Current fed inverter bridge with lossless snubbers

An inverter adapted to be operated from a source of direct voltage, comprising:

• a source of direct current adapted to be coupled to first and second terminals of a source of direct voltage for being energized thereby, for producing a substantially constant current between first and second terminals of said source of direct current;
• a first series combination of first and second controllable switches including a first intermediate terminal therebetween, said first series combination being coupled across said first and second terminals of said source of direct current;
• a second series combination of third and fourth controllable switches including a second intermediate terminal therebetween, said second series combination being coupled in parallel with said first series combination;
• a load coupled to said first and second intermediate terminals for receiving alternating current from said first, second, third and fourth switches in response to operation of said switches;
• control means coupled to said first, second, third and fourth switches for, in recurrent sequence of four sequential intervals, controlling said first, second, third and fourth switches for (a) maintaining said first and fourth switches conductive and said second and third switches nonconductive during first intervals of said recurrent sequence, (b) maintaining said first, second, third and fourth switches conductive during second intervals of said recurrent sequence, (c) maintaining said first and fourth switches nonconductive and said second and third switches conductive during third intervals of said recurrent sequence, and (d) maintaining said first, second, third and fourth switches conductive during fourth intervals of said recurrent sequence;
• first, second, third and fourth snubbers coupled to said first, second, third and fourth switches respectively, each of said first, second, third and fourth snubbers including the serial combination of a diode and capacitance means, said capacitance means being coupled for being charged in a first polarity by current bypassing the associated switch during its turn-off, whereby the snubber capacitor becomes charged in said first polarity and must be discharged in preparation for receiving current during the next turn-off cycle;
• first, second, third and fourth energy recovery means coupled to said capacitance means of said first, second, third and fourth snubbers, respectively;
• said first, second, third and fourth energy recovery means including first, second, third and fourth transformer means, each of said transformer means including a primary and a secondary winding, said secondary windings of each of said transformer means being coupled in series with rectifying means across said first and second terminals of said source of direct voltage, said primary windings of said first, second, third and fourth transformer means each being coupled in series with further rectifying means to form first, second, third and fourth primary circuits, one end of each of said first, second, third and fourth primary circuits being coupled to the junction of said diode and said capacitance means of said first, second, third and fourth snubbers, respectively, the other end of said first and third primary circuits being coupled to said first terminal of said source of direct voltage, the other end of said second and fourth primary circuits being coupled to said first terminal of said source of direct current, for transforming current pulses from said capacitance means of said snubbers to said source of direct voltage for recovering energy while discharging said capacitance means of said snubbers in preparation for receiving current during the next turn-off cycle;
• clamp capacitance means including first and second electrodes, said second electrode being coupled to said second terminal of said source of direct voltage;
• first, second, third and fourth unidirectional current conducting means, each including an anode terminal and a cathode terminal, one of said anode terminals and said cathode terminals of said first, second, third and fourth unidirectional current conducting means being coupled to said first electrode of said clamp capacitance means, and other one of said anode terminals and said cathode terminals of said first and third unidirectional current conducting means being coupled to said first terminal of said source of direct current, the other one of said anodes and said cathodes of said second and fourth unidirectional current conducting means being coupled to said first and second intermediate terminals, respectively;
• and
• voltage control means coupled to said first terminal of said source of direct voltage and to said first electrode of said clamp capacitance means for establishing a clamp voltage on said clamp capacitance means whereby voltage spikes across any of said first, second, third and fourth switches which exceed said clamp voltage cause the corresponding one of said first, second, third and fourth unidirectional current conducting means to conduct and thereby limit the magnitude of the voltage spike.

A direct-voltage to alternating-current inverter, comprising:

