ADJUSTABLE STABLE POWER SUPPLY FOR CONTINUOUS WAVE MAGNETRON
United States Patent 3723806
A power supply circuit for a continuous wave magnetron provides adjustable stable power levels. A first direct voltage source is connected across the magnetron and a second direct voltage source from a common alternating voltage source is connected across the magnetron coil in series with a direct current amplifier. Current feedback to the input of the amplifier is derived from the magnetron by an adjustable voltage divider connection to the first voltage source. Another portion of the voltage divider provides a third direct voltage connection between the second source and amplifier input, the amplifier input current being the sum of current from the third source and adjustable current from the magnetron.
US Patent References:
Control system for regulating the current in a magnetron tube
Bluk et al. - January 1967 - 3302060
Amplitude of h.f. oscillator stabilized by rectified output of l.f. osc.
Alford - September 1967 - 3344364
Control system for regulating the current in a magnetron tube
Bluk et al. - January 1967 - 3302060
Amplitude of h.f. oscillator stabilized by rectified output of l.f. osc.
Alford - September 1967 - 3344364
Inventors:
Poussereau, Maurice Francois (Conflans Saint Honorine, FR)
Denis, Marcel (Paris, FR)
Denis, Marcel (Paris, FR)
Application Number:
05/239529
Publication Date:
03/27/1973
Filing Date:
03/30/1972
View Patent Images:
Export Citation:
Assignee:
International Standard Electric Corporation (New York, NY)
Primary Class:
Other Classes:
331/86, 330/47
International Classes:
H02M7/06; H03B9/10; H05B6/68; H03B9/00; H01J25/50
Field of Search:
315/39.51 328/230 331/86 330/47
Primary Examiner:
Kominski, John
Claims:
What is claimed is
1. A power supply circuit for a continuous wave magnetron capable of delivering microwave power that is adjustable and stable for each adjustment, comprising:
2. The power supply circuit according to claim 1 including a resistor connected between the amplifier input and said third source, said voltage dividing connection including an adjustable resistor in series with said third low voltage source between the positive terminal of said first high voltage source and ground, said adjustable resistor being selected so that the negative feedback resistance is very much higher than the sum of the amplifier input resistance and said resistor from said amplifier input to the third source.
3. The power supply circuit according to claim 2 wherein said first, second and third source voltages are unstabilized and are supplied by a common primary AC source, said first and second sources including respective rectifiers and filters, and a source of counter electromotive force that is stable and has a very low internal resistance connected in series with said third low voltage source.
4. The power supply circuit according to claim 3 wherein said amplifier is a common-emitter NPN type power transistor, said voltage of said third source being produced by an adjustable voltage dividing connection from the voltage of said second source, said source of counter electromotive force includes a Zener diode, said adjustable resistor being an adjustable potentiometer connected in series between said third voltage source and ground, and the potentiometer slide contact being connected to the positive terminal of said first source.
5. The power supply circuit according to claim 4 including a high value capacitor connected between said transistor base and ground.
6. The power supply circuit according to claim 4 including a power diode connected in parallel with said coil, the backward diode direction being that of the coil current.
1. A power supply circuit for a continuous wave magnetron capable of delivering microwave power that is adjustable and stable for each adjustment, comprising:
2. The power supply circuit according to claim 1 including a resistor connected between the amplifier input and said third source, said voltage dividing connection including an adjustable resistor in series with said third low voltage source between the positive terminal of said first high voltage source and ground, said adjustable resistor being selected so that the negative feedback resistance is very much higher than the sum of the amplifier input resistance and said resistor from said amplifier input to the third source.
3. The power supply circuit according to claim 2 wherein said first, second and third source voltages are unstabilized and are supplied by a common primary AC source, said first and second sources including respective rectifiers and filters, and a source of counter electromotive force that is stable and has a very low internal resistance connected in series with said third low voltage source.
4. The power supply circuit according to claim 3 wherein said amplifier is a common-emitter NPN type power transistor, said voltage of said third source being produced by an adjustable voltage dividing connection from the voltage of said second source, said source of counter electromotive force includes a Zener diode, said adjustable resistor being an adjustable potentiometer connected in series between said third voltage source and ground, and the potentiometer slide contact being connected to the positive terminal of said first source.
5. The power supply circuit according to claim 4 including a high value capacitor connected between said transistor base and ground.
6. The power supply circuit according to claim 4 including a power diode connected in parallel with said coil, the backward diode direction being that of the coil current.
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to improvements in power supplies for continuous-wave magnetrons and particularly to a device providing an adjustable stable magnetron power supply.
