Electronic microwave oven power supply

A power supply for a magnetron is adapted to be powered from a regular power line and comprises an inverter means operable to provide an AC voltage of relatively high frequency. This relatively high-frequency AC voltage is applied to a step-up transformer, which transformer exhibits a relatively high leakage inductance between its input and output windings. A capacitor is connected across this transformer's output winding and effectively resonates with the internal inductance thereof. A rectifier and filter means is connected in parallel circuit with this capacitor, and provides an output of current-limited substantially constant-magnitude DC voltage for application to the magnetron. As a result, the magnetron is efficiently powered with a nearly constant DC voltage, as contrasted with the pulsed DC voltage normally used for powering magnetrons.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to electronic means for powering the 
magnetron in microwave ovens, particularly to means of a type that 
provides the magnetron with a substantially constant current derived from 
a programmably controllable inverter-type power supply. 
2. Description of Prior Art 
The magnetron in presently available microwave ovens is typically powered 
by a 60 Hz pulsed voltage provided by way of a conventional coil-and-core 
current-regulated transformer and a combination voltage doubler and 
rectifier means. In fact, it is believed that in all presently available 
microwave ovens, the magnetron is powered by way of the methods and 
circuits described in U.S. Pat. No. 3,396,342 issued to Feinberg and 
assigned to Advance Transformer Co. of Chicago, Ill. 
To the best of my knowledge, no inverter-type power supply for microwave 
oven magnetrons is presently available for purchase or described in 
published literature. 
RATIONALE RELATED TO THE INVENTION 
With the power supply presently used in microwave ovens, the magnetron is 
being supplied with a current-regulated voltage that is being pulsed in 
the fashion of a rectified sinewave at some relatively low frequency 
(typically 60 or 120 Hz). As a result of this sinewave-pulsed and 
therefore highly variable voltage and current, the magnetron does not 
operate all the time at its most efficient operating point. In a typical 
case, the overall conversion efficiency for a magnetron operated with the 
indicated low-frequency sinewave-pulsed current-regulated voltage may be 
about 55%. If that same magnetron were to be operated at a fixed voltage 
and a steady continuous current, the conversion efficiency could instead 
be about 70%--for some 27% improvement in magnetron efficiency, which 
implies a 33% reduction of magnetron losses at the same level of power 
output. 
Moreover, with a sinewave-pulsed (and thereby gradually varying) supply 
voltage, the magnetron goes through a range of oscillatory modes, and 
consequentially generates spurious oscillations that may be significantly 
detrimental in their effect. For instance, with presently existing 
microwave ovens, these spurious oscillations are causing serious problems 
in terms of interfering with the operation of the family TV set. On the 
other hand, with a constant magnetron voltage, these spurious oscillations 
would be greatly minimized--as would then also be the TV interference 
problem. 
With the presently used low-frequency power supplies, it is not practicable 
to provide for a substantially constant magnetron voltage. The required 
high-voltage filtering means would simply be far too large and costly. 
Never-the-less, when perceiving the various direct and indirect benefits 
associated with providing the magnetron with a substantially constant 
supply voltage, and by coupling this perception with the recognitions 
that: 
(a) the requisite high-voltage filtering capacitor would not represent a 
significant size or cost issue if the power supply were to operate at a 
frequency substantially higher than 60 or 120 Hz (like in the range from 
20 to 40 kHz), and 
(b) is is possible to design an inverter-type power supply that can meet 
the requirements associated with powering a microwave oven magnetron from 
a regular household electric power receptacle, 
it may be understood that--contrary to expectations--there is considerable 
economic justification for using an inverter-type power supply for 
powering the magnetron in microwave ovens, even if such a power supply 
were to represent a significant direct cost-penalty as compared with the 
conventional low-frequency power supply. 
