Inverter drive circuit - Patent Review 4204266

What is claimed is:

1. An inverter circuit having controllable semiconductors connected for a controllable output alternating voltage, comprising, in combination:

a control circuit connected to control the conduction time of said semiconductors, said control circuit having controllable input means connected to control the conduction time of said semiconductors;

means establishing a voltage reference signal;

means connected to sense the amount of output current from said semiconductors and to develop a current signal;

rectifier means having a plurality of outputs connected to rectify a signal proportional to said current signal;

a current limit setting potentiometer connected to said reference voltage signal to establish a reference signal representative of a maximum value of inverter output current;

a first current limit means including

(i) an operational amplifier connected as a comparator and having first and second inputs;

(ii) means connecting said comparator first input to one of the outputs of said rectifier means; and

(iii) means connecting said current limit setting potentiometer reference signal to said second input of said comparator; and

means connecting the output of said comparator to said controllable input means of said control circuit whereby the output voltage of said inverter is decreased upon the output of said comparator changing state when said rectified signal exceeds said reference signal.

2. An inverter circuit as set forth in claim 1, including capacitor means connectively coupled to said one of the outputs of said rectifier means to quickly charge upon increase of said current signal, and means connecting the output of said capacitor means to said first input of said comparator.

3. An inverter circuit as set forth in claim 2, including a large value resistance connected in parallel with said capacitor means to provide a slow discharge of said capacitor means.

4. An inverter circuit as set forth in claim 2, including diode means connected to pass the voltage at said one output of said rectifier means to said capacitor means,

and second current limit means including a comparator connected to another of the outputs of said rectifier means and connected to said control circuit to control same with a response time substantially less than that of said first current limit means.

5. An inverter circuit as set forth in claim 4, wherein said controllable input means includes an inhibit input connected to terminate output of said semiconductors upon the inhibit input changing logic states,

and means connecting the output of said comparator included in said second current limit means to said inhibit input.

6. An inverter circuit as set forth in claim 1, including capacitor means connected to said one of the outputs of said rectifier means to quickly charge upon increase of said current signal,

and a voltage follower connected to follow the voltage of said capacitor means and having the output thereof connected to said first input of said comparator.

7. An inverter circuit as set forth in claim 1, wherein said reference voltage is connected to said second input of said comparator,

and the output of said rectifier means is connected to said first input so that the output of said comparator changes logic states upon said rectified signal exceeding said reference voltage.

8. An inverter circuit comprising, in combination, at least first and second semiconductors connected for full wave alternating output on output terminals from DC input terminals,

a signal source having an alternating signal voltage,

first and second drive transformers and first and second capacitors connected to the control electrodes of said first and second semiconductors, respectively,

turn-on means connected to be controlled by said signal voltage and connected through said drive transformers to control the alternate conduction of said first and second semiconductors,

turn-off means including said first and second capacitors,

means associated with said turn-on means and connected to establish a voltage across said first and second capacitors,

and said turn-off means connected to be controlled by said signal voltage and connected to apply the voltage of said capacitors to the control electrode of the respective semiconductor in a direction to supply reverse bias thereto to turn off said respective semiconductor.

9. An inverter circuit as set forth in claim 8, wherein said turn-off means includes gate means connected to said alternating signal voltage.

10. An inverter circuit as set forth in claim 8, wherein said turn-on means includes first and second turn-on transistors connected to supply a voltage in accordance with said signal voltage to a primary of each of said drive transformers.

11. An inverter circuit as set forth in claim 8, wherein said drive transformers are square loop transformers and supply a signal to the respective semiconductors to establish conduction thereof.

12. An inverter circuit as set forth in claim 8, wherein said means associated with said turn-on means includes bias means connected in parallel with said capacitors,

said bias means conducting voltage to the control electrode of the respective semiconductor during turn-on of the semiconductor,

and said bias means establishing the voltage across said capacitors for said turn-off means.

13. An inverter circuit as set forth in claim 12, wherein said bias means includes at least one diode for unidirectional current flow therethrough.

