Abstract:
A starting circuit for an inverter includes, at least, a control winding provided in connection with a saturable magnetic core of an input transformer of the inverter, a transistor and a capacitor. When a DC voltage source is activated, a discharge current of the capacitor flows through the collector-emitter circuit of the transistor and the control winding so as to start the switching operation of the inverter. A control circuit is provided for detecting a secondary voltage of the inverter so as to cut off the operation of the starting circuit after the inverter starts.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a starting circuit for an inverter, and more particularly to a starting circuit for an inverter by which it can be certain that the inverter is started. 
     2. Description of the Prior Art 
     A DC-DC converter including a starting circuit 10, as shown in FIG. 1, is well known. In FIG. 1, an AC voltage of a predetermined level is applied to a pair of input terminals 1a and 1b, and it is rectified by a rectifying circuit 2 which is constituted, for example, by a diode-bridge. A positive voltage is led out in a line 4a, while a negative voltage is led out in another line 4b. Smoothing capacitors C 1  and C 2  are connected in series with each other between the lines 4a and 4b. The line 4a is connected to one end of a parallel circuit of a capacitor C 5  and a resistor R 7 , to the anode side of a thyristor 8 and to the collector electrode of a transistor Q 1 . 
     The other end of the parallel circuit of the capacitor C 5  and the resistor R 7  is connected to the gate terminal of the thyristor 8. The cathode side of the thyristor 8 is connected through a parallel circuit of a current limiting resistor R 8  and a speed-up capacitor C 6  to the base electrode of the transistor Q 2 . The collector electrode of the transistor Q 2  and the emitter electrode of the transistor Q 1  are connected to each other, and they are connected to one end of the secondary winding 11c of an input transformer 11 having a saturable magnetic core, and to one end of another secondary winding 11b of the input transformer 11. The other end of the secondary winding 11b is connected to the base electrode of the transistor Q 1 . A further secondary winding 11d is connected to the transistor Q 2 . The secondary windings 11b and 11d are wound round the saturable magnetic core of the input transformer 11 in opposite directions or in opposite polarities to each other. The secondary winding 11c is used for positive feedback, and so it works to increase voltages induced at the secondary windings 11b and 11d so as to compensate the base voltages of the transistors Q 1  and Q 2  when the inverter is highly loaded. The other end of the secondary winding 11c is connected through the primary winding 5a of the output transformer 5 to the connection point between the capacitors C 1  and C 2 . The base electrode of the transistor Q 2  is connected to one end of the secondary winding 11d of the input transformer 11. The other end of the secondary winding 11d  is connected to the emitter electrode of the transistor Q 2  and to the line 4b. Accordingly, the transistor Q 2  is put into the conductive state initially with the current supplied to the base electrode of the transistor Q 2  through the thyristor 8, and then with the voltage induced at the secondary winding 11d. 
     Both ends of a secondary winding 5b of the output transformer 5 are connected to a rectifying circuit 12 formed by a diode bridge to apply the output voltage of the output transformer 5 to the rectifying circuit 12. The output voltage is rectified by the rectifying circuit 12, so that a DC voltage of a predetermined level is obtained from output terminals 14a and 14b. Another secondary winding 5c of the output transformer 5, as a feedback winding, is connected through a resistor R 6  to the primary winding 11a of the input transformer 11. Thus, a part of the output obtained from the output transformer 5 is fed back through the resistor R 6  to switch on and off alternately the transistors Q 1  and Q 2 . 
     Next operation of the above described circuit will be described. 
     With the application of the AC voltage to the input terminals 1a and 1b of the rectifying circuit 2, a charging current flows into the capacitor C 5  from the line 4a to put the thyristor 8 into the conductive state. Accordingly, a current from the line 4a is supplied through the thyristor 8 and the capacitor C 6  to the base electrode of the transistor Q 2  to put the latter into the conductive state. Accordingly, as the capacitor C 6  is charged, namely the current flowing through the capacitor C 6  decreases, the current flowing through the resistor R 8  increases. Thus, a collector current of the transistor Q 2  flows from the capacitor C 2  to the line 4b through the primary winding 5a of the output transformer 5, the secondary winding 11c of the input transformer 11, and the transistor Q 2 . As a result, an output voltage having a width corresponding to the time for which the transistor Q 2  is put in the conductive state, is obtained from the secondary winding 5b of the output transformer 5. 
