Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
MCP-76-17
10~i9184
This invention relates to regulated adjuætable electrical
power supplie and, in a re specific context, to a regulated
aajustable power supply for a microwave magnetron of a microwave
oven.
The microwave oven is a well-known appliance used to heat
or cook foods by exposure to microwave energy. For this purpose,
conventional microwave ovens employ an electronic vacuum tube,
known as a magnetron. Simply stated, the magnetron is a device
having unidirectional current carrying characteristics which
converts DC power into energy within the microwave frequency
range, such as the frequency permitted by United States law for
that purpose, namely 2,450 megahertz. To provide that DC voltage,
various additional electrical components are included as a power
supply to convert normal household line voltage, typically 120 or
240 volts at 50 to 60 hertz into the high voltages, in the order
of 3,000 to 4,000 volts DC, required in the operation of pre-
sently available magnetrons. In its essentials, a typical
microwave oven power supply contains a tran~former for stepping
up the 120 volt ~C to the level of 3,000 to 4,000 volts,
depending upon the particular voltage requirement of any specific
model of microwave magnetron, a rectifying means, or a voltage
doubler-rectifier, and the magnetron itself. Moreover, a source
of low voltage is provided for the heater of the magnetron.
Microwave energy generated by the magnetron is taken from the
magnetron output and transmitted either directly, or through a
waveguide, into the oven chamber.
The average power supplied to the magnetron is set
within limits by the design of both the power supply and the
magnetron and i~ generally directly related to the microwave
output power generated thereby. It is known that the adjustment
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~ MCP-76-17
1(~6~184
of the microwave power can be made within limits by adjustment
of the DC current level through the magnetron. Present microwave
oven power supplies generally employ a high leakage reactance
transformer, in combination with a modified half-wave voltage
doubler, known also aq a "Villard" circuit, to rectify and double
the voltage output of the high voltage transformer, and apply a
high voltage DC to the magnetron. Exampleq of such circuits
appear in United States Patents Nos. 3,396,342, 3,651,371; and
3,684,978. These circuit~ provide satisfactory operation and
use a minimum number of components.
Recent practice i9 to provide additional elements within
the oven power supply which permit the user to adjust the average
power of the magnetron. This has been accomplished as either a
two-step "high" or "low" power arrangement or as an adjustable
level device allowing continuou~ adjustment. In the first type
of device, the value of the capacitance in the voltage doubler
circuit was made variable in order to permit adjusting the
current, see United States Patent No. 3,684,978. Another
expedient is to employ a triac control in the primary circuit of
the transformer, in order to regulate the average of current into
the power supply, but this circuit requires a separate filament
transformer because of the interrelated voltage into the primary
of the high voltage transformer, so that the expedient of having
the filament winding combined on the same transformer core upon
the high voltage transformer, but as a separate winding, cannot
be employed. Additionally, the use of pulse techniques, inherent
in this known method, in the primary winding of the tran~former
creates additional stresses on the transformer insulation, which
should preferably be avoided.
In the case of current control in the secondary winding
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~. .
MCP-76-17
~069184
circuit, the approach set forth in United States Patent No.
3,684,978 does not provide sufficient variety of adjustment, and
an adjustable resistor approach, disclosed in United States
Patent No. 3,760,291, appears somewhat impractical and wasteful,
in that a resistor consumes electrical energy as it generates
heat.
The present invention relates to the control of the
average power output level of a magnetron by controlling the
voltage in the secondary winding of the transformer. More
particularly, the invention provides a control which can be used
to allow the user to selectively adjust the power level of the
magnetron within a certain range, or, in an alternative applica-
tion, which may be established adjusted to a fixed level at the
factory. Then, the filament voltage may be supplied with power
from an additional winding on the same high voltage transformer
as that which is used to provide the high voltage to the magne-
tron. The primary pulse techniques of the above-mentioned known
technique is avoided, the reliability of the transformer i9
improved, and any surges caused by lightning hitting the input
line, as might destroy semiconductor type control devices
connected in the primary circuit, are minimized.
The invention provides a power supply of the type having
a transformer for generating high voltages AC in a secondary
winding, having a capacitor in series with the secondary winding
and one terminal of a unidirectionally current-conducting load,
such as a magnetron, and a diode connected in shunt of the load,
or magnetron, electrically oppositely poled thereto. Means are
included electrically in series with the diode to selectively
control the average current through the diode during the half-
cycle of AC in which the magnetron is not conducting current and
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:. . : . .~ . . ..
MCP-76-17
~069184
.~ .
thus control the level of charge and voltage on the capacitor.
In this way, the level of voltage and current applied to the
magnetron on alternate half-cycles, during which the magnetron
conducts current, may be selectively adjusted in level.
