Power-supply circuit

A power-supply circuit for a load, comprising:

• a transformer having a primary winding and a secondary winding,
• a switching transistor having a control electrode and first and second main electrodes which define a main current path of the switching transistor, said main current path being connected in series with the primary winding,
• a first resistor connected between the first main electrode and a first terminal of the secondary winding,
• a first diode connected in series with the load between a second terminal of the secondary winding and the first terminal of the secondary winding,
• a series arrangement of a first capacitor and a second resistor connected between the second terminal and the control electrode,
• a third resistor connected between the control electrode and a supply terminal, and
• threshold means for limiting the voltage on the control electrode, wherein the threshold means comprise;
• a first transistor having an emitter connected to the first terminal, a collector coupled to the control electrode of the switching transistor and a base coupled to the second terminal of the secondary winding,
• a second transistor having an emitter connected to the first terminal, a collector coupled to the base of the first transistor and a base connected to the first terminal via a further diode, and
• a voltage divider connected across the load and having a tap connected to the base of the second transistor.

A power-supply circuit for a load, comprising:

• a transformer having a primary winding and a secondary winding,
• a switching transistor having a control electrode and first and second main electrodes which define a main current path of the switching transistor, said main current path being connected in series with the primary winding,
• a first resistor connected between the first main electrode and a first terminal of the secondary winding,
• a first diode connected in series with the load between a second terminal of the secondary winding and the first terminal of the secondary winding,
• a series arrangement of a first capacitor and a second resistor connected between the second terminal and the control electrode,
• a third resistor connected between the control electrode and a supply terminal, and
• threshold means for limiting the voltage on the control electrode, wherein the threshold means comprise;
• a first transistor having an emitter connected to the first terminal, a collector coupled to the control electrode of the switching transistor and a base coupled to the second terminal of the secondary winding via a series resistor,
• a second transistor having an emitter connected to the first terminal, a collector coupled to the base of the first transistor via the series resistor, and a base,
• a third transistor of an opposite conductivity type and having an emitter connected to the first terminal, a collector coupled to the base of the first transistor, and a base, and
• a voltage divider connected across the load and having a tap connected to the base of the second transistor and to the base of the third transistor.

A power-supply circuit for a load, comprising:

• an input terminal for connection to a source of supply voltage for the power supply circuit,
• a transformer having a primary winding and a secondary winding,
• a switching transistor having a control electrode and first and second main electrodes which define a main current path of the switching transistor,
• first means for coupling the primary winding, the main current path of the switching transistor and a first resistor in a first series circuit between the input terminal and a first terminal of the secondary winding,
• a first diode connected in series with the load between the first terminal of the secondary winding and a second terminal thereof,
• second means for coupling a first capacitor and a second resistor in a second series circuit between said second terminal of the secondary winding and the control electrode of the switching transistor,
• a third resistor coupling the control electrode to a supply terminal, and
• voltage threshold means connected between the control electrode and the first terminal for limiting the voltage on the control electrode, wherein the voltage threshold means comprise a voltage threshold element and a second diode connected in series and with the second diode polarized to conduct when the control voltage of the switching transistor is being limited.