EP2854479A1 - Microwave oven with delayed activation of high voltage - Google Patents

Abstract

The control circuit for a microwave oven has a push-pull output stage (T1, T2) for driving a heating transformer (15), with which the cathode heating of the magnetron (3) is operated. For the generation of the high voltage, a separate high-voltage transformer (14) is provided, which is fed by a bridge circuit (T3 - T6). The control unit (13) of the device is designed to pre-heat the cathode in a preheat phase before the high voltage is switched on. It measures the current through the push-pull output stage (T1, T2) to ground and the power of the cathode heater by varying the width of the pulses generated by the push-pull output stage (T1, T2). As soon as the current stops changing, the high voltage is switched on. In this way, the duration of the preheating phase can be kept low.

Description

Field of the invention

The invention relates to a microwave oven with a magnetron comprising a cathode, an anode and a cathode heater and with a drive circuit for the magnetron. The invention also relates to a method for operating such a microwave oven.

background

Normally, a microwave oven has a transformer with two secondary windings. The one secondary winding is used to drive the cathode heater of the magnetron, while the other secondary winding is used to generate the high voltage between the cathode and anode. If such a device is activated, an AC voltage is applied to the primary side of the transformer. This causes the high voltage already applied before the cathode has reached operating temperature, resulting in a poorly controlled startup process. In particular, the voltage across the magnetron before ignition is greatly increased, so that the corresponding components must be dimensioned with excessively high dielectric strength.

In US 4,742,442 Therefore, a device is described in which the drive circuit has a disconnectable from the high voltage generator Heizstromgenerator. In particular, two separate transformers for the heating current and the high voltage are provided. After activating the device, an AC voltage is first applied to the heating current transformer to heat the cathode. After a certain time, For example, five seconds, then an AC voltage for the high voltage transformer is generated so that the high voltage is applied after preheating the cathode to the magnetron.

A disadvantage of this solution is the additional time that is required for preheating and can delay the switch-on of the device.

Presentation of the invention

It is therefore the object to provide a microwave oven and a method of the type mentioned, in which a fast, yet controlled switch-on is possible. This object is solved by the subject matter of the independent claims.

Accordingly, the drive circuit of the magnetron in a conventional manner has a high voltage generator for generating the high voltage between the anode and the cathode, and a Heizstromgenerator for generating the heating current for the cathode heater. Furthermore, a controller is provided which controls these components. The controller is designed in such a way that, to activate the device (i.e., to switch on the magnetron), the heating current generator is first activated. Only later (i.e., after activation of the heater current generator) is the high voltage generator activated. As a result, as described above, a defined switch-on process is achieved.

Furthermore, a measuring circuit is provided which is designed to determine a parameter dependent on the electrical resistance of the cathode heating. The controller is configured to set the switch-on time (ie the time of activation) of the high voltage generator in dependence on said parameter. In other words, that's the time activating the high voltage generator is a function of the measured parameter and varies depending on how the parameter changes.

1. (A) Erzeugen des Heizstroms durch die Kathodenheizung. Dies erfolgt in einer Vorheizphase während Vorheizdauer.
2. (B) Erst nach Ablauf der Vorheizdauer, Anlegen der Hochspannung zwischen der Anode und der Kathode.

Accordingly, the invention also relates to a method for operating a microwave oven, wherein the microwave oven comprises a magnetron having a cathode, an anode and a cathode heater. To activate the microwave oven, at least the following steps are performed:

1. (A) Generating the heating current through the cathode heater. This is done in a preheat phase during preheat time.
2. (B) Only after expiration of the preheating time, applying the high voltage between the anode and the cathode.

In addition, a parameter dependent on the electrical resistance of the cathode heating is measured, and depending on said parameter, the preheating time, i. the time at which the high voltage is applied between the cathode and the anode is determined.

The invention is based on the idea that the resistance of the cathode heating depends on the temperature of the cathode heating. By now measuring a parameter dependent on the resistance, it is thus possible to determine a measure of the cathode temperature. This allows the preheat duration to be selected dependent on the cathode temperature, which provides a basis for turning on the high voltage when a sufficient cathode temperature is reached. This makes it possible to reduce the preheating time when the initial cathode temperature is already relatively high. In addition, the switching on of the high voltage can take place at a well-defined cathode temperature, so that a precisely defined behavior and oscillation of the magnetron are made possible with very high probability.