Suitably, in accordance with a specific feature of the
; invention, means are provided to monitor the current through the
magnetron or other unidirectionally current-conducting load and
means responsive to the level of current during the one half-
cycle of AC for controlling the average value of current through
the diode during the other AC half-cycle. Ideally, the load
current is held at or near the selected current level. Thus,
when the current thr~ugh the magnetron falls below a certain
~i selected level, the control means permits the current through
the diode to increase, so as to permit a greater charge to be
stored in the capacitor during one, i.e. the first, half-cycle,
to thereby cause greater current to the magnetron on each
alternate, i.e. second, half-cycle. Conversely, if the level of
current through the magnetron is above a selected level, the
control means reduces the average value of current through the
diode on the other AC half-cycle to lessen the charge on the
capacitor and thus to reduce the level of voltage which appears
across the magnetron during the next succeeding alternate half-
cycle, to thereby cause a lesser current through the magnetron.
Suitably, the reference level can be adjusted by the user to any
desired level so as to permit the user to control the power
supplied to the magnetron. Alternatively, the reference level
can be adjusted at the factory to one level or for two separate
levels to be obtained, depending on the alternative positioning
of a switch accessible to the user.
In a more specific aspect of the invention, the control
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,: . . . - : . . ~ - .
MCP-76-17
10t;~184
,
means comprises a semiconductor controlled switch, such as a
triac, means for providing, i.e. setting, a reference signal
representative of the desired magnetron current level, means
for comparing the magnitude with that set as the reference and
providing an output to the gate of the semiconductor controlled
switch at a time before or during the next AC half-cycle com-
puted to properly charge the series capacitor.
In a still more detailed aspect of the invention, the
means for determining the magnetron current level comprises a
first resistance means connected in an electrical series circuit
with said magnetron. In addition, a series circuit comprising a
resistor and a capacitor is connected across said first resis-
tance means, to form a conventional RC circuit. Preferably, the
RC circuit has a time constant T, which is the product of capa-
citance (farads) multiplied by resistance (ohms), (secondswhich is approximately 1/2F seconds), where F is the frequency
in cycles per second of the AC high voltage appearing across
the transformer secondary winding. A comparator, such as an
operational, or differential amplifier, including a noninverting
input and an inverting input forms part of the circuit. The
reference voltage is applied to the noninverting input of the
comparator and the voltage across the capacitor in the RC net-
work is applied to the inverting input thereof. When the
voltage across the timing capacitor exceeds a certain level, the
output of the comparator switches from low to high, to shut off
the triac. Thus, during the alternate half-cycle in which the
voltage applied to the diode is correctly poled, so as to nor-
mally conduct current, current is blocked since the triac is not
gated on. However, at some time during such half-cycle, the
voltage across the timing capacitor discharges to a level less
MCP-76-17
1069~84
than the reference level, which, in turn, causes the comparator
to switch to its "on" state and provides a low gate pulse to the
triac. Once the triac conducts current, current flows through
the capacitor and the diode during the remainder of that AC
half-cycle to thus charge the capacitor. It is noted that, on
the next succeeding AC half-cycle, the triac is essentially
shut off, no current flows through the diode, and the magnetron
is properly poled with respect to the applied voltage and again
conducts current. The voltage across the magnetron at that half-
cycle equals the sum of the voltage across the capacitor and the
AC voltage across the secondàry winding of the transformer.
The invention will become better understood from the
following detailed description of one embodiment thereof, when
taken in conjunction with the drawings, wherein:
Figure 1 illustrates one embodiment of a power supply
circuit for a magnetron, in accordance with the
invention,
Figure 2 illustrates characteristic ideal wave-shapes of
voltages or currents inherent in the operation
of the circuit of Figure 1, and
Figure 3 is a power supply circuit for a magnetron in
accordance with another embodiment of the
invention.
The embodiment illustrated in Figure 1 includes a trans-
former T having a primary winding Tp, a high voltage secondary
winding TSl, a low voltage secondary winding TS2, a capacitor Cl,
a magnetron M, a rectifier diode D2, a triac TR1, a comparator
A1, resistors R4 and R5, a capacitor C3, a diode Dl, a capacitor
C2, further resistors Rl and R3 and a potentiometer resistor R2.