Moreover and unexpectedly, there are some significant additional benefits 
that may be obtained in connection with using an inverter-type power 
supply for the magnetron--benefits such as: improved controllability of 
magnetron power (without the need for using a Triac for switching the line 
voltage applied to the magnetron power supply ON or OFF); availability of 
power from the inverter for other loads in the microwave oven even during 
periods when the inverter is not powering the magnetron; reduced 
requirements of the blower used for cooling the magnetron (because of the 
increased efficiency of the magnetron); and/or more cooking power 
available from a given size magnetron. 
Yet, even with the improved magnetron efficiency, to provide the normally 
required 700 Watt of microwave cooking power will still require about 1000 
Watt of current-regulated real power input to the magnetron. With 1000 
Watt of real power being provided by an inverter in a current-regulated 
fashion (as, for instance, with the current being limited by an inductive 
means), the implication is that the inverter itself will have to handle 
4000 to 6000 Volt-Amp; which implies the need for using extremely 
high-powered and therefore costly transistors for efficiently handling a 
Volt-Amp product of that kind of magnitude; which further implies that 
inverters may be more costly than could reasonably be justified by the 
value advantages they would provide to the microwave oven. 
However, the perception that a series-resonant circuit coupled across the 
voltage output of the inverter can be used for achieving, not only the 
requisite current-regulated constant voltage for the magnetron, but also a 
drastic reduction in the magnetude of the Volt-Amp product that need be 
handled by the transistors, provides for a correspondingly drastic 
reduction in the requirements of the inverter and hence its cost; which 
then does appear to permit the cost of the inverter power supply to be 
reduced to a level that is commensurate with the values derived from it in 
the context of a microwave oven. 
SUMMARY OF THE INVENTION 
1. Objects of the Invention 
A first object of the present invention is that of providing an 
inverter-type power supply for the magnetron in a microwave oven. 
A second object is that of providing a magnetron power supply that is 
substantially reduced in weight and volume as compared with presently 
available power supplies. 
A third object is that of providing a magnetron power supply that powers 
the magnetron with a substantially constant current as compared with the 
pulsed current of present power supplies. 
A fourth object is that of providing a magnetron power supply the output of 
which is easier to control than is the case with present power supplies. 
A fifth object is that of providing a magnetron power supply that is 
particularly suited for integration with a microwave oven and its 
controls. 
A sixth object is that of providing a magnetron power supply that is more 
efficient that is the case with presently available power supplies. 
A seventh object is that of providing for a microwave oven a combination 
electronic power supply and a programmable control means. 
An eighth object is that of providing a magnetron power supply that permits 
the magnetron to operate in a mode of substantially higher efficiency. 
A ninth object is that of providing a high-frequency inverter power supply 
for a magnetron wherein high-frequency power is being extracted from the 
inverter at a relatively high power factor. 
These as well as other objects, features and advantages of the present 
invention will become apparent from the following description and claims. 
2. Brief Description 
The present invention relates to an electronic power supply for the 
magnetron in a microwave oven of the type adapted to be powered from a 
standard household electric power receptacle. 
In this electronic power supply, an inverter is used for generating a 
relatively high frequency AC voltage of moderate voltage magnitude. A 
transformer connected at the output of this inverter provides isolation 
from the power line as well as step-up transformation of this relatively 
high frequency AC voltage, thereby providing the high-magnitude 
power-line-isolated voltage required for the magnetron. (Because of the 
relatively high frequency--being 30 kHz or so--this transformer is very 
small and light compared with the transformer presently used for providing 
high voltage for the magnetron.) 
To optimize magnetron power conversion efficiency, while at the same time 
minimizing its spurious responses, this high-magnitude voltage is 
rectified and filtered before being applied across the magnetron; which 
magnetron is thus being operated at a substantially constant DC voltage. 
(Because of the relatively high frequency, the size and cost of the 
filtering means are very modest compared to what they would have had to be 
at a frequency of 60 Hz.) 
