MAGNETRON POWER SUPPLY
The present invention relates to a power supply for a magnetron, in particular
but not exclusively for use with a magnetron powering a lamp.
It is known that magnetrons can change mode unexpected, that is to say that
unexpectedly they can stop oscillating at one frequency and start oscillating at
another. Under these conditions, they can exhibit negative impedance. This can
result in damagingly high current flow. For this reason, it is known that
constant/controlled voltage power supplies are not suitable for magnetrons;
constant/controlled current power supplies are usually used for powering them.
Anode voltages in magnetrons are high and measurement of both anode
voltage and anode current are difficult.
In a previous power supply invented by the present inventor, measurement of
both voltage applied to a converter in a magnetron power supply and current through
the converter was utilised in a conjunction with a microcomputer to provide real time
control of power supplied to a magnetron. The microcomputer was programmed to
compute:
1. Power being consumed,
2. A difference from a desired power and
3. A difference between the power difference and the measured current.
This second difference signal was used to control the converter. It should be noted
that these three steps were executed in software. Unexpectedly, this power supply
still suffered from a degree of instability, causing perceivable flickering of the light
produced by its magnetron powered lamp.
Experience has now shown that the eye is extremely sensitive to light flicker
in a magnetron powered plasma lamp. It has now been appreciated that the limited
speed and resolution of the microprocessor output capability aggravated the perceived
flickering. Additionally, two of the inputs to the microprocessor, namely the voltage
applied to the converter and the current drawn through the converter are liable to be
noisy and multiplication of the two noisy signals is believed to have contributed to the
instability.
Simply filtering of the noise from the microprocessor reduces unacceptably
the reaction time of the control circuit and contributes to instability, bearing in mind
that fast reaction to changed magnetron conditions can be required. Accordingly a
new approach was required.
The object of the present invention is to provide an improved power supply for
;netron.
According to the invention there is provided a power supply for a magnetron
comprising:
• a DC voltage source;
• a converter for raising the output voltage of the DC voltage source, the
converter having:
• a capacitative-inductive resonant circuit,
• a switching circuit adapted to drive the resonant circuit at a variable
frequency above the resonant frequency of the resonant circuit, the
variable frequency being controlled by a control signal input to provide an
alternating voltage,
• a transformer connected to the resonant circuit for raising the alternating
voltage,
• a rectifier for rectifying the raised alternating voltage to a raised DC
voltage for application to the magnetron;
• means for measuring the current from the DC voltage source passing through
the converter;
• a microprocessor programmed to produce a control signal indicative of a
desired output power of the magnetron; and
• an integrated circuit arranged in a feed back loop and adapted to apply a
control signal to the converter switching circuit in accordance with a
comparison of a signal from the current measuring means with the signal
from the microprocessor for controlling the power of the magnetron to the
desired power.
Provision of the integrated circuit as a discrete element separate from the
microprocessor provides a fast control loop, which is not limited by the speed of the
microprocessor. (The latter is liable to be slow due to economic constraints on it
specification.) Thus the power supply of the invention is inherently more stable and
provides less flicker-prone illumination.
Whilst it can be envisaged that the integrated circuit could be a digital device,
in the interests of economy, it is preferably an analogue device. In the preferred
embodiment, the integrated circuit is an operational amplifier.
In the preferred embodiment the operational amplifier is arranged as an
integrator with a feedback capacitor whereby its output voltage is adapted to control a
voltage to frequency circuit for controlling the converter.
Preferably the microprocessor is programmed to filter noise from the desired
converter current signal. Alternatively a filter circuit can be provided between the
microprocessor and the operational amplifier.
In the preferred embodiments, the switching circuit is adapted to control the
frequency of the converter in accordance with a variable voltage signal output from
the operational amplifier. In this technique, an increase in frequency corresponds to a
reduction in magnetron drive voltage and microwave output.
Alternatively the switching circuit can be adapted to control the duty cycle of
the converter in accordance with the operational amplifier output, whereby reduction
in the duty cycle corresponds to a reduction in magnetron drive voltage and
microwave output.
In the preferred embodiments, the converter is a zero voltage switching
device; although it could be a zero current switching device.
Normally the switching circuit will have its own oscillator; however, it can be
envisaged that it could be timed from a clock in the microprocessor.
In one embodiment, the integrated circuit is adapted and arranged for the
comparison to be direct between the measured current signal and the desired power
signal, the integrated circuit being connected to receive these signals only, whereby
the converter current is controlled in accordance with the desired power independent
of transient changes in the voltage of the DC voltage source. This embodiment
controls the average power to be constant over voltage source ripple cycles.