Transformer T is of any conventional type, as it has a
MCP-76-17
1069184
laminated iron core, with the primary and secondary windings
located thereon and with the primary formed from a predetermined
number of turns of insulated wire adapted for connection to a
source of current, suitably 120 volt 60 hertz. The high voltage,
secondary winding TSl is formed of a large number of turns of
relatively fine insulated wire, so as to define a large turns
ratio between the secondary TSl and primary Tp and place the
secondary in a step-up voltage relationship with the primary,
while the other secondary winding TS2 is of a relatively few
number of turns of insulated wire, defining a turns ratio with
the primary less than one to place the secondary in a voltage
step-aown relationship with the primary, to provide the low
voltage suitable for application to a magnetron heater. A tap 3
in secondary winding TSl provides a low voltage relative to the
reference point to be obtained. The transformer may be one in
which the primary winding is "loosely" coupled to the high
voltage secondary, known as a high leakage reactance type trans-
former found in existing commercial ovens. It is noted,
however, that the transformer may be a ferrite core transformer
used in electrical inverter-type power supplies providing high-
voltage high-frequency power in the order of 20 kilohertz or
above.
Secondary winding TS2 is connected across the two heater-
cathode terminals of the magnetron. One terminal 1 of secondary
winding TSl is connected electrically in series to one terminal
; of capacitor Cl and the other terminal of the capacitor is
connected in circuit to one of the heater-cathode terminals of
; magnetron M. The magnetron anode is connected to one terminal
of resistor R5. The other terminal of resistor R5 is connected
to ground potential, as shown. The other terminal 2 of secondary
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MCP-76-17
9184
TSl is also connected to ground. Tap 3 is located at winding TSl
nearer the terminal 2, so as to be at a low voltage point rela- -
tive to ground. Tap 3 is connected to the cathode terminal of
rectifier diode Dl. The anode terminal of rectifier diode Dl
is connected to one terminal of capacitor C2 and, in turn, the
remaining terminal of capacitor C2 is connected to electrical
ground potential to form a rectifier-filter circuit. Resistors
Rl, R2 and R3 are connected in electrical series circuit between
the ungrounded~terminal of capacitor C2 and ground potential, to
form a resistive voltage divider network. Potentiometer resistor
R2 contains a conventional movable tap R2t, by means of which the
resistance value between an end terminal and the tap may be
selectively adjusted in value. This tap is connected electrically
to the noninverting input of comparator Al as indicated by the
plu8 (+) sign. The V- power input terminal of comparator Al is
connected in circuit with the ungrounded terminal of capacitor
C2 and the other power terminal of comparator Al is connected to
ground. The output terminal of comparator Al is connected to the
gate of triac TRl and the inverting input of comparator Al, as
represented by the minus (-) qymbol, i8 connected in circuit
between resistor R4 and capacitor C3. Resistor R4 and capacitor
C3 are connected in electrical 3eries circuit and that series
circuit is connected in circuit across resistor R5.
The anode of rectifier diode D2 is connected in circuit
with the heater-cathode terminal of the magnetron and the
cathode of diode D2 i~ connected to one power terminal of triac
TRl. The remaining triac power terminal is connected to ground
potential. The magnetron is situated in circuit between capa-
citor Cl and ground potential, so that, since the magnetron has
self-rectifying properties, passing DC current in a direction
` MCP-76-17
1069184
from its anode to its cathode-heater, the circuit has unidirec-
tional current-conducting properties. The diode D2, similarly,
can pass current only in a direction from its anode to cathode.
Since the diode is connected in a second electrical circuit that
shunts the first circuit, this second circuit has unidirectional
current-conducting characteristics also, but, inasmuch as the
diode anode is connected to capacitor Cl, this second circuit
conducts current only in a direction opposite to the current
direction in the first circuit and i8 therefore in essence
"oppositely electrically poled".
With a source of AC voltage, ~uch as I20 volts 60 cycle
AC, connected to primary winding Tp of the transformer, by
transformer action, the primary voltage is ~tepped up and appears
across secondary winding TSl as a high voltage AC. By design,
this AC voltage i9 less than that specified for operation of the
magnetron. Additionally, also by transformer action, the primary
voltage is stepped down and appears across secondary winding TS2
as a low AC voltage which, by design, is needed for the magnetron
heater which heats the magnetron cathode to make it more electron
emi~sive, as is known to those skilled in the art.
A low voltage AC voltage is also produced at tap 3 of the
high voltage secondary winding TSl. This voltage is rectified
; by rectifier Dl and the resultant DC current charges capacitor
C2 to a predetermined low voltage level to form a low voltage DC
power supply for the control circuits. This DC voltage is
applied from the ungrounded side of capacitor C2 to the V- input
of comparator Al. By selection of the suitable tap location on
the transformer secondary winding TSl, the DC voltage established
; is of the proper level to provide DC power for the comparator Al.