To achieve a high degree of magnetron power regulation, while also 
minimizing the Volt-Amp product required to be supplied by the inverter, 
the power to the magnetron is provided by way of a series-resonant circuit 
arrangement. This resonant circuit is used in a so-called Q-multiplier 
arrangement; which implies that the Q-multiplication factor increases as 
the load decreases; which, due to the highly non-linear voltage-current 
characteristics of the magnetron, further implies that, as the magnetron 
voltage is reduced, the circuit Q-factor (and thereby the Q-multiplication 
factor) increases; which tends to restore the magnetron voltage and 
thereby to maintain magnetron power. 
In other words, the series-resonant circuit arrangement effectively imparts 
to this magnetron power supply the characteristic normally associated with 
a current source. 
To provide a high degree of power supply controllability, the inverter is 
provided with input terminals by which its output characteristics can be 
significantly varied. In particular, means are provided by which the 
inversion frequency can be varied over a wide range. Thus, at or near the 
resonant frequency, with the help of the Q-multiplication factor, the 
magnetron receives its requisite current-regulated high voltage; while at 
a substantially lower frequency, well removed from resonance, the effect 
of the Q-multiplication factor is not present and the voltage supplied to 
the magnetron falls to such a low level as not to give rise to any 
significant magnetron current. Yet, the inverter is still in operation and 
can be used for furnishing voltages that are needed for other uses within 
the microwave oven, such as for powering the programming circuits.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a block diagram of the preferred embodiment of an electronic 
microwave oven power supply constructed in accordance with the present 
invention. 
In FIG. 1, a source of ordinary household AC voltage is represented by S. 
This AC voltage is provided across a pair of input terminals 1a and 1b to 
the overall power supply 1. Terminals 1a and 1b are connected with input 
terminals 2a and 2b of power factor correction means 2, the output of 
which is provided across input terminals 3a and 3b to rectifier and filter 
means 3. The output from rectifier and filter means 3 is a substantially 
constant unidirectional voltage; which voltage is applied across input 
terminals 4a and 4b of inverter 4. 
The output from inverter 4, which is provided across output terminals 4x 
and 4y, is an AC voltage of a relatively high frequency (such as in the 
range of 20 kHz to 40 kHz), which AC voltage is provided across input 
terminals 5a and 5b of auxiliary power transformer 5, and also across 
input terminals 6a and 6b of main power transformer 6. 
Auxiliary power transformer 5 has two output terminals 5x and 5y, which 
output terminals are electrically isolated from input terminals 5a and 5b. 
These output terminals 5x and 5y are connected with a pair of auxiliary AC 
power output terminals 1ACx and 1ACy of overall power supply 1, which 
terminals are adapted to accept connections with various loads external of 
the overall power supply. 
Output terminals 5x and 5y are also connected to rectifier and filter means 
5R, the output from which is connected with a pair of auxiliary DC power 
output terminals 1DCx and 1DCy of overall power supply 1, which terminals 
are adapted to accept connections with various loads external of the 
overall power supply. 
Main power transformer 6 has a primary winding 6p, a secondary winding 6s 
and a tertiary winding 6t, which windings are electrically isolated from 
each other. Primary winding 6p is magnetically coupled to secondary 
winding 6s and tertiary winding 6t by way of a magnetic core 6c, which 
core provides for substantial magnetic flux shunting between said primary 
winding and said secondary and tertiary windings. 
The output from secondary winding 6s is a relatively high voltage, which 
high voltage is provided across secondary output terminals 6sx and 6sy. 
The output from tertiary winding 6t, which is provided across tertiary 
output terminals 6tx and 6ty, is a relatively low voltage that is 
proportional in magnitude to the voltage output across secondary terminals 
6sx and 6sy. 
Secondary output terminals 6sx and 6sy are connected across capacitor 7 as 
well as to input terminals 8a and 8b of high-voltage rectifier and filter 
means 8. The output from rectifier and filter means 8 is a substantially 
constant high-magnitude DC voltage, which voltage is provided across 
output terminals 1x and 1y of overall power supply 1. 
Tertiary output terminal 6tx is connected with output terminal 1x and 
tertiary output terminal 6ty is connected to output terminal 1z of the 
overall power supply 1. Across tertiary output terminals 6tx and 6ty is 
connected a voltage limiting means 9. 
