Over-temperature protected triac and protection method
View Patent ↗A triac circuit comprises a triac having first and second main terminals (MT 1 ,MT 2 ) and a gate terminal and a thyristor connected between one of the main terminals (MT 1 ,MT 2 ) and a control terminal of the triac circuit. The thyristor is used to prevent turn on of the triac when it has turned on by temperature induced leakage currents.
1. A triac circuit comprising:
a triac having first and second main terminals and a gate terminal, for receiving a triac trigger current; and
a thyristor, having a holding current which is less than the triac trigger current, transitioning to an on-state in response to a temperature, and remaining in the on-state in response to the triac trigger current without the temperature, connected between one of the main terminals and the gate terminal of the triac.
2. A triac circuit as claimed in claim 1 , wherein the thyristor is adapted such that it will be turned on by its own thermally generated leakage current when raised to the temperature.
3. A triac circuit as claimed in claim 1 , wherein the thyristor comprises an SCR.
4. A triac circuit as claimed in claim 1 , wherein the thyristor is connected between one of the main terminals and a control terminal, further comprising a voltage drop element between the control terminal and the gate terminal.
5. A triac circuit as claimed in claim 1 , wherein the voltage drop element comprises a diode.
6. A triac circuit as claimed in claim 5 , wherein the control terminal is connected to the cathode of the diode and a cathode of the thyristor.
7. A triac circuit as claimed in claim 1 , wherein a gate terminal of the thyristor is open circuit.
8. A triac circuit as claimed in claim 1 , wherein the thyristor is adapted to turn on as a result of leakage currents when the temperature is in the range of about 100° C. to about 200° C.
9. A method of providing temperature protection for a triac, comprising;
forming a triac circuit including the triac, for receiving a triac trigger current, and a thyristor, having a holding current which is less than the triac trigger current, transitioning to an on-state in response to a temperature, and remaining in the on-state in response to the triac trigger current without the temperature, in a path between a gate terminal of the triac circuit and a main terminal of the triac.
10. A method as claimed in claim 9 , wherein the thyristor is connected between the main terminal and a control terminal, further comprising providing a voltage drop element between the control terminal and the gate terminal.
11. A method as claimed in claim 10 , wherein the voltage drop element comprises a diode.
12. A method as claimed in claim 9 , comprising leaving a gate terminal of the thyristor open circuit.
13. A method as claimed in claim 9 , wherein the thyristor is adapted such that it will be turned on by its own thermally generated leakage current when raised to the temperature.
14. A method as claimed in claim 9 , wherein the thyristor is designed to turn on as a result of its own thermally generated leakage currents when the temperature is in the range of about 100° C. to about 200° C.
15. A method as claimed in claim 9 , wherein the thyristor comprises an SCR.