2. Description of the Prior Art
In a previous U.S. Pat. application Ser. No. 179,384, filed Sept. 10, 1971 and assigned to the same assignee as the instant application, there is described a power supply for a continuous-wave magnetron wherein the control induction field is produced by means of a coil through which passes the sum I B of magnetron current intensity I and of current intensity I a produced by an additional source of voltage V a supplying a series resistor R and the coil resistor R B . Magnetron high voltage V remaining constant, applied power P = V I may be adjusted continuously or by steps by varying V a and/or R.
The microwave power supplied to the load is equal to P m with: P m = kP = kVI; where k is the magnetron efficiency, and from experience it appears that, if the load is rather well matched, k is substantially constant throughout the useful range of the tube characteristics.
Generally, the magnetron characteristics may, in the normal utilization range, be represented by the linear relation (1)
I = (V/r) - a I B ( 1)
where r is the internal magnetron resistance and a is an amplification coefficient of high value. In other words the sensitivity I/I B is equal to a and adjustments are unstable when the supply is directly applied to the coil by providing the current I B instead of generating I B by adding I and I a as proposed in the above mentioned prior application.
With the combined supply for the coil, the characteristics are represented by the linear relation:
(2) I = (V/aR - I a ; and here sensitivity is written: I/I a = 1;
If V a and R are used (2) becomes
I = V/ar [1 + (R B /R)] - (V a /R) (3)
The produced microwave power is
p m = k VI = k [ V 2 /ar 1 + (R B /R - (V a V/R)] (4)
A simple calculation shows that, if V a and V are obtained by rectifying a single AC voltage source U, the relative microwave power variation d P m/P m is equal to 2 dU/U, for each adjustment of the power P m .
An important purpose of the invention described in the above prior application is to obtain a satisfactory stabilization of P m by suitably amplifying instabilities of V a so as to compensate for instabilities of V.
To do so, the additional source of voltage V a is replaced by a source of voltage V a ' that supplies a non-linear circuit comprising serially a counter electromotive force (c.e.m.f.) element E o , that is very stable and has a very low internal resistance, the resistor R and the coil of resistance R B . V a 'is given by the relation:
V a ' = V a + E o
E o is easily obtained by using a Zener diode.
Still according to the above application, a stabilization effect is obtained for a fixed high-voltage V when, for each value of I, and therefore of V a ' and/or R, E o , R and I are associated by the relation (5):
E o = 2 RI (5)
By suitably associating a rheostat providing R with a variable autotransformer supplying V a ', it is possible to obtain the preceding relation for each desired value of P m and in a continuous manner.
Under normal conditions of operation for a power supply of the above type, there are two drawbacks that have to be overcome. First, it appears that the current of intensity I B through the coil heats it and, for a relatively long time, the coil resistance increases to stabilize at a heated coil value higher by 20-30 percent than the cold coil value. As a result, as indicated by the formula (4), there is a slow drift for P m , that is sometimes troublesome; the lower the value of R, the higher the value of P m , and the greater the power drift.
Second, there is a drawback due to transient phenomena appearing when turning the apparatus on. At that time, it is necessary to be certain that the voltage V a ' has a value such that the current I a through the coil is higher than the current (I B ) o that blocks the magnetron. In that case, it is possible to apply the high-voltage V and to manually reduce V a ' until the occurrence of the desired current I.
In a normal installation, such precautions represent constraints which users tend to neglect. But, if when applying high-voltage V which is assumed to occur instantaneously, voltage V a ' is not sufficient and provides a current I a substantially lower than coil current (I B ) o that blocks the magnetron, there results a transient mode with rapid increase of current I. Since the coil self-inductance is not negligible, there occurs at the coil terminals a c.e.m.f. L(dI/dt) that is subtracted from the auxiliary voltage V a ' and results in a reduction of the auxiliary current I a which may change its direction negatively, and thus further increase I. The phenomenon is unstable and the increase of I is limited only by safety switches or non-linear characteristics of the involved components such as the self-inductance L, Zener diode, magnetron, and rectifiers delivering voltage V a '. The current I increases with a time-constant L/R. This troublesome phenomenon could be avoided only if the increase of current I, after having applied high-voltage V, would be performed with a time-constant at least equal to L/R.
In any case, such exponentially increasing current is the most damaging to the equipment or, in less unfavorable cases, makes it difficult to operate.
SUMMARY OF THE INVENTION
Therefore, an object of this invention is to provide an additional coil current source of adjustable current I B , such that the value I B is substantially independent of the coil characteristics R B and L.
Another object of the invention is to provide an additional source in such a manner that, when high-voltage power is turned on, no transitory unstable phenomenon occurs.
A further object of this invention is to provide an adjustable power supply wherein the sensitivity I/I B is substantially lower than a.
