Oscillator circuit

Abstract: A magnetron is powered via an output transformer TR whose primary winding (a) is connected in series with a resonant converter oscillator circuit comprising an inductor (L) and a capacitor (C3) and switching transistors (T₁, T₂) which are connected to positive and negative power supply terminals and are switched by respective current transformers (CT1, CT2). The switching points of the switching transistors are varied so as to vary the output of the circuit by varying the positive and negative flux excursions in the cores of the transformers (CT1, CT2) such that the net flux excursions are sufficient to saturate the transformers and switch off the switching transistors at a predetermined point in each half cycle. The relative positive and negative flux excursions in the transformer cores are controlled by providing additional winings (g) which are connected to a control circuit which rectifies the output of these windings and controls the rectified DC voltage appearing across them.

Description: The present invention relates to oscillator circuits, and also relates to a magnetron powered by the rectified AC output of an oscillator circuit. Referring to Figure 1, which shows a series resonant circuit comprising an inductor L and a capacitor C3, it can be shown that the current in a resistive load LD, which is connected in parallel with capacitor C3, is independent of the load resistance when the current I is oscillating at the resonant frequency and is therefore in phase with the voltage V X-Y . In such a case the current flowing in load LD can be expressed as V X-Y /ωL where ω= 2 πf where f is the resonant frequency. Thus, at resonance, such a circuit acts as a constant current source and is particularly suitable (after rectification) for powering non-linear loads such as a magnetron which has a zener characteristic. A resonant converter is disclosed in EP-A-121,917 and is used to energise a discharge tube. The principal of operation of this type of resonant converter is illustrated in the accompanying Figures 2A to 2F which are circuit diagrams illustrating the oscillating current flows in this type of circuit. Figures 2A to 2F have been simplified by omitting the transformers utilised to control switching transistors T₁and T₂ and by omitting the load which is connected in parallel with capacitor C3. Referring to Figures 2A to 2F, it will be seen that the circuit comprises an inductor L and a capacitor C connected in series and gate-controlled switching means, namely MOSFET's T₁ and T₂ connected to the free terminal of inductor L. The free terminal of capacitor C3 is connected to the positive power supply terminal via a capacitor C1 and to the negative power supply terminal by a capacitor C2. Switching transistor T1 is bypassed by a parasitic diode D1 and switching transistor T2 is bypassed by a parasitic diode D2, both parasitic diodes being poled in the opposite sense to the polarity of the power supply terminals. In each of Figures 2A to 2F the flow of current 1 is indicated. Current flow from right to left as shown in Figure 2A is taken to be flow in the forward direction and accordingly, current flow as shown in Figure 2D is flow in the reverse direction. If switching transistors T₁ to T₂ are switched at the zero crossing points of the current wave form, the circuits oscillates at the resonant frequency as shown in Figure 4 which is a plot of the current wave form I and voltage wave form V X-Y (as defined in Figure 1). It will be seen that the positive half cycles correspond to the current flow shown in Figure 2A and that the negative half cycles corresponds to the current flow shown in Figure 2D. In the more general case, as illustrated in Figure 5, which is a plot of current I and voltage V X-Y of the circuit of Figures 2A to 2F, the switching transistors T₁ and T₂ are not switched at the zero crossing point of the current wave form and accordingly there is a phase difference between the current wave form and the voltage wave form V X-Y . The periods corresponding to the current flows shown in Figures 2A, 2B, 2D and 2E are shown in Figure 5. Referring now to Figures 2A to 2F, when switching transistor T₁ is switched on, current flow in the "forward" direction as shown in Figure 2A occurs. On switching off transistor T₁, this current flow is continued in a loop through inductor L, capacitor C3, capacitor C2 and parasitic diode D2 as shown in Figure 2B. The current flow in inductor L and capacitor C3 then reverses. If transistor T₂ is switched on as soon as the zero-crossing point of the current wave form is reached, then the current flow as shown in Figure 2D occurs. However, in practice there is a slight delay (assumed to be negligible in Figure 5), so that the initial reversal occurs through a loop consisting of capacitor C1, capacitor C3, inductor L and parasitic diode D1 as shown in Figure 2C. The current flow shown in Figure 2D is followed by a current flow in the forward direction through a loop consisting of capacitor C1, capacitor C3, inductor L and parasitic diode D1 as shown in Figure 2E, as a result of transistor T₂ being switched off. This current flow gradually dies away until the zero-crossing point of the current wave form is reached. Assuming that transistor T₁ is switched on as soon as this zero-crossing point is reached, the forward current flow as shown in Figure 2A then occurs. However, in practice, transistor T₁ will be switched on shortly after the zero-crossing point and in this intermediate condition between the zero-crossing point and the instant at which transistor T₁ is switched on, current flow in a loop as shown in Figure 2F occurs, namely through inductor L, capacitor C3, capacitor