Single-ended self-oscillating dc-dc converter for intermittently energized load having VBE responsive current limit circuit

Claim 2

Apparatus for delivering current to an intermittently energized load comprising: a. a coupled inductor including a primary winding a secondary winding and a feedback winding; b. a switching transistor having base, collector and emitter terminals and a base-emitter junction and being coupled in series with said primary winding for switching between conductive and non-conductive states to control the flow of current through said primary winding; c. a positive drive supply coupled between said feedback winding and the base of said switching transistor and energized by said feedback winding for providing a positive bias voltage to said switching transistor to switch into and maintain said switching transistor in the conductive state to transfer energy into said inductor; d. a current limiting circuit for removing the positive bias voltage from said switching transistor when the primary winding current reaches a predetermined value to thereby switch said switching transistor out of the conductive state and into the non-conductive state, said current limiting circuit including i. a semiconductor switch having a control lead with a threshold voltage; ii. a voltage divider coupled across the base-emitter junction of said switching transistor for applying a scaled output voltage to the control lead of said semiconductor switch to cause said semiconductor switch to conduct when the base-emitter voltage of said switching transistor reaches a predetermined current limit voltage representative of a preselected switching transistor collector current limit; iii. said semiconductor switch shunting the positive bias voltage away from the base of said switching transistor when the voltage on the control lead rises above the threshold voltage to switch said transistor out of the conductive state and into the non-conductive state; and e. a load coupled to said secondary winding for receiving energy stored in said inductor after said switching transistor is switched into the non-conductive state by said current limiting circuit.

Claim 3

The apparatus of claim 1 wherein said semiconductor switch comprises a first transistor switch having base, collector and emitter terminals and wherein the base terminal of said first transistor switch functions as the control lead of said semiconductor switch.

Claim 4

The apparatus of claim 1 further including positive and negative voltage input terminals and wherein the emitter terminal of said switching transistor is coupled directly to the negative voltage input terminal.

Claim 5

The apparatus of claim 1 wherein said positive drive supply further includes means for isolating said positive drive supply from the base of said switching transistor while said transistor is maintained in the non-conductive state.

Claim 6

The apparatus of claim 4 wherein said isolating means includes a diode.

Claim 7

The apparatus of claim 1 wherein said load includes a strobe flash lamp.

Claim 8

The apparatus of claim 6 further including a sensor for sensing the output voltage on said secondary winding and for activating said current limiting circuit when the output voltage reaches a predetermined level.

Claim 9

The apparatus of claim 6 further including a trigger pulse generator and a timing generator coupled to said trigger pulse generator for periodically energizing said strobe flash lamp.

Claim 10

The apparatus of claim 1 further including a negative drive supply coupled between said feedback winding and the base of said switching transistor for supplying a constant negative bias voltage to the base of said switching transistor independent of variations in the voltage across said feedback winding.

Claim 11

The apparatus of claim 9 wherein the constant negative bias voltage from said negative drive supply is set at a level less than the base-emitter avalanche voltage of said switching transistor.

Claim 12

The apparatus of claim 10 wherein said negative drive supply includes a voltage regulator.

Claim 13

The apparatus of claim 11 wherein the turns ratio of said secondary winding with respect to said feedback winding yields a feedback winding voltage which is always greater than said negative drive supply.

Claim 14

The apparatus of claim 9 wherein said positive drive supply further includes means for isolating said positive drive supply from the base of said switching transistor while said negative drive supply provides the negative bias voltage to said switching transistor.

Claim 15

The apparatus of claim 13 wherein said isolating means includes a diode.