• a source of direct current adapted to be coupled to a source of direct voltage for being energized thereby, for producing a substantially constant current between first and second terminals;
• a first series combination of first and second controllable switch means including a first intermediate terminal therebetween, said first series combination of first and second controllable switch means being coupled to said first and second terminals of said source of direct current for, when closed, conducting at least portion of said substantially constant direct current;
• a second series combination of third and fourth controllable switch means including a second intermediate terminal therebetween, said second series combination of third and fourth controllable switch means being coupled in parallel with said first series combination of first and second controllable switch means for, when closed, conducting at least a portion of said substantially constant direct current;
• a load coupled to said first and second intermediate terminals for having applied thereacross voltages which depend upon the combination of said first, second, third and fourth controllable switch means which are conducting;
• capacitance means coupled across said load to form a parallel capacitance-load circuit;
• inductance means serially coupled with said capacitance-load circuit;
• switch control means coupled to said first, second, third and fourth controllable switch means for, in recurrent sequence of four intervals, (a) maintaining said first and fourth controllable switch means conductive and said second and third controllable switch means nonconductive during a first interval of said recurrent sequence, for directing said substantially constant direct current through said inductance means and said load in a first polarity, and for charging said capacitance means in a first polarity, (b) for rendering said second and third controllable switch means conductive during a second interval of said recurrent sequence, whereby said first, second, third and fourth controllable switch means are conductive, thereby reducing the voltage between said first and second intermediate terminals, and between said first and second terminals of said source of constant current to zero, and creating paths including said first, second, third and fourth controllable switch means by which said inductance means can ring with said capacitance means to form a first ringing current such that the current flow in said first and fourth controllable switch means is decreased by said ringing current, (c) for rendering said first and fourth controllable switch means nonconductive at a time when the current therethrough is less than one-half said substantially constant direct current to initiate a third interval of said recurrent sequence, whereby only said second and third controllable switch means are conductive during said third interval of said recurrent recurring sequence, whereby a voltage appears between said first and second terminals of said source of direct current and a voltage appears across said first and second intermediate terminals which is poled to cause said substantially constant direct current to flow through said inductance means and said load in a second polarity, and for charging said capacitance means in a second polarity, (d) for rending said first and fourth controllable switch means conductive during a fourth interval of said recurrent sequence, whereby said first, second, third and fourth controllable switch means are conductive, thereby reducing said voltage between said first and second intermediate terminals, and between said first and second terminals of said source of direct current to zero, and creating paths including said first, second, third and fourth switch means by which said inductance means can ring with said capacitance means to form a second ringing current in such a manner that the current flow in said second and third switch means is decreased by said ringing current, and (e) for rendering said second and third controllable switch means nonconductive at a time when the current therethrough is less than one-half said substantially constant direct current, whereby only said first and fourth controllable switch means are conductive to begin another said first interval of said recurrent sequence, whereby the rendering nonconductive of said controllable switch means when a nonzero current flows therethrough may cause switch stress and unwanted power dissipation;
• first, second, third and fourth snubbing means coupled across said first, second, third and fourth controllable switch means, respectively, each of said snubbing means comprising the series combination of a diode and a capacitor, for diverting said nonzero current flowing in said first, second, third and fourth controllable switch means, respectively, at the moment said first, second, third and fourth controllable switch means, respectively, are rendered nonconductive, whereby energy is transferred into said capacitors of said second and third snubbing means at the beginning of said first interval of said recurrent sequence, and energy is transferred into said capacitors of said first and fourth snubbing means at the beginning of said third interval of said recurrent sequence;
• first energy recovery means coupled to said first snubbing means, said first energy recovery means comprising the series combination of the primary winding of a first transformer and a first rectifier coupled between a terminal of said source of direct voltage and the junction of said diode and said capacitor of said first snubbing means, and also comprises the series combination of a second rectifier and a secondary winding of said first transformer connected across said source of direct voltage for transforming a current pulse from said capacitor of said first snubbing means to said source of direct voltage at the beginning of said first interval of said recurrent sequence for recovering energy from said first snubbing means;
• second energy recovery means coupled to said second snubbing means, said second energy recovery means comprising the series combination of the primary winding of a second transformer and a third rectifier coupled between said first terminal of said source of direct current and the junction of said diode and said capacitor of said second snubbing means, and also comprises the series combination of a fourth rectifier and a secondary winding of said second transformer connected across said source of direct voltage for transforming a current pulse from said capacitor of said second snubbing means to said source of direct voltage at the beginning of said second interval of said recurrent sequence for recovering energy from said second snubbing means;
• third energy recovery means coupled to said third snubbing means, said third energy recovery means comprising the series combination of the primary winding of a third transformer and a fifth rectifier coupled between a terminal of said source of direct voltage and the junction of said diode and said capacitor of said third snubbing means, and also comprises the series combination of a sixth rectifier and a secondary winding of said third transformer connected across said source of direct voltage for transforming a current pulse from said capacitor of said third snubbing means to said source of direct voltage at the beginning of said third interval of said recurrent sequence for recovering energy from said third snubbing means;
• and
• fourth energy recovery means coupled to said fourth snubbing means, said fourth energy recovery means comprising the series combination of the primary winding of a fourth transformer and a seventh rectifier coupled between said first terminal of said source of direct current and the junction of said diode and said capacitor of said fourth snubbing means, and also comprises the series combination of an eighth rectifier and a secondary winding of said fourth transformer connected across said source of direct voltage from transforming a current pulse from said capacitor of said fourth snubbing means to said source of direct voltage at the beginning of said fourth interval of said recurrent sequence for recovering energy from said fourth snubbing means.

A method for operating a bridge inverter driven from a source of substantially constant direct current which in turn is energized from a source of direct voltage to apply alternating current to a load, comprising, in the following order, the steps of:

• maintaining closed first and fourth switches of said bridge and maintaining open second and third switches of said bridge during a first interval of a recurrent sequence of four intervals for coupling said load for receiving said substantially constant direct current in a first polarity;
• closing all switches of said bridge a first time, thereby short-circuiting said load and said source of substantially constant direct current to initiate a second interval of said recurrent sequence of four intervals;
• simultaneously with said closing of all switches of said bridge a first time, discharging a second snubber capacitor and transferring the energy thereof to said source of direct voltage;
• opening said first and fourth switches of said bridge to initiate a third interval of said recurrent sequence of four intervals for coupling said load for receiving said substantially constant direct current in a second polarity;
• simultaneously with said opening of said first and fourth switches, charging first and fourth snubber capacitors respectively with a current diverted from said first and fourth switches, respectively, due to the opening thereof;
• simultaneously with said opening of said first and fourth switches, discharging a third snubber capacitor and transferring the energy thereof to said source of direct voltage;
• closing all switches of said bridge a second time, thereby short-circuiting said load and said source of substantially constant direct current to initiate a fourth interval of said recurrent sequence of four intervals;
• simultaneously with said closing of all switches of said bridge a second time, discharging said fourth snubber capacitor and transferring the energy thereof to said source of direct voltage;
• opening said second and third switches to initiate said first interval of said recurrent sequence of four intervals, thereby coupling said load for receiving said substantially constant direct current in said first polarity;
• simultaneously with said opening of said second and third switches, charging said second and third snubber capacitors, respectively, with current diverted from said second and third switches, respectively, due to the opening thereof;
• and
• simultaneously with said opening of said second and third switches, discharging said first snubber capacitor and transferring the energy thereof to said source of direct voltage.