Browse Prior Art Database

MICRO POWER ISOLATED SWITCH WITH COMMON MODE REJECTION

IP.com Disclosure Number: IPCOM000007510D
Original Publication Date: 1995-Nov-01
Included in the Prior Art Database: 2002-Apr-02
Document File: 2 page(s) / 87K

Publishing Venue

Motorola

Related People

H. D. Bush: AUTHOR

Abstract

The customer requirement was a solid state relay which was isolated from the control circuitry and did not require any connections other than the two switch closure connections, and the connections were to be polarity independent. Minimum power for oper- ation was also desired for long battery life of the control circuitry Solution: (Refer to the schematic) The approach used in the design was to generate the necessary gate operating voltage for the TMOS FETs switch from the control circuit power source (3.3 volts) by generating a complementary square wave to be used to drive a voltage doubler via capacitors. ICla is a divide by two flip flop which generates a square wave From a one Hertz, very short duration pulse source. IClb is a latch and holds the desired state of the switch so the microprocessor which controls the switch can be shut down when desired, reducing system power usage. When the switch is to be turned on, IClb is loaded with D, and thus Q, being high. IC2a and IC2b transfer the complementary square wave to Cl and C2.

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MOlYlROLA Technical Developments

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MICRO POWER ISOLATED SWITCH WITH COMMON MODE REJECTION

by H. D. Bush

  The customer requirement was a solid state relay which was isolated from the control circuitry and did not require any connections other than the two switch closure connections, and the connections were to be polarity independent. Minimum power for oper- ation was also desired for long battery life of the control circuitry

  Solution: (Refer to the schematic) The approach used in the design was to generate the necessary gate operating voltage for the TMOS FETs switch from the control circuit power source (3.3 volts) by generating a complementary square wave to be used to drive a voltage doubler via capacitors. ICla is a divide by two flip flop which generates a square wave From a one Hertz, very short duration pulse source. IClb is a latch and holds the desired state of the switch so the microprocessor which controls the switch can be shut down when desired, reducing system power usage. When the switch is to be turned on, IClb is loaded with D, and thus Q, being high. IC2a and IC2b transfer the complementary square wave to Cl and C2.

  If we assume that Ql is a short, then the circuit is a voltage doubler. C2 is charged through D3 on the negative swing of C2. This charge is dumped into Cl via Dl on the next half cycle, and into C6 via D2 on the following half cycle. When the volt- age on C6 reaches the threshold of Q4 and Q5, the output turns on, with higher voltages producing lower switch resistance. The diodes are hot carrier to min- imize the forward voltage drop.

  When the switch is to be turned off, IClb is loaded with D, and...