Output terminals 1x, 1y, and 1z are connected respectively with input 
terminals 10a, 10b and 10c of magnetron 10, which magnetron is the source 
of microwave cooking power in a microwave oven. The magnetron's anode is 
connected with input terminal 10b; its cathode is connected with input 
terminals 10a and 10c. 
Auxiliary output terminals 1DCx and 1DCy of overall power supply 1 are 
connected to power input terminals 11a and 11b of programmer means 11. An 
output from this programmer means is connected to control input terminals 
1Ca and 1Cb of overall power supply 1, which input terminals are in turn 
connected to control input terminals 4Ca and 4Cb of inverter 4. 
Details of inverter 4 are illustrated by FIG. 2, which shows an inverter of 
basically known type, as described in detail in U.S. Pat. No. 4,279,011 
issued to Ole K. Nilssen. 
In FIG. 2, the numeral 4 refers to the overall inverter; which inverter is 
powered by a source of substantially constant DC voltage applied across 
inverter power input terminals 4a and 4b--with 4a being the positive or B+ 
terminal. The B+ voltage is applied to the center-tap 41c of the single 
winding on auto-transformer 41. In addition to the center-tap, this single 
winding has two connection points 41a and 41b. 
The inverter has two transistors 42 and 43 with collectors 42c and 43c, 
bases 42b and 43b, and emitters 42e and 43e, respectively. Emitters 42e 
and 43e are connected together and thence to inverter input terminal 4b, 
which is the negative side of the DC input voltage. Diodes 42D and 43D are 
connected between the bases and emitters of transistors 42 and 43 
respectively. 
The inverter has a saturable feedback transformer 44 with a first primary 
winding 44p1 and a second primary winding 44p2; Transformer 44 also has a 
secondary winding 44s with output terminals 44sx and 44sy. 
Auto-transformer connection points 41a and 41b are respectively connected 
by way of primary windings 44p1 and 44p2 to collectors 42c and 43c. These 
connection points are also connected with overall inverter output 
terminals 4x and 47. 
Across the secondary winding of saturable feedback transformer 44 is 
connected an inverter frequency control means 45, which consists of a 
series-combination of a saturable inductance means 46 and an electrically 
actuatable switch means 47. Switch means 47 has two actuating electrical 
input terminals 47a and 47b. 
The overall inverter 4 has two control input terminals 4Ca and 4Cb which 
are connected with input terminals 47a and 47b of switch means 47. 
The operation of inverter 4 is described in said U.S. Pat. No. 4,279,011 
except for the action imparted by frequency control means 45. The function 
of control means 45 is that of providing an electrically actuatable means 
by which the saturable inductor 46 can be connected in parallel with the 
secondary winding 44s of the saturable feedback transformer 44. However, 
for saturable inductor 46 to have an effect on the inverter oscillation 
frequency, it is necessary that the Volt-second product required by 
inductor 46 to reach saturation be smaller than the Volt-second product 
required by secondary winding 44s of feedback transformer 44 to reach 
saturation. But, as long as such is indeed the case, the inverter 
oscillation frequency will increase when saturable inductor means 46 is 
connected in parallel with secondary winding 44s of saturable feedback 
transformer 44. 
Thus, when freqeuncy control means 45 is in the actuated state, the 
inverter oscillation frequency is higher than it is when control means 45 
is in the non-actuated state. The degree to which the frequency is then 
higher depends on the degree to which saturable inductor 46 requires a 
smaller Volt-second product for saturation than does secondary winding 
44s. 
Upon actuation of control means 45, and if a direct short circuit is 
substituted for saturable inductor 46, instead of increasing in 
oscillation frequency, the inverter will simply cease oscillating. In this 
case, however, it is necessary to have another way of powering control 
means 45 than by way of the inverter auxiliary output, which of course 
disappears whenever the inverter stops oscillating. A resolution to this 
problem can be accomplished by using a battery for powering the 
programming means, and thereby the inverter control means. This battery 
should preferably be of a rechargeable type--with recharging done by way 
of the inverter when oscillating. 
