CA1053760A - Power controller for microwave magnetron - Google Patents
Power controller for microwave magnetronInfo
- Publication number
- CA1053760A CA1053760A CA268,943A CA268943A CA1053760A CA 1053760 A CA1053760 A CA 1053760A CA 268943 A CA268943 A CA 268943A CA 1053760 A CA1053760 A CA 1053760A
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- voltage
- magnetron
- secondary winding
- circuit
- triac
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Abstract
ABSTRACT OF THE DISCLOSURE
A microwave magnetron operating circuit includes a high-leakage reactance transformer having a secondary winding, a capacitor connected in series with the secondary winding and means connecting the series combination of capacitor and secondary winding across the heater-cathode and the anode terminal of a magnetron.
A controlled semiconductor switch, such as a triac, is connected electrically in shunt of the magnetron.
Control means are provided to operate the triac to its current conducting state, so as to conduct current in shunt of the magnetron. The control means includes a positionable control member, accessible to the user, for varying the time at which the controlled semicon-ductor device is placed in its electrically conductive state, so as to permit the user to adjust or set the power supplied to the magnetron to various different power levels.
A microwave magnetron operating circuit includes a high-leakage reactance transformer having a secondary winding, a capacitor connected in series with the secondary winding and means connecting the series combination of capacitor and secondary winding across the heater-cathode and the anode terminal of a magnetron.
A controlled semiconductor switch, such as a triac, is connected electrically in shunt of the magnetron.
Control means are provided to operate the triac to its current conducting state, so as to conduct current in shunt of the magnetron. The control means includes a positionable control member, accessible to the user, for varying the time at which the controlled semicon-ductor device is placed in its electrically conductive state, so as to permit the user to adjust or set the power supplied to the magnetron to various different power levels.
Description
i.~S3760 This invention relates to adjustable power control circuits for microwave magnetrons of the type used in microwave ovens.
The microwave oven is the famiLiar appliance used to heat or cook foods by exposure of the food to microwave energy. For this purpose, conventional micro-wave ovens employ an electronic vacuum tube known in the art as a magnetron. Simply stated, the magnetron is a device having unidirectional current-carrying characteristics. It converts DC voltage and current into energy in the microwave frequency range, such as for example 2,450 megahertz. This DC energy is provided by a power supply which converts normal household line voltages, typically 120 or 240 volts AC, into the needed normal operating voltages in the order of 3,000 to 4,000 volts DC, required in the operation of existing microwave magnetrons. In its essentials, known microwave oven power supplies contain a transformer for stepping up the 120 volts AC to the level of less than the required 3,000 to 4,000 volts, and have a voltage doubler-rectifier which provides the required DC voltage for the magnetron and a source of low voltage for the heater of the magnetron.
As is known, microwave energy generated by a magnetron is taken from the magnetron output and trans-mitted either directly or indirectly into the oven chamber.The average power supplied to the magnetron is set within ~CP-76-18 ~0537~0 Limits by the design of both the power suppLy and the magnetron, and is generally directly related to the microwave output power generated by the magnetron. It is known that adjustment of the microwave power can be made within limits by adjustment of the DC voLtage and, hence, current level applied to the magnetron. Most microwave ovens in commercial use contain power supplies that employ a high-leakage reactance transformer in combination with a modified half-wave voltage doubler, known also as a Villard circuit, to rectify and double the voltage output of the high-voltage transformer, and apply a high voltage DC to the magnetron. Examples of such circuits appear in United States Patents Nos. 3,396,342;
3,651,317 and 3,684,978.
Recent practice is to provide additional elements within the oven supply that permit the microwave oven user to adjust the average power generated by the magnetron to high or to low power levels, or which include a mode of operation in which adjustment to any power level is possible. One example of a high-to-low power adjustment in this combination is illustrated in the above-cited U. S. Patent No. 3,684,978 in which the series capacitance is changed from one value to another. Still another method is to employ a controlled semiconductor switching device, such as the bidirectional triac, in the primary circuit of the transormer, so aæ to control the ratio of "on" to "off" periods in pulsating current flow in the transf~rmer primary, to thereby regulate the average magnitude of current from the power supply. It is noted that in some of these circuits, a separate filament heater ~CP-76-18 ~0537~
transformer is required, because of the Limited current into the primary of the high-voltage transformer, so that the expedient of having the fiLament winding mounted upon the common core, but as a separate winding, upon the high-voltage transformer cannot presently be employed.
AdditionaLly, the use of pulse techniques applied to the primary winding of the transformer inherent in these circuits creates additional voltage stresses on the transformer insulation that should be avoided. Another method of controlling the current appears described in the U. S. Patent No. 3,760,291 in which a variable re-sistor is incLuded in the current path to the magnetron, so that the magnetron operating current can be varied as a function of LeveL of resistance. This circuit appears L5 impracticaL in that rather expensive resistors are empLoyed which consume current and generate heat.
The present invention reLates to a controL
circuit for adjusting the average power output of the oven magnetron by controlLing the voltage in the secondary circuit of the transformer. More particularly, the invention provides a simple control that allows the user to adjust the power leveL from a magnetron to a certain range. The fiLament voltage may then be supplied from a secondary winding on the same transformer structure, namely the core associated with the high-voltage secondary winding. The primary pulse technique of known circuits is thus avoided. The invention, therefore, increases the reliability of the transformer. Any line voltage surges caused by lightning on the input line, which might destroy semiconductor type control devices connected in the primary circuit, are believed to be prevented from doing damage to circuit components.
1(~537~;0 Briefly, the present invention includes in a power supply of the type having a power transformer, more particularly a high-leakage reactance transformer for providing high-voltage AC in its secondary winding, a capacitor in series with the secondary winding and one terminal of the magnetron, and a controlled semi-conductor switching device, such as a triac, is connected electrically in shunt of the magnetron. Adjustable means are provided to supply gating signaLs to the switching device so as to turn the triac into its current conducting condition, so as to conduct current in shunt of the magnetron.
In one specific aspect of the invention, the adjustable control is responsive to the phase of the AC
voltage in the secondary winding and supplies voltage to the gate of the switching device at a predetermined phase angle during each AC half-cycle.
In a second aspect of the invention, the control means contains str~cture to provide a gating voltage only at a predetermined phase during each aLternate haLf-cycle.
And in accordance with another aspect, a muLtivibrator is used to provide a gating voLtage. The multivibrator provides its output as the input to the triac for predetermined periods which may be Less than a half-cycle of AC or for a period covering many AC cycLes.
The foregoing obJects and advantages of the invention, as well as the structure characteristic of the invention, briefly summarized above, will become better understood by giving consideration to the following detailed description of various embodiments of the 1C~537~0 invention, with reference to the drawings, wherein:
Figure 1 schematically ilLustrates one embodiment of a circuit in accordance with the invention;
Figure 2 schematically iLlustrates a modification for the embodiment of Figure L;
Figure 3 schematicaLly illustrates a second embodiment of the invention; and Figure L~ schematic~ly illustrates still another L0 embodiment of the invention.
The embodiment of Figure 1, as well as the additional embodiments of the invention presented, are illustrated in the form of a schematic electrical circuit diagram, using conventional symboLs for known elements to iLlustrate the invention clearly without the necessity of detailed and less informative mechanicaL iLLustrations. As wilL be noted, various eLements of a microwave oven well known in the art are not iLLustrated, inasmuch as they wouLd not contribute to an understanding of the structure of the invention or its mode of operation.
In Figure L, a transformer TL, which is suitably of the high-leakage reactance type, incLudes a primary winding Pl, a high-voLtage secondary winding Sl and a Low-voltage secondary winding S2, often referred to as the heater winding or filament winding. A conventional magnetron M contains a heater-cathode assembly, an anode, and an output.
The magnetron is suitably of the type for C.W.
operation at a microwave frequency of perhaps 2450 MHZ.
1~537~i0 A capacitor Cl, a triac TRl having two main current terminals and a gate terminal, a diac Dl, an adjustable resistor RL having a positionabLe tap as ilLustrated connected by means of a shaft, indicated by a dashed line to a control knob Kl, and a capaci$or C2 are included. Heater winding S2 is connected to the heater terminals of the magnetron M, so as to provide a closed electrical current-conducting circuit through the magnetron filament. The magnetron anode is electricalLy connected to ground potential and to one end, i.e. terminal 1, of transformer secondary winding Sl.
The capacitor Cl is connected in circuit between the other terminal 3 of secondary winding S1 and one of the heater cathode terminals of magnetron M, so as to form a series circuit of magnetron M, capacitor C1, and secondary winding S1. As is con-ventional, the capacitor C1 is of a reactance greater than the leakage reactance of transformer T1.
Triac TRl is connected across the magnetron with one of its main current terminals being connected in circuit with one of the magnetron heater terminals and with the other terminal being connected to 'he magnetron anode and, hence, to ground, thereby to place triac TRl electrically in shunt of the magnetron. One terminal of the variable resistor Rl is connected to a low-voltage tap 2 on secondary winding Sl and the other terminal of the resistor is connected to one terminal of diac Dl and also to one terminal of capacitor C2.