14. An inverter circuit as set forth in claim 8, wherein said means associated with said turn-on means includes unidirectional conducting means to establish current flow to the control electrode of the respective semiconductor for turn-on thereof, said unidirectional conducting means establishing a voltage across said capacitors.

15. An inverter circuit as set forth in claim 8, wherein said signal source includes first and second output conductors,

said alternating signal voltage comprising alternate pulses on said output conductors,

and said turn-off means being connected to said output conductors to be responsive to the incidence of an absence of pulses on both said output conductors.

16. An inverter circuit as set forth in claim 8, wherein said turn-off means includes a turn-off transistor connected to said signal source and connected to be rendered conducting upon the incidence of an absence of signal voltage from said signal source.

17. An inverter circuit as set forth in claim 16, wherein said turn-off transistor is connected to a winding on the respective transformer to effectively short circuit said winding upon conduction of said turn-off transistor.

18. An inverter circuit as set forth in claim 17, wherein the short circuit on a particular transformer winding is reflected to the secondary thereof,

and said secondary being connected to the control electrodes of said first and second semiconductors, respectively, as part of said turn-on means.

19. An inverter circuit as set forth in claim 18, wherein said capacitors are connected in circuit with said secondary to supply a reverse bias to the respective semiconductor upon the effective short circuit on the respective transformer secondary.

20. An inverter circuit comprising, in combination, at least first and second semiconductors connected for full wave alternating output on output terminals from DC input terminals,

a signal source having an alternating signal voltage,

first and second drive transformers connected to the control electrodes of said first and second semiconductors, respectively,

set means connected to be controlled by said signal voltage and connected through said drive transformers to control the alternate conduction of said first and second semiconductors and to drive said drive transformers from a first saturated flux level to a second flux level,

and reset means connected to be controlled by said signal voltage and connected to said transformers to apply a reset current thereto during the non-conduction period of the respective semiconductor to reset the flux of the core of the transformers to said first flux level.

21. An inverter circuit as set forth in claim 20, wherein said reset means includes a reset winding on each of said transformers,

and said reset means supplies current to each of said reset windings to drive the flux in the core of the respective transformer to said first flux level to reset said transformer.

22. An inverter circuit as set forth in claim 20, including a constant current source connected in circuit in said reset means to establish substantially constant reset current to the respective transformer.

23. An inverter circuit as set forth in claim 20, wherein said reset means is connected to establish reset current in the respective transformer during substantially the same time period as the normal conduction period of the opposite semiconductor supplying current to the inverter output terminals.

24. An inverter circuit as set forth in claim 20, wherein said set means drives said drive transformers to said second flux level whose magnitude is less than the magnitude of said first flux level.

25. An inverter circuit having at least a first controllable semiconductor connected to supply an alternating voltage from DC terminals comprising, in combination;

a drive transformer having a secondary,

means including a decoupling unidirectional conducting device connecting said secondary to the control electrode of said first semiconductor,

turn-on means acting on said drive transformer to cause conduction through said decoupling device to said control electrode to turn on said first semiconductor,

turn-off means acting on said drive transformer to effectively terminate the turn-on signal on said secondary to terminate conduction of said first semiconductor,

and said decoupling device having a recovery time to achieve reverse blocking capability longer than that of said first semiconductor to thus prevent reverse bias being applied on the control electrode of said first semiconductor.

26. An inverter circuit as set forth in claim 25, wherein said decoupling device is a diode.

27. An inverter circuit as set forth in claim 25, wherein said decoupling device is a diode poled to conduct toward said control electrode.

28. An inverter circuit as set forth in claim 25, including reset means connected to apply a voltage to reset the flux in the core of said drive transformer to saturation in the opposite direction from turn-on with said decoupling device being thus reverse biased to prevent reverse bias being applied to the control electrode of said first semiconductor.

29. An inverter circuit as set forth in claim 25, wherein said decoupling device is included in a bias circuit connected to said control electrode.