In another embodiment, the integrated circuit is adapted and arranged for the
comparison to be not only between the measured current signal and the desired power
signal but also taking account of transient changes in the voltage of the DC voltage
source, a signal indicative of the voltage of the voltage source also being input to the
integrated circuit, whereby the converter current is controlled such that the power
passing through the converter is controlled in accordance with the desired power. This
embodiment controls the instantaneous power constant to be constant through voltage
source ripple cycles.
Normally the switching circuit will have its own oscillator; however, it can be
envisaged that it could be timed from a clock in the microprocessor.
To help understanding of the invention, a specific embodiment thereof will
now be described by way of example and with reference to the accompanying
drawings, in which:
Figure 1 is a block diagram of a pri or power supply for a magnetron;
Figure 2 is a similar block diagram of a power supply in accordance with the
invention;
Figure 3 is a more detailed circuit diagram of the power supply of Figure 2;
Figure 4 is a diagrammatic view of a lamp powered by a magnetron having a
power supply of the invention;
Figure 5 is a circuit diagram of a second embodiment of the invention;
Figure 6 is details of the voltage divider of the embodiment of Figure 5;
Figure 7 is a spectral diagram of magnetron output comparing that of the
embodiments of Figure 3 and 5; and
Figure 8 is a circuit diagram of a third embodiment of the invention.
Referring first to Figure 1, there is shown diagrammatically a prior power
supply having an oscillator 1 connected to power a magnetron 2 and controlled by a
microprocessor 3. An augmented mains voltage, DC voltage source 4 supplies
typically 400 volts on line 5 to the oscillator . This feeds alternating current to a
transformer 6 and rectifier 7 from which 4000 DC volts is applied on line 8 to the
magnetron. The oscillator, transformer and rectifier are referred to as a "high voltage
converter". Power being supplied to the magnetron is measured in terms of the
voltage across a resistor 9 in the earth return of the converter. The voltage is
indicative of the current in the resistor 9 and is proportional to the power applied to
the magnetron, assuming constant voltage from the voltage source 4. The resistor
voltage is one input on line 10 to the microprocessor. Another input on line 11
applies the voltage on line 5 to the microprocessor. A desired power control value 12
is set externally or as a manual input to microprocessor.
The microprocessor is programmed to perform the steps of :
1. Multiplication of the voltage on line 5 with current in resistor 9 to compute the
power being supplied to the magnetron, assuming high efficiency;
2. Comparison of the computation of the power being consumed with the desired
power and thence a computation of the current that should be being consumed
(the intended current);
3. Comparison of the intended current with the measured current and application to
the power supply of an incrementally higher voltage to drive the converter at a
higher frequency if the current is high or any incrementally lower voltage if the
current is to low. It should be noted that if the converter operates at a higher
frequency, the resultant voltage across the magnetron drops.
As already mentioned, this circuit proved in use to be too unstable for flicker
free operation of the magnetron as a light source.
Turning now to Figure 2, a power supply of the invention comprises the
following similar components connected in the same manner:
• oscillator / high voltage converter 101;
• magnetron 102;
• transformer 106
• rectifier 07
• resistor 109.
A microprocessor 103 is also included, but it operates quite differently. It
merely divides a desired power control value 112 by the augmented mains DC voltage
on the line 105, and provides a required current signal on line 121 indicative of the
desired current through the converter 101 to operate the magnetron at the desired
power. The signal on line 121 is fed to one input of an operational amplifier
122/EA1. Its other input has a line 110 to it from the resistor 109, indicating the
actual current passing through the converter. The operational amplifier is connected
as an integrating error signal magnifier.
Turning on now to Figure 3 there is shown a fuller circuit diagram of the
power supply of Figure 2. Central to it is a quasi-resonant oscillator 101 of a high
voltage converter, having MOSFET field effect switching transistors T1,T2. These
are switched in a manner to be described below by an integrated circuit oscillator IC .
An inductance LI and primary coil of the transformer 106 are connected in series to
the common point of the transistors T1,T2. Capacitors C3,C4 complete the series
resonant circuit. The inductances and the capacitors determine a resonant frequency,
above which the converter is operated, typically around 70kHz, whereby it appears to
be primarily an inductive circuit as regards the down-stream magnetron circuit. This
comprises four half bridge diodes D3,D4,D5,D6 and smoothing capacitors C5,C6,
connected to the secondary winding of the transformer and providing DC current to
the magnetron 102. The windings ratio of the transformer is 10:1, whereby voltage of
the order of 4000 volts is applied to the magnetron, the augmented mains DC voltage
on line 105 typically being 400 volts.