C2 and parasitic diode D2. It will be noted that the circuit disclosed in EP-A-121,917 is not controllable and can oscillate only at the resonant frequency as shown in Figure 4 (already referred to). However, a paper by Sebastian et al entitled "Regulating Self-Oscillating Resonant Converters", 2nd European Conference on Power Electronics and applications held at Grenoble, France 22 - 24 September 1987 discloses in Figure 6 a somewhat complex circuit which automatically switches the switching transistors in such a manner as to achieve non-resonant oscillation as shown in accompanying Figure 5 (already referred to). By controlling the switching of transistors, the phase difference between the voltage and current wave forms can be controlled and this in turn controls the power output of the circuit. The circuit disclosed by Sebastian utilises saturable transformers whose primary windings are connected in series with the inductor and capacitor of the circuit and whose secondary windings are connected across the gate and source of the switching transistors. The drive is removed from the gates of the switching transistors when the saturable transformers saturate. The saturable transformers each incorporate additional windings through which a controlled direct current is fed. This controlled current is generated by an external circuit and generates an additional magnetising force in each transformer core. It is implicit in the circuits disclosed by Sebastian et al that each transformer core saturates during each half cycle at an instant which is determined by the control current in the additional windings. By varying the control current flowing in the additional windings, the voltage gain and power output of the circuit can be controlled. An object of the present invention, which is defined in the appended claims, is to provide an oscillator circuit arrangement in which an externally powered control circuit of the type disclosed in Sebastian et al is not required. The invention also provides a magnetron system comprising a magnetron energised by the rectified AC output of the above oscillator circuit arrangement. The invention will now be described by way of example only with reference to Figures 1 - 8 of the accompanying drawings, of which: Figure 1 is a circuit diagram showing a load energised from a oscillator circuit, Figures 2A - 2F show successive current flows in an oscillator circuit, Figure 3 is a circuit diagram of an oscillator circuit which illustrates the principle of the present invention, Figure 4 is a plot of current and voltage wave form against time in one variant of the circuit shown in Figures 2A - 2F, Figure 5 is a plot of current and voltage wave form against time for another variant of the circuit shown in Figures 2A - 2F, Figure 6 shows plots of flux density B against magnetising force H in the transformer cores of the circuit arrangement of the present invention, Figure 7 is a partial circuit diagram of a magnetron system in accordance with the present invention, and Figure 8 is a circuit diagram showing the novel control circuitry of the present invention which is utilised to control the magnetron system of Figure 7. Figures 1, 2, 4 and 5 have already been refered to. The control circuit of the present invention differs fundamentally from the control circuit disclosed by Sebastian et al in that it is arranged to control the relative flux excursions in the saturable transformer cores, such that when the ratio of forward and reverse flux excursions is not equal to unity, net flux excursions occur in successive cycles until the saturable transformer core saturates in alternate half cycles only. This is illustrated in Figures 3 and 6. Referring to Figure 3, it will be noted that the circuit shown corresponds generally to the circuit shown in Figures 2A to 2F but includes current transformers CT1 and CT2 whose primaries are connected in series with inductor L and capacitor C3 and whose secondary windings are connected across the gate and source of switching transistors T₁ and T₂. Furthermore, in order to initiate oscillation in the circuit, the capacitor C4 is charged via a resistor R1 and fires a diac DI which is connected between the junction of C4 and R1 and the gate of switching transistor T₂. Furthermore, a load LD (such as a magnetron) is shown connected in parallel with capacitor C3. The windings of transformers CT1 and CT2 are wound in such a sense that when current I is positive (i.e. flowing from right to left in Figure 3) transistor T₁ is switched on and transistor T₂ is switched off, whereas when the current is flowing in the reverse direction, transistor T₁ is switched off and transistor T₂ is switched on. The secondary winding of each transformer is provided with a pair of zenor diodes ZD connected back to back in series. The flux  in each transformer core is proportional to the integral: where V is the voltage across the secondary winding and t is time. If the forward voltage drop across each pair of zenor diodes ZD is V F and the corresponding reverse voltage drop is V B , then the corresponding flux excursions  F and  B cancel in each cycle. This is illustrated in Figure 6(a). Accordingly, the saturation level is never reached and each transformer switches off its associated transistor T₁ or T₂ only at the zero crossing points of the current waveform. The voltage and current wave forms are shown in Figure 4, which also shows the forward and reverse voltage drops across the second windings, namely V F and V B and the corresponding time intervals T F and T B during which these voltage drops occur. In accordance with the present invention, it is contemplated that V F and V B may be made unequal. In this