Many types of electrical equipment contain IC (integrated circuit) devices which are vulnerable to damage from high voltage transients.
In a television receiver the anode of the image producing kinescope is typically biased at a high potential, e.g., 25,000 volts. High voltage transients may be produced when the high voltage anode of the kinescope is rapidly discharged to points at lower potentials. Such high voltage transients have positive and negative peaks often in excess of 100 volts and may last several microseconds. High voltage transients may also be produced when electrostatic charges are discharged as a user contacts the controls of the television receiver. High voltage transients may be coupled to the terminals of IC's employed in the television receiver for video and audio signal processing. Accordingly, these IC's may be damaged by high voltage transients.
Copending patent application entitled "Protection Circuit for Integrated Circuit Devices," Ser. No. 212,534, filed in the name of the present inventor Dec. 3, 1980 and assigned to RCA Corporation, discloses a protection device for protecting an IC from negative voltage transients that exceed the most negative power supply potential applied to the IC. Another copending patent application entitled "Protection Circuit for Integrated Circuit Devices," Ser. No. 230,357 filed also in the name of the present inventor Jan. 30, 1981, and also assigned to RCA Corporation, discloses a protection circuit which is triggered at one forward biased diode voltage drop above the positive supply voltage for protecting an IC from positive voltage transients that exceed the most positive power supply potential applied to the IC.
However, in a television receiver, particular signals applied to an IC may have positive voltage excursions which in normal operation exceed the positive supply potential. For example, a typical television horizontal/vertical regulator IC requires a feedback connection from the kinescope deflection coils to one of its input terminals. While the power supply for the IC is typically +10 volts, the peak feedback voltage from the deflection coils is typically +27 volts. Therefore, it is desirable to provide a positive transient protection circuit for such IC's that permits normal signal voltages to exceed the power supply potential without activating such protection circuit and nevertheless protects the IC from excessively large transients.