Overcurrent protection for the series fuse

• US 20020027758A1
• Filed: 12/15/2000
• Published: 03/07/2002
• Est. Priority Date: 10/22/1999
• Status: Active Grant

• Current Assignee: Motorola Solutions, Inc.
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Burrus, Philip Henry IV, Oglesbee, John Wendell
• Granted Patent: US 6,600,641 B2
• Time in Patent Office: Days
• US Class Current: 361/104
• CPC Class Codes: G05F 1/565 sensing a condition of the system or its load in addition to means responsive to deviations in the output of the system, e.g. current, voltage, power factor G05F1/563 takes precedence
• CPC Class Codes: G05F 1/613 using semiconductor devices in parallel with the load as final control devices G05F1/461 takes precedence
• CPC Class Codes: H02H 1/04 Arrangements for preventing response to transient abnormal conditions, e.g. to lightning or to short duration over voltage or oscillations; Damping the influence of dc component by short circuits in ac networks
• CPC Class Codes: H02H 5/044 using a semiconductor device to sense the temperature
• CPC Class Codes: H02H 6/005 using digital thermal images
• CPC Class Codes: H02H 7/18 for batteries; for accumulators
• CPC Class Codes: H02J 7/00304 Overcurrent protection
• CPC Class Codes: H02J 7/0031 using battery or load disconnect circuits H02J9/002 takes precedence
• CPC Class Codes: H02J 7/007192 in response to temperature
• CPC Class Codes: H03K 2017/0806 against excessive temperature

Abstract

This invention includes a circuit that approximates the thermal behavior of a fuse or other electronic device that is coupled in series with the circuit. In one preferred embodiment, the circuit protects a fuse coupled in series with a rechargeable battery from clearing during soft short conditions. Thus, when the instantaneous current is temporarily above the current rating of the fuse, yet the root mean squared current is below the current rating of the fuse, the circuit works to estimate the heating of the fuse element and limit the current to a root mean squared value that is less than the current rating of the fuse. One embodiment includes a programmable comparator that actuates a counter which, in turn, increments to estimate heating of the element when the current exceeds a predetermined threshold.

Claims

10. A circuit approximating the thermal characteristics of an electrical device, comprising:

a. at least one means for sensing current, the at least one means actuating a first signal when the current exceeds a predetermined threshold; and

b. at least one clock coupled to the means for sensing current, such that the at least one clock is enabled when the first signal is actuated.

1. An electronic circuit that approximates the thermal characteristics of an electronic device.

2. The circuit of claim 1, wherein the electronic device is selected from the group consisting of a fuse and a transistor.

3. The circuit of claim 2, wherein the electronic device is a fuse.

4. The circuit of claim 3, wherein the circuit is coupled in series with a fuse.

5. The circuit of claim 4, wherein the circuit reduces current delivered to the fuse so as to prevent the fuse from clearing so long as the root mean squared value of current is below the fuses current rating.

6. The circuit of claim 5, wherein the circuit is manufactured on a semiconductor die.

7. The circuit of claim 6, wherein the circuit is coupled to a rechargeable battery cell.

8. The circuit of claim 7, wherein the circuit comprises:

a. a comparator to sense when the current flowing through the fuse exceeds a predetermined threshold;

b. at least one clock coupled to the comparator, the at least one clock being actuated by the current exceeding the predetermined threshold; and

c. a counter having at least one output bit, the counter incrementing when the current exceeds the predetermined threshold and decrementing when the current is below the predetermined threshold.

9. The circuit of claim 8, wherein the circuit further comprises:

a. an digital to analog converter having an output and an input, such that the input coupled to at least one output bit of the counter; and

b. a linear current regulator coupled to the digital to analog converter such that the linear current regulator limits current as the output of the digital to analog converter increases.

11. The circuit of claim 10, further comprising:

a. clock select logic, wherein when one of the at least one means for sensing current is actuated, a corresponding clock is selected; and

b. a counter with a increment input coupled to clock select logic and a decrement input coupled to the clock select logic.

12. The circuit of claim 11, further comprising:

a. a digital to analog converter coupled to the counter; and

b. a linear current regulator coupled to the digital to analog converter,

wherein the linear current regulator limits current proportionally to an output of the digital to analog converter.

13. The circuit of claim 12, wherein the circuit is coupled in series with a fuse.

14. The circuit of claim 13, wherein the circuit limits the current so as to prevent the fuse from clearing so long as the root mean squared current flowing in the fuse is less than the current rating of the fuse.

15. A method of protecting a fuse coupled in series with an electronic circuit, the method comprising the steps of:

a. providing a means for comparing the current in the fuse with a predetermined threshold;

b. providing a means for determining the time at which the current is above, and the time at which the current is below the predetermined threshold;

c. sensing the current flowing in the fuse; and

d. reducing the current to a root mean squared level that is less than the current rating of the fuse.

15-1. The method of claim 15, wherein the fuse is coupled in series with a rechargeable battery cell.