A still further object of this invention is to provide an additional coil current source in such a manner that, for each adjustment of the coil current I B , the magnetron current I and the delivered microwave power P m are not dependent upon instabilities of the magnetron high-voltage V and on an additional source of voltage.
According to a feature of this invention, coil current intensity I B is provided at the output of a current amplifier, the dynamic output resistance of which is substantially higher than the coil resistor R B .
According to another feature of this invention, amplifier input current is equal to the sum of an adjustable control current I e and of a part q of magnetron current I, I e being lower than coil current I B , so that when high-voltage V is applied to the magnetron, it induces current I. From that feature and from the above mentioned formula (1), it is seen that magnetron current is given by the formula:
I = [ V/r (1 + a q)] - (a/1 + a q) I e ( 6)
Thus, the device sensitivity may be written as:
dI/d(I e ) = (a/1 + a q)
If a q >> 1, sensitivity is equal to 1/q, that is independent of a and, consequently, of the characteristics of the magnetron. By adequately selecting q, it is possible to have for the sensitivity any desired value, and particularly, the value 1, as in the above mentioned application. However, excessive transient starting currents are now avoided due to feedback action proportional to qI, which occurs without substantial delay.
According to another feature of this invention, input current I e is provided by a low voltage source V e serially mounted with a negative feedback voltage equal to r 1 I, r 1 being a low value resistor. In such conditions, magnetron current I, at constant high-voltage, and consequently microwave power P m , may be adjusted by varying V e and/or r 1 . The sensitivity I/(I e ) is equal to (r e + r 2 / r 1 ), r 2 and r e being the amplifier input resistor and the sum of internal resistance of low-voltage source V e and of other resistors possibly serially connected with r e , respectively.
According to another feature of this invention, a stable c.e.m.f. source e o with very low internal resistance is serially connected with source of low voltage V e ', such that V e ' = V e + e o , e o being related to r 1 and I by the formula: e o = 2 r 1 I. In such conditions, V e being constant, microwave power P m is adjusted between desired limits by varying r 1 and the power value is given by the very simple formula:
P m = k VI = k V (e o /2 r 1
When the relation e o = 2 r 1 I is fulfilled and when magnetron supply high-voltage V, low-voltage V B of amplifier output DC supply and low-voltage V e ' of amplifier input DC supply are provided from a rectified single AC primary source, power P m remains stable for any adjustment. The major advantage of such an arrangement is to provide an excellent control of delivered microwave power by using only a non-stabilized high-voltage source and two low-voltage sources, which is much less costly than power supplies used in the prior art.
According to another feature of this invention, the amplifier is a common-emitter power transistor. In such conditions, it is possible to use only one low-voltage source V B , with V e ' being obtained from V B through an adjustable potentiometer divider.
Other purposes and features of the present invention will appear more clearly from the following description in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic diagram of a power supply circuit for a continuous-wave magnetron according to this invention, and
FIG. 2 shows a specific embodiment of a power supply using a power transistor as an amplifier.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in FIG. 1, a magnetron 1 is provided with DC high-voltage V from source 2 which is assumed to be made of a first AC current rectifier of a standard type and of filter circuits, not shown. The anode of the magnetron 1 is connected to ground. Control induction field for the magnetron is provided by coil 3 through which current I B is passing; 3 has a resistor value equal to R B .
The bottom terminal of 3 is connected to a positive terminal (+) of a source 4 of DC low-voltage V B which is assumed to be made of a second AC current rectifier and filter circuits, not shown. The top terminal of 3 is connected to the output of a DC amplifier 5 having an output resistance equal to R i .
Input circuit of amplifier 5 comprises serially connected from ground: a resistor 6 of value r 1 through which current I of magnetron 1 passes, the positive terminal (+) of source 2 being connected to the top terminal of 6, a source 7 of adjustable DC low-voltage V e ' which is assumed to be made of a third AC current rectifier and of filter circuits not shown, a source 8 of c.e.m.f. e o which is very stable and has a very low internal resistance, a resistor 9 of value r 2 , and the input resistance r e of amplifier 5.
It will be assumed that sources 2, 4 and 7 have low internal resistances compared to those of the circuits to which they are supplying current; and also, it will be assumed that r 1 << r 2 + r e .