In addition, the DC is applied across the voltage divider formed
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MCP-76-17
~06~184
by resistors Rl, R2 and R3. The DC current through the resistors
produces a voltage or IR drop thereacross and a portion of the
voltage drop appears between tap R2t and ground. The potenti-
ometer tap R2t is movable to any one of various locations along
the resistor R2 and hence allows selection of different levels of
reference voltage. The tap location may be connected by means
of a shaft, indicated by the dashed line, to a rotatable knob
that may be located on the control panel of the microwave oven.
A calibration legend to inform the operator of the various power
levels which may be attained in the oven through rotation of the
knob to one position or any other position may be conveniently
provided. On the other hand, the tap may be adjusted to a
selected position at the factory for a predetermined power level
and fixed in that position. It is noted that a pushbutton
selected resistor network or equivalent may be substituted for
the potentiometer arrangement.
Magnetron M conducts electrical current only when the
voltage at its heater-cathode assembly is negative with respect
to the magnetron anode and then only when the magnitude of that
voltage reaches a predetermined triggering level, such as 3,500
volt~ by way of example, as in the case of a specific model
magnetron that has a normal operating voltage of about 4,000
volts. Thus, the circuit including magnetron M has unidirectional
current-carrying characteristics. Diode D2 conducts current in
a direction from its anode to its cathode and blocks current from
passing in the opposite direction. Moreover, since diode D2 is
in series with the triac TRl, current flows only when the triac
is switched into its current-conducting condition. A triac is a
well-known semiconductor switching device having two current-
carrying terminals and a gate terminal. Functionally, the triac
MCP-76-17
1069~84
conducts only after a high positive or negative voltage relative
to the grounded terminal is applied to its gate electrode and
thereafter continues to conduct current even after removal of the
potential from its gate electrode, as long as the current through
the device does not reduce to zero, i.e. as long as current flows
through it.
When the current does 90 reduce to zero, the triac
reverts, i.e. switches, to its noncurrent-conducting condition
and can return to its current-conducting condition only in
re~ponse to another inîtiating voltage applied to its gate
electrode. Thus, the series circuit of diode D2 and triac TRl
is switchable and has unidirectional current-conducting charac-
teristics. When the AC voltage at winding terminal 1 i8 in the
negative AC half-cycle, as is shown theoretically and graphically
in Figure 2a, relative to the other winding terminal considered
as the voltage reference, the AC voltage at the terminal of
capacitor Cl which i8 connected to diode D2 and to the magnetron
i~ negative.
Assuming for the moment that the output of operational
amplifier Al applied to the gate of triac TRl i9 voltage low,
that the AC voltage at winding terminal 1 is positive, and that
the triac TRl i8 in its "on" or current-conducting condition,
then current flows from the ~econdary winding through capacitor
Cl, through diode D2, through the triac to ground and thus to
the other terminal, i.e. terminal 2, of the transformer secondary
winding TSl. This current charges capacitor Cl to +V. Within
one half-cycle of alternating current, the current and voltage
reach a peak and then decrease to zero. At zero current, the
triac TRl restores to its "off" or noncurrent-conducting condi-
tion and remains off during the next half-cycle. As is noted,
--- MCP-76-17
1069184
the current through the secondary circuit has charged capacitor
Cl to a positive potential +V, theoretically speaking to the peak
level of voltage which appeared across secondary winding TSl.
On the next or alternate half-cycle, the AC voltage across
winding TSl reverse~ and makes the voltage at terminal 1 nega-
tive with respect to the grounded terminal 2. As this occurs,
the voltage on the winding is in additive relationship with the
voltage on capacitor Cl and, theoretically speaking, assuming no
electrical load, the voltage measured between the anode of diode
D2 and ground would attain a level of twice the peak voltage
across secondary winding TSl or -2V. mi8 is recognized as a
voltage multiplication effect. However, in reality, as the
voltage across ~econdary winding TSl is building up in the reverse
direction, at some point in time the voltage level between the
capacitor C2 and ground exceeds that required to initiate current
conduction in magnetron M. Inasmuch as the voltage across the
magnetron is properly poled, i.e. biased, 90 that the cathode iq
negative with respect to the grounded anode, current flows during
this half-cycle in a path from winding terminal 2, ground,
resistor R5, the magnetron, capacitor Cl, winding terminal 1,
and through winding TSl to winding terminal 2.
The magnetron, as is known, converts the DC energy into
high frequency microwave energy which is taken from output Mo and
conventionally routed to the cooking chamber of a microwave oven,
not illustrated, the details of which are not necessary to the
understanding of the present invention.