Advantageously, a power regulator is provided, which is designed so that the of the cathode heating Recorded power can be controlled to a setpoint. The control is designed in this case to regulate the heating power during the preheating phase to a predetermined, in particular fixed, first setpoint. This has the advantage over a constant heating voltage or a constant heating current that the power consumption is not too large - since the cathode heater at low temperatures has a lower resistance than at high temperatures, it could otherwise with cold cathode to excessive power consumption and thus to a Damage to the components come.

Advantageously, in this case, from the end of the preheating phase, the heating power is reduced to a predetermined, in particular fixed, second (lower) setpoint value, since power is supplied to the cathode as a result of the operation of the magnetron. Therefore, without reducing the power of the cathode heater, the cathode temperature would continue to rise, affecting the life of the cathode heater.

The measured parameter is advantageously the current through the cathode heating, or the duration of the power-controlled heating pulses. These sizes are a good measure of the heating resistance.

In a particularly advantageous embodiment, the controller is configured to terminate the preheat phase when the rate of change of the parameter drops below a change threshold. In other words, preheating is stopped and the high voltage is turned on when the temperature of the cathode is substantially unchanged. This procedure has the advantage that it is based on a measurement of relative values and is independent of absolute values, eg the absolute resistance of the cathode heating, so that scattering and aging effects of the components have hardly any influence on the operation.

Short description of drawings

• Fig. 1 einen Schnitt durch die im vorliegenden Zusammenhang wichtigsten Teile eines Mikrowellenofens,
• Fig. 2 ein vereinfachtes Schaltungsdiagramm des Mikrowellenofens und
• Fig. 3 ein Diagramm einiger Signale der Ansteuerschaltung für die Kathodenheizung.

Further embodiments, advantages and applications of the invention will become apparent from the dependent claims and from the following description with reference to FIGS. Showing:

• Fig. 1 a section through the most important parts of a microwave oven in the present context,
• Fig. 2 a simplified circuit diagram of the microwave oven and
• Fig. 3 a diagram of some signals of the drive circuit for the cathode heater.

Ways to carry out the invention definitions:

In the present context, high voltage is understood to mean a voltage which is required as anode-cathode voltage for operation of the magnetron. In practice, this voltage is in most cases at least 1 kV, usually several kilovolts.

A push-pull output stage is a series connection of two electronic components, which can be alternately switched continuously, so that at the center tap of the two components, a time-varying voltage.

A half-bridge circuit is a circuit with exactly one push-pull final stage.

A full-bridge circuit (H-circuit, H-bridge) is a circuit with two push-pull output stages connected in parallel, with the load between the center taps of the two push-pull final stages.

Grunda Uba:

The invention relates to a microwave oven, as exemplified in Fig. 1 is shown. The microwave oven has a cooking chamber 1 for receiving the food to be heated, which can be closed to the user by a user door 2. In the device, a magnetron 3 is also arranged, which is connected via a Holleiter 4 with the cooking chamber 1 in combination. A controller 5 controls the function of the device.

Fig. 2 shows the most important components of the controller 5 in the present context.

The mains voltage of e.g. 230 volts at 50 Hz is rectified in a rectifier 10. The first intermediate voltage Uz thus produced is then slightly filtered via a first capacitor C1, the capacitor C1, however, being dimensioned so that, under load, the value of the first intermediate voltage Uz varies by at least 50% with twice the mains frequency. The intermediate voltage Uz is also tapped via a diode D1 and further filtered via a second capacitor C2 to form a second intermediate voltage Uz '.

The first intermediate voltage Uz is supplied to a high voltage generator 11, with which, as described below, the high voltage for driving the magnetron 3 is generated. The second intermediate voltage Uz 'is supplied to a heating current generator 12, with which, as described below, the heating current for the cathode heating of the magnetron 3 is generated.

