RU2575166C2 - Power supply source for magnetron - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: power supply source for magnetron includes high-voltage converter (101), microprocessor (103) and resistor (109). High-voltage converter comprises integrated-circuit oscillator IC1, switching transistors T1, T2, inductance coils LI, transformer (106) and rectifier (107). Power supply source (4) supplies high direct-current voltage to converter (101). Made as error signal amplifier with integrating capacitor C7 and resistor R9 operational amplifier (122) compares control signal from microprocessor (103) and resistor (109) and supplies output signal to oscillator IC1. Oscillator IC1 controls switching transistors T1, T2, which output is connected to inductance coil LI and primary winding of transformer (106). Secondary winding of transformer (106) is connected to diodes D3, D4, D5, D6 of half-bridge and capacitors C5, C6, which deliver direct current from transformer to magnetron (102).

EFFECT: higher control accuracy.

17 cl, 8 dwg

Description

The present invention relates to a power source for a magnetron, in particular, but not exclusively, for use with a magnetron supplying a lamp.

It is known that magnetrons can change the mode unexpectedly, that is, they can suddenly stop generating at one frequency and start generating at another. Under such conditions, they may exhibit negative impedance. This can lead to destructively high current. For this reason, it is known that DC / DC voltage sources for magnetrons are not suitable; DC / DC power supplies are usually used to power them.

The anode voltages in magnetrons are high and measuring both the anode voltage and the anode current are complex.

In the previous power source invented by the present inventor, the measurement of both the voltage supplied to the converter in the magnetron power source and the current through the converter were used together with the microcomputer to provide real-time control of the power supplied to the magnetron. The microcomputer has been programmed to calculate:

1. Power consumption

2. The difference from the desired power and

3. The difference between the difference in power and the measured current.

This second difference signal was used to control the converter. It should be noted that these three steps were performed by software. Suddenly, this power source still suffered from some instability, causing a noticeable flicker of the light produced by the lamp fed by its magnetron.

Experience has now shown that the eye is extremely sensitive to flickering light in a plasma lamp powered by a magnetron. Now it has become clear that the limited speed and resolution of the load capacity of the microprocessor exacerbated perceived flicker. In addition, two of the input signals of the microprocessor, namely the voltage applied to the converter and the current conducted through the converter, are prone to noise, and it is believed that the multiplication of two noisy signals contributed to the instability.

Simple filtering of noise from a microprocessor unacceptably reduces the response time of the control circuit and contributes to instability, given that a quick response to changing magnetron states may be required. Accordingly, a new approach was required.

An object of the present invention is to provide an improved power source for a magnetron.

In accordance with the invention, a power source for a magnetron is provided, comprising:

• constant voltage source;

• a converter for increasing the output voltage of a constant voltage source has:

• capacitive inductive resonance circuit,

• a switching circuit adapted to excite the resonant circuit at a variable frequency above the resonant frequency of the resonant circuit, the variable frequency is controlled by an input control signal to provide an alternating voltage,

• a transformer connected to the resonant circuit to increase the alternating voltage,

• a rectifier for rectifying an increased alternating voltage into an increased direct voltage for application to a magnetron;

• means for measuring current from a constant voltage source passing through the converter;

• a microprocessor programmed to create a control signal characterizing the desired output power of the magnetron; and

• an integrated circuit arranged in a feedback loop and adapted to apply a control signal to the converter switching circuit in accordance with a comparison of the signal from the current measuring instrument with the signal from the microprocessor to control the magnetron power to the desired power.

Providing an integrated circuit as a discrete element separate from the microprocessor provides a fast control loop that is not limited by the speed of the microprocessor (the latter is prone to slowness due to economic restrictions on its specification). Thus, the power supply of the invention is inherently more stable and provides lighting less prone to flicker.

Although it may be provided that the integrated circuit could be a digital device, it is preferably an analog device in order to save. In a preferred embodiment, the integrated circuit is an operational amplifier.

In a preferred embodiment, the operational amplifier is configured as an integrator with a feedback capacitor by which its output voltage is adapted to control the voltage to frequency conversion circuit for controlling the converter.

Preferably, the microprocessor is programmed to filter out noise from the desired converter current signal. Alternatively, a filter circuit may be provided between the microprocessor and the operational amplifier.

In preferred embodiments, the switching circuit is adapted to control the frequency of the converter in accordance with an alternating voltage signal output from the operational amplifier. In such a method, an increase in frequency corresponds to a decrease in the excitation voltage of the magnetron and the microwave output.