30. An inverter circuit as set forth in claim 29, wherein said bias circuit includes a capacitor,

an impedance connected in parallel with said capacitor,

and said turn-on means acting to pass current through said paralleled capacitor and impedance to charge said capacitor.

31. An inverter circuit as set forth in claim 30, wherein said turn-off means utilizes the stored charge on said capacitor to apply a reverse bias voltage on said control electrode to terminate conduction of said first semiconductor.

32. An inverter circuit as set forth in claim 30, wherein said impedance is bias diode means poled to conduct in the same direction as said decoupling device.

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BACKGROUND OF THE INVENTION

Many inverter circuits have previously been used including those with pulse width modulation to establish variable output power. The prior art has known circuits with feedback from the output of the inverter to control the input drive signal in accordance with some electrical output condition such as voltage or current. The prior art has known a drive transformer used with two alternately conducting semiconductors so that when the drive transformer was energized of one polarity a first semiconductor conducted and when energized in the opposite polarity a second semiconductor in the inverter conducted. The difficulty with such a unit was that the drive transformer had a core material exhibiting a rectangular hysteresis loop and this core material was capable of being saturated. A variable amount of saturation in accordance with variable conduction times, for pulse width modulation, meant that the operation of the circuit was erratic, especially under rapidly changing conditions, being dependent upon the amount of conduction time in the previous cycle because of the variable degree of saturation.

The prior art inverter circuits have also included those of relatively high current carrying capability, but where this has been attempted to be combined with high frequency of operation, the semiconductors used, for example transistors, have been of the type which exhibited a relatively high forward voltage drop yet a relatively low reverse blocking voltage. This has made the operation of such an inverter circuit subject to potential failure if the control voltage on a semiconductor should be too high in the reverse direction. Also such inverter circuits have often been ones wherein it was difficult to achieve a turn-off of a particular semiconductor at the proper time in order to control the width of the pulses in the pulse width modulated inverter. Still further in such prior art inverters, especially during transient conditions, a suddenly applied load might tend to cause the inverter to have a greatly increased output which could overload the current carrying capabilities of the inverter semiconductors.

SUMMARY OF THE INVENTION

The problem to be solved is therefore how to construct a pulse width modulated inverter circuit to overcome these disadvantages of the prior art. This problem is solved by an inverter circuit having controllable semiconductors connected for a controllable output alternating voltage, comprising, in combination; a control circuit connected to control the conduction time of said semiconductors, said control circuit having controllable input means connected to control the conduction time of said semiconductors, means establishing a voltage reference source, means connected to sense the amount of output current from said semiconductors and to develop a current signal, rectifier means having a plurality of outputs connected to rectify a signal proportional to said current signal, a current limit setting potentiometer connected to said reference voltage source to establish a reference voltage, a first current limit means including an operational amplifier connected as a comparator and having first and second inputs; means connecting said comparator first input to one of the outputs of said rectifier means; and means connecting said current limit setting potentiometer reference signal to said second input of said comparator; and means connecting the output of said comparator to said controllable input means of said control circuit whereby the output voltage of said inverter is decreased upon the output of said comparator changing state when said rectified signal exceeds said reference signal.

The invention also contemplates use of an inverter circuit comprising, in combination, at least first and second semiconductors connected for full wave alternating output on output terminals from DC input terminals, a signal source having an alternating signal voltage, first and second drive transformers and first and second capacitors connected to the control electrodes of said first and second semiconductors, respectively, turn-on means connected to be controlled by said signal voltage and connected through said drive transformers to control the alternate conduction of said first and second semiconductors, turn-off means including said first and second capacitors, means associated with said turn-on means and connected to establish a voltage across said first and second capacitors, and said turn-off means connected to be controlled by said signal voltage and connected to apply the voltage of said capacitors to the control electrode of the respective semiconductor in a direction to supply reverse bias thereto to turn off said respective semiconductor.