The operation of the high voltage generator 11 and the heating current generator 12 is controlled by a control unit 13, e.g. in the form of a microprocessor, controlled.

An analog-to-digital converter of the control unit 13 is supplied via a voltage divider R5, R6 a proportional to the intermediate voltage Uz value so that it can determine the intermediate voltage Uz.

High voltage generator:

The high voltage generator 11 comprises a full bridge circuit with four electronic switching elements T3 - T6, in particular in the form of IGBT transistors, each with a freewheeling diode. The switching elements T3 - T6 are arranged in a known manner in two branches T3 and T4 or T5 and T6, wherein the switching elements of each branch are respectively arranged in series between the first intermediate voltage Uz and ground. Between the switching elements of each branch, a center tap is provided in each case, wherein the two center taps are connected to the two terminals of the primary winding of a high-voltage transformer 14. The high voltage transformer 14 has a secondary winding with a much higher number of turns than the primary winding for generating the high voltage. The high voltage is rectified via two diodes D2 and D3, doubled and filtered by means of two capacitors C3 and C4. The high voltage Uh thus generated is applied between the cathode K and the anode A of the magnetron 3.

For driving the switching elements T3 - T6, a drive circuit 16 is provided, which is controlled by the control unit 13. The drive circuit 16 generates the control voltages (gate or base voltages) UG3 - UG6 for the switching elements T3 - T6. The control unit 13 is designed to switch the two branches of the full bridge circuit T3 - T6 alternately. The control is done so that during a switching cycle, the primary winding of high voltage transformer 14 is not permanently between the first intermediate voltage Uz and ground, but that the primary winding is decoupled during a time to be selected by the control unit 13 from the intermediate voltage Uz, ie the circuit is with Pulse width modulation clocked so that the value of the high voltage Uh can be controlled.

To monitor the high voltage Uh this can be divided by a voltage divider R10 - R13 and R14 and fed to an optocoupler 17 whose output signal is forwarded to the control unit 13. For example, a lack or non-ignition of the magnetron can be detected in this way.

Furthermore, a resistor R20 is provided between the two branches T3, T4 or T5, T6 and a fixed reference potential, in particular ground. The initial increase in the voltage drop across this resistor at the beginning of a current pulse is a measure of the anode current of the magnetron 3 and is supplied via an amplifier 18 to the control unit 13 for measurement purposes.

heating current:

The Heizstromgenerator 12 is formed in the present embodiment of a half-bridge with two operated as push-pull final stage switching elements T1 and T2. The switching elements T1 and T2, which in turn are e.g. can be configured as IGBT transistors and which are each equipped with a freewheeling diode, are arranged in series between the second intermediate voltage Uz 'and ground.

The center tap between the two switching elements T1, T2 is connected to one terminal of the primary winding of a heating transformer 15. The second terminal of the primary winding of the heating transformer 15 is connected to the center tap of a capacitive voltage divider of two capacitors C5 and C6. The two capacitors C5 and C6 are connected in series between the second intermediate voltage Uz 'and ground.

The diode D1 prevents current from being discharged from the capacitors C5, C6 when the high voltage generator 11 connected to the intermediate voltage Uz draws current.

The secondary winding of the heating transformer 15 is connected to the cathode heater, i. connected to the filament, the magnetron 3 and supplies them with electricity.

For driving the switching elements T1 and T2, a drive circuit 20 is provided, which is controlled by the control unit 13. The drive circuit 20 generates the control voltages (gate or base voltages) UG1, UG2 for the switching elements T1 and T2. The type of control will be described in detail below.

Between the push-pull output stage, formed by the switching elements T1, T2, and the ground (or another fixed reference potential), a resistor R21 is arranged, through which the current from the push-pull output stage T1, T2 through the heating transformer to ground (or. the reference potential). The voltage drop across this resistor is a measure of the current flowing from the second intermediate voltage Uz 'through the primary coil of the high voltage transformer 15 to ground (or reference potential). It is tapped by an amplifier 21 and fed to an analog-to-digital converter of the control unit 13.