Claim 16

Apparatus for delivering current to an intermittently energized load comprising: a. a coupled inductor including a primary winding a secondary winding and a feedback winding; b. a switching transistor having base, collector and emitter terminals and a base-emitter junction and being coupled in series with said primary winding for switching between conductive and non-conductive states to control the flow of current through said primary winding; c. a starting voltage source coupled to the base of said switching transistor for continuously applying a starting bias voltage to said switching transistor to periodically switch said switching transistor into the conductive state; d. a positive drive supply coupled between said feedback winding and the base of said switching transistor and energized by said feedback winding for providing a positive bias voltage to said switching transistor to maintain said switching transistor in the conductive state to transfer energy into said inductor, the magnitude of the positive bias voltage exceeding the magnitude of the starting bias voltage; e. a current limiting circuit for removing the positive bias voltage from said switching transistor when the primary winding current reaches a predetermined value to thereby switch said switching transistor out of the conductive state and into the non-conductive state, said current limiting circuit including i. a semiconductor switch having a control lead with a threshold voltage; ii. a voltage divider coupled across the base-emitter junction of said switching transistor for applying a scaled output voltage to the control lead of said semiconductor switch to cause said semiconductor switch to conduct when the base-emitter voltage of said switching transistor reaches a predetermined current limit voltage representative of a preselected switching transistor collector current limit; iii. said semiconductor switch shunting both the starting bias voltage and the positive bias voltage away from the base of said switching transistor when the voltage on the control lead rises above the threshold voltage to switch said transistor out of the conductive state and into the non-conductive state; and f. a load coupled to said secondary winding for receiving energy stored in said inductor after said switching transistor is switched into the non-conductive state by said current limiting circuit.

Claim 17

Apparatus for delivering current to an intermittently energized load comprising: a. a coupled inductor including a primary winding a secondary winding and a feedback winding; b. a switching transistor having base, collector and emitter terminals and a base-emitter junction and being coupled in series with said primary winding for switching between conductive and non-conductive states to control the flow of current through said primary winding; c. a starting voltage source coupled to the base of said switching transistor for continuously applying a starting bias voltage to said switching transistor to periodically switch said switching transistor into the conductive state; d. a positive drive supply coupled between said feedback winding and the base of said switching transistor and energized by said feedback winding for providing a positive bias voltage to said switching transistor to maintain said switching transistor in the conductive state to transfer energy into said inductor, the magnitude of the positive bias voltage exceeding the magnitude of the starting bias voltage; e. a current limiting circuit for removing the positive bias voltage from said switching transistor when the primary winding current reaches a predetermined value to thereby switch said switching transistor out of the conductive state and into the non-conductive state, said current limiting circuit including i. a semiconductor switch having a control lead with a threshold voltage; ii. a voltage divider coupled across the base-emitter junction of said switching transistor for applying a scaled output voltage to the control lead of said semiconductor switch to cause said semiconductor switch to conduct when the base-emitter voltage of said switching transistor reaches a predetermined current limit voltage representative of a preselected switching transistor collector current limit; iii. said semiconductor switch shunting both the starting bias voltage and the positive bias voltage away from the base of said switching transistor when the voltage on the control lead rises above the threshold voltage to switch said transistor out of the conductive state and into the non-conductive state; and f. a load coupled to said secondary winding for receiving energy stored in said inductor after said switching transistor is switched into the non-conductive state by said current limiting circuit.

Claim 18

The apparatus of claim 15 wherein said apparatus includes a voltage input lead and wherein said starting voltage source includes a power input terminal coupled to said voltage input lead.

Claim 19

The apparatus of claim 15 further including a negative drive supply coupled between said feedback winding and the base of said switching transistor for supplying a negative bias voltage to the base of said transistor as energy is transferred from said inductor into said load.

Claim 20

The apparatus of claim 17 wherein said negative drive supply supplies a constant negative bias voltage to the base of said switching transistor as energy is transferred from said inductor into said load, wherein the negative bias voltage remains constant as the voltage across said feedback winding varies.

Claim 21

The apparatus of claim 18 wherein said semiconductor switch comprises a first transistor switch having base, collector and emitter terminals and wherein the base terminal of said first transistor switch functions as the control lead of said semiconductor switch.

Claim 22

The apparatus of claim 19 wherein said load includes a strobe flash lamp.