case the corresponding flux excursions  F and  B in successive half cycles will also be unequal, as illustrated in Figure 6(b). As shown, this results in a net increment in flux during each successive cycle, and after a few cycles the saturation flux density is reached, at which time the voltage in the secondary windings falls to zero. This situation is illustrated in Figure 5 (already referred to) which shows the forward voltage drop V F and the reverse voltage drop V B (assumed to be less than V F ) and the corresponding time periods t F and t B . Figure 5 also indicates that the transformer cores become saturated at the end of each period A in the cycle, period A being illustrated in Figure 2A. It will be seen that the phase shift between the voltage and current wave forms is dependent upon the ratio t B ;t F which is in turn dependent on the ratio V B :V F ; since V F . t F = V B . t B . It will be noted that this mode of operation is quite different to the operation of the Sebastian circuit in which a predetermind biasing flux is generated in the transformer core by means of a constant current passed through an additional winding. Such a biasing flux will have no effect on the net flux excursion in each cycle. In a particularly simple form of the invention, it is envisaged that at least one zener diode ZD associated with each transformer CT1 and CT2 would be of the externally controllable type, such that its zener voltage drop is controllable. Figures 7 and 8 show a preferred embodiment of the invention in which the control circuit which controls the forward and reverse flux excursions in the transformer cores requires no external power supply. The arrangement shown in Figure 7 comprises a bridge rectifier BR which is fed with AC from the mains conductors LN and feeds current to an oscillator circuit comprising inductor L, capacitor C3, MOSFET switching transistors T₁ and T₂ and additional capacitors C1 and C2. The resonant frequency of the circuit is determined principally by the inductance (typically 160 microhenries) of the inductor L and the capacitance (typicaly 0.47 microfarads) of the capacitor C3. The values of C1 and C2 are suitably 3.3 microfarads. The output of the above oscillator circuit is fed to the primary winding a of an output transformer TR and the output of the secondary winding c of this transformer is fed to magnetron M in a conventional voltage doubling circuit comprising capacitor C6 and diodes D4 and D5. The heater H of the magnetron is energised by a separate secondary winding b of transformer TR. The oscillator circuit includes two current transformers CT1 and CT2 which are connected in series with inductor L and capacitor C3. These current transformers each incorporate a saturable core of magnetisable material having a square hysteresis loop, such as "orthogonol" or "HCR" for example. It should be noted that when the circuit oscillates at the resonant frequency, the transformer cores do not saturate however. The arrangement includes a starter circuit of a coventional type, comprising a 47 microfarad capacitor C5 which is charged by a diode D3 and a series - connected resistor R1 and capacitor C4 which are connected between the junction of C5 and D3 and the negative pole of the power supply. A series - connected resistor R2 and diode D8 are connected between the junction of R1 and C4 and the source of transistor T₁. Furthermore, a diac DI is connected between the junction of R1 and C4 and to series - connected primary windings f of transformers CT1 and CT2 respectively. When the circuit is energized, a capacitor C4 charges up gradually at a rate determined by the RC constant of R1 and C4 and when its voltage reaches a predetermined level it fires diac DI which sends a pulse of current through windings f which are poled in such a manner as to generate a voltage across the gate and source of switching transistor T₁ which turns on this transistor. This initiates oscillation in the manner already described above with reference to Figures 2A - 2F. Switching transistors T₁ and T₂ are each bypassed by parasitic diodes D1 and D2. These diodes are indicated in dashed lines because they are not discrete components but are incorporated within T₁ and T₂ respectively. After oscillation has been started, transformers CT1 and CT2 are energised by current flowing in their primary windings e. An output voltage is generated in secondary windings h and it will be noted that the output voltage in CT1 is of opposite plurality to the output voltage of CT2, because primary windings e are wound in opposite senses whereas secondary windings h are wound in the same sense. Each secondary winding h incorporates two back - to back zener diodes ZD connected across its terminals and also incorporates two series - connected resistors (R3, R4 in the case of CT1 and R5, R6 in the case of CT2), the respective junctions of these pairs of resistors being connected to the gates of switching transistors T₁ and T₂ respectively. The drive circuits to the switching transistors are completed by connections between one end of each secondary winding h and the source of transistor T₁ (in the case of CT1) and the source of T₂ (in the case of CT2). Accordingly, when current is flowing from terminal X to terminal Y, the flux generated by primary windings e tends to turn on transistor T₁ and to turn off the transistor T₂. This situation is of course reversed when current flows from terminal Y to terminal X. The zener diodes ZD limit the voltage drops across respective secondary windings h to approximately 20 volts in either direction, and accordingly, the cores of transformers CT1 and CT2 do not saturate when the flux excursions in them during successive half