Filtering circuit and battery protection circuit using same

Details Filtering circuit and battery protection circuit using same Filtering circuit and battery protection circuit using same

2002-09-28

2004-01-06

Nguyen, Matthew V. (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S274000

Reexamination Certificate

active

06674273

ABSTRACT:

BACKGROUND
1. Technical Field
This invention relates generally to thermal regulation circuits for power dissipating electronic circuit elements, and more specifically to power regulation and thermal management for power transistors as, for example, in battery charging applications.
2. Background Art
FIG. 1
illustrates a simple battery charger
100
that is well known in the art. The charger
100
consists of a power supply
101
, a linear regulator
102
, a pass element
103
and a battery cell
104
. The power supply
101
provides voltage and current to the battery cell
104
. The voltage and current must be regulated by the pass element
103
so as to avoid charging the battery cell
104
too rapidly. The linear regulator
102
performs this regulation by dissipating as heat the difference between the power generated by the power supply
101
and the power stored by the battery cell
104
.
The problem with this prior art solution is that the pass element
103
can overheat. This is best explained by way of example. For a typical single-cell, lithium battery application, a fully charged battery cell
104
typically registers about 4.1 volts. Thus, to fully charge the battery cell
104
, and to give enough headroom for parasitic power losses in the pass element
103
and connecting circuitry, the power supply must be capable of supplying at least 5 volts. A typical battery cell
104
will charge optimally at a current of roughly 1 amp.
The problem arises with the battery cell
104
is fully discharged. A discharged battery cell
104
may register only 2 volts. As the power supply
101
would supply energy at a rate of 5 volts at 1 amp, or 5 watts, and the battery cell
104
stores energy at a rate of 2 volts at 1 amp, or 2 watts, the pass element
103
must dissipate energy at a rate of 3 watts. As typical pass elements
103
may come in a TO-220package, 3 watts for extended periods of time may make the pass element
103
quite warm. Extended periods of heat my actually jeopardize reliability by approaching—or surpassing—the threshold junction temperature of the pass element
103
.
The problem is exacerbated when an incompatible power supply
101
is coupled to the circuit. For example, if someone accidentally couples a 12-volt supply to the charger, the pass element
103
may have to dissipate 10 watts! This can eventually lead to thermal destruction of the pass element
103
.
One solution to this problem is recited in U.S. Pat. No. 5,815,382, issued to Saint-Pierre et al. entitled “Tracking Circuit for Power Supply Output Control”. This solution provides a means of reducing the output voltage of a power supply when the battery is in a discharged state, thereby reducing the total output power of the power supply. This, in turn, reduces the amount of power a pass element would need to dissipate.
While this is a very effective solution to the problem, it requires a power supply that both includes a feedback input and is responsive to the input by changing the output voltage. The electronics associated with an adjustable power supply can be more expensive that those found is a simple linear transformer power supply.
There is thus a need for an improved means of regulating temperature in a power-dissipating element like those employed as pass elements in battery charging applications.


REFERENCES:
patent: 4885522 (1989-12-01), Konopka
patent: 5744939 (1998-04-01), Leppo
patent: 6441594 (2002-08-01), Connell et al.
patent: 6492792 (2002-12-01), Johnson, Jr. et al.
patent: 6559623 (2003-05-01), Pardoen

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