A simple calculation shows that input current is:
I e = (r 1 I +V e ' - e o /r e + r 2 ) (7)
Considering the formula (1) of magnetron characteristics:
I = (V/r) - a I B
if the current gain of amplifier 5 is unity and if output resistor R i of 5 is very high, by combining (1) and (7) the following formula is obtained for current I:
I = [ V (r e + r 2 )/r (r e + r 2 + a r 1 ] - a (V e ' - e o /r e + r 2 + a r 1 ) (8)
From (8), it appears that when there is no feedback of current I to input of amplifier 5 (r 1 = 0), the transconductance I/V e ' is equal to (a/r e + r 2 ), while with current feedback, it is
(a/r e + r 2 + a r 1 )
and, thus, is reduced by the ratio (r e + r 2 /r e + r 2 + a r 1 ).
If a r 1 is very much higher than r e + r 2 , the transconductance becomes approximately 1/r 1 and, consequently, is independent of a; if the sources are stable, current I and microwave power P m provided by the magnetron are independent of possible instabilities of static magnetron or amplifier characteristics, such as for example resulting from temperature variations.
From 8), microwave power delivered by the magnetron is:
P m = kP = kVI = (kV/r e + r 2 + a r 1 ) [V (r e + r 2 )/r] - a (V e ' - e o ) (9)
For constant high-voltage V, P m varies within the desired range by adjusting V e ' and/or r 1 .
In practice, additional sources 4 and 7 will be first turned on, before applying high-voltage V to source 2. In such conditions, since there is no magnetron current feedback to the input of amplifier 5, current I B in coil 3 is:
I B = (V e ' - e o /r e + r 2 )
Such an intensity is then lower then that passing through coil 3, with the same voltage V e ', when magnetron 1 is under operation; but, in the present power supply, there is no risk of overcurrent when magnetron high-voltage is applied and whatever the value of V e is at that time, since, due to negative feedback through resistor r 1 , any abnormal magnetron current increase is immediately throttled. It is to be noted that this result is completely independent of the presence or not of the source 8 of c.e.m.f. e o , the purpose of which will now be examined.
Source 8 permits stabilization of microwave power P m when DC voltages of sources 4 and 7 are subjected to relative equal variations, particularly when voltages V, V B and V e ' are produced by rectifying a single source of AC current after suitable filtering. For each adjustment of P m and therefore, of V e ' and/or r 1 , as a result of combination of formulas (8) and (9), P m remains constant if V e ' and V are subjected to the same relative instabilities: d V e '/V e ' = dV/V, when I, [ r 1 + (r e + r 2 /a )] and e o are bound by the formula
2 I [ r 1 + (r e + r 2 /a )] = e o .
Thus, to adjust power P m , it is necessary only to vary the value of r 1 in accordance with the formula (10).
It is to be noted that voltage V B of source 4 as well as variations thereof do not interfere with the problem of stabilizing P m . That results from the fact that dynamic output resistance R i of amplifier 5 is assumed to be high when compared to R b , or, in other words, amplifier 5 operates as a constant current source.
It is of interest to use the power supply with values of r 1 such as: a r 1 >> r e + r 2 .
In such conditions, the formula (10) becomes:
2 I r 1 = e o (10')
and electrical power applied to the magnetron is stated in a very simple manner by the formula:
P = VI = Ve o /2r 1 (with P m = k (Ve o /a r 1 ), (11)
P and P m being no longer depending on stable elements (e o and r 1 ).
From a combination of (8) and (10), still assuming that a r 1 >> r e + r 2 , the value of voltage V e ' of source 7 is substantially constant and equal to:
V e = (e o /2 ) + [V (r e + r 2 )/ar]
Considering again the formula (1), V/ar measures the coil current (I B ) o that precludes magnetron 1 from operating when voltage of source 2 is equal to V; thus
V e ' = (e o /2 ) + (I B ) o x (r e + r 2 ) (12)
The amplifier may include a suitable silicon or germanium power transistor of an NPN type.
The c.e.m.f. e o of the source 8 may be provided by a Zener diode.
An example using practical figures will make the device more readily understandable. The following gives various values for a magnetron power supply operating in continuous mode and able to provide, in addition to the maximum microwave power (P m ) o , any value between (P m ) o and 1/4 (P m ) o , each value being adjusted and regulated according to this invention.
The magnetron high-voltage is 5 kV and current intensity providing maximum power (P m ) o is 1 A. With an efficiency ratio k of 70 percent, (P m ) o = 3.5 kW. The four power levels are obtained for current values 1, 0.75, 0.5 and 0.25 A. The value of (I B ) o is 1.3 A and magnetron figure a is within the range of 35; in other words, current I B corresponding to current I equal to 1 A is 1.27 A. Amplifier current gain is about 30. Power-transistor emitter-base resistance r e for a base current of 40 mA is lower than 15 ohms. Since this resistance varies in response to temperature changes, it is useful to mask it in the sum (r e + r 2 ) by having the value of r 2 substantially higher, i.e. 100 ohms.