The invention is also included in an inverter circuit comprising, in combination, at least first and second semiconductors connected for full wave alternating output on output terminals from DC input terminals, a signal source having an alternating signal voltage, first and second drive transformers connected to the control electrodes of said first and second semiconductors, respectively, set means connected to be controlled by said signal voltage and connected through said drive transformers to control the alternate conduction of said first and second semiconductors and to drive said drive transformers to a forward flux level, and reset means connected to be controlled by said signal voltage and connected to said transformers to apply a reset current thereto during the non-conduction period of the respective semiconductor to reset the flux of the core of the transformers to saturation in the opposite direction.

The invention is also contained in an inverter circuit having at least a first controllable semiconductor connected to supply an alternating voltage from DC terminals comprising, in combination; a drive transformer having a secondary, means including a decoupling unidirectional conducting device connecting said secondary to the control electrode of said first semiconductor, turn-on means acting on said drive transformer to cause conduction through said decoupling device to said control electrode to turn on first semiconductor, turn-off means acting on said drive transformer to effectively terminate the turn-on signal on said secondary to terminate conduction of said first semiconductor, and said decoupling device having a recovery time to achieve reverse blocking capability longer than that of said first semiconductor to thus prevent reverse bias being applied on the control electrode of said first semiconductor.

An object of the invention is to provide a pulse width modulated inverter drive circuit which provides for extremely rapid current limit in view of rapidly increasing transient output currents.

Another object of the invention is to provide an inverter circuit with an improved turn-on and turn-off means for controllable semiconductors.

Another object of the invention is to provide an improved inverter circuit with drive transformers controlling the conduction of first and second semiconductors wherein the drive transformers are reset to saturation in the opposite direction between each power pulse of the semiconductors.

Another object of the invention is to provide an improved inverter circuit with a decoupling diode so as to prevent a too high reverse bias being applied on the control electrode of the semiconductor.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a part of a drive circuit for an inverter,

FIG. 2 is a schematic diagram of the remainder of the drive circuit plus the inverter power circuit,

FIG. 3 is a schematic diagram of a modified bias circuit for the semiconductors,

FIG. 4 is a schematic diagram of a circuit usable in the control portion of the drive circuit of FIG. 1; and

FIGS. 5 and 6 are graphs of voltage and current pulses in the circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2, side by side, illustrate an inverter circuit 11 which includes in general an inverter power circuit 12 in FIG. 2 and a drive circuit 13 which generally is the remainder of FIGS. 1 and 2.

The inverter power circuit may include a center tapped transformer or separate decoupled transformers plus first and second controllable semiconductors 14 and 15, or a half bridge with two semiconductors, or it may be a three phase system with at least three controllable semiconductors. However, as shown, the power circuit 12 includes an inverter bridge circuit 18 which includes the first and second controllable semiconductors 14 and 15 as well as third and fourth controllable semiconductors 16 and 17, respectively. In this preferred embodiment of FIG. 1 these semiconductors 14-17 are transistors. The inverter bridge 18 operates from positive and negative DC input terminals 19 and 20, respectively, and has an alternating current output on terminals 21 and 22. The drive circuit 13 controls the inverter bridge 18 such that the first and second transistors 14 and 15 conduct alternately, and since this is a bridge circuit, transistors 14 and 17 conduct simultaneously and transistors 15 and 16 conduct simultaneously. A high frequency switching transistors satisfactory for use in this high voltage, e.g. 300 volts, circuit is a type TIP 563.

A control circuit 26 is connected to regulate the conduction time of the semiconductors 14-17. This control circuit is one which controls the width of the output pulses of this inverter bridge 18.