Having described the non-conventional part of the operation of inverter 4, 
the operation of the overall power supply circuit of FIG. 1 may now be 
explained as follows. 
Ordinary 120 Volt/60 Hz household power line voltage is applied to the 
power supply's input terminals 1a and 1b, from where this voltage is 
applied to rectifier and filter means 3 by way of a 
power-factor-correction means 2. This power-factor-correction means may 
consist of a series-connected inductor followed by a shunt-connected 
capacitor; its main function being that of widening the charging pulses 
supplied to rectifier and filter means 3. 
Rectifier and filter means 3 is constructed in a conventional manner, and 
provides as an input to inverter 4 a DC voltage of substantially constant 
magnitude. 
In well known manner, inverter 4 converts this input DC voltage into a 
substantially squarewave AC voltage of a relatively high frequency; which 
squarewave voltage is used for two purposes: (a) to provide for various 
auxiliary power outputs, as accomplished by way of transformer 5; and (b) 
to provide for magnetron power, as accomplished by way of transformer 6. 
Irrespective of inverter oscillation frequency, an AC voltage of 
substantially constant magnitude is provided at the output of auxiliary 
transformer 5; which voltage in turn is provided across output terminals 
1ACx and 1ACy to be used for various auxiliary needs for AC power in a 
microwave oven--such as for fluorescent lighting inside the oven cooking 
chamber. 
Similarly irrespective of inverter oscillation frequency, a DC voltage of 
substantially constant magnitude is provided at the output of rectifier 
and filter means 5R; which voltage in turn is provided across output 
terminals 1DCx and 1DCy to be used for various auxiliary needs for DC 
power in a microwave oven--such as for the indicated programming means. 
However, in case of transformer 6, because of the relatively loose coupling 
between the windings, and because the transformer loading is variable with 
frequency, the AC voltage outputs provided are not of constant magnitude 
irrespective of inverter oscillation frequency. 
In particular, the voltage across secondary winding 6s (that is, the 
voltage between output terminals 6sx and 6sy) will indeed be sensitive to 
the frequency of inverter oscillation. 
When the inverter frequency is relatively close to the natural resonance 
frequency of the effective output inductance of winding 6s as interacting 
with capacitor 7, the output voltage will be relatively high--higher than 
the transformer open-circuit output voltage by a factor determined by the 
operating-Q of the loaded circuit. By virtue of the particular and highly 
non-linear current-voltage characteristics of the magnetron, the circuit 
operating-Q will be relatively high whenever the output voltage is 
relatively low (because dispropotionately little magnetron current will 
then flow); but the circuit operating-Q will be relatively low whenever 
the output voltage is relatively high (because disproportionately heavy 
magnetron current will then flow). Thus, the magnitude of the output 
voltage will in effect be determined by the voltage-magnitude required by 
the magnetron to cause significant flow of anode current; which implies 
that magnetron anode current will automatically tend to stay constant 
regardless of relatively small variations in the magnitude of the AC 
voltage supplied by the inverter. In other words, changes in the magnitude 
of the magnetron current that might be caused by variations in the 
magnitude of the power line input voltage will tend to be cancelled by 
virtue of the circuit Q-multiplying effect. 
However, when the inverter frequency is substantially higher than said 
natural resonance frequency, the output voltage across winding 6s will be 
relatively low--so low as not to cause any significant magnetron anode 
current to flow. In other words, by increasing the inverter frequency 
significantly beyond circuit resonance, the power flow to the magnetron 
can be reduced to a negligible amount. Thus, magnetron power can be 
controlled by way of controlling the inverter oscillation frequency. 
While both secondary winding 6s and tertiary winding 6t are relatively 
loosely coupled with primary winding 6p, the coupling between 6s and 6t is 
relatively tight; which implies that the voltage present across the output 
of 6t is proportional in amplitude to that present across the ouput of 6s. 