The remaining terminal of capacitor C2 is connected to ground.
~537tjO
The remaining terminal of diac DL is connected to the gate terminaL of triac TRl.
As wiLl be noted, the circuit comprising re-sistor Rl and capacitor C2 is a conventional phase-shifting circuit in which the degree, i.e. magnitude of phase shift is a function of the resistance value of resistor Rl and the capacitance of capacitor C2. Hence, an AC voltage applied at tap 2 to one terminal of resistor Rl is electrically out-of-phase with the voltage appearing across capacitor C2, constituting the phase-shifted output.
The phase difference can be adjusted by positioning knob Kl, accessible to the user, which sets the re-sistor tap to a selected position along the resistance element. The positionable tap, as shown, is of the type which shunts, i.e. short circuits parts of the resistance element, so as to reduce the effective length of the re-sistor, but other types of adjustable resistors may obviously be substituted.
In operation, primary winding Pl is connected to a suitable AC source, suitabLy 120 voLts 60 hertz, that varies cyclically and sinusoidally in level with time.
By transformer action, the line voltage applied to the primary winding is stepped down in level to a low AC
voltage which appears across the secondary winding S2 and is stepped up in level to both a high AC voltage which appears across secondary winding Sl and a some-what lower high AC voltage appearing at secondary tap 2.
The voltage across heater secondary S2 is applied to the filament of the magnetron M, so as to pass current through the filament and, after a sufficient period of time, the lns3760 heating effect warms the magnetron cathode to the temperature at which sufficient electron emission occurs in the magnetron. The output voltage across winding Sl is less than the normal operating voltage specified by the manufacturer for magnetron M. And a voltage doubling effect of the circuit is necessary in order to build up sufficient voltage across the magnetron to allow the magnetron to conduct current.
It is recalled that the magnetron is a unidirectional current-conducting device, like a diode, as it conducts current only in a direction from its anode to its cathode.
At some predetermined phase of the AC voltage appearing at tap 2 of secondary winding Sl, the voltage across capacitor C2 is of the predetermined level to cause the diac Dl, a very sensitive semiconductor voltage breakdown device, to switch into its current-conducting state, placing a voltage at the gate of triac TRl and causing discharge current to flow from capacitor C2 through the diac Dl to the gate electrode of triac TRl. Once the current through the diac re-duces to zero, the diac returns to its "off" condition.
In response to the triggering voltage and current at its gate, triac TRl is thus placed into its current-conducting condition. Considering the voltage on themagnetron filament, to be positive relative to ground and terminal 1 of transformer winding Sl, triac TRl conducts current in a path from terminal 3 of secondary winding Sl, through capacitor Cl, the triac, to the 10537ti0 other terminal 1 of secondary winding Sl. As a result, the current charges up capacitor Cl to a high positive voltage at the ungrounded side or, v~ed another way, charges the capacitor Cl to a high negative voltage at the terminal connected to the magnetron heater.
Thereafter, the AC voltage across winding Sl reverses in polarity and commences to build up to an instantaneous level. The voltage across winding Sl is additive with respect to the voltage appearing across recharged capacitor Cl. Hence, the DC voltage applied across the magnetron is essentially increasing to a level which is higher than that across secondary winding Sl.
The polarity of the voltage across the magnetron now becomes positive at the anode with respect to the heater cathode assembly. As soon as the voltage builds up to an appropriate level, the magnetron conducts current. Current then passes from the magnetron anode to its heater-cathode assembly, through the capacitor Cl, and through secondary winding Sl, which current discharges capacitor Cl.
In a conventional manner, the magnetron converts such DC current to microwave energy which is taken at the output thereof and coupled to the microwave oven cavity, not illustrated. At some time during this second half-cycle of AC, the voltage appearing across capacitor C2again increases in level sufficient to again switch diac Dl to its "on" condition which in turn triggers triac TRl into its "on", i.e. current-conducting state.
Triac TRl thus conducts current in a shunt path around magnetron M and during this phase, the magnetron no longer ~CP-76-L8 537~0 generates microwave energy. This process repeats itself quickly.
By adjustment of knob K~, the position of the short circuit on variabLe resistor Rl and, hence, the effective resistance value of r~sistor Rl is changed to vary the phase at which diac Dl and, hence, triac TRl is placed ln its electrically conductive state, so that on one half-cycLe the magnitude of charge supplied to capacitor Cl is varied and the duration of current to magnetron M during the proceeding, and then each aLternate half-cycle is similarly varied in duration.
By the above-described operation, the average of power effectively passed by the magnetron and converted into microwave energy is varied by the simple adjustment of L5 potentiometer, i.e. resistor tap to vary the microwave output power Level.
Triac TRl has another useful property in this circuit. The triac is inherently self-protective against high-voltage transients. Thus, if the voltage across its main terminaLs exceeds a certain level, known as the breakdown level of the triac, the triac switches into its current-conducting condition, irrespective of the lack of a trigger voltage at the triac's gate input.
The triac then conducts current thereafter until the current through the terminals is reduced to zero, at which time the triac again becomes nonconductive. Thus, the high voltage which appears across the magnetron is dissipated by the triac to protect the transformer and other components from high voltage spikes. In lOS37~0 accordance with the invention, the breakdown voltage of the triac seLected is within the range of LL0~ to 200~
greater than the normaL operating voltage of magnetron M.
These high voltage transients occur usually during the warm up period of the magnetron filament, i.e. the heater. In order to have sufficient electrons within the magnetron for fuLL operation, the magnetron heater must be raised to a temperature level specified by the manufacturer and in practice a magnetron does not reach its operating temperature for a period of perhaps 1 to 1-1/2 seconds after heater current to the magnetron heater has started to flow.
The power technique in which the high voltage is started to be applied to the magnetron simultaneously with the beginning of the application of the heater current to the magnetron filament is known as "coLd-start"
operation and appears in the embodiments of this invention. However, at that starting time, the magnetron filament is incapable of generating the degree of electron emission necessary for proper operation of the magnetron.
Thus, although the voltage supplied between the anode and the heater-cathode assembly of the magnetron is of the proper polarity and level, the magnetron does not conduct current~ As the magnetron's filament hçats up, a greater number of electrons becomes available although still less than that required for full operation.
During such time, the magnetron conducts current only until the supply of electrons is exhausted and then ceases conducting. This occurs over a time period of 1~537~0 less than one-half AC cycle during which the W voltage is applied across the magnetron. The sudden halt in the magnetron current generates high transient voltages, a phenomenon of the magnetron termed moding. These transient voltages are very high, possibly at the level of three to four times as high as the normal operating voltage of the magnetron.
In the prior, known power supply constructions, a diode is connected in shunt of the magnetron which is replaced in the embodiment of the invention shown in Figure 1 by the triac TRl. The diode used had reverse breakdown voltages in the order of 12,000 to 20,000 volts, so that the diode would not conduct in the reverse direction when the voltage transients appeared.
By selecting the triac to have a voltage breakdown of between lO~o greater than the normal operating voltage of the magnetron and twice the normal operating voltage of the magnetron, it is seen that the triac breaks down and conducts current to completely shunt the magnetron and divert any current, which would otherwise be supplied by the source, through the series combination of secondary winding Sl and capacitor Cl. Thus, if the magnetron does mode during the warm-up period, any transient high voltage spike is dissipated, i.e. diverted through the triac and all current is shunted around the magnetron. This process re-occurs until the magnetron filament is up to its full operating temperature when the transients no longer appear.
1~537~0 It is noted in connection with the foregoing circuit that if power control is not desired, but transient voltage suppression is desired, the conventional half-wave doubler circuit of the prior art may be modified by sub-stituting a reverse voltage breakdown characteristicsubstantially less than the 15,000 to 20,000 volts ~ reverse breakdown voltage of diodes currently empLoyed in the power supplies in accordance with the foregoing principles. Thus, a triac type device should be used that is capable of carrying the forward current in one direction and which has a reverse brea~down voltage in the order of 1.1 to 2 times the normal operation voltage of the magnetron specified by the magnetron manufacturer.
Figure 2 illustrates the combination of a diode D2 and a silicon controlled rectifier SCR
connected electricaLly in series and electrically poled in the same direction. This combination of elements may be substituted in the circuit of Figure 1 for triac TRl, as well as in other embodiments in which a triac is employed. Other equivalent semiconductor switching devices may be substituted with or without minor circuit modifications. As is apparent, current can pass through this circuit only in a direction from the anode of diode D2 to the cathode through the anode of the SCR and the cathode thereof. When SCR has the appropriate voltage applied at its gate, current cannot flow in the reverse direction. With such a substitution, the self-protecting, because transient-suppressing characteristics, inherent in the embodiment of Figure 1, ~537~0 due to the triac, are not available. Moreover, in the operation of a circuit having the elements of Figure 2 substituted for triac TRl in Figure L, current flows through capacitor Cl to charge capacitor Cl only on 5 every alternate AC half-cycle. Thus, when the voltage at the filament of the magnetron is positive with respect to the anode of the magnetron in these half-cycles of AC, the magnetron does not normaLly conduct current. In contrast to the mode of operation described for the circuit of Figure L, the combination of diode D2 and siLicon controLLed rectifier SCR cannot operate to shunt current from the magnetron during the AC half-cycl~ in which the voltage at the anode is positive with respect to the heater-cathode.