If the current I is of 1 A and value of r 1 of 1.5 ohms, condition a r 1 >> r e + r 2 is amply fulfilled. It is still better fulfilled when r 1 is increased to provide lower currents I.
The formula (10') gives e o = 3 V (a value that is readily provided by a Zener diode) and the formula (12) gives: V e ' = 1.5 + 1.3(115/30) ≠≠ 6.5 V.
Resistor 6, which in this case must continuously vary from 1.5 to 6 ohms, is made of a rheostat.
FIG. 2 shows a detailed circuit embodiment of the invention using a common-emitter power-transistor amplifier of the NPN type having the same reference numeral 5 as in the FIG. 1. Other like reference numerals will be used for the same components already shown in FIG. 1.
Rectifier 10 followed by filter stage 11 provides voltage V B that is applied through resistor R B of coil 3 to the collector of transistor 5.
Voltage V e ' is produced by means of a voltage divider 12, having a relatively low resistance, serially connected with resistor of rheostat 6, to provide resistor values r 1 suitable for current feedback and stabilization according to the invention. Voltage V e ' from slide contact of 12 is applied through Zener diode 8 and resistor 9 to the base of transistor 5. Capacitor 13, connected between the transistor base and ground, removes AC signals which remain at the output of filter 11.
Divider 12 has a division ratio that is slightly adjustable so as to adjust V e ' at a suitable constant value, either at an initial adjustment or, from time to time, during maintenance operations. Again, it is noted that resistor r 2 is equal to the sum of resistor 9, the divider resistor between the top point and rheostat, and the rheostat resistor.
Power diode 16 is connected in parallel with coil 3 in a backward direction with respect to I B . Its purpose is to avoid damaging transistor 5 in the case of a power supply break-down. If such an event occurs, the magnetic energy stored in coil 3 is dissipated in 16, so as to protect transistor 5.
High-voltage V of magnetron 1 is provided by rectifier 14 followed by filter stage 15. Rectifiers 10 and 14 are supplied by a single primary AC source, not shown, that, according to the invention may be a non-stabilized source.
While the principles of this invention have been described above in relation to several embodiments, it is to be clearly understood that this has been only made by way of example and does not limit the scope of the invention, as set forth in the appended claims.
1. Field of the Invention
This invention relates to improvements in power supplies for continuous-wave magnetrons and particularly to a device providing an adjustable stable magnetron power supply.
2. Description of the Prior Art
In a previous U.S. Pat. application Ser. No. 179,384, filed Sept. 10, 1971 and assigned to the same assignee as the instant application, there is described a power supply for a continuous-wave magnetron wherein the control induction field is produced by means of a coil through which passes the sum I B of magnetron current intensity I and of current intensity I a produced by an additional source of voltage V a supplying a series resistor R and the coil resistor R B . Magnetron high voltage V remaining constant, applied power P = V I may be adjusted continuously or by steps by varying V a and/or R.
The microwave power supplied to the load is equal to P m with: P m = kP = kVI; where k is the magnetron efficiency, and from experience it appears that, if the load is rather well matched, k is substantially constant throughout the useful range of the tube characteristics.
Generally, the magnetron characteristics may, in the normal utilization range, be represented by the linear relation (1)
I = (V/r) - a I B ( 1)
where r is the internal magnetron resistance and a is an amplification coefficient of high value. In other words the sensitivity I/I B is equal to a and adjustments are unstable when the supply is directly applied to the coil by providing the current I B instead of generating I B by adding I and I a as proposed in the above mentioned prior application.
With the combined supply for the coil, the characteristics are represented by the linear relation:
(2) I = (V/aR - I a ; and here sensitivity is written: I/I a = 1;
If V a and R are used (2) becomes
I = V/ar [1 + (R B /R)] - (V a /R) (3)
The produced microwave power is
p m = k VI = k [ V 2 /ar 1 + (R B /R - (V a V/R)] (4)
A simple calculation shows that, if V a and V are obtained by rectifying a single AC voltage source U, the relative microwave power variation d P m/P m is equal to 2 dU/U, for each adjustment of the power P m .
An important purpose of the invention described in the above prior application is to obtain a satisfactory stabilization of P m by suitably amplifying instabilities of V a so as to compensate for instabilities of V.
To do so, the additional source of voltage V a is replaced by a source of voltage V a ' that supplies a non-linear circuit comprising serially a counter electromotive force (c.e.m.f.) element E o , that is very stable and has a very low internal resistance, the resistor R and the coil of resistance R B . V a 'is given by the relation:
V a ' = V a + E o
E o is easily obtained by using a Zener diode.
