Multiple use of a single transistor for linear regulation...

Details Multiple use of a single transistor for linear regulation... Multiple use of a single transistor for linear regulation...

2000-07-19

2001-06-12

Wong, Peter S. (Department: 2838)

Electricity: battery or capacitor charging or discharging

Battery or cell discharging

Regulated discharging

Reexamination Certificate

active

06246214

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to battery charging technology, and more specifically to the use of a safety field effect transistor (FET) to regulate charging of a rechargeable battery.
BACKGROUND OF THE INVENTION
There is a universal problem associated with rechargeable battery systems known as the “key chain” problem. It is known as the “key chain” problem because when a battery with exposed terminals is placed in a pocket of a user, any conductive object, such as a key chain for example, that comes in contact with the terminals can provide a short circuit. Such a short circuit can produce large currents that may cause the key chain or other conductive object to increase in temperature.
To prevent such a phenomenon from occurring, prior art battery systems have typically included a series element, such as a fuse or positive temperature coefficient (PTC) resistor, coupled between one external terminal and the cell. When large currents flow, either the fuse clears or the PTC becomes an open, thereby stopping the flow of current.
More modern systems employ transistors coupled in series between a terminal and the cell.
FIG. 1
shows such a prior art system. In the prior art system of
FIG. 1
, it may be necessary to protect the battery cell from excessive current flow in either direction. For example, a “key-chain” short between terminals
108
and
109
may cause too much current flow in the cell discharge direction, with excessive current flowing from cell terminal
101
to the external terminal
108
. Also, a cell may be overcharged from too much current flow in the cell charge direction, with excessive current flowing from the external terminal
108
to the cell terminal
101
. To protect against excessive current flow in both directions two field effect transistors (FETs) are used due to the parasitic diode component associated with each. When power semiconductor FETs are manufactured, the body substrate, coupled with the method of manufacture create a parasitic diode. Referring to
FIG. 1
, diode
102
is a parasitic diode component of transistor
103
.
This parasitic diode causes problems in battery circuit designs. While transistor
103
can prevent current from flowing from the cell
101
to the positive terminal
108
, the transistor
103
cannot prevent current from flowing in the opposite direction due to the diode. In other words, if transistor
103
is open, and diode
102
becomes forward biased, current will flow through the diode
102
.
To prevent bidirectional current flow, designers must use another transistor
106
that has its parasitic diode
107
aligned in the opposite direction. In such a fashion the charging transistor
106
opens to prevent current from flowing from the terminal
108
to the cell
101
, and the discharge transistor
103
opens to prevent current from flowing from the cell
101
to the terminal
108
. In each case, the parasitic diode is reverse biased preventing current flow.
In addition, as known in the prior art, transistor
106
may be used to regulate current flow in the charge direction. In this case, the FET
106
is used in its linear ohmic operating mode to adjust the desired charging current. A charge regulator system
105
may be used to accomplish this operation, as is common in the art. Likewise, transistor
102
may be used to regulate current flow in the discharge direction, with the FET
102
used in its linear ohmic operating mode to adjust the desired discharge current. A discharge regulator system
104
may be used to accomplish this operation, as is common in the art. The linear ohmic operating mode is understood herein to include also the states where the transistor being controlled may be completely “on” (saturated), or completely “off” (cutoff), or the transistor may be in any ohmic conduction region between these extremes. Because of the prior art application and usage of these two transistors, transistor
106
may be typically referred to as the “Charge FET,” and transistor
102
may be typically referred to as the “Safety FET.”
The use of two transistors to regulate and control cell current flow in either direction from a battery cell is an expensive and bulky solution to the problem of cell safety protection. There is a need for an improved battery circuit to provide a charging system that reduces the number of components and complexity of the circuit, and still provides desirable safety features.


REFERENCES:
patent: 6137265 (1998-10-01), Cummings et al.
patent: 6160381 (1998-10-01), Peterzell

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