Reference is now made to the embodiment shown in Figure 3. For convenience, where the same elements used in the embodiment of Figure 1 are employed, the same designations are used to identify the same eLements and in the interest of brevity, these eLements are not described again. As is apparent from a com-parison of the two embodiments, the embodiment of Figure 3 includes a diode D3 and a resistor R2 connected in series between tap 2 on secondary winding Sl and one end terminal of potentiometer-type resistor Rl and the phase-shifting circuit.
Diode D3 is electrically poled so that its anode is connected directly to the secondary winding tap 2.
Resistor R2 is inserted in series circuit as shown to constitute a rough adjustment, as it adds to the 1 ~537~0 resistance of resistor RL. Diode D3 allows current to pass through the series circuit of resistors R2 and Rl to charge capacitor C2. This occurs only during those alternate half-cycles of AC current which appear at tap 2 of transformer secondary winding Sl and with positive polarity with respect to ground. Accordingly, capacitor C2 is charged sufficiently on alternate ones of the AC half-cycles to "fire" or cause diac Dl to breakdown and conduct current. As described above in connection with Figure 1, L0 the operation of diac Dl discharges capacitor C2 through the gate of triac TRl and thereby renders triac TRl conductive only on these same AC half-cycles. The phase angle of the AC at which triac TRl switches to its current-conducting condition is a function of tap L5 position of adjustable resistor Rl, as adjusted by the oven user by manipulation of knob Kl. Thus, on the one-half cycle of AC as appears across secondary winding Sl when the voltage at the winding terminal 3 is positive relative to the grounded terminal 1, current passes through triac TRl, secondary winding Sl and capacitor Cl to charge capacitor Cl to a high negative voltage, limited in level to the level of voltage appearing across secondary winding Sl, at the terminal of the capacitor Cl connected to one magnetron heater terminal. Triac TRl, once it is in the "on"
condition, continues to conduct current until the current level reduces to ~ero, after which the triac returns to its nonconducting condition. The triac then remains in the "off" condition until a trigger voltage is applied to its gate electrode to repeat the process.
!MCP-76-L8 1~537~:i0 By varying the phase angle at which the triac TRl is placed in its current conducting condition, the charge and hence the voltage across capacitor Cl may be adjusted in LeveL. SuitabLy, this adjustment is most pronounced at the Line phase angles of between 70 degrees and L20 degrees so tha~ there is insufficient time for the capacitor Cl to charge to high voltages. The described operàtion and structure has its roots, in principle, in the clamper circuits described in Chapter 5, pages 65 through 71 of the boo~ entitled "Semi-conductor Pulse Circuits" Mitchell, published by Holt-Rinehart &
Winston, Inc. 1970, which illustrates the principle of varying voltage levels in a general sense, as applied to pulse type circuits.
In the next succeeding AC haL~-cycle, the voLtage appearing across secondary winding Sl rises sinusoidally in level, but is opposite in polarity from the voltage in the preceding half-cycle. The voltage across winding Sl is added to the voltage across capacitor Cl and the sum of these voltages is applied between the anode and the heater-cathode assembly of magnetron M. As the voltage across the magnetron rises to the proper level and is of the proper polarity, the magnetron conducts current through the path consisting of the magnetron, the main capacitor CL and the secondary winding Sl. The current cLosed through this path discharges the capacitor Cl and the direct current through the magnetron is converted into high frequency microwave energy. Clearly, the DC current 10537~0 through the magnetron depends upon the charge in capacitor Cl. The charge applied to capacitor Cl through operation of the triac in succeeding half-cycles is adjustable in leveL, hence also the current to the magnetron and the power output thereof is similarly adjustable.
As in the above-described embodiment, the triac TRl is preferably selected so as to have a breakdown voltage, i.e. a voltage at which the triac L0 will conduct even though no trigger voltage is applied to its gate eLectrode, of between 110% and 200% of the normal operating voltage Vn specified by the manufacturer for proper operation of magnetron M, to thus provide the transient voltage suppressing characteristic in the L5 power suppLy during the initial warm-up period of the magnetron, as described above.
It is noted that, in connection with circuit of Figure 2, because of the mode of operation, the effective impedance of the secondary winding, as viewed from the primary winding, may change and become more reactive, so ~hat the power factor of the circuit drops to an extent resulting in an increase in the primary current. It is believed that this primary current is still at an acceptable level. Nonetheless, various compromises may be made in the circuit in pro-portioning and optimizing the various elements so that the primary power factor remains within acceptable levels.
For example, if the specific model of transformer found in various ~nown microwave ovens is employed, as welL as l~S~o the existing capacitor and magnetron, the application of the phase-controlled triacs to that specific assembly results in a high peak current and full charge of capacitor Cl, if the triac is turned "on"
early in the first quadrant (90~ of the AC cycle, due to the inherent effects of resonance like inter-action between the capacitance of the capacitor and the leakage reactance of the transformer, so that power variation may be adjusted within a range of 70 to 90 degrees of the first quadrant. This leads to the desired result of adjustability. It is obvious, however, that to eliminate this resonance effect, the transformer design is modified to increase the leakage inductance of the transformer, as by the addition of a larger magnetic shunt between the primary and secondary windings, as is ~nown to those skilled in the art.
Having the disclosed invention before him, one skilled in the art obviously can attend to the details of optimization of design for any specific embodiment of the invention.
It will be clear that the above-described circuits provide power control with relatively few components. Inasmuch as power controL is obtained through electronic operations on the secondary side of the power transformer, the power controL actions do not seriously affect *he voltages appearing across the heater winding, as occurs in those earlier power control designs in which the current applied to the primary was controlled or interrupted, which would ~537~0 necessariLy result in interruptions of heater current, had the heater winding been supplied by the same trans-former in those preceding designs, which in fact required heater transformers separate from the high voltage trans-former . Moreover, a degree of isolation is provided,forthe electronic components against line voltage surges occurring in the primary winding Pl due to ambient events, such as lightning striking the power lines from the eLectrical utility company.
Reference is now made to the embodiment of Figure 4 which includes a high-leakage reactance trans-former T2 having a primary winding P2 adapted to be connected to a source of 120 volts 60 hertz AC, as availabLe from a locaL power utility company, a low-voltage secondary winding S4 and a high-voltage secondary winding S3. A magnetron M has its fiLament connected in circuit with the secondary winding S4,.
The magnetron anode is connected to one terminal of secondary winding S3 which is considered to be at electrical ground potential, as indicated by the symbol. A capacitor C3 is connected in circuit between one of the magnetron heater-cathode assembly terminals and the other terminal of the secondary winding. This is the basic circuit described above in connection with Figures 1 and 3.
A diode D4 and a triac TR2 are connected electrically in series between the heater-cathode terminal of the magnetron and the grounded anode terminal of the magnetron. Diode D4 is electrically poled so that ~CP-76-L8 ~1~5371~0 ~ts anode is connected directly in circuit with the heater-cathode terminal of the magnetron, so as to place diode D4 in a reverse poLarity reLationship with the magnetron. A multivibrator MV is provided which may be of any conventional structure. The multivibrator incLudes a controL knob K2 that is accessible to the microwave oven user. SuitabLy, the muLtivibrator may be powered by any convenient electrical power source and preferably is operated from voLtage supplied by a -10 tap on the secondary winding S3, as indicated by the dashed line. The multivibrator output is connected to the gate terminal of triac TR2.
Preferably, the multivibrator provides a square wave output voltage, with a positive output pulse having a duration of greater than many AC half-cycles, such as for example one second. The multivibrator is adjustable to provide various durations of output voltage. For example, a given setting of the multivibrator may permit selecting an output which is positive for one second and which then is zero for another second and which then repeats itself, so as to enable the gate electrode of triac TR2 for one second intervals, separated by identical intervals in which triac TR2 remains in its noncondutive state. As the orientation of control knob K2 is adjusted, the output voltage duration changes, let us say to another position of knob K2 with which the output pulse will be at a duration of one-half second. As in the above-described examples, in essence the circuit functions in the described manner.
h~P-76-18 1~537tj0 The Line voLtage applied to the primary winding P2 is stepped down to the low AC voltage appearing across low-voltage secondary winding S4 and is stepped up to the high AC voltage appearing across secondary winding S3, typically Less than the high voltage which would be the normal operating voltage specified for operation of magnetron M. Moreover, a low voltage is provided at the tap. It is assumed that the magnetron is at operating temperature and the voltage at the upper terminal of winding S3, as seen in Figure 4, is positive reLative to ground. If then the output pulse of multivibrator MV is applied to the gate electrode of triac TR2 to place the triac into its eLectricaLLy conductive condition and to maintain it in this state L5 for the duration of the pulse, current flows from the upper terminal of winding S3 through capacitor C3, diode D4, triac TR2, and back to the secondary winding, so as to allow capacitor C3 to become fully charged.