In another case, the switching circuit may be adapted to control the duty cycle of the converter in accordance with the output of the operational amplifier, whereby a decrease in the duty cycle corresponds to a decrease in the excitation voltage of the magnetron and the microwave output.

In preferred embodiments, the converter is a zero voltage switch, although it could be a zero current switch.

Typically, the switching circuit has its own generator; however, it may be provided that it could be synchronized from the clock in the microprocessor.

In one embodiment, the integrated circuit is adapted and arranged so that the comparison is directly between the measured current signal and the desired power signal, the integrated circuit is connected to receive only these signals, so that the converter current is controlled in accordance with the desired power, independent of short-term voltage changes DC voltage source. This embodiment controls the average power so that it is constant during the ripple cycles of the voltage source.

In another embodiment, the integrated circuit is adapted and arranged so that the comparison is not only between the measured current signal and the signal of the desired power, but also taking into account short-term changes in the voltage of the DC voltage source, a signal characterizing the voltage of the voltage source is also supplied to the integrated circuit, whereby the current of the converter is controlled so that the power passing through the converter is controlled in accordance with the desired power. This embodiment controls the instantaneous power constant so that it is constant during the ripple cycles of the voltage source.

Typically, the switching circuit has its own generator; however, it may be provided that it could be synchronized from the clock in the microprocessor.

To help understand the invention, its specific embodiment will now be described, for example and with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a prototype power source for a magnetron;

FIG. 2 is a similar block diagram of a power source in accordance with the invention;

FIG. 3 is a more detailed circuit diagram of the power supply shown in FIG. 2;

FIG. 4 is a schematic illustration of a lamp powered by a magnetron having the power supply of the invention;

FIG. 5 is a schematic diagram of a second embodiment of the invention;

FIG. 6 is a detailed diagram of a voltage divider of the embodiment of FIG. 5;

FIG. 7 is a spectral diagram of the magnetron output comparing it with the embodiments of FIG. 3 and 5; and

FIG. 8 is a schematic diagram of a third embodiment of the invention.

Turning first to FIG. 1, a prototype power supply is represented schematically, having a generator 1 connected to power a magnetron 2, and controlled by a microprocessor 3. The amplified voltage of the mains DC source 4 usually delivers 400 V through line 5 to generator 1. This supplies alternating current to transformer 6 and rectifier 7, from which 4000 V DC are applied via line 8 to the magnetron. The generator, transformer and rectifier are called “high voltage converter”. The power supplied to the magnetron is measured in terms of the voltage across the resistor 9 in the circuit through the converter ground. The voltage characterizes the current in the resistor 9 and is proportional to the power supplied to the magnetron, under the condition of constant voltage from the voltage source 4. The voltage across the resistor is a single input on line 10 to the microprocessor. Another input on line 11 applies voltage on line 5 to the microprocessor. The control value 12 of the desired power is set externally or as a manual input into the microprocessor.

The microprocessor is programmed to complete the steps:

1. The multiplication of the voltage on line 5 by the current in the resistor 9 to calculate the power supplied to the magnetron, assuming high efficiency;

2. Comparison of the calculation of power consumption with the desired power, and hence the calculation of the current that should be consumed (estimated current);

3. Comparison of the estimated current with the measured current and applying a step-by-step higher voltage to the power supply to excite the converter at a higher frequency if the current is strong, or any step-by-step lower voltage if the current is too weak. It should be noted that if the converter operates at a higher frequency, the resulting voltage on the magnetron drops.

As already mentioned, in practice this scheme has proved to be too unstable for the flicker-free operation of the magnetron as a light source.

Turning now to FIG. 2, the power supply of the invention contains the following similar components connected in the same way:

• generator / high-voltage converter 101;

• magnetron 102;

• transformer 106;

• rectifier 107;

• resistor 109.

The microprocessor 103 is also contained, but it works in a completely different way. It only divides the control value 112 of the desired power by the amplified direct voltage of the network on line 105 and provides a signal of the required current on line 121 characterizing the desired current through the converter 101 to control the magnetron at the desired power. The signal on line 121 is applied to one input of an operational amplifier 122 / EA1. Its other input has a line 110 to it from resistor 109, indicating the actual current passing through the converter. The operational amplifier is connected as an integrating error signal amplifier.