A feature of the converter circuit is that when the transistors Tl ,T2 are
switched ON and then OFF sequentially in turn, the energy stored in the inductance
LI inverts the voltage across it. This drives down the voltage at the common point C
before TR2 switches on and drives up the common point voltage before TR1 switches
on. Thus switching occurs at zero or close to zero volts across the transistor about to
be switched on, that is in ZVS mode (Zero-Voltage-Switching mode). This
contributes to reliability and longevity.
At high switching frequency (i.e. above resonance), the voltage at the common
point between the capacitors C3,C4 is substantially constant at half the voltage on line
105, whereby on transistor switching, a substantially triangular wave form ramp
current flows through the inductance LI. This is passed to the transformer and thence
ultimately to the magnetron.
Lowering the frequency to operation closer to resonance increases the voltage
swing at D away from half the voltage on line 105 and increases the voltage at the
magnetron, its current and its microwave output.
The current through the converter is measured at resistor 109/R1, typically
lOOmQ, and a voltage indicative of it is passed via feed-back resistor R5, typically
470, to one input 123 of the operational amplifier 122. The microprocessor 103, via
a voltage divider R3,R4, receives the voltage from the line 105. A required power
setting is set via a manual input 112. The microprocessor is programmed to divide the
required power by the line voltage and apply to the other input 125 of the operational
amplifier a voltage indicative of the converter current required for the required
magnetron, via a 6kQ resistor R10. The operational amplifier has an integrating
capacitor C7, typically 470nF in series with a resistor R9 1. The ratio of the
resistors R9,R10 determines the gain of the operational amplifier. This again set to
suppress mains voltage flicker as much as feasible. The amplifier passes an integrated
voltage indicative of the required power to a frequency control circuit 126 for the
oscillator IC1, which is a voltage to frequency circuit, typically Texas Instruments
IRS2153 or ST Thomson L6569. The circuit comprising resistor R2 18kQ, capacitors
C1,C2, both 470pF, and diodes D1,D2 operates to control the frequency of the
converter. When the operational amplifier's output is zero, the capacitor CI is in
parallel with C2 and the lowest frequency is obtained. This corresponds to maximum
magnetron power. On the other hand, when the output is maximum the diodes never
conduct and the frequency is controlled by C2 alone. Maximum frequency and
minimum power - of the order of one tenth of the maximum - is supplied. At
intermediate voltages, CI has an intermediate effect and the frequency and power is
controlled accordingly.
Thus the magnetron can be controlled to operate at the desired power input to
the microprocessor. The microprocessor is susceptible to flicker inducing variations
in the voltage on line 105. However, the signal to RIO can be filtered internally by
software or externally by a non-shown RC filter. Should the magnetron power
consumption shift, as it can do as its magnets heat up and its resistance changes, the
operational amplifier reacts fast to the change in current measured at the resistor Rl
and adjusts the frequency of the converter and hence corrects the power consumption
by the magnetron independently of the signal on line 125 from the microprocessor.
That said, if there is flicker on the voltage source line, the power of the
magnetron will be constant only when averaged over the flicker period. There does
tend to be double mains frequency flicker on the voltage source line, due to the cost of
large smoothing capacitors.
It should be noted that the above described power supply is particularly suited
to control of the LER magnetron powered lamp as described in WO 2009/063205. It
enables the light output of the lamp to be controlled at will as and when required from
low level for background light to full power full illumination.
Shown in Figure 4 is a simplified representation of a lamp driven by the
magnetron. It has a lucent crucible 201 with a Faraday cage 202. A void 203 in the
crucible has a fill 204 of excitable material. The magnetron 205is arranged to project
its microwaves into a waveguide / transition 206 from which they exit on a coaxial
connection 207 to an antenna 208 releasing them into the crucible. Powering of the
magnetron by a power supply 209 of the invention causes the excitable material to
emit light. It is this light that the power supply of the invention is advantageous for in
avoiding flickering.
Turning now to Figure 5, there is shown an improved high voltage converter,
also in accordance with the invention. It takes account not only of variations in the
converter current, and hence the magnetron current, but also mains frequency ripple -
or more precisely twice main frequency ripple on the output of the voltage source.
This ripple does not cause perceptible flicker in the light from the LER, but does
induce bandwidth spreading in the output of the magnetron.