The magnetron, which exhibits diode-Like characteristics as is known, does not conduct current during those half-cycles in which the voltage at its heater-cathode terminal is positive with respect to the anode. In the alternate half-cycles, the voltage across winding S3 reverses in poLarity and sinusoidaLLy buiLds up in level, just like in the above-described embodiments. The voltage across winding S3 is additive to the voltage appearing across capacitor C3, a result of the charge given the capacitor during the preceding AC half-cycle. When the sum of the voltages _22-:
,~
10537~V
reaches the level necessary to cause the magnetron to conduct, the magnetron conducts current from its anode to its heater-cathode, the circuit incLuding secondary winding S3, magnetron M and capacitor C3, to discharge the capacitor and to convert the DC current through the magnetron into microwave frequency energy which is extracted by an output circuit, not iLLustrated.
At this moment, the electricaL poling of diode D4 prevents current Erom passing through the circuit consist;ng of diode D4 and triac TR2, inasmuch as the voltage at the anode terminal of diode D4 in this AC half-cycle is negative with respect to the cathode. The operation continues with the magnetron conducting during one-half AC cycles and the circuit consisting of diode D4 and triac TR2 conducting on alternate AC half-cycles to charge the capacitor C3, untiL the output of multivibrator MV drops off and ceases for an interval. It is apparent that, if the current through the magnetron continues for one second and then ceases for one second, the magnetron is operated at essentiaLLy haLf power, over a given period of time.
As to modifications to this circuit, various equivaLents may be substituted for the triac, such as a series of sLave-triggered silicon controlLed rectifiers (not iLlustrated) of the type described on page L64 of the SCR ManuaL, 5th edition, pubLished by the General Electric Co. in 1972. The purpose in so doing is to use a series of silicon controlled _23-1~537~0 rectifiers having reLatively Low voltage breakdown characteristics so that the reverse breakdown characteristic of the circuit is the sum of the individuaL breakdown characteristic. Thus, should a triac be unavaiLable having a suitable reverse voltage breakdown characteristic necessary to prevent current conduction during the alternate half-cycles as a result of any high-voltage transients generated in the circuit during moding, a series string of lower voltage silicon controlled rectifiers may be used instead. It is also noted that various multivibrators may be employed including a multivibrator having a variable time base, as well as a variable pulse width.
It is believed that the foregoing description.
L5 of various embodiments of the invention is sufficient in detail to enable one skilled in the art to make and use same. It is expressly understood that the invention is not limited to these details presented in connection with the above description, inasmuch as various substitutions of equivalents, additions, and improvements will become apparent to those skiLled in the art upon reading this specification, all of which may be embodied with the invention. Accordingly, the invention is to be broadly construed within the full spirit and scope of the appended claims.
^24-
The microwave oven is the famiLiar appliance used to heat or cook foods by exposure of the food to microwave energy. For this purpose, conventional micro-wave ovens employ an electronic vacuum tube known in the art as a magnetron. Simply stated, the magnetron is a device having unidirectional current-carrying characteristics. It converts DC voltage and current into energy in the microwave frequency range, such as for example 2,450 megahertz. This DC energy is provided by a power supply which converts normal household line voltages, typically 120 or 240 volts AC, into the needed normal operating voltages in the order of 3,000 to 4,000 volts DC, required in the operation of existing microwave magnetrons. In its essentials, known microwave oven power supplies contain a transformer for stepping up the 120 volts AC to the level of less than the required 3,000 to 4,000 volts, and have a voltage doubler-rectifier which provides the required DC voltage for the magnetron and a source of low voltage for the heater of the magnetron.
As is known, microwave energy generated by a magnetron is taken from the magnetron output and trans-mitted either directly or indirectly into the oven chamber.The average power supplied to the magnetron is set within ~CP-76-18 ~0537~0 Limits by the design of both the power suppLy and the magnetron, and is generally directly related to the microwave output power generated by the magnetron. It is known that adjustment of the microwave power can be made within limits by adjustment of the DC voLtage and, hence, current level applied to the magnetron. Most microwave ovens in commercial use contain power supplies that employ a high-leakage reactance transformer in combination with a modified half-wave voltage doubler, known also as a Villard circuit, to rectify and double the voltage output of the high-voltage transformer, and apply a high voltage DC to the magnetron. Examples of such circuits appear in United States Patents Nos. 3,396,342;
3,651,317 and 3,684,978.
Recent practice is to provide additional elements within the oven supply that permit the microwave oven user to adjust the average power generated by the magnetron to high or to low power levels, or which include a mode of operation in which adjustment to any power level is possible. One example of a high-to-low power adjustment in this combination is illustrated in the above-cited U. S. Patent No. 3,684,978 in which the series capacitance is changed from one value to another. Still another method is to employ a controlled semiconductor switching device, such as the bidirectional triac, in the primary circuit of the transormer, so aæ to control the ratio of "on" to "off" periods in pulsating current flow in the transf~rmer primary, to thereby regulate the average magnitude of current from the power supply. It is noted that in some of these circuits, a separate filament heater ~CP-76-18 ~0537~
transformer is required, because of the Limited current into the primary of the high-voltage transformer, so that the expedient of having the fiLament winding mounted upon the common core, but as a separate winding, upon the high-voltage transformer cannot presently be employed.
AdditionaLly, the use of pulse techniques applied to the primary winding of the transformer inherent in these circuits creates additional voltage stresses on the transformer insulation that should be avoided. Another method of controlling the current appears described in the U. S. Patent No. 3,760,291 in which a variable re-sistor is incLuded in the current path to the magnetron, so that the magnetron operating current can be varied as a function of LeveL of resistance. This circuit appears L5 impracticaL in that rather expensive resistors are empLoyed which consume current and generate heat.
The present invention reLates to a controL
circuit for adjusting the average power output of the oven magnetron by controlLing the voltage in the secondary circuit of the transformer. More particularly, the invention provides a simple control that allows the user to adjust the power leveL from a magnetron to a certain range. The fiLament voltage may then be supplied from a secondary winding on the same transformer structure, namely the core associated with the high-voltage secondary winding. The primary pulse technique of known circuits is thus avoided. The invention, therefore, increases the reliability of the transformer. Any line voltage surges caused by lightning on the input line, which might destroy semiconductor type control devices connected in the primary circuit, are believed to be prevented from doing damage to circuit components.
1(~537~;0 Briefly, the present invention includes in a power supply of the type having a power transformer, more particularly a high-leakage reactance transformer for providing high-voltage AC in its secondary winding, a capacitor in series with the secondary winding and one terminal of the magnetron, and a controlled semi-conductor switching device, such as a triac, is connected electrically in shunt of the magnetron. Adjustable means are provided to supply gating signaLs to the switching device so as to turn the triac into its current conducting condition, so as to conduct current in shunt of the magnetron.
In one specific aspect of the invention, the adjustable control is responsive to the phase of the AC
voltage in the secondary winding and supplies voltage to the gate of the switching device at a predetermined phase angle during each AC half-cycle.
In a second aspect of the invention, the control means contains str~cture to provide a gating voltage only at a predetermined phase during each aLternate haLf-cycle.
And in accordance with another aspect, a muLtivibrator is used to provide a gating voLtage. The multivibrator provides its output as the input to the triac for predetermined periods which may be Less than a half-cycle of AC or for a period covering many AC cycLes.
The foregoing obJects and advantages of the invention, as well as the structure characteristic of the invention, briefly summarized above, will become better understood by giving consideration to the following detailed description of various embodiments of the 1C~537~0 invention, with reference to the drawings, wherein:
Figure 1 schematically ilLustrates one embodiment of a circuit in accordance with the invention;
Figure 2 schematically iLlustrates a modification for the embodiment of Figure L;
Figure 3 schematicaLly illustrates a second embodiment of the invention; and Figure L~ schematic~ly illustrates still another L0 embodiment of the invention.
The embodiment of Figure 1, as well as the additional embodiments of the invention presented, are illustrated in the form of a schematic electrical circuit diagram, using conventional symboLs for known elements to iLlustrate the invention clearly without the necessity of detailed and less informative mechanicaL iLLustrations. As wilL be noted, various eLements of a microwave oven well known in the art are not iLLustrated, inasmuch as they wouLd not contribute to an understanding of the structure of the invention or its mode of operation.
In Figure L, a transformer TL, which is suitably of the high-leakage reactance type, incLudes a primary winding Pl, a high-voLtage secondary winding Sl and a Low-voltage secondary winding S2, often referred to as the heater winding or filament winding. A conventional magnetron M contains a heater-cathode assembly, an anode, and an output.
