The present invention relates to a power supply and particularly,
but not exclusively, to a rechargeable power supply for powering electronic equipment.
Rechargeable power supplies generally consist of a number
of rechargeable batteries or cells arranged in series, in parallel or in a series/parallel
combination. It is common to provide Zener diodes connected in parallel across each
cell to prevent the recharging voltage supplied to the cell from exceeding a predetermined
level, which could result in damage to the cell, and to allow the power supply to
continue to supply power to any connected equipment in the event of one or more
of the cells failing in an open circuit condition.
In such an arrangement, the Zener diodes act as voltage
regulators whereby if the voltage applied to the cell exceeds a certain level, break
down of the Zener diode will occur causing current to leak through the Zener diode
in the reverse direction. As the applied voltage increases the-current leaking through
the Zener diode also increases. This has the effect of clamping the voltage over
the cell to a predetermined level.
To ensure that the cell is not subjected to a voltage higher
than its maxi-mum rating, the Zener diode used must be capable of leaking a reverse
current which is at least equal to the charging current of the power supply at a
voltage which is less than or equal to the cell's maximum voltage rating. However,
Zener diodes generally do not have a stepped on-off break down characteristic and
thus permit leakage current through the diode, to greater or lesser extent, over
a range of voltages. Thus, a Zener diode having a 2.5 mA leakage current at a breakover
voltage of 3.3 V may still permit 50% of that current to pass through at an applied
reverse-biased voltage of 3.2 volts. Consequently, once the power supply is fully
charged and the charging current removed, the leakage current through the Zener
diodes will continue until the voltage over the cell has been reduced to a point
where the Zener diode has negligible leakage current. During this time, the leakage
current through the Zener diode will cause discharging of the cell.
Since rechargeable cells are effective for only a limited
number of charge/discharge cycles, this number usually being a function of the percentage
depth of each cycle, the discharge effect of the Zener diode on the cell means that
the potential life of the cell, and therefore power supply as a whole, is significantly
reduced, both in terms of each charge/discharge cycle and the overall life of the
power supply. This is a particular problem in low power applications (for example
smoke detectors/alarms), where the leakage current through the Zener diodes is often
a high multiple of the supply current required to power the equipment. This has
the effect of reducing the life of the rechargeable power supply to a fraction of
its potential life.
The prior art document
discloses a protection circuit for a rechargeable cell, which comprises
a voltage detector for detecting the voltage on the supply line, and for closing
a switch when the voltage rises to a predetermined threshold or above.
However, when the battery is fully charged, and the charging voltage is removed,
the voltage detector still senses the battery voltage and the switch is maintained
in the closed state until the battery voltage drops below the threshold, regardless
of the presence or absence of an applied charging voltage. Hence, the switch remains
connected even when the power rail is de-energised. Where reverse leakage occurs
from the cell, the recharge frequency is inevitably increased significantly and
hence, the lifetime of the cell is reduced.
The present invention seeks to provide an improved power
supply having reduced reverse leakage current from a fully charged cell.
Accordingly, the present invention provides a rechargeable
power supply comprising:
- a rechargeable cell connectable to a power source;
- voltage protection means connected to said cell; and
- disconnecting means for selectively disconnecting said voltage protection means
from said cell thereby to reduce any discharge of said cell through said voltage
protection means, wherein said disconnecting means is operable for disconnection
of said voltage protection means from said cell, whenever said power source is de-energised.
The present invention will now be described, by way of
example only, with reference to the accompanying drawings in which:
- Figure 1 is an electrical circuit diagram of a first form of power supply according
to the invention;
- Figure 2 is an electrical circuit diagram of a second form of power supply according
to the invention;
- Figure 3 is an electrical circuit diagram of a third form of power supply according
to the invention; and
- Figure 4 is an electrical circuit diagram of a fourth form of power supply according
to the invention.
Referring to Figure 1, a preferred-form of power supply
according to the invention is shown generally at 10. The power supply 10 comprises
a single battery or cell 12, the positive terminal of the cell 12 being connected
to a power source or charging current source in the form of a power rail 14 via
a diode 15 and a parallel combination of a resistor 16 and a Schottky diode 18.