Consider now the operation of the control circuits and
the selective switching of triac TRl. The DC current flowing
thr~ugh the magnetron on one, i.e. a first, half-cycle AC also
passes through resistor R5 producing a voltage drop across
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1069184
resistor RS, such as might be represented by any of curves A, B
or C of Figure 2b. This voltage i9 of a pulsating nature having
a peak value equal to that representative of the peak current
through the magnetron.
This voltage is applied through resistor R4 to charge
capacitor C3 up to essentially peak voltage. Resistor R4 and
capacitor C3 are seen to form a well-known RC circuit. For
example, during the half-cycle when magnetron M is conducting
current, the voltaqe across resistor R5 creates a charging current
through resistor R4. During the alternate half-cycle in which
the magnetron is not conducting current, the charge on capacitor
C3 discharges through resistor R4 and resistor R5 in series.
The time constant of the circuit R4, RS and C3 is
preferably about equal to one AC half-cycle, so that the charge
on a capacitor C3 is partially discharged during the alternate
half-cycles in which magnetron M is not conducting current. For
example, the AC half-cycle may be 8.3 milliseconds and the time
constant may be 8 milliseconds. Thus, during an alternate half-
cycle, the level of voltage on capacitor C3 is representative of
the current level through magnetron M during the preceding half-
cycle. This voltage i9 ideally illustrated in Figure 2c.
Obviously, the greater the voltage drop across resistor R5
during the half-cycle in which the magnetron is conducting
current, the higher is the level of charge on capacitor C3, as
is evident by comparing curves A, B and C in Figure 2c. In
effect, the RC circuit stores the information on the current
level through the magnetron in the preceding half-cycle for use
during the subsequent half-cycle. The voltage from capacitor C3
is coupled to the inverting input indicated by a minus sym~ol
(-) of operational amplifier Al. It is noted that this time
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~- MCP-76-17
1069184
constant can be made longer or shorter than one half-cycle, with
concurrent change in value of other circuit components, to
obtain similar but less satisfactory results.
An operational amplifier is a well-known circuit device
and is here used as a comparator by means of which the voltage
level representative of current through the magnetron i9 com-
pared to the predetermined voltage level established at the
reference input thereof from the tap R2t of potentiometer
resistor R2. me operational amplifier or comparator, as may be
variously termed herein, is illustrated by a conventional symbol.
However, the circuit in fact is a somewhat complicated component
having numerous transistors, diodes and resistors on a single
integrated circuit chip. Such types of devices are available
from semiconductor manufacturers, one type being marketed under
the designation CA 741, although an equivalent may be used.
Briefly, in operation, the output V0 of the operational amplifier
is negative low when the level of voltage at its inverting input
is less than the voltage level at its reference input. When the
voltage at the inverting input exceeds the voltage level at the
reference input, the output is zero. As is shown, the output of
the comparator Al i8 coupled to the gate electrode of the triac
TRl. The triac blocks current flow through the path from one
terminal of capacitor Cl, diode D2, the triac to ground and
thence to the other side of the secondary TSl until the voltage
at the inverting input of comparator Al falls below the reference
voltage level.
Returning now to the state of charge on capacitor C3, it
is apparent that capacitor C3 was charged during the preceding
AC half-cycle to a certain voltage which is representative of the
current through the magnetron. This voltage is greater than the
~ MCP-76-17
1069~84
reference voltage applied to the reference input of the compar-
ator. However, during the half-cycle the voltage on capacitor
C3 reduces to below thi~ reference level and the output of
comparator Al turns negative and turns on the triac. This
occurs during the next AC half-cycle when magnetron M is not
conducting current and when voltage applied to the anode of
diode D2 is proper for conduction. Considering curve A of
Figure 2c as representative of the voltage on capacitor C3 at
this time, triac switching occurs when curve A and the dashed
line VREF inter~ect. With triac TRl in the "on" condition and
diode D2 properly biased, current commences to flow through that
circuit to charge the capacitor Cl to the level of voltage of AC
at the time the circuit conducts current.
Con~ider next the case where the current through the
magnetron is excessive. In that event, capacitor C3 is charged
to a higher voltage than in the preceding case as represented in
curve C of Figure 2c. Accordingly, during the alternate half-
cycle, the discharge of this capacitor takes a longer period of
time and does not fall to the reference level VREF until a later
time within the next alternate half-cycle, as compared to the
preceding case, as may be represented by curve C of Figure 2c.
Thus, amplifier Al switches on at a later time and allows triac
TRl to switch on at a later time to initiate current through the
path including diode D2 and triac TRl to charge capacitor Cl to
a lower voltage level.