Thus, voltage limiting means 9 (which is connected directly across the 
output of winding 6t) acts to limit the amplitude of the voltage across 
the output of winding 6s, as well as across winding 6t. 
The amplitude at which means 9 provides its voltage limiting effect is 
chosen such that the voltage across winding 6s will not significantly 
exceed the voltage level at which the magnetron will normally provide 
voltage limitation. Thus, during brief periods when the magnetron might 
not be operating in its normal voltage-limiting mode--such as during the 
very brief period required for its cathode to reach full operating 
temperature after initial power turn-on--voltage limiting means 9 will 
prevent the Q-multiplication effect from giving rise to destructively high 
output voltages across winding 6s. 
The relatively high-frequency AC voltage output across winding 6s is 
applied to a conventional high-voltage rectifier and filter means 8, the 
output of which is a relatively constant DC voltage of magnitude suitable 
for providing the required magnetron anode voltage--typically on the order 
of 5000 Volt. 
Due to the relatively high frequency of the AC voltage applied to rectifier 
and filter means 8, a high degree of filtering can be attained with a 
relatively modest size energy storing reactor. For instance, for a given 
amount of ripple on the output DC voltage, a filtering capacitor at 30 kHz 
would need to be only 1/500 of the capacitance it would have had to be at 
60 Hz. 
With most conventional inverter circuits, to prevent potentially 
self-destructive inverter operation, it is important to note that the 
inverter frequency should at no time be permitted to significantly lower 
than the natural resonance frequency of the transformer 6 output 
inductance as interacting with capacitor 7; which inductor-capacitor 
combination represents a series-tuned L-C circuit effectively connected 
directly across the inverter output. 
On the other hand, to provide for the best possible high-frequency power 
factor--which implies optimum inverter efficiency and minimum inverter 
components stress--it is important, when powering the magnetron, to have 
said natural resonance frequency be relatively close to the inverter 
oscillating frequency. 
Although the power supply herein disclosed would permit operating the 
magnetron at substantially any desired level of DC current input, thereby 
providing for any desired level of microwave oven cooking power, it is 
submitted that a better and more efficient method of regulating cooking 
power is that of turning the magnetron ON and OFF in duty-cycle fashion 
and in accordance with the amount of cooking power needed on an average 
basis. That way, the magnetron can be operated at its optimum point at all 
times--this optimum point being mainly determined on the basis of best 
energy conversion efficiency commensurate with a minimum of spurious 
radiation effects. 
By well known techniques, this ON-OFF duty-cycle function can readily be 
attained by way of programming means 11 in cooperation with inverter 
control means 45. Depending on the particular requirements of the 
application at hand, this ON-OFF control may be achieved either in the 
fashion of varying the inversion frequency (with the help of saturable 
inductor 46), or by turning the inverter ON and OFF (which will result 
when inductor 46 is replaced wih a direct short circuit). 
It is noted that there is no basic need to have a separate transformer 
means as an integral part of inverter 4--such as represented by 
auto-transformer 41. Rather, as would be well known in the art, 
transformer 41 could simply be combined with main power transformer 6 in 
the sense of being represented by its (now center-tapped) primary winding 
6s. 
Moreover, it is noted that there is no basic necessity for using a separate 
transformer for providing power for the indicated auxiliary loads. Rather, 
the output provided by transformer 5 can instead be provided by an 
additional output winding on transformer 6, which additional winding 
should be tightly coupled with primary winding 6p. 
It is also noted that the inverter-based microwave oven power supply herein 
described may itself constitute a source of Electro-Magnetic Interference 
(EMI), particularly in terms of sending EMI out on the power line. 
However, it is relatively simple to prevent this form of EMI by way of 
including an EMI filter in circuit with the power line input. In fact, 
power-factor-correction means 2 would naturally constitute such a filter. 