The control circuit 26 has output lines 27 and 28, and these may be considered a signal source to control the conduction times of the first through fourth semiconductors 14-17. The control circuit 26 may be a commercially available controller such as Motorola part number MC 3420, and has alternate output pulses on the output lines 27 and 28, as shown by the graphs of voltages 27A and 28A on FIGS. 5B and 5C, respectively. These pulses are a logic zero or low condition from a normally high output. The control circuit 26 has a variable width of these pulses, in order that the drive circuit 13 will provide a variable conduction time of the semiconductors 14-17. The width of these pulses as shown in the graph 27A and 28A cannot be made so wide that the pulses intersect or overlap, instead there is a minimum deadtime 32 as shown in FIG. 5A. This is established by a deadtime voltage 29 which intersects the triangular wave form 36 shown in this FIG. 5A. The triangular wave form is caused by an internal oscillator which, in this embodiment, causes a ramp up voltage from 2.0 volts and then a ramp down voltage from 6.0 volts. Resistors 30 and 31 are connected between a voltage reference terminal VR and ground, and the junction of these resistors is connected to a deadtime adjust terminal DT in order to set the length of this minimum deadtime 32.

The frequency of oscillation is set by a resistor 33 connected between a terminal RE and ground and also set by a capacitor 34 connected between a terminal CE and ground. The control circuit has controllable input means on conductors 37 and 38. Conductor 37 is connected to input terminal CV and conductor 38 is connected to inhibit terminal INH.

The control circuit 26 has the ability to control the width of the pulses on the output terminals 27 and 28. From a review of FIG. 5A, one will note that if the value of the control voltage on the conductor 37 is decreased, then the control voltage 37A intersects more of the tip of the triangular wave 36 and hence this would increase the width of the output pulses. The control voltage 37A is shown as making a sharp decrease at the location 37B to a low level below the triangular wave form 36 in order to illustrate a maximum output condition of the inverter drive circuit 13. If the normally high voltage on the inhibit terminal to which conductor 38 is connected goes low, then this forces the output on the output lines 27 and 28 to go high.

The control circuit terminal VR is a constant voltage reference from the internally generated voltage reference signal and this is applied through an optional voltage follower circuit 42 in order to increase the power capabilities of this constant voltage reference. The voltage follower circuit 42 has an output terminal 43 with a low impedance and this is supplied through a potentiometer 44 to ground. The potentiometer 44 supplies a controllable reference voltage to an output voltage regulator circuit 45. Terminals 46 and 47 are connected to all or preferably a portion of the output of the inverter at the AC output terminals 21 and 22. This voltage is rectified by rectifier 48 and filtered by an inductor 56 and by a capacitor 49 in parallel with resistors 50 and 51. The inductor input filter senses the average voltage rather than peak voltage across the output terminals 21 and 22. The wiper of the potentiometer 44 is connected to ground through a capacitor 52 and is also connected through a resistor 53 to the inverting terminal of a comparator 54. The non-inverting terminal of the comparator is connected to the junction of resistors 50 and 51. The voltage at the junction of resistors 50 and 51 is a positive voltage which normally slightly exceeds the voltage from the wiper of potentiometer 44. As a result the comparator 54 normally has a small positive voltage output. This might be about +5 volts, for example, as shown in the control voltage 37A of FIG. 5A. If the output voltage increases somewhat, for example, as caused by a decreased load current, this will increase the error signal which is the output of comparator 54 and this is passed by a diode 55 onto conductor 37 and the control voltage terminals CV. As shown in FIG. 5A an increase in the voltage 37A will narrow the width of the pulse output on the conductors 27 and 28, and as shown on the curves 27A and 28A of FIGS. 5B and 5C. This reduces the output voltage of the inverter power circuit 12 to return the inverter output to a stable condition.

The inverter drive circuit 13 also includes first and second current limit circuit 61 and 62. The first current limit circuit 61 has an input on terminals 63 and 64. These terminals are shown in FIG. 2 as being the output from a current transformer 65 connected in the output leads of the inverter bridge circuit 18. This current signal on the input terminals 63 and 64 in FIG. 1 is supplied to a rectifier 66, and the rectified signal is supplied to a pair of voltage divider resistors 67, 68 and then through a diode 69 to the non-inverting input of an amplifier connected as a voltage follower 70. On the non-inverting input of voltage follower 70 a peak charging circuit is provided which includes a small capacitor 71 in parallel with a resistor 72 to ground. In one practical circuit constructed in accordance with the invention, the capacitor 71 was 0.1 microfarads and the resistor 72 was 220,000 ohms. Such a combination means that the small capacitor 71 is quick to charge with a sudden increase in load current and the relatively high resistance of resistor 72 means that the capacitor 71 is relatively slow to discharge. Under these conditions, the peak charging circuit is stable and does not readily pass any AC signal therethrough, but instead follows the peak of the incoming current limit signal.