The modification of Figure 5 is the inclusion of a resistor R6, in the form of
two 1resistors in series, from the voltage source line to the operational amplifier
input 123 to which the feed back resistor R5 is connected. The resistors R6-R5 form
a voltage divider. The divider is such that the voltage across the resistor R5 is
substantially the same as the voltage across the current measuring resistor, typically
both of the order of lOOmV, giving 200mV at the operational amplifier input. The
actual voltage varies with both the actual current in the converter and the actual
voltage on the voltage source line. It will be appreciated that an increase in the
operational amplifier input of 200mV due to increase in the voltage source line will be
equivalent to an increase in the operational amplifier input 200mV due to increase in
the current. Both raise the integrated output voltage of the operational amplifier, with
the result that the controlled current is reduced.
The actual increase in operational amplifier input due to a 5% increase in the
voltage source voltage will be 5%, because the voltage across the current measuring
resistor is small compared to the voltage source voltage. Equally for a 5% increase in
current, the voltage across the current measuring resistor will be 5%. This will be
added to the voltage at the operational amplifier input. Thus for a 5% or other small
percentage increase in the voltage or the current, the current will be reduced by the
same percentage.
In turn this results in a 5% or other small percentage reduction in the power
being applied to the magnetron. Thus the arrangement acts to keep instantaneous
power constant. In this respect, instantaneous is used to mean that the power is kept
constant throughout the cycle of the voltage ripples for instance.
This operation can be explained mathematically as follows:
The power of the magnetron is the product of the voltage source voltage U and
the converter current I, i.e.
P = U I .
In terms of units of voltage and current, u and I :
P = (Cix u) x (C2x i)
P = K x (u x i)
With u and i having unit value, this formula can be rewritten as
P = K x (u + i) / 2.
This relationship remains approximately correct for small variations in voltage and
current, i.e. for u ± u, i ± ί .
The above equation can be rewritten as
Thus the power of the magnetron can be represented as a constant plus another
constant times any deviation of the actual voltage source from its nominal value plus
another constant times any deviation of the current from a nominal current. The
current deviation itself can be represented of the voltage across the current measuring
resistor.
With appropriate constants, and considering only the variations input to the
operational amplifier, it can be seen that the voltage divider does input the sum of the
two variations in voltage source voltage and converter current to the operational
amplifier. The only proviso is that the approximation
P = U x I K x (u + i) / 2
Is satisfied only if the voltage across R5 is approximately equal to that across Rl.
This is satisfied for the values:
U = 400volts
Rl = 0.1
R5 = 470
R6 = 2.
These resistors are shown in series in Figure 6, with indication of the relevant
voltages also shown.
It should be noted that because R6 is seven orders of magnitude greater than
Rl and R5 is four orders of magnitude greater, any change in U which create an
appreciable change of voltage at the operational amplifier input is unlikely to cause an
appreciable change of voltage across Rl, whose voltage is controlled only by the
current through it. Accordingly the voltage across Rl is added to that across R5 and
the sum is input to the operational amplifier.
It will be appreciated that this means of operation is not exactly linear, but it
does provide significant improvements. With reference to Figure 7, there is shown a
saddle shaped graph of the bandwidth of frequency of the magnetron's generation. Its
generation frequency is dependent on the current through it, it being a feature of a
magnetron that it has a characteristic akin to that of a zener diode in controlling the
voltage across it. Thus if more power is available to it, its current increases and with
its operating frequency is lowered. Where there is a mains voltage related ripple on
the voltage of the voltage source, the magnetrons frequency varies and the bandwidth
exhibits a slight saddle shape. By contrast, with the power control of the embodiment
of Figure 5, the bandwidth is much narrower and has a Gaussian distribution. This in
its turn is advantageous in causing much less interference with Bluetooth
communication networks and the like
Turning on to Figure 8, a multiplier circuit 301 is shown at the input to the
operational amplifier. This circuit is an analogue device, although a digital device is
conceivable, and has the mid-point of the common point of the R6-R7 potential
divider applied to one input and voltage signal from the current measuring resistor Rl
applied to the other input. The multiplier multiplies these two voltage and current
indicating signals together to produced and apply to the input of the operational
amplifier a signal indicative of the magnetron power. This embodiment is more
precise than that of Figure 5, but is more expensive in that multiplier circuits are little
used and tend to be expensive. We regard the embodiment of Figure 5 as being better
in that it is adequately accurate and at the same time is cheaper.