The magnetron is suitably of the type for C.W.
operation at a microwave frequency of perhaps 2450 MHZ.
1~537~i0 A capacitor Cl, a triac TRl having two main current terminals and a gate terminal, a diac Dl, an adjustable resistor RL having a positionabLe tap as ilLustrated connected by means of a shaft, indicated by a dashed line to a control knob Kl, and a capaci$or C2 are included. Heater winding S2 is connected to the heater terminals of the magnetron M, so as to provide a closed electrical current-conducting circuit through the magnetron filament. The magnetron anode is electricalLy connected to ground potential and to one end, i.e. terminal 1, of transformer secondary winding Sl.
The capacitor Cl is connected in circuit between the other terminal 3 of secondary winding S1 and one of the heater cathode terminals of magnetron M, so as to form a series circuit of magnetron M, capacitor C1, and secondary winding S1. As is con-ventional, the capacitor C1 is of a reactance greater than the leakage reactance of transformer T1.
Triac TRl is connected across the magnetron with one of its main current terminals being connected in circuit with one of the magnetron heater terminals and with the other terminal being connected to 'he magnetron anode and, hence, to ground, thereby to place triac TRl electrically in shunt of the magnetron. One terminal of the variable resistor Rl is connected to a low-voltage tap 2 on secondary winding Sl and the other terminal of the resistor is connected to one terminal of diac Dl and also to one terminal of capacitor C2.
The remaining terminal of capacitor C2 is connected to ground.
~537tjO
The remaining terminal of diac DL is connected to the gate terminaL of triac TRl.
As wiLl be noted, the circuit comprising re-sistor Rl and capacitor C2 is a conventional phase-shifting circuit in which the degree, i.e. magnitude of phase shift is a function of the resistance value of resistor Rl and the capacitance of capacitor C2. Hence, an AC voltage applied at tap 2 to one terminal of resistor Rl is electrically out-of-phase with the voltage appearing across capacitor C2, constituting the phase-shifted output.
The phase difference can be adjusted by positioning knob Kl, accessible to the user, which sets the re-sistor tap to a selected position along the resistance element. The positionable tap, as shown, is of the type which shunts, i.e. short circuits parts of the resistance element, so as to reduce the effective length of the re-sistor, but other types of adjustable resistors may obviously be substituted.
In operation, primary winding Pl is connected to a suitable AC source, suitabLy 120 voLts 60 hertz, that varies cyclically and sinusoidally in level with time.
By transformer action, the line voltage applied to the primary winding is stepped down in level to a low AC
voltage which appears across the secondary winding S2 and is stepped up in level to both a high AC voltage which appears across secondary winding Sl and a some-what lower high AC voltage appearing at secondary tap 2.
The voltage across heater secondary S2 is applied to the filament of the magnetron M, so as to pass current through the filament and, after a sufficient period of time, the lns3760 heating effect warms the magnetron cathode to the temperature at which sufficient electron emission occurs in the magnetron. The output voltage across winding Sl is less than the normal operating voltage specified by the manufacturer for magnetron M. And a voltage doubling effect of the circuit is necessary in order to build up sufficient voltage across the magnetron to allow the magnetron to conduct current.
It is recalled that the magnetron is a unidirectional current-conducting device, like a diode, as it conducts current only in a direction from its anode to its cathode.
At some predetermined phase of the AC voltage appearing at tap 2 of secondary winding Sl, the voltage across capacitor C2 is of the predetermined level to cause the diac Dl, a very sensitive semiconductor voltage breakdown device, to switch into its current-conducting state, placing a voltage at the gate of triac TRl and causing discharge current to flow from capacitor C2 through the diac Dl to the gate electrode of triac TRl. Once the current through the diac re-duces to zero, the diac returns to its "off" condition.
In response to the triggering voltage and current at its gate, triac TRl is thus placed into its current-conducting condition. Considering the voltage on themagnetron filament, to be positive relative to ground and terminal 1 of transformer winding Sl, triac TRl conducts current in a path from terminal 3 of secondary winding Sl, through capacitor Cl, the triac, to the 10537ti0 other terminal 1 of secondary winding Sl. As a result, the current charges up capacitor Cl to a high positive voltage at the ungrounded side or, v~ed another way, charges the capacitor Cl to a high negative voltage at the terminal connected to the magnetron heater.
Thereafter, the AC voltage across winding Sl reverses in polarity and commences to build up to an instantaneous level. The voltage across winding Sl is additive with respect to the voltage appearing across recharged capacitor Cl. Hence, the DC voltage applied across the magnetron is essentially increasing to a level which is higher than that across secondary winding Sl.
The polarity of the voltage across the magnetron now becomes positive at the anode with respect to the heater cathode assembly. As soon as the voltage builds up to an appropriate level, the magnetron conducts current. Current then passes from the magnetron anode to its heater-cathode assembly, through the capacitor Cl, and through secondary winding Sl, which current discharges capacitor Cl.
In a conventional manner, the magnetron converts such DC current to microwave energy which is taken at the output thereof and coupled to the microwave oven cavity, not illustrated. At some time during this second half-cycle of AC, the voltage appearing across capacitor C2again increases in level sufficient to again switch diac Dl to its "on" condition which in turn triggers triac TRl into its "on", i.e. current-conducting state.
Triac TRl thus conducts current in a shunt path around magnetron M and during this phase, the magnetron no longer ~CP-76-L8 537~0 generates microwave energy. This process repeats itself quickly.
By adjustment of knob K~, the position of the short circuit on variabLe resistor Rl and, hence, the effective resistance value of r~sistor Rl is changed to vary the phase at which diac Dl and, hence, triac TRl is placed ln its electrically conductive state, so that on one half-cycLe the magnitude of charge supplied to capacitor Cl is varied and the duration of current to magnetron M during the proceeding, and then each aLternate half-cycle is similarly varied in duration.
By the above-described operation, the average of power effectively passed by the magnetron and converted into microwave energy is varied by the simple adjustment of L5 potentiometer, i.e. resistor tap to vary the microwave output power Level.
Triac TRl has another useful property in this circuit. The triac is inherently self-protective against high-voltage transients. Thus, if the voltage across its main terminaLs exceeds a certain level, known as the breakdown level of the triac, the triac switches into its current-conducting condition, irrespective of the lack of a trigger voltage at the triac's gate input.
The triac then conducts current thereafter until the current through the terminals is reduced to zero, at which time the triac again becomes nonconductive. Thus, the high voltage which appears across the magnetron is dissipated by the triac to protect the transformer and other components from high voltage spikes. In lOS37~0 accordance with the invention, the breakdown voltage of the triac seLected is within the range of LL0~ to 200~
greater than the normaL operating voltage of magnetron M.
These high voltage transients occur usually during the warm up period of the magnetron filament, i.e. the heater. In order to have sufficient electrons within the magnetron for fuLL operation, the magnetron heater must be raised to a temperature level specified by the manufacturer and in practice a magnetron does not reach its operating temperature for a period of perhaps 1 to 1-1/2 seconds after heater current to the magnetron heater has started to flow.
The power technique in which the high voltage is started to be applied to the magnetron simultaneously with the beginning of the application of the heater current to the magnetron filament is known as "coLd-start"
operation and appears in the embodiments of this invention. However, at that starting time, the magnetron filament is incapable of generating the degree of electron emission necessary for proper operation of the magnetron.
Thus, although the voltage supplied between the anode and the heater-cathode assembly of the magnetron is of the proper polarity and level, the magnetron does not conduct current~ As the magnetron's filament hçats up, a greater number of electrons becomes available although still less than that required for full operation.
During such time, the magnetron conducts current only until the supply of electrons is exhausted and then ceases conducting. This occurs over a time period of 1~537~0 less than one-half AC cycle during which the W voltage is applied across the magnetron. The sudden halt in the magnetron current generates high transient voltages, a phenomenon of the magnetron termed moding. These transient voltages are very high, possibly at the level of three to four times as high as the normal operating voltage of the magnetron.
In the prior, known power supply constructions, a diode is connected in shunt of the magnetron which is replaced in the embodiment of the invention shown in Figure 1 by the triac TRl. The diode used had reverse breakdown voltages in the order of 12,000 to 20,000 volts, so that the diode would not conduct in the reverse direction when the voltage transients appeared.
By selecting the triac to have a voltage breakdown of between lO~o greater than the normal operating voltage of the magnetron and twice the normal operating voltage of the magnetron, it is seen that the triac breaks down and conducts current to completely shunt the magnetron and divert any current, which would otherwise be supplied by the source, through the series combination of secondary winding Sl and capacitor Cl. Thus, if the magnetron does mode during the warm-up period, any transient high voltage spike is dissipated, i.e. diverted through the triac and all current is shunted around the magnetron. This process re-occurs until the magnetron filament is up to its full operating temperature when the transients no longer appear.