     When the output voltage is obtained from the one secondary winding 5b of the output transformer 5, another output voltage is obtained from the other seecondary winding 5c of the output transformer 5, and it is applied through the resistor R 6  to the primary winding 11a of the input tranformer 11. Accordingly, secondary voltages are induced at the secondary windings 11b and 11d in opposite polarities to each other. With the application of the induced voltage of the secondary winding 11d, the transistor Q 2  continues to be put into the conductive state. And with the increase of the voltage applied to the primary winding 11a, the magnetic core of the input transformer 11 is saturated to put the induced voltages of the secondary windings 11b and 11d nearly into zero, so that the transistor Q 2  is suddenly put into the non-conductive state. The current to the primary winding 5a of the output transformer 5 from the capacitor C 2  is suddenly interrupted. Thus, a higher voltage is induced at the secondary winding 5c in the reverse polarity to the previously induced voltage, and it is applied to the primary winding 11a of the input transformer 11. The magnetic core of the input transformer 11 becomes unsaturated, and so voltages are induced at the secondary windings 11b and 11d in the reverse polarities to the previously induced voltages, respectively. As a result, the transistor Q 1  is now put into the conductive state with the application of the induced voltage of the secondary winding 11b to the base electrode of the transistor Q 1 . A current now flows from the line 4a to the capacitor C 1  through the transistor Q 1 , the secondary winding 11c and the primary winding 5a of the output transformer 5. The output is obtained from the secondary winding 5b of the output transformer 5. The other output from the secondary winding 5c is applied to the primary winding 11a of the input transformer 11. As above described, the magnetic core of the input transformer 11 is again saturated to suddenly put the transistor Q 1  into the non-conductive state. And the transistor Q 2  is again put into the conductive state. Thus, the transistors Q 1  and Q 2  are alernately put into the conductive or non-conductive state. A continuous pulse output of a predetermined frequency and level is obtained from the secondary winding 5b of the output transformer 5. The continuous pulse output is rectified by the rectifying circuit 12. A DC voltage of a predetermined level is obtained from the output terminals 14a and 14b, and it is supplied to a load circuit (not shown). 
     However, in the above described circuit, the starting operation by the thyristor 8 is very unstable, and the switching operation of the transistors cannot often be smoothly effected. 
     A part of the current flowing through the thyristor 8 is shunt to the secondary winding 11d of the input transformer 11. Accordingly, the base current to the transistor Q 2  is not sufficient enough to put the transistor Q 2  into the conductive state, in some cases. Thus, the starting operation is very unstable. Even when the switching operation of the transistors Q 1  and Q 2  is smoothly started, the conduction and non-conduction of the thyristor 8 are repeated in accordance with the time constant of the parallel circuit of the capacitor C 6  and resistor R 8 , and so the current flowing through the parallel circuit becomes load or loss in operation. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide a starting circuit for an inverter which overcomes the above-described disadvantages of the conventional starting circuit. 
     Another object of this invention is to provide a starting circuit for an inverter which can be cut off from the inverter circuit after the operation of the inverter starts. 
     A further object of this invention is to provide a starting circuit for an inverter which can be controlled in operation with the detection of the secondary voltage of the inverter. 
     In accordance with one aspect of the present invention, a starting circuit for an inverter is provided which includes: 
     a DC voltage source having first, second and voltage reference terminals; an input transformer having a primary winding, a pair of secondary windings and a saturable magnetic core; an output transformer having a primary winding, a secondary winding and a feedback winding; first and second transistors each having base, emitter and collector electrodes, the collector-emitter circuits of which are connected, through the primary winding of the output transformer, between the first and voltage reference terminals of the DC voltage source, and between the second and voltage reference terminals thereof, respectively, and the base-emitter circuits of which are connected across the corresponding secondary windings of the input transformer respectively; a circuit for connecting the primary winding of the input transformer to the feedback winding of the output transformer; a DC voltage output circuit connected across the secondary winding of the output transformer; a first control circuit including at least a control winding provided in connection with the saturable magnetic core of the input transformer, and flowing an exciting current through the control winding so as to put one of the first and second transistors in its conductive state when the DC voltage source is operated; and a second control circuit for detecting the secondary voltage of the output transformer and stopping the operation of the first control circuit when the secondary voltage reaches a predetermined value. 
    
    
     The above and other objects, features and advantages of this invention will become apparent from the following detailed description of illustrative embodiment shown in the accompanying drawing. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a prior art inverter including a conventional starting circuit; and 
     FIG. 2 is a circuit diagram of an inverter including a starting circuit according to one embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of this invention, applied to a starting circuit for an inverter, will be described with reference to FIG. 2. Parts in FIG. 2 which correspond to the parts in FIG. 1, are denoted by the same referencenumerals, which will not be described in detail. 
     In FIG. 2, a DC-DC converter includes a starting circuit 20 according to this invention, instead of the conventional starting circuit 10 having thethyristor 8 of FIG. 1. 
     A DC source voltage of a predetermined level is supplied through a resistorR 1  to the starting circuit 20. One end of the resistor R 1  is connected to resistors R 2  and R 5 , a capacitor C 3  and further to an emitter electrode of a PNP transistor Q 3 . The other endof the resistor R 5  is connected to the base electrode of the transistor Q 3  to decide a base voltage therefor, and further to the collector electrode of a transistor Q 4 . A resistor R 3  is connected in series with the resistor R 2 . The resistors R 2  and R 3  function as a voltage-divider. The connecting point between the resistors R 2  and R 3  is connected through a resistor R 4  to the base electrode of the transistor Q 4  and to a collector electrode of the transistor Q 3 . Further, the base electrode of the transistor Q 4  is connected through diodes D 1  and D 2  to one end of the feedback winding 5c of the output transformer 5. A smoothing capacitor C 4  is connected between the base and emitter electrodes of the transistor Q 4 . 