Still according to the above application, a stabilization effect is obtained for a fixed high-voltage V when, for each value of I, and therefore of V a ' and/or R, E o , R and I are associated by the relation (5):
E o = 2 RI (5)
By suitably associating a rheostat providing R with a variable autotransformer supplying V a ', it is possible to obtain the preceding relation for each desired value of P m and in a continuous manner.
Under normal conditions of operation for a power supply of the above type, there are two drawbacks that have to be overcome. First, it appears that the current of intensity I B through the coil heats it and, for a relatively long time, the coil resistance increases to stabilize at a heated coil value higher by 20-30 percent than the cold coil value. As a result, as indicated by the formula (4), there is a slow drift for P m , that is sometimes troublesome; the lower the value of R, the higher the value of P m , and the greater the power drift.
Second, there is a drawback due to transient phenomena appearing when turning the apparatus on. At that time, it is necessary to be certain that the voltage V a ' has a value such that the current I a through the coil is higher than the current (I B ) o that blocks the magnetron. In that case, it is possible to apply the high-voltage V and to manually reduce V a ' until the occurrence of the desired current I.
In a normal installation, such precautions represent constraints which users tend to neglect. But, if when applying high-voltage V which is assumed to occur instantaneously, voltage V a ' is not sufficient and provides a current I a substantially lower than coil current (I B ) o that blocks the magnetron, there results a transient mode with rapid increase of current I. Since the coil self-inductance is not negligible, there occurs at the coil terminals a c.e.m.f. L(dI/dt) that is subtracted from the auxiliary voltage V a ' and results in a reduction of the auxiliary current I a which may change its direction negatively, and thus further increase I. The phenomenon is unstable and the increase of I is limited only by safety switches or non-linear characteristics of the involved components such as the self-inductance L, Zener diode, magnetron, and rectifiers delivering voltage V a '. The current I increases with a time-constant L/R. This troublesome phenomenon could be avoided only if the increase of current I, after having applied high-voltage V, would be performed with a time-constant at least equal to L/R.
In any case, such exponentially increasing current is the most damaging to the equipment or, in less unfavorable cases, makes it difficult to operate.
SUMMARY OF THE INVENTION
Therefore, an object of this invention is to provide an additional coil current source of adjustable current I B , such that the value I B is substantially independent of the coil characteristics R B and L.
Another object of the invention is to provide an additional source in such a manner that, when high-voltage power is turned on, no transitory unstable phenomenon occurs.
A further object of this invention is to provide an adjustable power supply wherein the sensitivity I/I B is substantially lower than a.
A still further object of this invention is to provide an additional coil current source in such a manner that, for each adjustment of the coil current I B , the magnetron current I and the delivered microwave power P m are not dependent upon instabilities of the magnetron high-voltage V and on an additional source of voltage.
According to a feature of this invention, coil current intensity I B is provided at the output of a current amplifier, the dynamic output resistance of which is substantially higher than the coil resistor R B .
According to another feature of this invention, amplifier input current is equal to the sum of an adjustable control current I e and of a part q of magnetron current I, I e being lower than coil current I B , so that when high-voltage V is applied to the magnetron, it induces current I. From that feature and from the above mentioned formula (1), it is seen that magnetron current is given by the formula:
I = [ V/r (1 + a q)] - (a/1 + a q) I e ( 6)
Thus, the device sensitivity may be written as:
dI/d(I e ) = (a/1 + a q)
If a q >> 1, sensitivity is equal to 1/q, that is independent of a and, consequently, of the characteristics of the magnetron. By adequately selecting q, it is possible to have for the sensitivity any desired value, and particularly, the value 1, as in the above mentioned application. However, excessive transient starting currents are now avoided due to feedback action proportional to qI, which occurs without substantial delay.
According to another feature of this invention, input current I e is provided by a low voltage source V e serially mounted with a negative feedback voltage equal to r 1 I, r 1 being a low value resistor. In such conditions, magnetron current I, at constant high-voltage, and consequently microwave power P m , may be adjusted by varying V e and/or r 1 . The sensitivity I/(I e ) is equal to (r e + r 2 / r 1 ), r 2 and r e being the amplifier input resistor and the sum of internal resistance of low-voltage source V e and of other resistors possibly serially connected with r e , respectively.
According to another feature of this invention, a stable c.e.m.f. source e o with very low internal resistance is serially connected with source of low voltage V e ', such that V e ' = V e + e o , e o being related to r 1 and I by the formula: e o = 2 r 1 I. In such conditions, V e being constant, microwave power P m is adjusted between desired limits by varying r 1 and the power value is given by the very simple formula:
P m = k VI = k V (e o /2 r 1
When the relation e o = 2 r 1 I is fulfilled and when magnetron supply high-voltage V, low-voltage V B of amplifier output DC supply and low-voltage V e ' of amplifier input DC supply are provided from a rectified single AC primary source, power P m remains stable for any adjustment. The major advantage of such an arrangement is to provide an excellent control of delivered microwave power by using only a non-stabilized high-voltage source and two low-voltage sources, which is much less costly than power supplies used in the prior art.
