Publishing Venue
Motorola
Related People
Authors:
David Soo
•
Steve McKinnis
Abstract
A power MOSFET current limiting circuit for monitoring the current flowing through a current sensing field effect transistor (SENSFET), and turning off the SENSFET if the current reaches a selected maximum magnitude is shown in the figure. A SENSFET M4, is connected to drive a load L, with a gate drive circuit pro- viding the gate voltage to turn the SENSFET on and off. A portion of the load current flowing through M4 is fed back to a monitoring circuit. Since the portion of current being fed back is indicative of the full load current, it can be used to determine when the load current has exceeded a maximum allowable magnitude.
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MOTOROLA Technical Developments Volume 8 October 1988
CURRENT LIMITING CIRCUIT
by David Soo and Steve McKinnis
A power MOSFET current limiting circuit for monitoring the current flowing through a current sensing field effect transistor (SENSFET), and turning off the SENSFET if the current reaches a selected maximum magnitude is shown in the figure. A SENSFET M4, is connected to drive a load L, with a gate drive circuit pro- viding the gate voltage to turn the SENSFET on and off. A portion of the load current flowing through M4 is fed back to a monitoring circuit. Since the portion of current being fed back is indicative of the full load current, it can be used to determine when the load current has exceeded a maximum allowable magnitude.
Field effect transistors Ml, M2, and M3 provide a source current for the current monitoring circuit. Bipolar transistor Q2 has an emitter area that is N times the emitter area of bipolar transistor Ql. Field effect transistor M2 is matched to field effect transistor M3 so that the current flowing in Ql is equal to the current flowing in Q2. Because the emitter area of 02 is greater than that of Ql, Ql will have a greater current density and a delta V, develops between the emitters of Ql and Q2. As a result, a reference current equal to delta V,/R is established by the current flowing through Q2.
The portion of the load current that is fed back to the current monitoring circuit is added to the reference current. When the porti...