The negative terminal of the cell 12 is connected to the earth or zero volt rail
17 of the charging current source.
The cell 12 has a series combination of voltage protection
means, in the form of a Zener diode 20, and disconnecting means, in the form of
a field effect transistor (FET) 22 connected in parallel across the cell 12. The
Zener diode 20 is connected across the cell 12 in a reverse biased direction, i.e.
with its cathode connected to the positive terminal of the cell 12. The anode of
the Zener diode 20 is connected to the drain electrode of the FET 22 whilst the
source electrode of the FET 22 is connected to the negative terminal of the cell
It will be appreciated that any electrically equivalent
connection of the Zener diode and FET, for example with the anode of the Zener diode
being connected to the negative terminal of the cell 12, the cathode of the Zener
diode being connected to the source of the FET and the drain of the FET being connected
to the positive terminal of the cell, will function equally well.
The gate electrode of the FET 22 is connected directly
to the power rail 14 of the charging current source.
In operation, when a voltage is applied to the power rail
14 of the charging current source, thereby to charge the cell, current flows through
the diode 15 and resistor 16 and through the cell 12 to earth. The purpose of the
resistor 16 is to limit the current through the cell to a value appropriate for
charging the cell. At the same time, the voltage on the power rail 14 is applied
to the gate electrode of the FET 22 which is thereby switched on. This connects
the Zener diode 20 across the cell 12.
As the cell 12 is charged, the voltage across the cell
rises. If the voltage across the cell 12, and thus across the Zener diode 20, rises
above the breakover voltage of the Zener diode 20, the Zener diode 20 will break
down and begin to conduct leakage current therethrough. The flow of leakage current
through the Zener diode 20 prevents the voltage across the cell 12 from rising further
and thus the voltage is effectively clamped at or around the breakover voltage of
the Zener diode. It is normal, therefore, for the breakover voltage of the Zener
diode to be chosen to correspond substantially to the maximum voltage rating of
the cell. It will be appreciated, therefore, that the presence of the Zener diode
prevents overcharging of the cell 12.
When the cell is sufficiently charged and the voltage on
the power rail 14 from the charging current source is switched off, current flows
from the charged cell 12 through the Schottky diode 18 to the power rail 14 and
out to any connected electronic equipment or circuit. Current is prevented from
returning to the charging current source by the diode 15. The Schottky diode 18
is included to provide a low impedance path between the cell 12 and the connected
equipment or circuit. However, it is not essential to the invention and its inclusion
is entirely optional.
As described above, however, with the Zener diode 20 connected
across the cell 12, leakage current will continue to flow through the Zener diode
20 thus gradually discharging the cell 12, possibly at a rate greater than that
caused by the electronic equipment itself. To prevent this, therefore, when the
voltage on the power rail 14 from the charge current source is switched off, the
voltage applied to the gate electrode of the FET 22 is reduced to zero such that
the FET 22 is switched off and presents an open circuit. The Zener diode 20 is therefore
effectively disconnected from the cell 12 and thus no leakage current can flow therethrough.
All current from the cell 12 is thus applied to the electronic equipment or circuit
and any unwanted discharging to the Zener diode 20 is effectively eliminated.
Figure 2 illustrates a similar arrangement but where the
power supply comprises three, series-connected cells 12 , hereafter termed collectively
as the "battery". In this embodiment, each cell 12 in the battery has a Zener diode
20 connected in parallel with it and a FET 22 connected in series with the Zener
diode. It is clear from the drawings that the arrangement within the dashed box
10 in Figure 1 is repeated for each cell in the battery of Figure 2.
In this embodiment, when the voltage across any one of
the cells 12 in the battery exceeds the breakover voltage of the cell's respective
Zener diode 20, leakage current will be passed through the Zener diode and prevent
the voltage on the cell from rising further.