The output of the voltage follower 70 is supplied to resistors 73 and 74 and the junction thereof supplies an output to the non-inverting input of an op-amp 75 connected as a comparator. This op-amp has the inverting input connected through a resistor 76 to the wiper of a potentiometer 77 which is connected to the voltage reference source at terminal 43. The setting of the wiper on the potentiometer 77 sets the value of the current limit and this is normally set at a higher magnitude of voltage on the inverting input of op-amp 75 than the output on the non-inverting input thereof from the voltage follower 70. Accordingly, upon an increase in current output of the inverter bridge circuit 18 such that the signal on the non-inverting input of op-amp 75 exceeds that reference current limit point on the inverting input, the normally negative output of op-amp 75 changes to a positive output and this is passed through a diode 78 to the conductor 37. This increasing positive signal acts, as will be seen in FIG. 5A, to decrease the width of the output pulse and hence decrease the output of the inverter bridge circuit 18. Accordingly, current limiting is effected.

The second current limit circuit 62 includes a comparator 81 which has the non-inverting input thereof connected to the same wiper of potentiometer 77 so that it is controlled by the same current limit point as the first current limit circuit 61. The inverting input of the comparator 81 is connected to the junction of resistors 67 and 68 to receive the current limit signal from rectifier 66. A capacitor 82 is connected to ground from this inverting input. Normally the voltage at the non-inverting input exceeds the magnitude of the voltage at the inverting input of comparator 81 and both are positive voltages so that the output of comparator 81, connected to conductor 38, is normally positive. The capacitor 82 is a very small value capacitor, for example, in one practical circuit embodying the invention this was 100 picofarads. As a result this second current limit circuit 62 is an extremely rapidly operating circuit. Should the current output of the inverter bridge circuit 18 suddenly increase, for example, as in a short circuit, then this second current limit circuit 62 acts much more rapidly than the first current limit circuit 61.

In the first circuit 61 the capacitor 71 must charge before that first circuit 61 can act and a typical response time might be 50 microseconds. In the second current limit circuit 62, since the capacitor 82 is a very small capacitor, the response time might typically be two to five microseconds. This would be less than one cycle of operation at the frequency of the regulator circuit which might be anywhere from 2 KHZ to 100 KHZ. Upon this rapidly increased current output from the inverter, e.g. from a short circuit, the inverting input on comparator 81 would exceed that of the non-inverting input to reverse the output thereof causing it to go from a positive or logic one condition to a low or logic zero condition. This low on the inhibit terminal INH of the control circuit 26 immediately forces the output on both lines 27 and 28 to go high to cause cessation of all output of the inverter bridge circuit 18. Thus the extremely rapid operation of the second current limit circuit 62 is used to protect the entire inverter circuit 11 from damage and especially to protect the semiconductors 14-17 from burning out. It also permits the various components to be of a smaller power capability rating than would otherwise be the case. For example, not only the transistors 14-17 may be smaller in current rating, but other components carrying power thereafter may be smaller. For example, the output terminals 21 and 22 may supply some form of a rectifier. The components therein also may be made smaller in current carrying capacity because of the protection afforded by the very rapid action of the second current limit circuit 62.

In the first current limit circuit 61 the voltage follower 70 has the advantage of having a satisfactory voltage output to drive the resistors 73 and 74, yet it has a high input impedance so that it does not load the resistor 72 and capacitor 71. Accordingly, the time constant of resistor 72 and capacitor 71 will not be affected.