cycles are equal and opposite. As previously noted in connection with Figures 2A - 2F and Figures 4 and 5, this condition occurs when the oscillator is running at the resonant frequency. In order to control the power output of the oscillator circuit, two additional secondary windings g are provided on current transformers CT1 and CT2 which are wound in opposite senses and connected in series. The junction of these windings is connected to a conductor 1 and the free ends of these windings are connected to conductors 2 and 3 respectively which are connected to a control circuit (Fig. 8). The control circuit of Figure 8 is also connected to the positive and negative output of a rectifier circuit which is powered by a secondary winding d of power output transformer TR. It will be seen that the positive terminal of this circuit is connected to the junction of two diodes D6 and D7 and that the negative terminal is connected to the mid point of winding d. The operation of this control circuit will now be described with reference to Figure 8. The conductors 2 and 3 from the free ends of additional windings g are connected to diodes D8 and D9 and a reservoir capacitor C7 is connected between conductor 1 and the junction of diodes D8 and D9. Accordingly, the voltage appearing across capacitor C7 is the voltage appearing across windings g of the current transformers CT1 and CT2. The flux excursion in the cores of transformers CT1 and CT2 is in turn dependent upon the product Vt in the additional windings g and accordingly the ratio of the successive flux excursions in the transformer cores due to forward and reverse current flow is dependent upon the voltage appearing across reservoir capacitor C7 (Figure 8). This voltage is controlled by a regulator circuit comprising a TL431 shunt regulator IC1 which controls a power transistor T3. Transistor T3 discharges the capacitor C7 to a preset voltage determined by the ratio of variable resistor VR1 and resistor R7, the junction of these resistors being connected via a further resistor R8 to the control terminal of IC1. A resistor R9 is connected between the base and omitter of transistor T3. Variable resistor VR1 is variable manually and controls the power output of transformer TR. It will be noted that the power output may be reduced to zero by setting VR1 to a value such that the phase difference between the voltage appearing across terminals X and Y (Figure 7) and the current flowing between these terminals is 90°. In order to limit the power output of the transformer TR when the heater circuit H of the magnetron M is warming up, the conductors from secondary winding d are connected to the control terminal of IC1 (Figure 8) via a diode D10 which is connected to the junction of a series connected resistors R10 and R11. Until the magnetron heater H has warmed up and electrons emitted, the magnetron M is an open circuit and high voltages appear across the transformer primary a. These voltages are sensed by winding 'd', attentuated by resistors R10 and R11 and fed into the control input of ICI, turning on T3 and reducing the power fed into the resonant circuit, keeping the system under control.

Claims: An oscillator circuit arrangement comprising an inductor (L) connected in series with a capacitor (C3), positive and negative power supply terminals, first gate-controlled switching means (T1) arranged to conduct current from said terminals through said inductor and capacitor in the forward direction and second gate-controlled switching means (T2) arranged to conduct current from said terminals through said inductor and capacitor in the reverse direction, the gates of said switching devices being coupled to the secondary windings (h) of respective saturable transformers (CT1,CT2) which are arranged to switch on the switching devices alternately, each saturable transformer having a primary winding (e) which carries the oscillating current produced by said circuit arrangement and having control means comprising an additional winding (g) coupled to said saturable transformer core, the control means being further characterised by rectifying means (D8,D9) connected in series with said additional winding, and regulating means connected to the output of said rectifying means and arranged to set a DC voltage drop in said additional winding and thereby to set the ratio of forward to reverse magnetic flux excursion in the saturable transformer cores. An oscillator circuit arrangement as claimed in claim 1, wherein said regulating means is energized solely by the output of said additional winding. An oscillator circuit arrangement as claimed in claim 1 or claim 2 wherein each of said switching means (T1,T2) is bypassed by a respective diode (D1,D2). A magnetron system comprising a magnetron energized by the rectified AC output of an oscillator circuit arrangement as claimed in any preceding claim. A magnetron system as claimed in claim 4, wherein the magnetron is energised from a transformer whose primary winding is connected in parallel with said capacitor (C3). A magnetron system as claimed in claim 4 or claim 5, as dependent upon claim 1 or claim 2, wherein further control means coupled to the heater circuit of the magnetron is arranged to generate a control signal which temporarily limits the output of said oscillator circuit arrangement while the heater of the magnetron is warming up. A magnetron system as claimed in any of claims 4 to 6, wherein said positive and negative power supply terminals are energised by the output of a rectifying circuit.

Inventors: GURWICZ DAVID, GURWICZ, DAVID
Applicants: ELECTROLUX AB, AKTIEBOLAGET ELECTROLUX
Application year: 1990
Main classification: H02M7
IPC codes: H05B6, H02M3, H02M7