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OVERTEMPERATURE PROTECTION OF FETs

IP.com Disclosure Number: IPCOM000005872D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2001-Nov-13
Document File: 3 page(s) / 97K

Publishing Venue

Motorola

Related People

John Pigott: AUTHOR [+2]

Abstract

In applications where high currents are being controlled it is often desirable to use acurrent-sensing FET(SENSEFET* FET) to both control the current and monitor the load current. There is a simple circuit which will protect the SENSEFET FET from damage due to overtemperature which takes advantage of the SENSEFET FET's changing characteristics with increasing temperature.

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MOTOROLA Technical Developments March 1990

OVERTEMPERATURE PROTECTION OF FETs

by John Pigott and John Qualich

   In applications where high currents are being controlled it is often desirable to use acurrent-sensing FET(SENSEFET* FET) to both control the current and monitor the load current. There is a simple circuit which will protect the SENSEFET FET from damage due to overtemperature which takes advantage of the SENSEFET FET's changing characteristics with increasing temperature.

   In Figure 1, the voltage VsENsE is impressed across the sensing resistor RSENSE. For example, Figure 2 shows the sense voltage characteristics for Motorola's MTP40N06M. The voltage across a sense resistor of 10 ohms will vary from 140mV to 16OmV (14%) while that across a 100 ohm resistor will vary from 300mV to 500mV (66%) when the temperature is varied from 25°C to 125°C.

   In a system in which two SENSEFET FETs (FETl and FET2) are paralleled, the circuit of Figure 3 can be used to provide overtemperature protection for the SENSEFET FETs. Figure 4A shows the expected variation of sense voltage related voltages V, and V, for the FETs in Figure 3.

   In Figure 3, the voltage V, across the 100 ohm resistor will have a greater temperature coefficient than the voltage VsI across the 10 ohm resistor. The buffer amplifier gains are chosen so their output voltages V, and V, are equal at the desired maximum temperature of 125°C. The signal Vor having a positive logic (high) output state indicates...