Control of the heating current generator:

The following is based on Fig. 3 describes how the control unit 13 controls the switching elements of the heating current generator 12. The figure shows the course of the voltages UG1 and UG2, which are applied to the control inputs of the switching elements T1 and T2, as well as the course of the voltage Uih, which drops across the resistor R21.

The control unit 13 is designed to switch the two switching elements T1 and T2 cyclically alternately. A typical cycle period Tz is advantageously in the range of 10 - 50 μs.

The periods in which one of the switching elements T1 or T2 is turned on are referred to below as heating phases H1 and H2, respectively, and are shown in FIG FIG. 3 drawn, wherein in the heating phase H1, the first switching element T1 and H2 in the heating phase, the second switching element T2 is turned on. Between the heating phases H1 and H2 or H2 and H1 both switching elements T1, T2 are turned off. The phases in which both switching elements T1 and T2 are turned off are referred to as resting phases R1 and R2 and are in Fig. 3 also marked. The heating phases have a duration th, the rest periods a duration tr.

The time th can be selected identically for both switching elements T1 and T2 in a simple embodiment, as well tr.

In this way, an alternating current is generated in the primary winding of the heating transformer 15, which is supplied (except for losses in the components, in particular in the heating transformer 15) as heating power of the cathode heater of the magnetron 3. The average magnitude of the heating power is a function of the duty cycle, i. of the quotient th / Tz.

How out Fig. 3 can be seen, after switching on one of the switching elements T1, T2, the current through the primary winding of the heating transformer 15 and thus the voltage drop Uih via resistor R21 and can be measured via the amplifier 21 from the control unit 13.

The voltage drop Uih forms a parameter that depends on the resistance of the cathode heater of the magnetron 3. Assuming that no losses occur in the heating transformer 15, Uih towards the end of the heating pulse is inversely proportional to the resistance of the cathode heater. Thus, resistor R21 together with amplifier 21 form a measuring circuit which is designed to determine a parameter dependent on the resistance of the cathode heating.

In Fig. 3 is a time tm plotted, to which the controller 13 measures the voltage drop Uih. This time tm is preferably just before the End tx of the respective heating phase H1 or H2, for example at most 1 μs before the end tx of the heating phase. Advantageously, a measurement takes place in each heating phase.

The control unit 13 is configured to keep the product P = Uz '· Uih (tm) · th constant by varying the duration th of the heating phases depending on the values of Uih (tm) and Uz'. The product P is at least approximately proportional to the power supplied to the cathode heater.

For the value of the intermediate voltage Uz 'approximately the value of the intermediate voltage Uz can be used, as it is determined by the control unit via the voltage divider R5, R6. As long as (in the preheating phase) the high-voltage generator 11 is not in operation, Uz 'corresponds to the value of Uz except for the voltage drop across D1. Although Uz 'is sometimes somewhat larger than Uz, the difference remains small if the components are dimensioned appropriately. If Uz 'is to be determined exactly, in addition or as an alternative to R5, R6, a second voltage divider may be provided, which supplies the second intermediate voltage Uz' to the measurement of the control unit 13.

Preferably, P is averaged over a filter time which is at least half a clock period of the line voltage, i. at least 10 ms. An adaptation of the pulse width th occurs only after the filter time has expired.

P is a direct measure of the power that the push-pull output stage T1, T2 delivers, and thus (neglecting the power losses, especially in the heating transformer 15) and a measure of the heating power of the cathode heater of the magnetron 3. Thus, the control unit 13 forms a power regulator, with which the power absorbed by the cathode heater power can be controlled to a desired value.

Business:

If the user activates the microwave oven, i. has given the order to supply energy to the food in the oven, the controller 13 starts a preheat first. In this preheating phase, the switching elements T3 - T6 all remain switched off, so that no high voltage is applied to the magnetron 3. The preheating phase is then followed by an operating phase in which the switching elements T3-T6 are alternately put into operation in order to apply the high voltage to the magnetron and to generate the desired microwave radiation.

The switching elements T1 and T2 are in the in Fig. 3 operated and described above, both in the preheating phase and (with slightly different operating parameters) in the subsequent operating phase.

