| 摘要 |
<p>PURPOSE:To reduce a withstand voltage and a power loss of a transistor while the fracture of a drive circuit, which uses a MOS transistor, due to surges is prevented. CONSTITUTION:A Zener diode ZD1 with the cathode thereof connected to a power supply VDD is connected in series with resistors R1 and R2. A surge voltage which is more than the maximum rated power supply voltage is applied to the power supply VDD, and a current which exceeds a withstand voltage of the Zener diode D1 flows through the resistors, thereby resulting in a rise in electric potential of a connecting point X1. As a result of this, a transistor Q1 with the base thereof connected to the connecting point X1 and with the collector thereof connected to gates of transistors Tr1 and Tr4 is turned on, and hence the transistors Tr1 and Tr4 are turned off. In the same manner, transistors Tr2 and Tr3 are turned off by means of a Zener diode ZD2, resistors 3 and 4 and a transistor Q2, whereby the fracture of a drive circuit is prevented. Hence, it is possible to reduce a withstand voltage per one transistor of the transistors Tr1-Tr4 to a half of the maximum rated power supply voltage.</p> |
