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Quasi-Constant Current Circuit for Battery Charging

IP.com Disclosure Number: IPCOM000105941D
Original Publication Date: 1993-Sep-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 2 page(s) / 70K

Publishing Venue

IBM

Related People

Gladstein, L: AUTHOR [+2]

Abstract

Disclosed is a circuit establishing a quasi-constant current source for battery charging, in which a power FET (Field Effect Transistor) is used to switch the battery current, while a control FET, through which a current flows at a level proportional to the current flowing through the power FET, is used as an input to a comparator which controls the switching of both FET devices [*]. In this way, the current used for control purposes is much smaller than the current being switched.

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Quasi-Constant Current Circuit for Battery Charging

      Disclosed is a circuit establishing a quasi-constant current
source for battery charging, in which a power FET (Field Effect
Transistor) is used to switch the battery current, while a control
FET, through which a current flows at a level proportional to the
current flowing through the power FET, is used as an input to a
comparator which controls the switching of both FET devices [*].  In
this way, the current used for control purposes is much smaller than
the current being switched.

      Referring to the Figure, a direct-current input voltage, such
as 15 volts, from a power source (not shown), is provided at an input
terminal 10 of the circuit, while a load, such as a battery 12 to be
charged, is connected to output terminal 14 of the circuit.  The
power source may be, for example, an adapter circuit attached to line
voltage or to a vehicle battery.  An inductor 15 is connected between
input terminal 10 and a capacitor 16, the other side of which is
connected to an inductor 17, and to output terminal 14 through a
high-current diode 18 and a filter network, comprised of capacitors
20 connected to electrical ground and an inductor 21.  The
application of an input voltage to input terminal 10 causes a current
to flow through inductors 15 and 17.

      A one-Mhz oscillator 22 sets latch 24, which in turn provides
an input to driver circuit 26, applying a signal to the gates of
power FET 28 and control FET 30, to turn on both these FET devices.
Each of these devices actually comprises a large group of field
effect transistors connected in parallel, with power FET 28 being
sized to carry 4300 times as much current as control FET 30.  When
these FET devices are t...