1~537~0 It is noted in connection with the foregoing circuit that if power control is not desired, but transient voltage suppression is desired, the conventional half-wave doubler circuit of the prior art may be modified by sub-stituting a reverse voltage breakdown characteristicsubstantially less than the 15,000 to 20,000 volts ~ reverse breakdown voltage of diodes currently empLoyed in the power supplies in accordance with the foregoing principles. Thus, a triac type device should be used that is capable of carrying the forward current in one direction and which has a reverse brea~down voltage in the order of 1.1 to 2 times the normal operation voltage of the magnetron specified by the magnetron manufacturer.
Figure 2 illustrates the combination of a diode D2 and a silicon controlled rectifier SCR
connected electricaLly in series and electrically poled in the same direction. This combination of elements may be substituted in the circuit of Figure 1 for triac TRl, as well as in other embodiments in which a triac is employed. Other equivalent semiconductor switching devices may be substituted with or without minor circuit modifications. As is apparent, current can pass through this circuit only in a direction from the anode of diode D2 to the cathode through the anode of the SCR and the cathode thereof. When SCR has the appropriate voltage applied at its gate, current cannot flow in the reverse direction. With such a substitution, the self-protecting, because transient-suppressing characteristics, inherent in the embodiment of Figure 1, ~537~0 due to the triac, are not available. Moreover, in the operation of a circuit having the elements of Figure 2 substituted for triac TRl in Figure L, current flows through capacitor Cl to charge capacitor Cl only on 5 every alternate AC half-cycle. Thus, when the voltage at the filament of the magnetron is positive with respect to the anode of the magnetron in these half-cycles of AC, the magnetron does not normaLly conduct current. In contrast to the mode of operation described for the circuit of Figure L, the combination of diode D2 and siLicon controLLed rectifier SCR cannot operate to shunt current from the magnetron during the AC half-cycl~ in which the voltage at the anode is positive with respect to the heater-cathode.
Reference is now made to the embodiment shown in Figure 3. For convenience, where the same elements used in the embodiment of Figure 1 are employed, the same designations are used to identify the same eLements and in the interest of brevity, these eLements are not described again. As is apparent from a com-parison of the two embodiments, the embodiment of Figure 3 includes a diode D3 and a resistor R2 connected in series between tap 2 on secondary winding Sl and one end terminal of potentiometer-type resistor Rl and the phase-shifting circuit.
Diode D3 is electrically poled so that its anode is connected directly to the secondary winding tap 2.
Resistor R2 is inserted in series circuit as shown to constitute a rough adjustment, as it adds to the 1 ~537~0 resistance of resistor RL. Diode D3 allows current to pass through the series circuit of resistors R2 and Rl to charge capacitor C2. This occurs only during those alternate half-cycles of AC current which appear at tap 2 of transformer secondary winding Sl and with positive polarity with respect to ground. Accordingly, capacitor C2 is charged sufficiently on alternate ones of the AC half-cycles to "fire" or cause diac Dl to breakdown and conduct current. As described above in connection with Figure 1, L0 the operation of diac Dl discharges capacitor C2 through the gate of triac TRl and thereby renders triac TRl conductive only on these same AC half-cycles. The phase angle of the AC at which triac TRl switches to its current-conducting condition is a function of tap L5 position of adjustable resistor Rl, as adjusted by the oven user by manipulation of knob Kl. Thus, on the one-half cycle of AC as appears across secondary winding Sl when the voltage at the winding terminal 3 is positive relative to the grounded terminal 1, current passes through triac TRl, secondary winding Sl and capacitor Cl to charge capacitor Cl to a high negative voltage, limited in level to the level of voltage appearing across secondary winding Sl, at the terminal of the capacitor Cl connected to one magnetron heater terminal. Triac TRl, once it is in the "on"
condition, continues to conduct current until the current level reduces to ~ero, after which the triac returns to its nonconducting condition. The triac then remains in the "off" condition until a trigger voltage is applied to its gate electrode to repeat the process.
!MCP-76-L8 1~537~:i0 By varying the phase angle at which the triac TRl is placed in its current conducting condition, the charge and hence the voltage across capacitor Cl may be adjusted in LeveL. SuitabLy, this adjustment is most pronounced at the Line phase angles of between 70 degrees and L20 degrees so tha~ there is insufficient time for the capacitor Cl to charge to high voltages. The described operàtion and structure has its roots, in principle, in the clamper circuits described in Chapter 5, pages 65 through 71 of the boo~ entitled "Semi-conductor Pulse Circuits" Mitchell, published by Holt-Rinehart &
Winston, Inc. 1970, which illustrates the principle of varying voltage levels in a general sense, as applied to pulse type circuits.
In the next succeeding AC haL~-cycle, the voLtage appearing across secondary winding Sl rises sinusoidally in level, but is opposite in polarity from the voltage in the preceding half-cycle. The voltage across winding Sl is added to the voltage across capacitor Cl and the sum of these voltages is applied between the anode and the heater-cathode assembly of magnetron M. As the voltage across the magnetron rises to the proper level and is of the proper polarity, the magnetron conducts current through the path consisting of the magnetron, the main capacitor CL and the secondary winding Sl. The current cLosed through this path discharges the capacitor Cl and the direct current through the magnetron is converted into high frequency microwave energy. Clearly, the DC current 10537~0 through the magnetron depends upon the charge in capacitor Cl. The charge applied to capacitor Cl through operation of the triac in succeeding half-cycles is adjustable in leveL, hence also the current to the magnetron and the power output thereof is similarly adjustable.
As in the above-described embodiment, the triac TRl is preferably selected so as to have a breakdown voltage, i.e. a voltage at which the triac L0 will conduct even though no trigger voltage is applied to its gate eLectrode, of between 110% and 200% of the normal operating voltage Vn specified by the manufacturer for proper operation of magnetron M, to thus provide the transient voltage suppressing characteristic in the L5 power suppLy during the initial warm-up period of the magnetron, as described above.
It is noted that, in connection with circuit of Figure 2, because of the mode of operation, the effective impedance of the secondary winding, as viewed from the primary winding, may change and become more reactive, so ~hat the power factor of the circuit drops to an extent resulting in an increase in the primary current. It is believed that this primary current is still at an acceptable level. Nonetheless, various compromises may be made in the circuit in pro-portioning and optimizing the various elements so that the primary power factor remains within acceptable levels.
For example, if the specific model of transformer found in various ~nown microwave ovens is employed, as welL as l~S~o the existing capacitor and magnetron, the application of the phase-controlled triacs to that specific assembly results in a high peak current and full charge of capacitor Cl, if the triac is turned "on"
early in the first quadrant (90~ of the AC cycle, due to the inherent effects of resonance like inter-action between the capacitance of the capacitor and the leakage reactance of the transformer, so that power variation may be adjusted within a range of 70 to 90 degrees of the first quadrant. This leads to the desired result of adjustability. It is obvious, however, that to eliminate this resonance effect, the transformer design is modified to increase the leakage inductance of the transformer, as by the addition of a larger magnetic shunt between the primary and secondary windings, as is ~nown to those skilled in the art.
Having the disclosed invention before him, one skilled in the art obviously can attend to the details of optimization of design for any specific embodiment of the invention.
It will be clear that the above-described circuits provide power control with relatively few components. Inasmuch as power controL is obtained through electronic operations on the secondary side of the power transformer, the power controL actions do not seriously affect *he voltages appearing across the heater winding, as occurs in those earlier power control designs in which the current applied to the primary was controlled or interrupted, which would ~537~0 necessariLy result in interruptions of heater current, had the heater winding been supplied by the same trans-former in those preceding designs, which in fact required heater transformers separate from the high voltage trans-former . Moreover, a degree of isolation is provided,forthe electronic components against line voltage surges occurring in the primary winding Pl due to ambient events, such as lightning striking the power lines from the eLectrical utility company.
Reference is now made to the embodiment of Figure 4 which includes a high-leakage reactance trans-former T2 having a primary winding P2 adapted to be connected to a source of 120 volts 60 hertz AC, as availabLe from a locaL power utility company, a low-voltage secondary winding S4 and a high-voltage secondary winding S3. A magnetron M has its fiLament connected in circuit with the secondary winding S4,.
The magnetron anode is connected to one terminal of secondary winding S3 which is considered to be at electrical ground potential, as indicated by the symbol. A capacitor C3 is connected in circuit between one of the magnetron heater-cathode assembly terminals and the other terminal of the secondary winding. This is the basic circuit described above in connection with Figures 1 and 3.
A diode D4 and a triac TR2 are connected electrically in series between the heater-cathode terminal of the magnetron and the grounded anode terminal of the magnetron. Diode D4 is electrically poled so that ~CP-76-L8 ~1~5371~0 ~ts anode is connected directly in circuit with the heater-cathode terminal of the magnetron, so as to place diode D4 in a reverse poLarity reLationship with the magnetron. A multivibrator MV is provided which may be of any conventional structure. The multivibrator incLudes a controL knob K2 that is accessible to the microwave oven user. SuitabLy, the muLtivibrator may be powered by any convenient electrical power source and preferably is operated from voLtage supplied by a -10 tap on the secondary winding S3, as indicated by the dashed line. The multivibrator output is connected to the gate terminal of triac TR2.