Conversely, when the magnetron cQnducts less than the
desired current in one AC half-cycle, the voltage across resistor
R5, represented ideally by curve B of Figure 2b, is lower,
capacitor C3 is charged to a lower value, the voltage on capa- -
citor C3 in the next succeeding half-cycle will drop more rapidly,
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~ MCP-76-17
1069184
i.e. in a shorter time, to the reference voltage level during the
next half-cycle of AC, as ideally represented by curve B of
Figure 2c, and the operational amplifier Al switches on triac
TRl at an earlier time within the next ~ucceeding half-cycle, so
that capacitor Cl is charged to a higher voltage. It is of
course understood that current of greater magnitude will flow
through the magnetron during the next succeeding AC half-cycle,
since the voltage on the capacitor will be additive with the
voltage of winding TSl.
By a judicious choice of voltage levels and circuit time
constants, the current lével of the magnetron is monitored, i.e.
~ampled during each half-cycle in which the magnetron conducts.
mi8 information is used in the next AC half-cycle to gate or
ungate the shunt current path across the magnetron in order to
regulate the level of charge on the series capacitor Cl, which,
in turn, affects the current level of the magnetron in the next
succeeding half-cycle in which the magnetron again conducts
current.
In essence, there are a high-voltage secondary winding,
a series capacitor and a magnetron (which is a unidirectionally
current-conducting device) connected in a series circuit. There
is an additional circuit for conducting current in shunt of the
magnetron during the half-cycles in which the magnetron i9 not
properly poled, i.e. biased, for conducting current, so as to
provide a voltage doubling effect by charging up the capacitor
during these alternate half-cycles.
The shunt current path is effectively blocXed during the
half-cycles in which the magnetron is conducting and remains
blocked until the beginning of, or at some point in time within,
the next half-cycle of AC, when the control means again unblocks
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MCP-76-17
184
the shunt current path to allow the capacitor to receive some
electrical charge and the control means is responsive to the
level of current through the magnetron in the preceding half-
cycle as the determinant of the time at or within the next
succeeding half-cycle in which the control means unblocks the
current.
If the average current to the magnetron is increased, the
average power is increased. By adjusting the reference voltage
at the reference input terminal of the comparator Al, i.e. by
setting it for higher or lower power levels, the magnetron power
; level may be easily adjusted.
Within the spirit of the invention, and viewed in a
broader context, it is apparent that other electrical loads
having a unidirectional current-conducting characteristic can be
utilized or sub~tituted for the magnetron, to achieve the same
effect and advantage.
Moreover, other obviou~ variations are apparent simply
from Figure 1. As indicated above, the magnetron include~ a
heater, with the low-voltage secondary winding TS2 of the tr~ns-
former T supplying heater current. A~ is well known, however, asecond transformer or other means can be used for this purpose,
even though the~e alternatives are more expensive. Specifically,
the novel arrangement as applied to a magnetron permits the '
heater winding to be employed on the ~ame transformer core as
the high-voltage ~econdary winding. Similarly, while a DC source
i~ shown to supply the reference and power supply voltages for
operational amplifier Al as a rectifier-filter combination con-
~isting of diode Dl and capacitor C2 coupled to a low voltage
tap on secondary TSl, it is equally possible to accomplish this
function either by substituting for that a separate DC source
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MCP-76-17
106~184
or, alternatively, providing a low voltage AC winding separate
from the secondary TSl, such as is the case with the heater
winding, or providing a separate transformer entirely to provide
the low voltage AC into the rectifier Dl. These are obviously
more expensive alternatives. While the triac is used in the mode
where it initially blocks current, it is equally possible for a
device to be used in accordance with the teachings set forth
herein to normally allow current to flow initiaLly and then to
block the current. In effect, it is possible to implement a
reversal of the sequential arrangement of parts, as compared to
that illustrated, but for the same purpose of regulating the
magnitude of charge and the voltage to which the capacitor Cl is
charged during alternate half-cycles when the magnetron is not
poled, or biased, for conducting current.
Ideally, a single silicon controlled rectifier can be
substituted for the diode and the triac in Figure l, to perform
the same functions which can be achieved with a regular SCR
voltage, if available, for operation with a negative gating or
by incorporating an inverter in the circuit to invert the ampli-
fier output to a positive voltage. ~owever, this substitution
i8 presently permissible only in a low voltage power supply by
which some other electrical load, i.e. other than a magnetron,
is driven. A magnetron operates in the voltage range of 3,000 to
4,000 volts or more, 80 that currently available semiconductor
control devices are not acceptable, as they are not capable of
withstanding back voltages at that level, although diodes, such
as one useful as diode D2, are available which can withstand such
high voltages. Moreover, tne triac is somewhat self-protecting
against reverse voltage transients and i6 thus preferred in com-
parison to presently available silicon controlled rectifiers.
--19--
MCP-76-17
.