During the preheat phase, the controller gives a first setpoint for the cathode heater power. This is e.g. at 80 - 120 watts (the value to be chosen depends of course on the size and power of the magnetron).

During the preheating phase, the temperature of the cathode rises. Since the cathode heater is a PTC thermistor (i.e., a PTC resistor), its resistance increases with increasing temperature. This will reduce the voltage Uih. Since the control unit 13 tries to control the product P = Zu · Uih (tm) · th to a constant value, it increases the duration th of the heating phases so as to keep the product constant. As a result, the current pulses generated by the push-pull output stage T1, T2 become longer, and the power output remains approximately constant.

After some time, the cathode reaches a temperature equilibrium. This time depends primarily on the initial temperature of the cathode. When the state of equilibrium is reached, the measured value Uih (tm) no longer changes. Thus, the control unit can Recognize equilibrium state that the rate of change of the measured parameter Uih (tm) falls below a change threshold. The smaller this threshold is chosen, the better the balance, but the longer the preheat phase lasts. The threshold value can, for example, be specified as a percentage per 10 ms, ie a threshold value of x% / 10 ms is undershot if the measured parameter Uih (tm) does not change by more than x% over 10 ms. Advantageously, the threshold value is less than 10% / 10 ms (ie 10% over the half-wave time of 10 ms).

• Hierzu beginnt sie die Brückenschaltung T3 - T6 zu betreiben, indem sie in alternierend die Schaltelemente T3 und T6 bzw. T4 und T5 aktiviert, um so eine Wechselstrom in der Primärwicklung des Hochspannungstransformators 14 zu erzeugen, wodurch die Hochspannung über der Kathode und Anode des Magnetrons 3 aufgebaut wird.
• Weiter reduziert die Steuereinheit 13 den Sollwert für die Heizleistung der Kathodenheizung auf einen zweiten Sollwert, da der Kathode nun auch über die Hochspannung Leistung zugeführt wird. Vorteilhaft wird der Sollwert um mindestens 30% reduziert, z.B. auf 30 - 40 Watt.

As soon as the rate of change of the parameter falls below the said threshold value, which is the case depending on the starting temperature of the cathode, typically after two to eight seconds, the control unit 13 ends the preheating phase and initiates the operating phase:

• To this end, it starts to operate the bridge circuit T3-T6 by alternately activating the switching elements T3 and T6 or T4 and T5 so as to generate an alternating current in the primary winding of the high-voltage transformer 14, whereby the high voltage across the cathode and anode of the magnetron 3 is built.
• Further, the control unit 13 reduces the target value for the heating power of the cathode heater to a second desired value, since the cathode is now also supplied with power via the high voltage. Advantageously, the setpoint is reduced by at least 30%, for example to 30-40 watts.

For example, in the operating phase, the control unit may regulate or limit the high voltage by monitoring the signal of the optocoupler 17, and / or may regulate or limit the current through the bridge circuit T3-T6 to ground by measuring the signal of the amplifier 18. This regulation or limitation can be carried out independently of the control of the heating power.

Remarks:

In the above embodiment, the voltage drop Uih across R21 is used as a parameter depending on the resistance of the cathode heater. Alternatively, e.g. Also, the quotient Uih / Uz 'may be used as a parameter, since it is independent of variations of the second intermediate voltage Uz', or the value Uih may be e.g. are averaged over at least one half grid period to compensate for the corresponding periodic variations of Uz '.

In principle, any parameter may be used which depends on the resistance of the cathode heating, in particular the current through the cathode heating or (as in the above embodiment) the primary side current of the heating transformer 15. As a parameter, the duration th of the heating phases can be used, since these due to above described regulation is also dependent on the heating resistor.

The sequence control of the described method steps can be implemented as hardware and / or software in the control unit 13.

