A POWER SUPPLY ARRANGEMENT

Abstract: A magnetron (M) serving as microwave source in a microwave oven is driven by a Switch Mode Power Supply (SMPS). The resonance circuit of the Power Supply contains a transformer (Tr), the secondary side of which is connected to the magnetron (M) via a voltage multiplier in the shape of a rectifier and voltage doubler circuit (C3, C4, D3, D4). In order to obtain a feed-back signal, which is a measure of the power fed to the magnetron in order to enable regulation of this power, a current transformer (ST) is connected in series with one of the diodes (D3) in the rectifier and voltage doubler circuit. The output signal of the current transformer (ST) is compared with a reference signal and the result of the comparison is used to control the switch frequency and thereby the power.

Description: Field of the invention. The invention relates to a power supply arrangement in a microwave oven comprising a magnetron driven by a Switch Mode Power Supply having a resonance circuit fed from the mains via a mains rectifier and comprising a first capacitance, an inductance, a second capacitance and a transformer, which is connected to the magnetron via a voltage multiplier and feeds a driving voltage to the said magnetron and a controllable switch which is switched between a closed and open condition at a given switch frequency, the power delivered by the resonance circuit to the magnetron being dependent upon the switch frequency, and furthermore comprising a current transformer the output signal of which is led to a control circuit comprising a comparator and a voltage controlled oscillator where it is compared with a predetermined reference signal in order to control said switch frequency and thereby the power fed to the magnetron to a value determined by the reference signal. The output power of a magnetron has a linear relationship to the anode current as the anode voltage can be regarded as constant. As a measure of the magnetron power it is therefore possible to use the anode current. Then a current sensing device, for example, a current transformer producing a signal corresponding to the DC-mean value of the anode current is requiered. Background of the invention. A power supply arrangement according to the above is previously known from NL 7707605. The primary winding of the current transformer is included in the anode circuit of the magnetron. Accordingly the anode current is directly measured by the current transformer. However this envolves a great draw-back due to the fact that the anode current has a very irregular waveform and contains strong disturbances, which will make the utilization of the fee-back signal difficult and will require a filtering operation. Disturbances in the anode current may be caused by, for example, changes in the microwave impedance due to the fact that the anode current has a very irregular waveform and contains strong disturbances, which will make the utilization of the feed-back signal difficult and will require a filtering operation. Disturbances in the anode current may be caused by, for example, changes in the microwave impedance due to the character of the load or the position of the agitator. It is to be noted that DE Offenlegungschrift 2 217 691 discloses a voltage multiplier in the output stage of a SMPS magnetron of the kind used in the power supply arrangment of the invention. However, there is no feed-back signal from the voltage multiplier to regulate the switch frequency and thereby the power fed. As a further example of prior art DE C2 27 28 616 may be mentioned. The current flowing in the magnetron is sensed and used as a feed-back coupling. It is not shown in detail in what way the current is sensed, but the use of a current transformer connected into a branch of a voltage multiplier must be excluded due to the simple fact that no voltage multiplier is shown or proposed. Summary of the invention. The invention has for its object to modify a power supply arrangement of the kind as described in the opening paragraph such that a feed-back signal can be produced in a more simple manner not exhibiting the draw-back of the prior art power supply arrangement according to the above. The feed-back signal must fulfill the following requirements. 1. The signal strength of the feed-back signal has to correspond to the DC mean value of the anode current. 2. The feed-back signal must not be influenced by disturbances caused by irregularities in the anode current. According to the invention this is achieved thereby, that in a power supply arrangement of described kind, the current transformer is connected into a branch of the voltage multiplier connected in parallell with the magnetron. In a prefered power arrangement in which the voltage multiplier comprises a branch parallel to the magnetron comprising two diodes, the current transformer preferrably is connected in series with one of the diodes in said branch of the voltage multiplier. In another prefered power supply arrangement in which the voltage multiplier is a voltage doubler circuit included in a combined rectifier and doubler circuit including diod couplings, the arrangement is characterized in that the current transformer is connected in series with one of the diodes in the rectifier and voltage doubler circuit. The invention is based upon the recognition of the fact that the DC-mean value of the current in a voltage multiplier, as a rectifier and voltage doubler circuit, corresponds to the mean value of the anode current through the magnetron and that this current in the voltage multiplier has a low disturbance level and a regular and geometrically simple waveform, which makes it possible and favourable to connect the current transformer into a branch of the multiplier instead of the anode circuit of the magnetron. The transformer will automatically produce galvanic insulation and as a result of the regular and simple waveform of the current and absence from disturbances, its output signal can be used directly as a measure of the DC-level in spite of the fact that it only can transfer the AC-content of the current and not the initial DC-level. Brief description of the drawings. The invention is illustrated by means of example with reference to the accompanying drawings, in which figure 1 shows a simplified circuit diagram, partly drawn as a block diagram, of a power supply arrangement according to the invention, figure 2 shows some time diagrams in order to explain the function of the arrangement according to figure 1, and figure 3 shows three examples of the anode current of the magnetron. Description of embodiment. In figure 1 reference B designates a mains rectifier fed from the mains via the the terminals S1, S2 and followed by a filtering coil L1. The rectified and filtered voltage is fed to a resonance circuit consisting of a capacitance C1, an inductance L2, a DC-blocking capacitance C2 and the reactive impedances appearing at the primary side of a transformer Tr. The secondary side of the transformer is connected to a rectifier and voltage doubler circuit consisting of two capacitors C3, C4 and two high-voltage diodes D3, D4. The rectifier and doubler circuit delivers the operating voltage to a magnetron M. Two capacitors C5 and C6 act as tuning capacitances in the resonance circuit. Connected across the resonance circuit there is a controllable semiconductor switch D1 in series with a power diode D2. The setting moments of the switch are determined by a control circuit K connected to the control electrode of the switch via a drive stage S. The resonance circuit forms a parallel resonance circuit and the power transferred to the magnetron will increase with increasing switch frequency. According to the invention the power fed to the magnetron is sensed by means of a current transformer ST, the primary side of which is connected in series with one of the high-voltage diodes D3 in the rectifier and doubler circuit. The secondary side of the current transformer ST is connected to a control input of the control circuit K, so that a closed regulation loop with negative feed-back is formed. In the manner as described in the simultaneuosly filed patent application SE 8803662-9 a voltage proportional to the current from the transformer ST is compared with a reference voltage V ref in the control circuit K and the result of the comparison is used to control the frequency of a controllable oscillator determining the switch frequency, so that this frequency and thereby the power fed to the magnetron M is regulated to a value determined by V ref . It is appreciated that the arithmetic DC-mean value of a current through the high voltage diodes D3, D4 coincides with the mean value of the current through the magnetron M, being the magnitude to be sensed. Figure 2 shows the current 1 through the high voltage diodes in the rectifier and doubler circuit as function of the time t, on the one hand in case of low power (figure 2a) and on the other hand in case of high power (figuer 2b). It is evident from figure 2 that the current through the high voltage diodes of the rectifier and doubler circuit has a low disturbance level and a regular and geometrically simple waveform. According to the invention this is utilized thereby that a current transformer, which only can transfer the AC-content of the current, is used in order to get a measure of the dc-mean value of the current and thereby the power fed to the magnetron. The waveform shown in figure 2 makes it namely possible, only by using the shown current, to determine the DC-mean value without knowing the initial zero level. This is a condition for being able to use a current transformer for producing a feed-back signal, as the transformer cannot transfer the DC-level. Furthermore the current transformer has the great advantage to produce galvanic insulation. Figure 3 shows three examples of the anode current of the magnetron. As can be seen from the three examples the anode current has a very irregular waveform and contains strong disturbances. Every second pronounced peak is to be compared with the diode current peak of figure 2 which latter peaks show a quite more regular and non-disturbed character. Instead of the rectifier and voltage doubler circuit as shown other types of voltage multipliers built-up by diodes and capacitors can also be used, the current transformer being connected in series with one of the diodes in the voltage multiplier.

