Blocking oscillator type power supply with power foldback short circuit protection

Claim 2

A blocking oscillator type power supply with power foldback short circuit protection, comprising: a transistor having first and second main electrodes and a control electrode; a transformer having a primary winding, at least one output winding, and a feedback winding; means for coupling said primary winding in series with said main electrodes and a source of potential difference; a frequency determining capacitor; means for coupling said feedback winding in series with said capacitor and in circuit with the control electrode of said transistor to provide positive feedback from said first main electrode to said control electrode by inductive coupling between said primary and feedback windings; biasing means coupled between said source and said control electrode; threshold means coupled to said control electrode for limiting the potential applied to said control electrode to a predetermined value and for providing a current flow path for said capacitor through said feedback winding; means for connecting said output winding to a load; and means for (i) causing said capacitor to be repetitively charged through said threshold means to enable said oscillator to operate at a given frequency when said load has a given value, and for (ii) causing said capacitor to be repetitively charged through said biasing means to cause said oscillator to operate at a substantially lower frequency when said output winding is short-circuited by said load, thereby substantially decreasing the power delivered to said load and limiting the power dissipated in said oscillator to a safe operating value while said output winding is short-circuited.

Claim 3

The power supply according to claim 1, wherein said output winding comprises a secondary winding of said transformer distinct from said primary and feedback windings.

Claim 4

The power supply according to claim 2, further comprising rectifying means coupled to said secondary winding.

Claim 5

The power supply according to claim 3, wherein said rectifying means comprises a half wave rectifier polarized to draw current from said secondary winding only when said transistor is non-conductive.

Claim 6

The power supply according to claim 4, wherein said transistor is bipolar, and said first main electrode, second main electrode and control electrode are the collector, emitter and base electrodes of said transistor respectively.

Claim 7

The power supply according to claim 5, wherein said threshold means comprises a Zener diode.

Claim 8

The power supply according to claim 4, further comprising shunt regulating means coupled to said rectifying means for drawing sufficient current from said secondary winding to maintain the output voltage of said rectifying means at a desired value.

Claim 9

The power supply according to claim 7, wherein said regulating means comprises a switching regulator.

Claim 10

The power supply according to claim 1, further comprising regulating means for varying the frequency of said oscillator to maintain the output voltage of said rectifying means at a desired value.

Claim 11

The power supply according to claim 9, wherein said regulating means includes voltage sensing means coupled to said rectifying means and frequency control means coupled to said transistor.

Claim 12

The power supply according to claim 10, wherein said regulating means further includes optical coupling means comprising (i) a light source coupled to said voltage sensing means, and (ii) a light responsive element coupled to said fRequency control means.

Claim 13

The power supply according to claim 1, further comprising means for providing a periodic reference signal at the frequency of said oscillator.

Claim 14

The power supply according to claim 1, wherein said primary winding also serves as said output winding.

Claim 15

A blocking oscillator type power supply with power foldback short circuit protection, comprising: a transistor having collector, emitter and base electrodes; a transformer having a primary winding, at least one secondary winding, and a tertiary feedback winding; means for coupling said primary winding in series between said collector electrode and a first terminal of a source of potential difference; means for coupling said emitter electrode to a second terminal of said source; a capacitor; means for coupling said feedback winding in series with said capacitor between said base electrode and the second terminal of said source, the terminals of said primary and feedback windings being connected in such relative relationship as to provide positive feedback from said collector electrode to said base electrode by inductive coupling between said windings; a biasing resistor connected between said base electrode and a point having a potential relative to said second terminal which is of the same polarity as the potential of said first terminal relative to said second terminal; means for connecting said secondary winding to a load; a Zener diode connected between said base electrode and said second terminal, so that the capacitor is repetitively charged through said diode to permit said oscillator to operate at a given frequency when the load has a given value, said capacitor being repetitively charged through said biasing resistor to cause said oscillator to operate at a substantially lower frequency when the secondary winding is short-circuited by the load, thereby substantially decreasing the power delivered to said load and limiting the power dissipated in said oscillator to a safe operating value while said secondary winding is short-circuited; and rectifying means between said secondary winding and said load to provide unidirectional current to the load, said rectifying means including a half wave rectifier polarized to draw current from said secondary winding only when said transistor is nonconductive.