The error signal from comparator 54 is shown above to be that which controls the width of the pulses on the output lines 27 and 28 of the control circuit 26 and hence the width of the pulses which form the output of the inverter bridge circuit 18. The drive circuit 13, see FIG. 2, does control the conduction time periods of the transistors 14-17 and the drive circuit 13 includes a turn-on circuit 90 and a turn-off circuit 91.

The turn-on circuit 90 acts through first through fourth drive transformers 94-97 each connected in association with the first through fourth controllable semiconductors 14-17, respectively. The turn-on circuit 90 controls the set and reset of these transformers 94-97. These transformers may be ones with a substantially rectangular hysteresis loop so that the magnetizing current of the transformers is extremely small and therefore in effect the transformers are essentially transparent, merely giving a transformation of voltage and current in accordance with the turns ratio. A feature of the present invention is that a separate drive transformer is used for each of the controllable semiconductors 14-17, rather than the common prior art practice of one transformer for a pair of alternately conducting semiconductors. The drive transformers are substantially identical and in general only the transformers 94 and its associated circuitry with the first semiconductor 14 will be described in detail. The transformer 94 has a primary winding 99, a secondary winding 100, a reset winding 101 and a regenerative winding 102.

The turn-on circuit 90 acts through a drive transformer to the respective semiconductor. In the case of the first transformer 94 and first semiconductor 14, it acts through the secondary 100 and what may be termed a bias circuit to a control electrode of this semiconductor 14. The semiconductor is shown as an NPN transistor having base drive. The secondary 100 has terminals 104 and 105 with terminal 104 connected through a capacitor 106 to the base of the transistor 14. This capacitor is a part of the turn-off means. Bias means is connected across the capacitor 106 and in the preferred embodiment this bias means is an impedance which limits the voltage across the capacitor 106. This impedance in the preferred embodiment is a plurality of diodes 107 and 108 which are poled to conduct current from terminal 104 to the base of transistor 14. A resistor 109 is connected across the secondary 100. The collector of transistor 14 is connected to the positive DC terminal 19 and the emitter of transistor 14 is connected through the regenerative winding 102 to the collector of the next transistor in the bridge circuit 18, transistor 16, and is also connected to the AC output terminal 21.

The turn-on circuit 90 includes first and second turn-on transistors 111 and 112 shown in this preferred embodiment as being PNP type with the emitters connected together and through diodes 113 to a positive supply voltage at terminal 114. This positive supply voltage terminal 114 is connected through resistors 115 and 116 to the output lines 27 and 28, respectively, from the control circuit 26. This connection makes these lines normally high, until driven low by the pulse output on the respective line from the control circuit 26. Resistors 117 and 118 are connected in these lines 27 and 28, respectively, to convey the signal on lines 27 and 28 and to provide a proper current to the bases of the transistors 111 and 112. The collector of transistor 112 is connected to the anodes of a group of diodes 121-124 and the collector of transistor 111 is connected to the anodes of a group of diodes 125-128. Current limiting resistors 129 are individually connected to the cathodes of the diodes 123-126, respectively. Conductors 131-138 are connected to the cathodes of the diodes 121-128, respectively, with the resistors 129 interposed in such connection with respect to the diodes 123-126. These conductors 131-138 are connected to the various terminals on the primary windings and reset windings of the pulse transformers 94-97.

The turn-off circuit 91 includes first, second and third transistors 141, 142 and 143, respectively. The collector of NPN transistor 141 is connected through a resistor 144 to the positive supply terminal 114. The emitter of this transistor is connected to the emitter of PNP transistor 142, the collector of which is grounded. The base of the transistor 141 is connected through a diode OR circuit or gate formed by diodes 145 and 146 with the anodes connected to the base of transistor 141 and the cathodes connected to the output lines 27 and 28, respectively. The base of the transistor 142 is connected through another diode OR circuit consisting of diodes 147 and 148 to the output lines 27 and 28, respectively. A resistor 149 connects the positive supply voltage terminal to the base of transistor 141 and a resistor 150 connects the emitter of transistor 142 to the base thereof. This emitter is also connected to the base of the NPN transistor 143 with the emitter of this transistor connected through a plurality of biasing diodes 151 to ground. The collector of transistor 143 is connected through a diode OR circuit consisting of a group of diodes 153-156. The anodes of the diodes 153-156 are connected to the conductors 133-136, respectively. The emitter of transistor 143, in addition to being connected through the biasing diodes 151 to ground, is also connected to a clamping terminal 157 which in turn is connected by clamping conductors 158 to the lower terminal of each of the primaries on the transformers 94-97, such as primary winding 99.