1C~69184
The serious problems connected with the operation of
magnetrons is that of moding or mismoding. This means that, under
certain circumstances when power is applied to the magnetron, the
magnetron may go into oscillation at a frequency at which the
magnetron was not designed to operate, instead of the correct
frequency. This phenomenon is the cause for an inherent diffi-
culty in using magnetrons and occurs chiefly in operation of
magnetrons in the pulse mode, typically in radar systems where
the full voltage is applied almost instantaneously, such as like
a step function. If the anode voltage is applied gradually to
the desired level, such as at audio frequencies or below, the
phenomenon of moding does not usually occur. Conversely, were
the magnetron circuit to be gated on and off rapidly, as by a
device such as a silicon controlled rectifier or triac, which
have very fast operating times, namely in the order of nano-
seconds, a mismoding problem could arise. However, as is seen
in the circuit, the rapid turn on of the triac, or a substituted
semiconductor controlled rectifier, occurs during the charging
of the capacitor and does not directly gate on or off the magne-
tron. In this specific context, there is provided a uniquefeedback control circuit for regulating the current level of a
magnetron in which the rapid switching of current occurs during
the alternate half-cycles in which the magnetron is not operating.
It is anticipated that any voltage transients that may arise and
appear to cause difficulties can be cured by judicious insertion
of suitable protéctive devices, such as Zener diodes.
It is noted that with present magnetrons it is not
necessary to adjust the voltage applied to the magnetron over a
full range from zero volts to a maximum operating voltage, but
that only a limited range of voltage variation is satisfactory.
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` MCP-76-17
1069184
By way of example, a specific model of a magnetron may provide an
output of'600 watts with an applied voltage of 4,000 volts, but
may provide only 100 watts output with an applied voltage of
~,500 volts. mus, only 500 volts less result in a six-fold
reduction in power output.
Therefore, it is only necessary to reduce the charge or
voltage on the main capacitor by a small percentage to obtain a
large percentage reduction in power output.
Reference may be made to Chapter 5, Clamper Circuits,
pages 65-71 of the book entitled "Semi Conductor Pulse Circuits",
Mitchell, Holt, Rinehart ~ Winston, 1970, for background
principle relevant to the clamping of voltages to different
levels and which formed part of the helpful knowledge from which
the present invention evolves.
A second embodiment of the invention is presented in
Figure 3. For clarity, those elements in this embodiment which
are essentially the same as those described in connection with
Figure 1 are identified by the same reference character, to
identify the element. Moreover, inasmuch as most of the elements
in Figure 3 are connected in the same manner and have the same
function as in the above-described embodiment, they are not
described again, in the i,nterests of clarity and conciseness, so
that the following description of structure is confined to those
changes or modifications in the c~rcuit which are significant.
Thus, in the embodiment shown in Figure 3, the anode of
magnetron M is connected electrically to, ground. The terminal 2
of secondary winding TSl is connected in series with resistor R5
to ground. The high-voltage diode D2, used in Figure 1, is not
used in this embodiment. Diode Dl is connected in circuit with
its anode terminal to secondary winding tap 3 and the cathode
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., . ~.
MCP-76-17
~c~69~84
terminal to the circuit junction of capacitor C2 and resistor Rl
and is thus reversed in polarity from the diode circuit orienta-
tion in the embodiment of Figure 1, to rectify and produce a
positive polarity voltage at the circuit junction of the
capacitor C2 with resistor Rl and hence at the noninverting input
of operational amplifier Al.
The anode of a rectifier diode D3 is connected in circuit
with ~econdary winding terminal 2 and one terminal of resistor
R5. Re~istor R6 and capacitor C4 are connected electrically in
~erie~ circuit between the anode of diode D3 and ground. A
bleeder re~i~tor R7 is connected electrically in shunt of capa-
citor C4. The circuit junction of resistor R6 with capacitor C4
is connected to the inverting input, designated by the minus
symbol (-), of amplifier Al to apply any voltage on capacitor C4
to the amplifier inverting input.
Resistor R7 and capacitor C4 essentially perform the
function of an R-C circuit, which i9 that performed by resistor
R4 and capacitor C3 in the embodiment of Figure 1. However,
because rectifier diode D3 is included in this circuit and blocks
current flow in one direction, the resistor R7 is connected to
provide a path for current out of the capacitor, so that capaci-
tor C4 may be discharged.
The mode of operation of the circuit is the same as that
of the embodiment of Figure 1 in most aspects, except as follows.