Turning now to FIG. 3, a more complete circuit diagram of the power supply shown in FIG. 2. At its center is a quasi-resonant high-voltage converter generator 101 having MOS field-effect switching transistors T1, T2. They are switched in a manner that will be described below by an IC1 generator on an integrated circuit. The inductor LI and the primary winding of the transformer 106 are connected in series and connected to a common point of the transistors T1, T2. Capacitors C3, C4 complete the series resonant circuit. Inductors and capacitors determine the resonant frequency above which the converter operates, usually around 70 kHz, whereby it turns out to be mainly an inductive circuit with respect to the magnetron circuit below. It contains four half-bridge diodes D3, D4, D5, D6 and smoothing capacitors C5, C6 connected to the secondary winding of the transformer and providing the magnetron 102 with direct current. The ratio of the transformer windings is 10: 1, due to which a voltage of about 4000 V is applied to the magnetron, and the amplified direct voltage on line 105 is usually 400 V.

A characteristic feature of the converter circuit is that when the transistors T1, T2 are sequentially turned on and then turned off, the energy stored in the inductor LI changes the polarity of the voltage across it. This lowers the voltage at common point C before turning on TR2, and raises the voltage of common point before turning on TR1. Thus, the switching occurs at zero or close to zero voltage on the transistor, which will soon turn on, that is, in the PNI mode (switching mode at zero voltage). This contributes to reliability and durability.

At a high switching frequency (i.e., above the resonance), the voltage at the common point between the capacitors C3, C4 is substantially constant at half the voltage on line 105, so that after switching the transistor, the current with a substantially linearly varying triangular waveform flows through the inductor L1. It is transmitted to a transformer, and from there, ultimately, to a magnetron.

Lowering the frequency to work closer to resonance increases the voltage swing in D from half the voltage on line 105 and increases the voltage on the magnetron, its current and its microwave output.

The current through the converter is measured on a resistor 109 / R1, usually 100 mOhm, and the voltage characterizing it is transmitted through a feedback resistor R5, usually 470 Ohms, to one input 123 of the operational amplifier 122. The microprocessor 103 receives a voltage from the voltage divider R3, R4 with line 105. The required power setting is set by manual input 112. The microprocessor is programmed to divide the required power into line voltage and apply voltage characterizing the current to another input of the operational amplifier 125 To required for said magnetron through 6 kOhm resistor R10. The operational amplifier has an integrating capacitor C7, usually 470 nF, in series with a 1MΩ resistor R9. The ratio of resistors R9, R10 determines the gain of the operational amplifier. This gain should suppress power surges as much as possible. The amplifier transmits the integrated voltage characterizing the required power to the frequency control circuit 126 for generator IC1, which is a voltage to frequency conversion circuit, typically Texas Instruments IRS2153 or ST Thomson L6569. A circuit containing an 18 kΩ resistor R2, capacitors C1, C2, both 470 pF, and diodes D1, D2, works to control the frequency of the converter. When the output of the op-amp is zero, the capacitor C1 is parallel to C2 and the lowest frequency is obtained. This corresponds to the maximum power of the magnetron. On the other hand, when the output is maximum, the diodes do not conduct at all, and the frequency is controlled by one C2. The maximum frequency and minimum power are supplied - about one tenth of the maximum. At intermediate voltages, C1 has an intermediate effect and the frequency and power are controlled accordingly.

Thus, the magnetron can be controlled to operate at the desired power input to the microprocessor. The microprocessor is susceptible to flicker, causing changes in voltage on line 105. However, the signal at R10 can be filtered internally, by software, or externally, not shown by the RC filter. When the power consumed by the magnetron changes, how can it do this, when its magnets heat up and its resistance changes, the operational amplifier quickly responds to a change in the current measured on resistor R1, and adjusts the frequency of the converter and, therefore, corrects the power consumed by the magnetron regardless of the signal on line 125 from the microprocessor.

Moreover, if there is flicker on the line of the voltage source, the magnetron power will be constant only when averaged over the flicker period. On the line of the voltage source, a double flicker of frequency is actually manifested due to the cost of large smoothing capacitors.

It should be noted that the above power supply is particularly suitable for controlling an LER lamp powered by a magnetron, such as described in WO 2009/063205. It allows you to control the light output of the lamp as you wish, as and when required, from a low level for background light to full power of full illumination.

FIG. 4 is a simplified representation of a magnetron excited lamp. It has a transparent crucible 201 with a Faraday cage 202. The cavity 203 in the crucible has a charge 204 of excitable material. The magnetron 205 is adapted to direct its microwaves into the waveguide / junction 206, from which they exit through a coaxial connection 207 to an antenna 208 emitting them into a crucible. Powering the magnetron with the inventive power source 209 causes the excitable material to emit light. It is for this light that the power supply of the invention is advantageous in that it avoids flickering.