In Figure 3, the power supply is shown having a different
arrangement of cells 12. In this case, the battery is comprised of 4 cells 12 in
a two by two, series-parallel arrangement. Each pair of parallel connected cells
12 is provided with a respective Zener diode 20 and FET 22 connected in parallel
over the cell pair.
The mode of operation of the embodiment of Figure 3 is
similar to that of Figures 1 and 2 in that if the voltage over any one of the cells
12 rises above the breakover voltage of the respective Zener diode 20, then leakage
current through the Zener diode will prevent that voltage from rising any further.
In Figure 4, a stepped arrangement of voltage protection
is provided. In this embodiment, the battery of the power supply comprises three
cells 12a, 12b, 12c connected in series. The series combination of cells 12a and
12b has a first Zener diode 20(i) and FET 22(i) connected in parallel thereover.
The series combination of cells 12b and 12c has a second Zener diode 20(ii) and
FET 22(ii) connected in parallel thereover while the series combination of all three
cells 12a, 12b, 12c have a third Zener diode 20(iii) and FET 22(iii) connected in
As will be clearly understood by those skilled in the art,
if the voltage over the series combination of cells 12a and 12b exceeds the breakover
voltage of Zener diode 20(i), then leakage current through Zener diode 20(i) will
prevent the voltage over the cells from rising any further. Similarly, if the voltage
over cells 12b and 12c exceeds the breakover voltage of Zener diode 20(ii) then
leakage current through Zener diode 20(ii) will prevent the voltage over cells 12b
and 12c from increasing any further. Finally, if the combined voltage over all three
cells exceeds the breakover voltage of Zener diode 20(iii) then this diode will
break down and leakage current through Zener diode 20(iii) will prevent the voltage
over cells 12a, 12b and 12c from increasing any further.
In all illustrated embodiments, the voltage protection
provided by the Zener diodes 20 is only effective when the respective FET 22 connected
to the Zener diode is switched on thereby to connect the Zener diode across the
respective cell or cells. Each FET is switched on only when the power rail 14 of
the charge current source is energised, for example when the cells 12 are being
charged. When the power rail 14 is de-energised, each FET 22 will switch off thus
disconnecting the respective Zener diode from the respective cell or cells 12, thus
preventing any leakage current from passing through the Zener diode and thus avoiding
any unwanted discharging of the cell or cells.
It will be appreciated that the present invention allows
the safe charging of one or more rechargeable cells and avoids the disadvantageous
effects of both overcharging of one or more of the cells and excessive discharging
of the cells during normal operation.
It will also be appreciated, however, that a number of
modifications may be made to the invention as desired. For example, the Zener diode
20 may be replaced by resistive devices or any other devices which permit a selected
level of leakage current to pass therethrough. In addition, the FET's 22 could be
replaced by any other form of electronic switches or by any other device which upon
energising of the power rail 14, causes the voltage protection means to be connected
across one or more of the cells. It is even envisaged that a mechanical arrangement,
such as a manually operated switch, may be employed to selectively disconnect the
Zener diodes or other voltage protection means from the cell or cells.
It will be clear that the invention is not limited to any
particular kind of rechargeable cell which may be, for example, nickel-cadmium (Ni-Cd)
"NICAD" cells, nickel metal hydride (NiMH) cells or any other chemical rechargeable
cells or even solid cells (e.g. lithium polymer cells), capacitors or any other
type of rechargeable device capable of storing and delivering electrical energy.
It is also possible for the invention easily to be configured
for mechanical applications such as fuel cells for gas, liquid or other fuel or
pressure containers. In this case the voltage protection means may be replaced by
pressure release valves or other such devices.
In order to allow the power supply to operate in the event
of an open circuit failure of one or more of the cells, conventional diodes may
be permanently connected in parallel across each cell or cell pair to ensure a continuous
path around the failed cell or cells. Having negligible reverse leakage current,
these diodes will not discharge the power supply in any way. A possible arrangement
for such diodes 23 is shown in Figure 3 whilst in Figure 4, each FET 22(i, ii, iii)
is provided with an integral diode 24.