Diode Dl rectifies the low voltage AC that appears at
low voltage tap 3 of secondary TSl and provides a positive volt-
age to charge up capacitor C2 and thus the rectifier and
capacitor function as a positive DC voltage source. This volta~e
is applied to the Vl input of the comparator, i.e. operational
amplifier Al, to provide power to the amplifier and to the
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MCP-76-17
~0691B4
.
resistive voltage divider network Rl, R2 and R3, so that a
positive voltage is applied via potentiometer tap R2t to the
noninverting input + of the amplifier Al.
m e magnetron current flows in a series circuit including
capacitor Cl, secondary winding TSl, resistor R5, and ground.
Hence, the voltage drop across resistor R5 during the AC half-
cycle in which the magnetron conducts current is proportional
to the magnetron current, as in the first embodiment. The
polarity of the voltage drop is positive with respect to ground
and this is then applied through resistor R6 to charge up
capacitor C4.
During the alternate half-cycles in which magnetron M is
not conducting current, the voltage drop across resistor R5 is
representative essentially of current to charge capacitor Cl.
Since the latter current is opposite in direction to the current
through the magnetron in the preceding half-cycle, the voltage at
resistor R5 i9 negative in polarity. Diode D3 acts to block that
negative voltage from being applied to capacitor C4.
Thus, capacitor C4 is charged to a voltage of positive
polarity during one half-cycle and that voltage level is repre-
~entative of the DC current through magnetron M. During the
next half-cycle of AC during which the power circuit capacitor
Cl is to be charged, capacitor C4 commences to discharge through
resistor R7. At any given period of time during this half-cycle,
the voltage on C4 is a function of the time constant RC of the
circuit formed by resistor R7 and capaci~or C4, and the voltage
level to which capacitor C4 was initially charged. As in the
embodiment of Figure 1, this time constant is preferably equal to
1/2F, where F is the line voltage frequency, such as 8 milli-
seconds.
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MCP-76-17
~069184
As in the first embodiment, the voltage at the inverting
input of comparator Al decreases to a level equal to that of the
reference voltage applied via tap R2t to the inverting input.
At that time, the differential amplifier Al, used as the compara-
tor, switches it~ output from zero to a high positive voltage toapply an enabling input to the gate of triac TRl and the triac
switches into its current-conducting condition.
As in the embodiment of Figure 1, the point in time
during a half-cycle of AC at which the amplifier Al switches
shifts back and forth, depending on the magnetron current level
during the preceding half-cycle, to automatically ideally, by
virtue of this feedback mechani~m, adjust the voltage to which
capacitor Cl charges.
Setting of tap R2t, by means of knob K allow~ the same
adjustment as in the embodiment of Figure 1.
- It i8 noted that, since the embodiment of Figure 3
employs a po~itive output voltage to trigger triac TRl into its
"on" condition, as contrasted with a negative trigger voltage in
the embodiment of Figure 1, those electronic semiconductor switch-
ing devices which require a positive trigger voltage, such as asilicon controlled rectifier, or a series circuit of a rectifier
diode and a silicon controlled rectifier, both electrically poled
in the same direction, may be directly substituted for triac TRl.
By omitting, in the embodiment of Figure 3, the diode D2,
u~ed in the embodiment of Figure 1, and thus as a result of the
absence of a diode in series with the triac in the embodiment of
Figure 3, the risk of damage due to certain high-voltage tran-
sients is eliminated. The triac is selected to have a voltage
breakdown characteristic of between 1.1 to 2 times the magnitude
of the normal operating voltage of the magnetron, as specified by
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MCP-76-17
1069184
the magnetron manufacturer. This voltage breakdown is the
characteristic voltage at which the semiconductor switch goes
into its conducting state, irrespective of any gating voltage.
By way of example, it was found suitable to select a triac having
a breakdown voltage of 4,500 volts for a magnetron having a
normal operating voltage of 4,000 volts.
In the event that a high transient voltage commences to
develop and reaches 4,500 volts, the triac in Figure 3 will
break down and conduct the current, shunting the current from
the magnetron and the triac continues to conduct the current
until the current passing through it falls to zero. The triac
then restores to its "off", i.e. noncurrent-conducting state.
Thus, although in normal operation of the circuit the
triac conducts current in only one direction to charge the main
series capacitor, in the abnormal or voltage transient suppress-
ing de, the triac also conducts current in the opposite
direction to discharge the main capacitor and protect the
circuit again~t high voltage transients.
It is believed that the foregoing detailed description
of two embodiments of the invention is sufficient to enable one
skilled in the art to make and use the invention. However, it
is expressly understood that the specific details presented for
that purpose are not intended in any way to limit the invention,
inasmuch as, in accordance with the teachings contained herein,
numerous modifications, alterations or substitutions of equiva-
lents, such as those in part described herein! may be made by
those skilled in the art.
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.
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