     The other end of the capacitor C 3  is connected to one end of a starting winding 11e which is newly wound as a primary winding around the magnetic core of the input transformer 11. The other end of the starting winding 11e is connected to the line 4b. Accordingly, a discharge current of the capacitor C 3  flows through the starting winding 11e to induce secondary voltages at the secondary windings 11b and 11d. The other end ofthe starting winding 11e is connected further to one end of the primary winding 11a. The other end of the primary winding 11a is connected throughthe resistor R 6  to the feedback winding 5c of the output transformer 5to supply a part of the output of the output transformer 5. 
     Next, the operation of the above described circuit will be described. 
     With the application of the AC voltage source to the input terminals 1a and1b, a voltage of positive polarity is led out in the line 4a, while anothervoltage of negative polarity is led out in the line 4b. The voltage of positive polarity is applied through the resistor R 1  to the starting circuit 20. The capacitor C 3  is gradually charged, and its charging current becomes nearly zero after a predetermined time. At the same time, the voltage across the capacitor C 4  gradually rises up, and it is applied to the base electrode of the transistor Q 4 . When it rises above a certain level, for example, beyond 0.6 V, the transistor Q 4  is put into its conductive state. Accordingly, the collector voltage of the transistor Q 4  lowers to put the transistor Q 3  into the conductive state. When both the transistors Q 3  and Q 4  are thus put into the conductive state, the resistance between the one end of the capacitor C 3  and the line 4b becomes very low. Accordingly, the capacitor C 3  is instantaneously discharged through the emitter-base circuit of the transistor Q 3  and the collector-emitter circuit of thetransistor Q 4 . The discharge current flows through the starting winding 11e to induce a predetermined voltage at the secondary winding 11bof the input transformer 11. 
     The induced voltage is applied to the base electrode of the transistor Q 1  to put the latter into the conductive state. Accordingly, a current from the line 4a flows through the transistor Q 1 , the secondary winding 11c of the input transformer 11, and the primary winding5a of the output transformer 5 to the capacitor C 1 . Secondary voltagesare induced at the secondary windings 5b and 5c by the current flowing through the primary winding 5a. The output voltage of the secondary winding 5c is fed back to the primary winding 11a of the input transformer11. The magnetic core of the input transformer 11 is presently saturated, so that the induced voltage of the secondary winding 11b becomes zero, andthe transistor Q 1  is suddenly put into the non-conductive state. Accordingly, as above described in FIG. 1, the feedback voltage in the reverse polarity to the previous feedback voltage is applied to the primary winding 11a of the input transformer 11 to induce the voltage at the secondary winding 11d to put the transistor Q 2  into the conductive state. With the conduction of the transistor Q 2 , a currentfrom the capacitor C 2  flows through the primary winding 5a of the output transformer 5, the secondary winding 11c of the input transformer 11 and the transistor Q 2  to the line 4b. The output voltage of the secondary winding 5b is applied to the rectifying circuit 12. The magneticcore of the input transformer 11 is again saturated, and the transistor Q 2  is suddenly put into the non-conductive state. Next, the voltage is induced at the secondary winding 11b to again put the transistor Q 1  into the conductive state. 
     As above described, the transistors Q 1  and Q 2  are alternately putinto the conductive or non-conductive state, so that a continuous pulse output voltage is obtained from the secondary winding 5b of the output transformer 5. The pulse output voltage is rectified by the rectifying circuit 12. A DC output is led out from the output terminals 14a and 14b. 
     The output voltage of the other secondary winding 5c is rectified by the diodes D 1  and D 2  to obtain a negative voltage beyond a predetermined level at the base electrode of the transistor Q 4 . With the application of the negative voltage, the transistor Q 4  and accordingly, the transistor Q 3  are put into the non-conductive state.As a result, as soon as the ON-OFF operation of the transistors Q 1  andQ 2  is started, the operation of the starting circuit 20 is stopped. Inthe circuit according to this invention, the charging voltage of the capacitor C 3  to start the switching operation of the transistors Q 1  and Q 2  can be adjusted in accordance with the load by making the resistors R 2  and R 3  variable, so that the starting operationcan be surely effected. 
     According to this invention, for the inverter including a pair of transistors for oscillation, a starting circuit is provided which comprises a capacitor for imparting a starting voltage to one of the transistors, and a switching element for charging and discharging the capacitor, and a part of the output of the inverter is rectified and then applied to the starting circuit to put the latter into inactive condition.As soon as the inverter is put into its active condition, the starting circuit is put into the OFF-state with the application of a part of the output of the inverter. Accordingly, the starting circuit does not become a load to the inverter in operation. Moreover, the starting operation can be surely effected. 
     While there have been described preferred embodiments of the invention, obviously modifications and variations are possible in light of the above teachings. It is, therefore, to be understood that within the scope of theappended claims, the invention may be practiced otherwise than as specifically described.