Transistor-PTC circuit protection devices

Details Transistor-PTC circuit protection devices Transistor-PTC circuit protection devices

1998-10-22

2001-01-30

Sherry, Michael J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Circuit interruption by thermal sensing

C361S093800, C361S093900, C361S058000, C361S101000

Reexamination Certificate

active

06181541

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally relates to electronic circuits, and more particularly to circuit protection devices comprising a resistor with a positive temperature coefficient (PTC), referred to as PTC_r, operating in combination with a semiconductor device.
PTC_r circuit protection devices are well known. The device is placed in series with a load, and under normal operating conditions, is in a low temperature, low resistance state. However, if the current through the PTC_r device increases excessively, and/or the ambient temperature around the PTC_r device increases excessively, then the PTC_r device will be “tripped,” i.e., converted to a high resistance state such that the current is reduced substantially to a safe level. Generally, the PTC_r device will remain in the tripped state, even if the fault is removed, until the device has been disconnected from the power source and allowed to cool. After the current and/or temperature return to their normal levels, the PTC_r device will switch back to the low temperature, low resistance state.
An example of a PTC_r composition device is one that contains a PTC_r which is composed of a PTC conductive polymer. The largest steady state current which will not cause any of the devices in a batch of devices to trip is referred to as the “hold current” (I
hold
), and the smallest steady state current which will cause all of the devices to trip is referred to as the “trip current” (I
trip
). In general, the difference between I
hold
and I
trip
decreases slowly as the ambient temperature increases, and the higher the ambient temperature, the lower the hold current and the trip current.
PTC and semiconductor devices have been used together in electrical circuits. In some instances the PTC_r devices have been used to protect the semiconductor devices from overcurrent and/or overtemperature conditions. In other instances, the semiconductor and PTC_r devices have been used together to protect the circuits in which they reside from overcurrent and/or overvoltage conditions.
In many instances, it is desirable to use PTC_r devices in high voltage applications. However, in most cases, the PTC_r device that is operated at its normal current generally has a large size. PTC_r devices of large size are not suitable for portable electronic devices in which size and weight are important. PTC_r devices of large size are also not suitable for high density electronics circuits.
SUMMARY OF THE INVENTION
The present invention provides a circuit protection device that has a reduced size and weight and is particularly suitable for use, for example, in portable electronic devices and high density electronic circuits. The present invention also provides design flexibility by allowing adjustment of the ratio of the trip current and the minimum current that flows in the PTC_r device after tripping occurs. The device of the present invention comprises a three-terminal switch element having first, second and third terminals, the first terminal for series connecting to the electrical load; a positive temperature coefficient (PTC) resistor having first and second ends, the first end connected to the third terminal of the switch element; and a voltage divider circuit including first and second resistors, the first resistor connected between a voltage source and the second terminal of the switch element, and the second resistor connected between the second terminal of the switch element and the second end of the PTC resistor.
According to the present invention, the switch element may be a metal-oxide-semiconductor transistor (MOSFET) and the first, second and third terminals of the switch element are drain, gate and source electrodes, respectively. Alternatively, the switch element may be a junction field-effect transistor (JFET) and the first, second and third terminals of the switch element are drain, gate and source electrodes, respectively. The switch element may also be a bipolar junction transistor and the first, second and third terminals of the switch element are collector, base and emitter electrodes, respectively.
Various other embodiments of the invention are disclosed.


REFERENCES:
patent: 3708720 (1973-01-01), Whitney et al.
patent: 3884080 (1975-05-01), Chapman
patent: 4041276 (1977-08-01), Schwarz et al.
patent: 4281358 (1981-07-01), Plouffe et al.
patent: 4295088 (1981-10-01), Malchow
patent: 5381296 (1995-01-01), Ekelund et al.
patent: 5684663 (1997-11-01), Mitter
patent: 5763929 (1998-06-01), Iwata
patent: 2249463 (1975-05-01), None
patent: 94/11937 (1994-05-01), None
International Search Report for International Application No. PCT/US99/24716 dated Feb. 18, 2000.

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