Turning now to FIG. 5, an improved high voltage converter is shown, also in accordance with the invention. It takes into account not only changes in the current of the converter, and therefore, the magnetron current, but also the ripple of the network frequency or, more specifically, the double ripple of the network frequency at the output of the voltage source. These pulsations do not cause perceived flicker in the light from the LER, but still lead to an extension of the frequency range at the output of the magnetron.

New to FIG. 5 is the inclusion of the resistor R6 in the form of two consecutive 1 MΩ resistors from the voltage source line to the input 123 of the operational amplifier to which the feedback resistor R5 is connected. 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, both typically of the order of 100 mV, giving 200 mV at the input of the operational amplifier. The actual voltage varies both with the actual current in the converter and with the actual voltage on the line of the voltage source. It will be understood that an increase of 200 mV of the input of the operational amplifier due to an increase in the voltage source line will be equivalent to an increase of 200 mV of the input of the operational amplifier due to an increase in current. Both increase the integrated output voltage of the operational amplifier with the result that the controlled current decreases.

The actual increase in the input of the operational amplifier due to a 5% increase in the voltage of the voltage source will be 5%, since the voltage across the current measuring resistor is small compared to the voltage of the voltage source. Also, for a 5% increase in current, the voltage across the current measuring resistor will be 5%. This will add to the voltage at the input of the op amp. Thus, for a 5% or other small percentage increase in voltage or current, the current will be reduced by the same percentage.

In turn, this leads to a 5% or other small percentage reduction in the power applied to the magnetron. Thus, the device acts to maintain instantaneous power constant. In this regard, “instantaneous” is used to indicate that power is kept constant, for example, throughout the entire voltage ripple cycle.

This operation can be mathematically explained as follows.

The magnetron power is the product of the voltage U of the voltage source and the current I of the converter, i.e.

P = U x I.

In terms of units of voltage and current, u and I:

P = (C ₁ xu) x (C ₂ xi)

P = K x (u x i)

For a unit value of u and i, this formula can be rewritten in the form

P = K x (u + i) / 2.

This ratio remains approximately true for small changes in voltage and current, i.e. for u ± δu, i ± δi.

The above equation can be rewritten as:

P = K ₃ + K ₄ x δV + K ₅ x δv.

Thus, the magnetron power can be represented as a constant plus another constant multiplied by any deviation of the actual voltage source from its normal value, plus another constant multiplied by any deviation of the current from the rated current. The current deviation itself can be represented by the voltage across the current measuring resistor.

With the corresponding constants and considering only the changes applied to the operational amplifier, it can be seen that the voltage divider supplies the sum of two changes, the voltage of the voltage source and the converter current to the operational amplifier. The only condition is that the approximation

P = UxI ≈ K x (u + i) / 2

only performed if the voltage at R5 is approximately equal to the voltage at Rl. This is done for the values:

U = 400 V

R1 = 0.1 Ohm

R5 = 470 ohm

R6 = 2 megohms

These resistors are shown in FIG. 6 in sequence, an indication of the respective voltages is also shown.

It should be noted that, since R6 is seven orders of magnitude larger than Rl and R5 is four orders of magnitude larger, any change in U that creates a significant change in voltage at the input of the operational amplifier is unlikely to cause a significant change in voltage on Rl, whose voltage is controlled only by the current through it. Accordingly, the voltage at Rl is added to the voltage at R5 and the amount is supplied to the operational amplifier.

It will be understood that this mode of operation is not entirely linear, but still provides significant improvements. With reference to FIG. 7 provides a saddle-shaped graph of the width of the spectrum of the magnetron generation frequency. Its frequency of generation depends on the current through it, this is a sign of the magnetron that it has a property similar to that of the Zener diode with regard to controlling the voltage on it. Thus, if more power is available to him, his current increases, and his operating frequency decreases. Where there is ripple in the voltage of the voltage source related to the mains voltage, the magnetron frequency changes and the width of the spectrum shows a slightly pronounced saddle shape. In contrast, with a power controller in the embodiment of FIG. 5, the width of the spectrum is much narrower and has a normal distribution. This, in turn, is advantageous in that it causes less interference with Bluetooth networks, etc.

Turning to FIG. 8, a multiplication circuit 301 is shown at the input of the operational amplifier. This circuit is an analog device, although a digital device is also possible, and has a midpoint common point of the voltage divider R6-R7 applied to one input and a voltage signal from a current measurement resistor Rl applied to another input. A multiplier multiplies these two signals characterizing the voltage and current to create and apply to the input of the operational amplifier a signal characterizing the power of the magnetron. This embodiment is more accurate than that shown in FIG. 5, but also more expensive, since multiplication schemes are used little and, as a rule, are expensive. The embodiment of FIG. 5 is considered to be the best, since it is accurate enough and at the same time cheaper.