According to another feature of this invention, the amplifier is a common-emitter power transistor. In such conditions, it is possible to use only one low-voltage source V B , with V e ' being obtained from V B through an adjustable potentiometer divider.
Other purposes and features of the present invention will appear more clearly from the following description in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic diagram of a power supply circuit for a continuous-wave magnetron according to this invention, and
FIG. 2 shows a specific embodiment of a power supply using a power transistor as an amplifier.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in FIG. 1, a magnetron 1 is provided with DC high-voltage V from source 2 which is assumed to be made of a first AC current rectifier of a standard type and of filter circuits, not shown. The anode of the magnetron 1 is connected to ground. Control induction field for the magnetron is provided by coil 3 through which current I B is passing; 3 has a resistor value equal to R B .
The bottom terminal of 3 is connected to a positive terminal (+) of a source 4 of DC low-voltage V B which is assumed to be made of a second AC current rectifier and filter circuits, not shown. The top terminal of 3 is connected to the output of a DC amplifier 5 having an output resistance equal to R i .
Input circuit of amplifier 5 comprises serially connected from ground: a resistor 6 of value r 1 through which current I of magnetron 1 passes, the positive terminal (+) of source 2 being connected to the top terminal of 6, a source 7 of adjustable DC low-voltage V e ' which is assumed to be made of a third AC current rectifier and of filter circuits not shown, a source 8 of c.e.m.f. e o which is very stable and has a very low internal resistance, a resistor 9 of value r 2 , and the input resistance r e of amplifier 5.
It will be assumed that sources 2, 4 and 7 have low internal resistances compared to those of the circuits to which they are supplying current; and also, it will be assumed that r 1 << r 2 + r e .
A simple calculation shows that input current is:
I e = (r 1 I +V e ' - e o /r e + r 2 ) (7)
Considering the formula (1) of magnetron characteristics:
I = (V/r) - a I B
if the current gain of amplifier 5 is unity and if output resistor R i of 5 is very high, by combining (1) and (7) the following formula is obtained for current I:
I = [ V (r e + r 2 )/r (r e + r 2 + a r 1 ] - a (V e ' - e o /r e + r 2 + a r 1 ) (8)
From (8), it appears that when there is no feedback of current I to input of amplifier 5 (r 1 = 0), the transconductance I/V e ' is equal to (a/r e + r 2 ), while with current feedback, it is
(a/r e + r 2 + a r 1 )
and, thus, is reduced by the ratio (r e + r 2 /r e + r 2 + a r 1 ).
If a r 1 is very much higher than r e + r 2 , the transconductance becomes approximately 1/r 1 and, consequently, is independent of a; if the sources are stable, current I and microwave power P m provided by the magnetron are independent of possible instabilities of static magnetron or amplifier characteristics, such as for example resulting from temperature variations.
From 8), microwave power delivered by the magnetron is:
P m = kP = kVI = (kV/r e + r 2 + a r 1 ) [V (r e + r 2 )/r] - a (V e ' - e o ) (9)
For constant high-voltage V, P m varies within the desired range by adjusting V e ' and/or r 1 .
In practice, additional sources 4 and 7 will be first turned on, before applying high-voltage V to source 2. In such conditions, since there is no magnetron current feedback to the input of amplifier 5, current I B in coil 3 is:
I B = (V e ' - e o /r e + r 2 )
Such an intensity is then lower then that passing through coil 3, with the same voltage V e ', when magnetron 1 is under operation; but, in the present power supply, there is no risk of overcurrent when magnetron high-voltage is applied and whatever the value of V e is at that time, since, due to negative feedback through resistor r 1 , any abnormal magnetron current increase is immediately throttled. It is to be noted that this result is completely independent of the presence or not of the source 8 of c.e.m.f. e o , the purpose of which will now be examined.
Source 8 permits stabilization of microwave power P m when DC voltages of sources 4 and 7 are subjected to relative equal variations, particularly when voltages V, V B and V e ' are produced by rectifying a single source of AC current after suitable filtering. For each adjustment of P m and therefore, of V e ' and/or r 1 , as a result of combination of formulas (8) and (9), P m remains constant if V e ' and V are subjected to the same relative instabilities: d V e '/V e ' = dV/V, when I, [ r 1 + (r e + r 2 /a )] and e o are bound by the formula
2 I [ r 1 + (r e + r 2 /a )] = e o .
Thus, to adjust power P m , it is necessary only to vary the value of r 1 in accordance with the formula (10).
