Photo-coupler apparatus having adequate protection against exces

Details Photo-coupler apparatus having adequate protection against exces Photo-coupler apparatus having adequate protection against exces

1994-01-13

1995-04-18

Allen, Stephone B.

Radiant energy

Photocells; circuits and apparatus

Signal isolator

327514, G02B 2700, H03K 342

Patent

active

054081025

ABSTRACT:
A photo-coupler apparatus has a light emitting diode, a photo-diode array, an output MOSFET which is driven by the photoelectromotive force generated by said photo-diode array, and a circuit for limiting the gate voltage of said output MOSFET below a certain value. Said limiting circuit includes a first resistor for detecting output currents, an NPN transistor, a second resistor, and a PNP transistor. In this structure, the voltages developed across the first and second resistors are used to drive the NPN and PNP transistors respectively. In addition, the NPN transistor is connected to the gate of said MOSFET through said second resistor. According to this structure, the gate voltage of the output MOSFET is kept below a certain value by being controlled with the PNP transistor which is driven by the NPN transistor. Thus, stable protection against excessive current is achieved. In addition, a resistor having a small value can be used as the first resistor so as to reduce the heat loss of this apparatus.

REFERENCES:
patent: 4777387 (1988-10-01), Collins
patent: 4801822 (1989-01-01), Idaka et al.
patent: 4931656 (1990-06-01), Ehalt et al.
patent: 5057694 (1991-10-01), Idaka et al.
patent: 5105090 (1992-04-01), Miyajima et al.
patent: 5151602 (1992-09-01), Idaka

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