Claim 16

A blocking oscillator type power supply with power foldback short circuit protection, comprising: a transistor having collector, emitter and base electrodes; a transformer having a primary winding, at least one secondary winding, and a tertiary feedback winding; means for coupling said primary winding in series between said collector electrode and a first terminal of a source of potential difference; means for coupling said emitter electrode to a second terminal of said source; a capacitor; means for coupling said feedback winding in series with said capacitor between said base electrode and the second terminal of said source, the terminals of said primary and feedback windings being connected in such relative relationship as to provide positive feedback from said collector electrode to said base electrode by inductive coupling between said windings; a biasing resistor connected between said base electrode and a point having a potential relative to said second terminal which is of the same polarity as the potential of said first terminal relative to said second terminal; means for connecting said secondary winding to a load; a Zener diode connected between said base electrode and said second terminal, so that the capacitor is repetitively charged through said diode to permit said oscillator to operate at a given frequency when the load has a given value, said capacitor being repetitively charged through said biasing resistor to cause said oscillator to operate at a substantially lower frequency when the secondary winding is short-circuited by the load, thereby substantially decreasing the power delivered to said load and limiting the power dissipated in said oscillator to a safe operating value while said secondary winding is short-circuited; and rectifying means between said secondary winding and said load to provide unidirectional current to the load, said rectifying means including a half wave rectifier polarized to draw current from said secondary winding only when said transistor is nonconductive.

Claim 17

The power supply according to claim 14, further comprising regulating means for maintaining the voltage delivered to said load at a desired value by varying the frequency of said oscillator in response to variations in said voltage.

Claim 18

The power supply according to claim 14, further comprising regulating means for maintaining the current delivered to said load at a desired value by varying the frequency of said oscillator in response to variations in said current.

Claim 19

A blocking oscillator type power supply with power foldback short-circuit protection, comprising: a transistor; a transformer having primary, output and feedback windings; a first time constant circuit comprising a resistance-capacitance network; means for coupling said primary winding to said transistor and said feedback winding to said transistor and said time constant circuit ot provide positive feedback to cause said transistor to oscillate at a given frequency; means for connecting said output winding to a load; a second time constant circuit comprising a second resistance-capacitance network; a frequency-determining capacitor common to both of said resistance-capacitance networks; and means for causing said transistor to oscillate at a frequency determined by said second time constant circuit which is substantially lower than said given frequency when said output winding is short-circuited by said load, thereby substantially decreasing the power delivered to the load and limiting the power dissipated in said oscillator to a safe operating value while said output winding is short-circuited.

Claim 20

A blocking oscillator type power supply with power foldback short-circuit protection, comprising: a transistor; a transformer having primary, output and feedback windings; a first time constant circuit comprising a resistance-capacitance network; means for coupling said primary winding to said transistor and said feedback winding to said transistor and said time constant circuit ot provide positive feedback to cause said transistor to oscillate at a given frequency; means for connecting said output winding to a load; a second time constant circuit comprising a second resistance-capacitance network; a frequency-determining capacitor common to both of said resistance-capacitance networks; and means for causing said transistor to oscillate at a frequency determined by said second time constant circuit which is substantially lower than said given frequency when said output winding is short-circuited by said load, thereby substantially decreasing the power delivered to the load and limiting the power dissipated in said oscillator to a safe operating value while said output winding is short-circuited.

Claim 21

The poWer supply according to claim 17, further comprising rectifying means for drawing current from said output winding only when said transistor is nonconductive.