It is to be noted that voltage V B of source 4 as well as variations thereof do not interfere with the problem of stabilizing P m . That results from the fact that dynamic output resistance R i of amplifier 5 is assumed to be high when compared to R b , or, in other words, amplifier 5 operates as a constant current source.
It is of interest to use the power supply with values of r 1 such as: a r 1 >> r e + r 2 .
In such conditions, the formula (10) becomes:
2 I r 1 = e o (10')
and electrical power applied to the magnetron is stated in a very simple manner by the formula:
P = VI = Ve o /2r 1 (with P m = k (Ve o /a r 1 ), (11)
P and P m being no longer depending on stable elements (e o and r 1 ).
From a combination of (8) and (10), still assuming that a r 1 >> r e + r 2 , the value of voltage V e ' of source 7 is substantially constant and equal to:
V e = (e o /2 ) + [V (r e + r 2 )/ar]
Considering again the formula (1), V/ar measures the coil current (I B ) o that precludes magnetron 1 from operating when voltage of source 2 is equal to V; thus
V e ' = (e o /2 ) + (I B ) o x (r e + r 2 ) (12)
The amplifier may include a suitable silicon or germanium power transistor of an NPN type.
The c.e.m.f. e o of the source 8 may be provided by a Zener diode.
An example using practical figures will make the device more readily understandable. The following gives various values for a magnetron power supply operating in continuous mode and able to provide, in addition to the maximum microwave power (P m ) o , any value between (P m ) o and 1/4 (P m ) o , each value being adjusted and regulated according to this invention.
The magnetron high-voltage is 5 kV and current intensity providing maximum power (P m ) o is 1 A. With an efficiency ratio k of 70 percent, (P m ) o = 3.5 kW. The four power levels are obtained for current values 1, 0.75, 0.5 and 0.25 A. The value of (I B ) o is 1.3 A and magnetron figure a is within the range of 35; in other words, current I B corresponding to current I equal to 1 A is 1.27 A. Amplifier current gain is about 30. Power-transistor emitter-base resistance r e for a base current of 40 mA is lower than 15 ohms. Since this resistance varies in response to temperature changes, it is useful to mask it in the sum (r e + r 2 ) by having the value of r 2 substantially higher, i.e. 100 ohms.
If the current I is of 1 A and value of r 1 of 1.5 ohms, condition a r 1 >> r e + r 2 is amply fulfilled. It is still better fulfilled when r 1 is increased to provide lower currents I.
The formula (10') gives e o = 3 V (a value that is readily provided by a Zener diode) and the formula (12) gives: V e ' = 1.5 + 1.3(115/30) ≠≠ 6.5 V.
Resistor 6, which in this case must continuously vary from 1.5 to 6 ohms, is made of a rheostat.
FIG. 2 shows a detailed circuit embodiment of the invention using a common-emitter power-transistor amplifier of the NPN type having the same reference numeral 5 as in the FIG. 1. Other like reference numerals will be used for the same components already shown in FIG. 1.
Rectifier 10 followed by filter stage 11 provides voltage V B that is applied through resistor R B of coil 3 to the collector of transistor 5.
Voltage V e ' is produced by means of a voltage divider 12, having a relatively low resistance, serially connected with resistor of rheostat 6, to provide resistor values r 1 suitable for current feedback and stabilization according to the invention. Voltage V e ' from slide contact of 12 is applied through Zener diode 8 and resistor 9 to the base of transistor 5. Capacitor 13, connected between the transistor base and ground, removes AC signals which remain at the output of filter 11.
Divider 12 has a division ratio that is slightly adjustable so as to adjust V e ' at a suitable constant value, either at an initial adjustment or, from time to time, during maintenance operations. Again, it is noted that resistor r 2 is equal to the sum of resistor 9, the divider resistor between the top point and rheostat, and the rheostat resistor.
Power diode 16 is connected in parallel with coil 3 in a backward direction with respect to I B . Its purpose is to avoid damaging transistor 5 in the case of a power supply break-down. If such an event occurs, the magnetic energy stored in coil 3 is dissipated in 16, so as to protect transistor 5.
High-voltage V of magnetron 1 is provided by rectifier 14 followed by filter stage 15. Rectifiers 10 and 14 are supplied by a single primary AC source, not shown, that, according to the invention may be a non-stabilized source.
While the principles of this invention have been described above in relation to several embodiments, it is to be clearly understood that this has been only made by way of example and does not limit the scope of the invention, as set forth in the appended claims.