Claims (17)

1. A power source for a magnetron, containing:
constant voltage source;
a converter for increasing the output voltage of a constant voltage source, the converter has:
- capacitive inductive resonance circuit,
- a switching circuit adapted to excite the resonant circuit at a variable frequency above the resonant frequency of the resonant circuit, the variable frequency is controlled by an input control signal to provide an alternating voltage,
- a transformer connected to the resonant circuit, to increase the alternating voltage,
- a rectifier for rectification of the increased alternating voltage into an increased constant voltage for application to the magnetron;
means for measuring current from a constant voltage source passing through the converter;
a microprocessor programmed to create a control signal characterizing the desired output power of the magnetron; and
an integrated circuit included in the feedback circuit, while the integrated circuit has:
- inputs connected to a current measuring means and a microprocessor, and
- an output connected to the converter, and
the integrated circuit is adapted to apply a control signal to the switching circuit of the converter in accordance with a comparison of the signal from the current measuring means with the signal from the microprocessor to control the magnetron power to the desired power. 2. The power source according to claim 1, characterized in that the integrated circuit is an analog device. 3. The power supply according to claim 2, characterized in that the integrated circuit is an operational amplifier connected as an error signal amplifier, the error signal is the difference between the signals characterizing the measurement of the converter current and the desired magnetron output power. 4. The power supply according to claim 1, 2 or 3, characterized in that the integrated circuit is designed as an integrator with a feedback capacitor, whereby its output voltage is used to control the voltage to frequency conversion circuit for controlling the converter. 5. The power supply according to claim 1, characterized in that the integrated circuit is applied and arranged so that the comparison is directly between the measured current signal and the desired power signal, the integrated circuit is connected to receive only these signals, due to which the converter current is controlled by in accordance with the desired power, independent of short-term changes in the voltage of the constant voltage source. 6. The power supply according to claim 5, characterized in that the current measuring means is a resistor connected in series with the converter, one end of the resistor is grounded, and the other connected to the input of the integrated circuit, preferably through a feedback resistor. 7. The power supply according to claim 4, characterized in that the integrated circuit is applied and arranged so that the comparison is not only between the measured current signal and the signal of the desired power, but also taking into account short-term changes in the voltage of the constant voltage source, a signal characterizing the voltage the voltage source is also fed to the integrated circuit, so that the current control of the converter is carried out so that the power passing through the converter is controlled in accordance with dance with the desired power. 8. The power source according to claim 4, characterized in that
- the current measuring means is a resistor connected in series with the converter, one end of the resistor is grounded, and
- a voltage divider is provided for entering the integrated circuit, the divider contains two separating resistors between the output bus of the DC voltage source and the non-grounded end of the current measuring resistor, and the common connection of the two separating resistors is connected to the input of the integrated circuit. 9. The power source according to claim 8, characterized in that
current measurement means is a resistor connected in series with the converter, one end of the resistor is grounded and provided:
- a voltage divider containing two separating resistors between the output bus of the DC voltage source and the zero voltage bus, and
- the multiplication circuit, the voltage across the current measuring resistor is applied to one input of the multiplier, the voltage across the common connection of the separating resistors is applied to the other input of the multiplier, and the output of the multiplier is applied to the integrated circuit for comparison with the microprocessor output. 10. The power source according to claim 9, characterized in that the microprocessor is programmed to filter out noise from the desired current signal of the converter. 11. The power source according to claim 9, characterized in that it includes a filter circuit provided between the microprocessor and the operational amplifier. 12. The power supply according to claim 11, characterized in that the switching circuit is used to control the frequency of the converter in accordance with an AC signal output from the operational amplifier, whereby an increase in frequency corresponds to a decrease in the excitation power of the magnetron and microwave output. 13. The power supply according to claim 12, characterized in that the switching circuit is used to control the duty cycle of the converter in accordance with the output of the integrated circuit, whereby a decrease in the duty cycle corresponds to a decrease in the excitation voltage of the magnetron and microwave output. 14. The power source according to claim 13, characterized in that the switching circuit is applied with the possibility of synchronization from the clock in the microprocessor. 15. The power source according to claim 13, characterized in that the switching circuit has its own generator. 16. The power source according to claim 15, characterized in that the converter is a switching device at zero voltage. 17. The power source according to claim 15, wherein the converter is a switching device at zero current.

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