It is additionally noted that, in most anticipated applications, it would 
be advantageous for programmer 11 to have a built-in time-base; which 
would then be used in establishing the indicated ON-OFF duty-cycle for the 
magnetron. 
It is also anticipated that programmer 11 constitute the basic cooking 
programmer for the microwave oven; in which case it is important that it 
be provided with a substantially non-interruptible source of power. 
Such a source of power may be a built-in battery, (which battery could be 
rechargeably powered from terminals 1DCx and 1DCy of power supply 1), but 
it could also be a separate power supply fed directly from the power line. 
It is yet also noted that the current-regulating characteristics of the 
subject power supply, in addition to its high-frequency 
power-factor-correction characteristics, is herein described as being 
attained by way of a series-resonant circuit effectively connected across 
the output of the inverter--with the magnetron being coupled in parallel 
circuit with one of the reactive elements of this series-resonant circuit. 
However, with other inverter configurations and/or with other high-voltage 
rectifier configurations, a parallel-resonant or a composite 
series/parallel-resonant circuit may be used. In any case, to attain the 
desired current-regulating benefits, it is desirable that the magnetron be 
connected such as to act in effect as a load that is parallel-connected to 
one of the reactive elements of the resonant circuit. That way, the 
circuit-Q would increase as the current supplied to the magnetron would 
decrease. 
Next, it is noted that high voltage rectifier and filter means 8 is herein 
anticipated to constitute an ordinary high-voltage bridge rectifier with a 
high-voltage filter capacitor connected across its output. However, it is 
understood that several other rectifier-filter arrangements can be used. 
One such other arrangement, which is well known in the art, involves the 
use after rectification of a pi-filter consisting of a relatively 
small-capacity shunt-capacitor followed by a series-inductor, which is 
then followed by another relatively large-capacity shunt-capacitor. This 
arrangement can at least in part accomplish the power-factor-correction 
which, in FIG. 1, is being accomplished with capacitor 7 in combination 
with the leakage inductance of transformer 6. In other words, at least 
some of the high-frequency power-factor-correction can be accomplished as 
part of the high-voltage rectifier and filter means 8. 
Similarly, in FIG. 1, instead of using power factor correction means 2 for 
separately accomplishing the desired correction of power factor as 
asociated with the flow of power from the power line to power supply 1, a 
similar pi-filter may be incorporated with rectifier and filter means 3. 
Finally, it is noted that "corrected power factor", "power factor 
correction means" or similar terms used in this patent application refer 
to the notion of a power factor that is substantially improved 
over-and-above what it would have been without overt use of power factor 
correction means. 
For instance, the power factor normally associated with the power flowing 
from a conventional household power line and directly into an ordinary 
full-wave rectifier and filter means is typically about 50%. With even a 
high-efficiency microwave oven requiring a total power input of over 1000 
Watt, the Volt-Amp product resulting with a 50% power factor is over 2000 
VA; which is simply too high to be accommodated from an ordinary household 
power receptacle, which is typically fused at 15 Amp. 
On the other hand, using power factor correction means as herein 
prescribed, the power factor associated with the power flowing into 
rectifier and filter means 3 can readily be made as high as 90%; although 
it would frequently be adequate if it were only about 70%. 
Correspondingly, in respect to the power factor associated with the flow of 
high-frequency power from inverter 4 and to high-voltage rectifier and 
filter means 8, and in view of the need to provide for a high degree of 
magnetron current regulation, it is noted that--without the overt use of 
correction means--this power factor would be very low. Typically, without 
correction, it would be about 30%; which would imply that the Volt-Amp 
product required to be handled by the inverter would have to be three 
times as large as the power delivered. With correction as shown, this 
power factor is improved to 70% or so--with corresponding improvement in 
inverter efficiency and reduction in component ratings. 
It is believed that the present invention and its several attendant 
advantages and features will be understood from the preceeding 
description. However, without departing from the spirit of the invention, 
changes may be made in its form and in the construction and 
interrelationships of its component parts, the form herein presented 
merely representing the preferred embodiment.