and hence a magnetron operating at a high duty ratio requires less power to be supplied to the cathode than one operating at a lower duty ratio.

One common use of the magnetron is in radar systems, and such systems are commonly arranged to operate in a number of different modes. Each of these will result in a different duty ratio and hence requires a different heater supply power. This power is commonly adjusted by varying the voltage by means of a tapped transformer. The disadvantage of this is that only a limited number of taps is possible for practical and economic reasons.

Further problems presented in the design of magnetron heater power supplies arise from the fact that the anode of the magnetron is usually at earth potential, with the result that the components of the power supply may be at a high voltage of, say, 15KV or more. In addition, the thermal inertia of the cathode, that is the time taken for temperature changes to take effect, also has to be taken into account.

It is an object of the invention to provide a magnetron heater power supply which will accommodate a wide range of duty ratios.

According to the present invention there is provided a magnetron heater power supply which includes a source of alternating voltage connected to the heater of the magnetron, regulating means connected in series with the said source and operable to control the number of half-cycles of the alternating voltage of one polarity which are applied to the heater during a predetermined time interval and to apply all half-cycles of the opposite polarity, and control means operable to apply a control signal to the regulating means in accordance with the required duty ratio of the magnetron.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates the operation of the invention; Figure 2 is a circuit diagram of a first embodiment of the invention; Figure 3 is a circuit diagram of part of Fig.

2; and Figure 4 is a circuit diagram of a second embodiment of the invention.

Referring now to Fig. 1, this shows the voltage waveform which is applied to the heater of the magetron. This is sinusoidal but, as shown, some of the negative half-cyces may be removed to reduce the power supplied to the heater. The range of power control is from the application of the complete voltage waveform, used when the magnetron is on stand-by, to the application of only the positive half-cycles of the voltage waveform, used when the magnetron is operating at its maximum duty cycle. Between these two limits there is a very wide variation of power possible, by the removal of any required number of negative half-cycles of the waveform.

Due to its thermal inertia, the cathode takes some time to change temperature as the power supplied to it varies. This time may be of the order of several seconds. If the frequency of the heater supply voltage is 400Hz, as is common, then 1200 cycles of the waveform occur in a 3-second period. It is most convenient to block the negative half-cycles for a measured time during the time period T, as shown in Fig. 1.

Fig. 2 shows a circuit for applying the invention. Referring to the drawing a magnetron 20 has a cathode 21 and a cathode heater 22. An alternating current source 23 delivers the appropriate voltage, say 6.3 volts at 400Hz to supply the heater and one side of this source is connected to the heater 22. The other side of the source is connected to the regulating means, comprising a switching FET 24 having its source connected to the heater 22 and its drain connected through a voltage correction network to the supply 23. The gate of the FET is connected to the control means 25. The voltage correction network comprises a resistor 26 with a diode 27 connected in parallel with it. The purpose of this network, which may not be necessary in every case, is to correct for the fact that the power in the half-cycle waveform is not half of the power in the full waveform.The diode 27 is poled so that current on the selected negative halfcycles of the waveform passes through it, whereas current on all positive half-cycles passes through the resistor 26.

The control means 25 connected to the gate of the FET is operable to apply a control voltage to the gate to turn the FET on and off at the appropriate times. Hence the control signal produced by the control means 25 will conveniently be a rectangular waveform having a, variable mark/space ratio. The control means produces the appropriate mark/space ratio in accordance with the designated duty ratio of the magnetron. The repetition rate of the control is dependent upon the thermal inertia of the heater, and is a predetermined time interval for any particular magnetron.

In operation, the FET will always pass halfcycles of the supply voltage of one polarity, say the positive half-cycles, even if the control signal to the gate effectively switches the FET off. The control signal only determines the times at which the FET will also pass halfcycles of the other polarity. Hence by switching the FET on and off the power supplied to the heater of the magnetron may be varied from half power to full power as required.

Although the control signal to the FET is required to be a simple rectangular waveform, the fact that the heater of the magnetron is at a very high voltage presents a problem in applying the signal. If a simple dc voltage was applied then the entire control circuitry would have to be insulated to the heater voltage.

Fig. 3 shows one way in which the problem may be solved using a pulse train and an isolating transformer. The control means comprises a pulse generator 30 to which the control signal is applied and which generator pulses at a frequency of, say, 60KHz. These pulses are switched on and off by the control signal, and are applied through an isolating pulse transformer 31 to a pulse detecting circuit 32. The output of the pulse detecting circuit is a dc voltage which is applied to the gate of the FET 24. The pulse transformer 31 provides the necessary isolation between the FET and the source of the control signal.

Other arrangements for applying the control voltage to the gate of the FET may be used if the necessary electrical isolation can be provided. Opto-electronic or magnetic coupling and isolating arrangements may be used, for example.

In place of the FET with its inherent diode coupling between source and drain it would be possible to use a transistor with a diode connected between collector and emitter so as to pass half-cycles of the supply waveform when the transistor is switched off.

The power supply control system described above is simple and effective, and does not require any complex timing arrangements.

Known devices such as thyristor-controlled circuits required zero-crossing detection arrangements which would make them costly for the application in question. Accurate control of the magnetron heater power supply is made possible to an extent which has not been possible before, allowing a magnetron to be used in a wide range of modes.

All references to positive and negative halfcycles of the alternating heater supply voltage may, of course, be reversed, assuming that the polarities of any diodes and transistors are changed accordingly.

Claims

1. A magnetron heater power supply which includes a source of alternating voltage connected to the heater of the magnetron, regulating means connected in series with the said source and operable to control the number of half-cycles of the alternating voltage of one polarity which are applied to the heater during a predetermined time interval and to apply all half-cycles of the opposite polarity, and control means operable to apply a control signal to the regulating means in accordance with the required duty ratio of the magnetron.

2. A power supply as claimed in Claim 1 in which the regulating means include a fieldeffect transistor.

3. A power supply as claimed in either of Claims 1 or 2 in which the control means is operable to apply a dc voltage to the regulating means.

4. A power supply as claimed in Claim 3 in which the control means includes means for generating a high-frequency pulse train under the control of the control signal and a pulse detecting circuit for converting the pulse train to a dc voltage.

5. A power supply as claimed in either of Claims 3 or 4 in which the control means includes voltage isolating means.

6. A power supply as claimed in Claim 5 in which the voltage isolating means includes a transformer.

7. A power supply as claimed in any one of the preceding claims which includes a voltage correction network connected in series with the source and the regulating means.

8. A magnetron heater power supply substantially as herein described with reference to the accompanying drawings.