| 摘要 |
A PNPN structure thyristor and a third P-type impurity region are formed on a semiconductor substrate, and a fixed current circuit using an NPN transistor 20 and a resistor 30 is connected to a cathode terminal K and a gate terminal G of a composite element 10 that has a P-channel MOS transistor Q 3 with the third P-type impurity region as a drain connected to a PNP transistor Q 1 of the thyristor. If a voltage applied to the anode terminal A of the composite element 10 rises from 0V, initially the thyristor is on and current flows, but once the threshold voltage of transistor Q 3 is reached transistor Q 3 turns on. A short circuit then exists between the base and emitter of transistor Q 3 of the thyristor and current of the composite element 10 itself is cut off. Because of the bipolar structure it is simple to configure a small integrated circuit with a high withstand voltage. In this way, a zero crossing detection circuit is provided that has high withstand voltage characteristics, but which can be made as a small sized integrated circuit.
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