A constant current source 162 is used in the preferred embodiment as a current source for the reset windings such as winding 101 on transformer 94. This constant current source is connected by conductors 163 to the upper end of the reset windings on each of the transformers 94-97 and this constant current source is also connected to ground. Protective diodes 164 are connected across the series combination of each of the transistors 14-17 and its regenerative winding, such as winding 102, in order to limit the reverse voltage applied to the respective transistors. A transient supressing capacitor 165 and resistor 166 are connected in series across the AC output terminals 21 and 22.

Operation

The output lines 27 and 28 from the control circuit 26 may be considered a signal source having an alternating signal voltage. As shown in FIGS. 5B and 5C, this alternating signal voltage is actually alternating pulses and the pulses are of variable width in order to control the conduction times of the semiconductors 14-17. FIG. 5 is a diagram of graphs of various portions of the circuit with FIG. 5D showing a graph 111A of the current conducted by transistor 111, and with FIG. 5E showing a graph 112A of the current conducted by the transistor 112. Transistor 143 may be considered a clamping transistor and FIG. 5F shows a graph 143A of the current conducted by this clamping transistor 143. FIG. 5G shows a graph 14A of the base current conducted by the transistor 14 and FIG. 5H shows a graph 15A of the base current conducted by transistor 15. FIG. 5I is a graph 21A of the output voltage appearing at the output terminals 21, 22. This is for the particular case of a resistive load, or the case of a load consisting of a rectifier and an inductive input filter. Referring to this graph 21A there is a sequence of three different operations for each transistor in one alternating current cycle. The first step is shown by the rising wave front 170 which is caused by the set of the transformer 94 and practically simultaneous turn-on of the respective transistor. The second step in the sequence is the turn-off of the respective transistor shown by the falling wave front 171. This is accomplished by the clamp established on the transformer 94 and this occurs during the horizontal portion 172 of the output voltage curve between positive and negative pulses. The third step in the sequence is the reset of the respective transformer 94 which occurs during the conduction period of the opposite transistor 15.

In more detail, the set of the particular transformer will be described with respect to the first transformer 94. The signal source on the control circuit lines 27 and 28 controls the turn-on of the transistors 14-17. At the time that the output line 27 goes low, as shown at portion 175 of graph 27A, this turns on transistor 111 because the base thereof goes low. Conduction of transistor 111 goes through the diode 125 and resistor 129 to conductor 135. This makes current flow in what will be termed the forward direction in the primary winding 99. This sets the transformer core. The transformer is not driven to forward saturation, instead, the flux level is partially changed from the flux level at reverse saturation condition, enough so that the secondary winding 100 emits a pulse out of the terminal 104 which is passed by the diodes 107 and 108 to the base of the transistor 14. This turns on this transistor almost simultaneously with the beginning of the set of the core of the first transformer 94.

FIGS. 5B through 5I show that at time t.sub.1 the control circuit output line 27 has a low output pulse, the transistor 111 turns on, the transistor 14 turns on, and the output of the entire inverter circuit 11 has a positive going output pulse, due to conduction of both transistors 14 and 17.

The pulse from transistor 111 into the primary winding 99 continues, but it is not necessary in this particular circuit because of the regenerative winding 102. As soon as the transistor 14 begins to conduct, then current flows through the regenerative winding 102 to supply current from the secondary winding 100 to keep transistor 14 turned on. This current flows through the bias means which is the diodes 107 and 108 and this will charge the capacitor 106 so that it is positive