Power supply fault protection circuit

A fault detection and protection arrangement of a power supply apparatus, comprising:

• a source of an input supply voltage;
• a first switching power transistor having first and second main current conducting electrodes and a control electrode;
• a source of a first signal at a given frequency coupled to said control electrode of said first switching power transistor for causing, during normal operation, said first switching transistor to operate at a switching frequency determined in accordance with said given frequency;
• a utilization circuit including an impedance coupled to said input supply voltage and to said first main current conducting electrode for developing an output supply in said utilization circuit, said impedance forming a current path to a switched main current that flows in said first switching power transistor for developing a second signal at a frequency that corresponds with that of said first control signal and that is developed between said first and second main current conducting electrodes of said first switching power transistor, said second signal being indicative of a level of said switched main current;
• a second impedance having a first terminal that is coupled to said first main current conducting electrode of said first switching power transistor;
• a second switching transistor having a control electrode that is coupled to said first signal and a main current conducting electrode that is coupled to a second terminal of said second impedance for developing a third signal at said main current conducting electrode of said second switching transistor when said first switching power transistor is conductive that is indicative of an amplitude of said switched main current, and when said first switching power transistor is nonconductive, said second switching transistor causes said third signal to be substantially smaller in value than when said first switching power transistor is conductive;
• and
• means responsive to said third signal for generating a control signal that is coupled to said control electrode of said first switching power transistor, said control signal causing a reduction in said current that flows in said first switching power transistor when said third signal exceeds a predetermined magnitude.

A fault detection and protection apparatus in a switching power supply, comprising:

• a source of an input supply voltage;
• a first switching power transistor having first and second main current conducting electrodes and a control electrode;
• a source of a first signal at a given frequency that is coupled to said control electrode of said first switching power transistor for causing, during normal operation, said first switching transistor to operate at a switching frequency that is determined in accordance with said given frequency;
• a first transformer winding having a first terminal that is coupled to said first main current conducting electrode of said first switching power transistor;
• a second switching power transistor having a first main current conducting electrode that is coupled to a second terminal of said first transformer winding that is remote from said first switching power transistor for conducting a second switching current in said second switching power transistor and having a control electrode that is responsive to a signal at a frequency that is related to that of said first signal such that said first and second switching power transistors operate in a push-pull manner;
• means coupled to said first main current conducting electrodes of said first and second switching power transistors for generating a third signal during first and second portions of a given period of said first signal that is obtained from voltages developed in respective internal impedances of said first and second switching power transistors, and that is proportional, during said first and second portions, to said first and second switching currents that flow therein, respectively;
• and
• means responsive to said third signal for generating a control signal when a magnitude of either one of said first and second switching currents is outside a normal operation range, that is indicative of an occurrence of a fault condition in said power supply, said control signal being coupled to said control electrode of at least one of said first and second switching power transistors for providing protection against said fault condition.

A fault detection and protection apparatus in a switching power supply, comprising:

• a source of an input supply voltage;
• a first switching power transistor having first and second main current conducting electrodes and a control electrode;
• a source of a first control signal at a given frequency that is coupled to said control electrode of said first switching power transistor for causing, during normal operation, said first switching transistor to operate at a switching frequency determined in accordance with said given frequency;
• a utilization circuit including an impedance coupled to said input supply voltage and to said first main current conducting electrode for developing an output supply voltage in said utilization circuit, said impedance forming a current path to a switched main current that flows in said first switching power transistor to develop a first current sensing signal at a frequency determined in accordance with said given frequency, and that is developed between said first and second main current conducting electrodes of said first switching power transistor when said first switching power transistor is conductive, said first current sensing signal being indicative of a level of said main current when said first switching power transistor is conductive;
• first means responsive to said first control signal and to said first current sensing signal and having an input terminal that is coupled to said first main current conducting electrode for generating at an output terminal a first portion of a second current sensing signal that is indicative of said level of said main current when said first switching power transistor is at a conductive state, and for decoupling said output terminal from said first switching power transistor when said first switching power transistor is at a nonconductive state to prevent a voltage that is developed at said first main current conducting electrode of said first switching power transistor when said power transistor is nonconductive from having a substantial contribution to an amplitude of said second current sensing signal;
• and
• second means responsive to said second current sensing signal for generating a second control signal, when a magnitude of said first portion of said second current sensing signal is outside a normal operation range indicating an occurrence of a fault condition in said power supply, said second control signal being coupled to said control electrode of said first switching power transistor for preventing said first switching power transistor from further conducting, wherein said first means comprises a second switching transistor having a control electrode that is coupled to said first control signal and a main current conducting electrode that is coupled via a second impedance to said first main current conducting electrode of said first switching power transistor for developing said second current sensing signal at said main current conducting electrode of said second switching transistor such that when said first switching power transistor is conductive said second switching transistor is nonconductive, and vice versa.