CLAIMS:
1. A power supply for a magnetron comprising:
• a DC voltage source;
• a converter for raising the output voltage of the DC voltage source, the
converter having:
• a capacitative-inductive resonant circuit,
• a switching circuit adapted to drive the resonant circuit at a variable
frequency above the resonant frequency of the resonant circuit, the
variable frequency being controlled by a control signal input to provide an
alternating voltage,
• a transformer connected to the resonant circuit for raising the alternating
voltage,
• a rectifier for rectifying the raised alternating voltage to a raised DC
voltage for application to the magnetron;
• means for measuring the current from the DC voltage source passing through
the converter;
• a microprocessor programmed to produce a control signal indicative of a
desired output power of the magnetron; and
• an integrated circuit arranged in a feed back loop and adapted to apply a
control signal to the converter switching circuit in accordance with a
comparison of a signal from the current measuring means with the signal
from the microprocessor for controlling the power of the magnetron to the
desired power.
2. A power supply as claimed in claim 1, wherein the integrated circuit is an
analogue device.
3. A power supply as claimed in claim 2, wherein the integrated circuit is an
operational amplifier connected as an error signal amplifier, the error signal being the
difference between signals indicative of a measurement of the converter current and
the desired output power of the magnetron.
4. A power supply as claimed in claim 1, claim 2 or claim 3, wherein the integrated
circuit is arranged as an integrator with a feedback capacitor, whereby its output
voltage is adapted to control a voltage-to-frequency circuit for controlling the
converter.
5. A power supply as claimed in any preceding claim, wherein the integrated circuit
is adapted and arranged for the comparison to be direct between the measured current
signal and the desired power signal, the integrated circuit being connected to receive
these signals only, whereby the converter current is controlled in accordance with the
desired power independent of transient changes in the voltage of the DC voltage
source.
6. A power supply as claimed in claim 5, wherein the current measuring means is a
resistor in series with the converter, one end of the resistor being grounded and the
other being connected to an input of the integrated circuit, preferably via a feed back
resistor.
7. A power supply as claimed in any one of claims 1 to 4, wherein the integrated
circuit is adapted and arranged for the comparison to be not only between the
measured current signal and the desired power signal but also taking account of
transient changes in the voltage of the DC voltage source, a signal indicative of the
voltage of the voltage source also being input to the integrated circuit, whereby the
converter current is controlled such that the power passing through the converter is
controlled in accordance with the desired power.
8. A power supply as claimed in claim 4, wherein:
• the current measuring means is a resistor in series with the converter, one end
of the resistor being grounded and
• a potential divider is provided for input to the integrated circuit, the divider
comprising two dividing resistors between an output rail of the DC voltage
source and the non-grounded end of the current measuring resistor, with the
common connection of the two dividing resistors being connected to an input
of the integrated circuit.
9. A power supply as claimed in claim 8, wherein
• the current measuring means is a resistor in series with the converter, one end
of the resistor being grounded and
• there is provided:
• a potential divider comprising two dividing resistors between an output rail
of the DC voltage source and a zero volts rail and
• a multiplier circuit, the voltage at the current measuring resistor being
applied to one multiplier input and the voltage at the common connection
of the dividing resistors being applied to the other multiplier input and the
multiplier output being applied to the integrated circuit for comparison
with the microprocessor output.
10. A power supply as claimed in any preceding claim, wherein the microprocessor is
programmed to filter noise from the desired converter current signal.
11. A power supply as claimed in any one of claims 1 to 9, including a filter circuit
provided between the microprocessor and the operational amplifier.
12. A power supply as claimed in any preceding claim, wherein the switching circuit
is adapted to control the frequency of the converter in accordance with a variable
voltage signal output from the operational amplifier, whereby an increase in
frequency corresponds to a reduction in magnetron drive power and microwave
output.
13. A power supply as claimed in any preceding claim, wherein the switching circuit
is adapted to control the duty cycle of the converter in accordance with the integrated
circuit output, whereby reduction in the duty cycle corresponds to a reduction in
magnetron drive voltage and microwave output.
14. A power supply as claimed in any preceding claim, wherein the switching circuit
is adapted to be timed from a clock in the microprocessor.
15. A power supply as claimed in any one of claims 1 to 13, wherein the switching
circuit has its own oscillator.
16. A power supply as claimed in any preceding claim, wherein the converter is a zero
voltage switching device.
17. A power supply as claimed in any one of claims 1 to 15, wherein the converter is a
zero current switching device..