Claims: A power supply arrangement in a microwave oven comprising a magnetron (M) driven by a Switch Mode Power Supply having a resonance circuit fed from the mains via a mains rectifier (B) and comprising a first capacitance (C1), an inductance (L2), a second capacitance (C2) and a transformer (Tr), which is connected to the magnetron (M) via a voltage multiplier (C3, C4, D3, D4) and feeds an operating voltage to the said magnetron (M) and a controllable switch (D1) which is switched between a closed and an open condition at a switch frequency, the power delivered by the resonance circuit to the magnetron being dependent upon the switch frequency, and furthermore comprising a current transformer (ST) for sensing the current through the magnetron the output signal of which current transformer (ST) is fed to a control circuit (K) comprising a comparator and a voltage controlled oscillator where it is compared with a predermined reference signal (Vref) in order to control said switch frequency and thereby the power fed to the magnetron (M) to a value determined by said predetermined reference signal, characterized in that the current transformer (ST) is connected into a branch of the voltage multiplier connected in parallel with the magnetron (M). A power supply arrangment as claimed in the claim 1, in which the voltage multiplier comprises a branch parallel to the magnetron comprising two diodes (D3, D4), characterized in that the current transformer (ST) is connected in series with one of the diodes (D3) in said branch of the voltage multiplier. A power supply arrangement as claimed in the claim 2, in which the voltage multiplier is a voltage doubler circuit included in a combined rectifier and voltage doubler circuit including diode couplings, characterized in that the current transformer is connected in series with one of the diodes (D3) in the rectifier and voltage doubler circuit

Inventors: BRAUNISCH, ECKART, OENNEGREN, JAN, Önnegren, Jan
Applicants: WHIRLPOOL EUROPE B.V.
Application year: 1989
Main classification: H05B6
IPC codes: H05B6