Preferably, the multivibrator provides a square wave output voltage, with a positive output pulse having a duration of greater than many AC half-cycles, such as for example one second. The multivibrator is adjustable to provide various durations of output voltage. For example, a given setting of the multivibrator may permit selecting an output which is positive for one second and which then is zero for another second and which then repeats itself, so as to enable the gate electrode of triac TR2 for one second intervals, separated by identical intervals in which triac TR2 remains in its noncondutive state. As the orientation of control knob K2 is adjusted, the output voltage duration changes, let us say to another position of knob K2 with which the output pulse will be at a duration of one-half second. As in the above-described examples, in essence the circuit functions in the described manner.
h~P-76-18 1~537tj0 The Line voLtage applied to the primary winding P2 is stepped down to the low AC voltage appearing across low-voltage secondary winding S4 and is stepped up to the high AC voltage appearing across secondary winding S3, typically Less than the high voltage which would be the normal operating voltage specified for operation of magnetron M. Moreover, a low voltage is provided at the tap. It is assumed that the magnetron is at operating temperature and the voltage at the upper terminal of winding S3, as seen in Figure 4, is positive reLative to ground. If then the output pulse of multivibrator MV is applied to the gate electrode of triac TR2 to place the triac into its eLectricaLLy conductive condition and to maintain it in this state L5 for the duration of the pulse, current flows from the upper terminal of winding S3 through capacitor C3, diode D4, triac TR2, and back to the secondary winding, so as to allow capacitor C3 to become fully charged.
The magnetron, which exhibits diode-Like characteristics as is known, does not conduct current during those half-cycles in which the voltage at its heater-cathode terminal is positive with respect to the anode. In the alternate half-cycles, the voltage across winding S3 reverses in poLarity and sinusoidaLLy buiLds up in level, just like in the above-described embodiments. The voltage across winding S3 is additive to the voltage appearing across capacitor C3, a result of the charge given the capacitor during the preceding AC half-cycle. When the sum of the voltages _22-:
,~
10537~V
reaches the level necessary to cause the magnetron to conduct, the magnetron conducts current from its anode to its heater-cathode, the circuit incLuding secondary winding S3, magnetron M and capacitor C3, to discharge the capacitor and to convert the DC current through the magnetron into microwave frequency energy which is extracted by an output circuit, not iLLustrated.
At this moment, the electricaL poling of diode D4 prevents current Erom passing through the circuit consist;ng of diode D4 and triac TR2, inasmuch as the voltage at the anode terminal of diode D4 in this AC half-cycle is negative with respect to the cathode. The operation continues with the magnetron conducting during one-half AC cycles and the circuit consisting of diode D4 and triac TR2 conducting on alternate AC half-cycles to charge the capacitor C3, untiL the output of multivibrator MV drops off and ceases for an interval. It is apparent that, if the current through the magnetron continues for one second and then ceases for one second, the magnetron is operated at essentiaLLy haLf power, over a given period of time.
As to modifications to this circuit, various equivaLents may be substituted for the triac, such as a series of sLave-triggered silicon controlLed rectifiers (not iLlustrated) of the type described on page L64 of the SCR ManuaL, 5th edition, pubLished by the General Electric Co. in 1972. The purpose in so doing is to use a series of silicon controlled _23-1~537~0 rectifiers having reLatively Low voltage breakdown characteristics so that the reverse breakdown characteristic of the circuit is the sum of the individuaL breakdown characteristic. Thus, should a triac be unavaiLable having a suitable reverse voltage breakdown characteristic necessary to prevent current conduction during the alternate half-cycles as a result of any high-voltage transients generated in the circuit during moding, a series string of lower voltage silicon controlled rectifiers may be used instead. It is also noted that various multivibrators may be employed including a multivibrator having a variable time base, as well as a variable pulse width.
It is believed that the foregoing description.
L5 of various embodiments of the invention is sufficient in detail to enable one skilled in the art to make and use same. It is expressly understood that the invention is not limited to these details presented in connection with the above description, inasmuch as various substitutions of equivalents, additions, and improvements will become apparent to those skiLled in the art upon reading this specification, all of which may be embodied with the invention. Accordingly, the invention is to be broadly construed within the full spirit and scope of the appended claims.
^24-
Claims (7)
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED
AS FOLLOWS:
1. An adjustable power operating circuit for a microwave magnetron in a microwave oven, said magnetron having an anode terminal and a pair of heater-cathode terminals, and possessing unidirectional current-conducting characteristics between the anode and cathode thereof, the magnetron being operative to generate microwave frequency energy responsive to the application of a voltage of magnitude Vn volts between the anode and cathode thereof in which the polarity of such voltage is negative at said cathode, comprising:
a high-leakage reactance transformer containing a primary winding, a low-voltage secondary winding and a high-voltage secondary winding;
said primary winding being adapted for connection to an AC voltage source, which voltage is sinusoidally varying in polarity at a period T;
said low-voltage secondary winding being connected in circuit with said heater terminals for supplying current thereto;
said high-voltage secondary winding being operable for stepping up the voltage of said primary to a high AC voltage Vs, where Vs is less than Vn volts, appearing thereacross;
said high-voltage secondary winding including a tap for providing a low AC voltage representative of the high AC voltage across said secondary winding;
capacitor means;
means connecting said secondary winding, said capacitor means and said magnetron in electrical series circuit;
electronic switching means, said electronic switching means having first and second main current-conducting terminals and a gate terminal, and possessing the characteristic of being responsive to a trigger voltage applied to its gate electrode for switching into an electrically conductive state substantially instantaneously relative to one-half the period of said AC voltage and which returns to the electrically non-conductive state responsive to the instantaneous value of current therethrough when reduced to zero;
said first and second main terminals being connected, respectively, in circuit with said anode and cathode terminals, respectively, of said magnetron to place said electronic switching means in shunt of said magnetron;
phase-shifting circuit means having an input and an output for introducing a phase shift between an AC signal as appears at said output, said phase shifting circuit including:
adjustable means for permitting selective adjustment of the magnitude of said phase shift;
an accessible positionable control member;
scale means being associated with said control member for visually representing power levels as a function of the position of said positionable control member;
means coupling said control member to said adjustable means to permit adjustment of said adjustable means as a function of the position of said control member;
means connecting said input of said phase-shifting circuit to said tap on said secondary winding for providing an AC voltage to said input; and triggering means coupled to said gate terminal and to said output of said phase-shifting network responsive to the output of said phase-shifting network attaining a predetermined level during each AC cycle at a time when the voltage at the magnetron heater-cathode is positive relative to the anode for providing a trigger voltage pulse at least once during each AC cycle to said gate terminal of said electronic switching means; whereby said capacitor may be charged to a voltage during the half-cycle of AC which voltage is additive with the voltage Vs in the next succeeding half-cycle and the sum of which voltages is at least equal to Vn volts.
a high-leakage reactance transformer containing a primary winding, a low-voltage secondary winding and a high-voltage secondary winding;
said primary winding being adapted for connection to an AC voltage source, which voltage is sinusoidally varying in polarity at a period T;
said low-voltage secondary winding being connected in circuit with said heater terminals for supplying current thereto;
said high-voltage secondary winding being operable for stepping up the voltage of said primary to a high AC voltage Vs, where Vs is less than Vn volts, appearing thereacross;
said high-voltage secondary winding including a tap for providing a low AC voltage representative of the high AC voltage across said secondary winding;
capacitor means;
means connecting said secondary winding, said capacitor means and said magnetron in electrical series circuit;
electronic switching means, said electronic switching means having first and second main current-conducting terminals and a gate terminal, and possessing the characteristic of being responsive to a trigger voltage applied to its gate electrode for switching into an electrically conductive state substantially instantaneously relative to one-half the period of said AC voltage and which returns to the electrically non-conductive state responsive to the instantaneous value of current therethrough when reduced to zero;
said first and second main terminals being connected, respectively, in circuit with said anode and cathode terminals, respectively, of said magnetron to place said electronic switching means in shunt of said magnetron;
phase-shifting circuit means having an input and an output for introducing a phase shift between an AC signal as appears at said output, said phase shifting circuit including:
adjustable means for permitting selective adjustment of the magnitude of said phase shift;
an accessible positionable control member;
scale means being associated with said control member for visually representing power levels as a function of the position of said positionable control member;
means coupling said control member to said adjustable means to permit adjustment of said adjustable means as a function of the position of said control member;
means connecting said input of said phase-shifting circuit to said tap on said secondary winding for providing an AC voltage to said input; and triggering means coupled to said gate terminal and to said output of said phase-shifting network responsive to the output of said phase-shifting network attaining a predetermined level during each AC cycle at a time when the voltage at the magnetron heater-cathode is positive relative to the anode for providing a trigger voltage pulse at least once during each AC cycle to said gate terminal of said electronic switching means; whereby said capacitor may be charged to a voltage during the half-cycle of AC which voltage is additive with the voltage Vs in the next succeeding half-cycle and the sum of which voltages is at least equal to Vn volts.