In summary, therefore, a control circuit for a microwave oven is described. This has a push-pull output stage T1, T2 for driving a heating transformer 1), with which the cathode heater of the magnetron 3 is operated. For the generation of the high voltage, a separate high-voltage transformer 14 is provided, which is fed by a bridge circuit T3 - T6. The control unit 13 of the device is configured to pre-heat the cathode in a preheat phase prior to the high voltage being turned on. It measures the current through the push-pull output stage T1, T2 to ground and the power of the cathode heater by varying the width of the pulses generated by the push-pull output stage T1, T2. As soon as the current stops changing, the high voltage is switched on. On In this way, the duration of the preheating phase can be kept low.

While preferred embodiments of the invention are described in the present application, it is to be understood that the invention is not limited thereto and may be embodied otherwise within the scope of the following claims.

Claims (14)

Microwave oven with a magnetron (3) comprising a cathode (K), an anode (A) and a cathode heater and with a drive circuit for the magnetron (3), the drive circuit comprising: a high voltage generator (11) for generating a high voltage between the anode (A) and the cathode (K), a Heizstromgenerator (12) for generating a heating current for the cathode heating and a controller (13), wherein the controller (13) is configured to activate the Heizstromgeneratcr (12) and only later the high voltage generator (11) after an activation of the microwave oven in a preheating during preheating first characterized by a measuring circuit (21, R21) adapted to determine a parameter dependent on a resistance of the cathode heating, the controller (13) being arranged to set a switch-on time of the high voltage generator (11) in dependence on said parameter. Microwave oven according to claim 1, comprising a power regulator (13, 21, R21) with which a heating power received by the cathode heater can be regulated, the controller (13) being designed to regulate the heating power during the preheating phase to a first desired value. Microwave oven according to claim 2, wherein the controller (13) is adapted to regulate the heating power from the end of the preheating phase to a second setpoint, wherein the second setpoint is lower than the first setpoint, in particular by at least 30% lower. Microwave oven according to one of the preceding claims, with a heating transformer (15) to which the cathode heating is connected on the secondary side is, and with a push-pull output stage with two series-connected, by the controller (13) alternately switchable switching elements (T1, T2), wherein the push-pull output stage is connected to the primary side of the heating transformer (15). Microwave oven according to claim 4, wherein the measuring circuit (21, R21) comprises a resistor (R21) through which the current from the push-pull final stage through the heating transformer (15) to a reference potential, in particular to ground, flows. Microwave oven according to claim 2 and claim 5, wherein the controller (13) is adapted to alternately turn on the two switching elements (T1, T2) during heating phases (H1, H2) and between them during rest periods (R1, R2) both switching elements (T1, T2 ), wherein the power controller (13, 21, R21) controls the duration (th) of the heating phases (H1, H2). The microwave oven according to claim 6, wherein the power controller is configured to maintain a product P = Uz '· Uih (tm) · th constant, where Uz' is a voltage applied to the push-pull output stage, Uih is a voltage dropped across the resistor R21, and th is the duration of the heating phases. A microwave oven according to any one of claims 6 or 7, wherein the parameter is the duration (th) of the heating phases. A microwave oven according to any one of claims 4 to 7, wherein the parameter is a current flowing from the push-pull final stage through the heating transformer (15) to a reference potential, in particular to ground. A microwave oven according to any one of the preceding claims, wherein the controller (13) is adapted to terminate the preheat phase when a rate of change of the parameter falls below a change threshold, and more particularly wherein the rate of change is below 10% / 10ms. Method for operating a microwave oven with a magnetron (3) comprising a cathode (K), an anode (A) and a cathode heater, wherein at least the following steps are carried out to activate the microwave oven: Generating a heating current through the cathode heater in a preheating during preheating and after the preheating time has elapsed, applying a high voltage between the anode (A) and the cathode (K), characterized in that a dependent of the resistance of the cathode heating parameter is measured and depending on said parameter, the preheating time is determined. The method of claim 11, wherein the heating power of the cathode heater is controlled during the preheating phase to a first setpoint. The method of claim 12, wherein the heating power is regulated from the end of the preheating phase to a second setpoint, wherein the second setpoint is lower than the first setpoint, in particular by at least 30% lower The method of any one of claims 11 to 13, wherein the preheat phase is terminated when a rate of change of the parameter falls below a change threshold, and more particularly wherein the change threshold is below 10% / 10ms.

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