Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Utilizing three or more electrode solid-state device
Reexamination Certificate
2000-08-14
2001-07-10
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Utilizing three or more electrode solid-state device
C327S434000
Reexamination Certificate
active
06259306
ABSTRACT:
The present invention relates to a control system for a bidirectional switch for current monitoring circuits, in particular current monitoring circuits suitable for controlling the charge and discharge of rechargeable batteries. More particularly, the present invention concerns a control system for a bidirectional switch formed of a pair of MOFSET power transistors connected in anti-series, i.e. source to source or drain to drain.
Circuits allowing the bidirectional flow of a current to be controlled are used in numerous applications. Such circuits are used in particular to control the charge and discharge of rechargeable batteries (lithium, lithium-ion batteries, etc.), assuring the power supply of portable devices of various types (telephones, lap-top computers, timepieces, etc.).
It is known that rechargeable batteries of this type require protection against inadequate operating conditions (overcharging or undercharging of the battery, too high charging or discharging currents, too high operating temperature, etc.) which can appear when a load or charger is connected to the battery or during a short-circuit of the battery terminals. This protection is necessary to assure that the battery performance is not adversely affected.
These current monitoring circuits thus typically include current interruption means which are placed in series in the battery current path and respond to signals indicative of inadequate operating conditions, in order to interrupt the continuation of the charging or discharging of the battery or, generally, in order to interrupt the flow of a current in the battery which could have an adverse affect on the performance thereof. These current interruption means are commonly formed of switches made by means of MOFSET power transistors and controlled by circuits monitoring the level of charge or discharge of the battery, the value of the current passing through it or the battery temperature.
In numerous applications, these interruption means are formed of a switch including a pair of MOFSET power transistors connected in anti-series, i.e. connected source to source or alternatively drain to drain.
FIGS. 1
a
to
1
c
illustrate three examples of such a bidirectional switch with two transistors.
FIG. 1
a
illustrates a pair of N-MOFSET transistors connected source to source,
FIG. 1
b
illustrates a pair of N-MOFSET transistors connected drain to drain, and
FIG. 1
c
illustrates a pair of P-MOFSET transistors connected source to source. Of course, a fourth solution, which is not illustrated here, consists in connecting a pair of P-MOFSET transistors drain to drain. This type of switch is commonly called a “bidirectional switch” since it allows the current to be blocked as well as to flow in two directions.
In normal operation, the two transistors are at the “ON” state and a current can flow in both directions between terminals X and Y through the switch. When an inadequate operating condition is detected, one or the other of the transistors is set to the “OFF” state in order to block the flow of current in the battery. It will be noted however that the current can still flow in the opposite direction through a parasitic diode or “body diode” formed between the drain and the source of each MOFSET power transistor, this parasitic diode being due to the fact that the substrate (“body”) and the source of the power transistor are connected together at the same potential.
Each transistor thus allows the current to be blocked in one direction while allowing a current to flow in the opposite direction through its parasitic diode. Used in a rechargeable battery charge and discharge monitoring circuit, one of the transistors thus allows the battery charging current to be interrupted, while the other transistor allows the discharge current in the opposite direction to be interrupted.
There are two determining factors when such a bidirectional switch is selected and made. One the one hand, one wishes to limit as much as possible the value of the series resistance (or conduction resistance) R
DS
—
ON
of the bidirectional switch. On the other hand, one wishes to minimise as much as possible the time necessary for making the bidirectional switch conduct, hereinafter referred to as conduction delay time.
These two purposes are in fact intimately linked and opposite. Indeed, in order to reduce series resistance R
DS
—
ON
of the bidirectional switch, the transistors need to be controlled with relatively high gate voltages, typically of the order of 10 to 15 volts between the gate and the source of the transistor. The power transistors must thus necessarily bear these gate voltages through the gate oxide which consequently has a relatively large thickness and thus a large gate capacitance. This high gate capacitance thus leads to relatively long transistor conduction delay times.
The shortest possible conduction delay time of the bidirectional switch allows a quick response to changes in conditions detected by the monitoring circuit. This is all the more important since in most applications, these circuits operate in a pulsed manner and must thus respond almost instantaneously to the detected conditions.
One object of the present invention is thus to provide a control system for a bidirectional switch with two transistors which, in particular, allows the two aforementioned opposite purposes to be answered.
The present invention thus concerns a control system for a bidirectional switch with two transistors, whose features are listed in claim
1
.
The present invention also concerns a control method for a bidirectional switch with two transistors, whose features are listed in claim
7
.
According to the present invention, advantage is taken of the fact that the bidirectional switch is formed of two power transistors mounted in anti-series and that only one of the transistors is set at the “OFF” state when one wishes to interrupt the flow of current through the battery, the other transistor always remaining at the “ON” state.
By coupling the power transistor gates at least temporarily when the transistor at the “OFF” state is made to conduct again, the charge present on the transistor which remained at the “ON” state is shared with the other transistor and it is thus possible to substantially reduce the transistor conduction delay time. It is thus possible to improve the dynamic features of the bidirectional switch, namely essentially its conduction delay time, despite the use of high gate voltages allowing a low series resistance to be obtained for the switch.
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Williams, Richard K., et al., The Bidirectional Power NMOS—A New Concept in Battery Disconnect Switching, Proceedings of 1995 International Symposium on Power Semiconductor Devices & ICs, (ISPSD), US, New York, IEEE, vol. SYMP 7, pp. 480 through 485.
Brülhart Marcel
Trillat Stéphane
EM Microelectronic
Robinson Richard K
Tran Toan
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