2. The circuit as defined in Claim 1, wherein said electronic switching means comprises a triac, wherein said triggering means comprises a diac and wherein said phase-shifting circuit includes capacitor means.
3. The invention as defined in Claim 2, wherein said magnetron has a normal operating voltage of the magnitude Vn and said triac has a voltage breakdown characteristic in the range of 1.1 to 2.0 times said normal operating voltage of said magnetron.
4. The circuit as defined in Claim 1, wherein said adjustable means comprises a variable resistor.
5. The circuit as defined in Claim 1, and further including diode rectifier means whose diode is connected in series between said tap on the high-voltage secondary winding and said input of said phase-shifting network for supplying AC voltage to said phase-shifting circuit only on those AC half-cycles of voltage when said magnetron is in the nonconducting state.
6. An adjustable power operating circuit for a microwave magnetron in a microwave oven, said magnetron having an anode terminal and a pair of heater-cathode terminals, and possessing unidirectional current-conducting characteristics between the anode and cathode thereof, the magnetron being operative to generate microwave frequency energy responsive to the application of a voltage of magnitude Vn volts between the anode and cathode thereof in which the polarity of such voltage is negative at said cathode, comprising:
a high-leakage reactance transformer con-taining a primary winding, a low-voltage secondary winding and a high-voltage secondary winding;
said primary winding being adapted for connection to an AC voltage source, which voltage is sinusoidally varying in polarity at a period T;
said low-voltage secondary winding being connected in circuit with said heater terminals for supplying current thereto;
said high-voltage secondary winding being operable for stepping up the voltage of said primary to a high AC voltage Vs, where Vs is less than Vn volts, appearing thereacross;
said high-voltage secondary winding including a tap for providing a low AC voltage representative of the high AC voltage across said secondary winding;
capacitor means;
means connecting said secondary winding, aid capacitor means and said magnetron in electrical series circuit;
triac switching means, said triac having first and second main current-conducting terminals and gate terminal, and possessing the characteristic of being responsive to a trigger voltage applied to its gate electrode for switching into an electrically conductive state substantially instantaneously relative to one-half the period of said AC voltage and which returns to the electrically nonconductive state responsive to the instantaneous value of current therethrough when re-duced to zero and further possessing a characteristic reverse breakdown voltage in the range of 1.2 Vn to 2.0 Vn;
said first and second main terminals being connected, respectively, in circuit with said anode and cathode terminals, respectively, of said magnetron to place said triac switching means in shunt of said magnetron;
phase-shifting circuit means having an input and an output for introducing a phase shift between an AC signal applied at said input and an AC
signal as appears at said output, said phase-shifting circuit including:
adjustable resistance means for permitting selective adjustment of the magnitude of said phase shift;
an accessible positionable control member;
scale means associated with said control member for visually representing power levels as a function of the position of said positionable member;
means coupling said control member to said adjustable resistance means for adjusting the resistance of said resistance means as a function of the position of said control member;
means connecting said input of said phase-shifting circuit to said tap on said secondary winding for providing an AC voltage to said input; and triggering means coupled to said gate terminal and to said output of said phase-shifting network responsive to the output of said phase-shifting network attaining a predetermined level during each AC cycle at a time when the voltage at the magnetron heater-cathode is positive relative to the anode for providing a trigger voltage pulse at least once during each AC cycle to said gate terminal of said triac switching means; whereby said capacitor may be charged to a voltage during the half-cycle of AC which voltage is additive with the voltage Vs in the next succeeding half-cycle and the sum of which voltages is at least equal to Vn volts.
a high-leakage reactance transformer con-taining a primary winding, a low-voltage secondary winding and a high-voltage secondary winding;
said primary winding being adapted for connection to an AC voltage source, which voltage is sinusoidally varying in polarity at a period T;
said low-voltage secondary winding being connected in circuit with said heater terminals for supplying current thereto;
said high-voltage secondary winding being operable for stepping up the voltage of said primary to a high AC voltage Vs, where Vs is less than Vn volts, appearing thereacross;
said high-voltage secondary winding including a tap for providing a low AC voltage representative of the high AC voltage across said secondary winding;
capacitor means;
means connecting said secondary winding, aid capacitor means and said magnetron in electrical series circuit;
triac switching means, said triac having first and second main current-conducting terminals and gate terminal, and possessing the characteristic of being responsive to a trigger voltage applied to its gate electrode for switching into an electrically conductive state substantially instantaneously relative to one-half the period of said AC voltage and which returns to the electrically nonconductive state responsive to the instantaneous value of current therethrough when re-duced to zero and further possessing a characteristic reverse breakdown voltage in the range of 1.2 Vn to 2.0 Vn;
said first and second main terminals being connected, respectively, in circuit with said anode and cathode terminals, respectively, of said magnetron to place said triac switching means in shunt of said magnetron;
phase-shifting circuit means having an input and an output for introducing a phase shift between an AC signal applied at said input and an AC
signal as appears at said output, said phase-shifting circuit including:
adjustable resistance means for permitting selective adjustment of the magnitude of said phase shift;
an accessible positionable control member;
scale means associated with said control member for visually representing power levels as a function of the position of said positionable member;
means coupling said control member to said adjustable resistance means for adjusting the resistance of said resistance means as a function of the position of said control member;
means connecting said input of said phase-shifting circuit to said tap on said secondary winding for providing an AC voltage to said input; and triggering means coupled to said gate terminal and to said output of said phase-shifting network responsive to the output of said phase-shifting network attaining a predetermined level during each AC cycle at a time when the voltage at the magnetron heater-cathode is positive relative to the anode for providing a trigger voltage pulse at least once during each AC cycle to said gate terminal of said triac switching means; whereby said capacitor may be charged to a voltage during the half-cycle of AC which voltage is additive with the voltage Vs in the next succeeding half-cycle and the sum of which voltages is at least equal to Vn volts.
7. A variable power operating circuit for a microwave magnetron of the type having an anode terminal and heater-cathode terminal, comprising:
a high-leakage reactance transformer having a primary and a secondary winding, said secondary winding including a low-voltage tap;
capacitor means;
means for connecting said capacitor-means and said secondary winding electrically in circuit across said magnetron;
triac means, said triac means having a gate electrode and two main current-conducting terminals;
means connecting each of said respective main terminals in circuit with a corresponding one of the anode terminal and heater-cathode terminal, respectively, of said magnetron to place said triac in shunt of said magnetron;
rectifier diode means, selectively adjustable resistance means, and capacitor means, connected in series circuit between said low-voltage tap on said secondary winding and an end of said secondary winding;
diac means connected in circuit between the junction of said resistance means and said capacitor means and said gate electrode of said triac means; and an accessible positionable control member coupled to said adjustable resistance means for setting said adjustable resistance means.
a high-leakage reactance transformer having a primary and a secondary winding, said secondary winding including a low-voltage tap;
capacitor means;
means for connecting said capacitor-means and said secondary winding electrically in circuit across said magnetron;
triac means, said triac means having a gate electrode and two main current-conducting terminals;
means connecting each of said respective main terminals in circuit with a corresponding one of the anode terminal and heater-cathode terminal, respectively, of said magnetron to place said triac in shunt of said magnetron;
rectifier diode means, selectively adjustable resistance means, and capacitor means, connected in series circuit between said low-voltage tap on said secondary winding and an end of said secondary winding;
diac means connected in circuit between the junction of said resistance means and said capacitor means and said gate electrode of said triac means; and an accessible positionable control member coupled to said adjustable resistance means for setting said adjustable resistance means.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA268,943A CA1053760A (en) | 1976-12-30 | 1976-12-30 | Power controller for microwave magnetron |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA268,943A CA1053760A (en) | 1976-12-30 | 1976-12-30 | Power controller for microwave magnetron |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1053760A true CA1053760A (en) | 1979-05-01 |
Family
ID=4107622
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA268,943A Expired CA1053760A (en) | 1976-12-30 | 1976-12-30 | Power controller for microwave magnetron |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1053760A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110663108A (en) * | 2017-05-03 | 2020-01-07 | 应用材料公司 | Method and apparatus for uniform heat distribution in a microwave cavity during semiconductor processing |
-
1976
- 1976-12-30 CA CA268,943A patent/CA1053760A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110663108A (en) * | 2017-05-03 | 2020-01-07 | 应用材料公司 | Method and apparatus for uniform heat distribution in a microwave cavity during semiconductor processing |
| CN110663108B (en) * | 2017-05-03 | 2024-03-12 | 应用材料公司 | Method and apparatus for uniform heat distribution in microwave cavity during semiconductor processing |
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