Note: Descriptions are shown in the official language in which they were submitted.
CA 02251448 1998-10-22
TITLE: BACKUP POWER CIRCUIT
FIELD OF THE INVENTION
The invention relates to a backup power circuit
providing temporary power to circuits in an electronic
communications terminal device when the main power signal is
lost.
BACKGROUND OF THE INVENTION
Terminal devices are located at remote sites and
exchange data with a central computer system. Common examples
of such devices include Point-of-Sale (POS) terminals,
electronic fund transfer (EFT) devices, electronic banking
machines and digital communications equipment (DCEs).
Terminal devices are generally powered by electricity
provided at the remote site generated by a power adapter. If
power is interrupted while the device is performing a critical
task, such as modifying data or transferring data to other
equipment, the validity and reliability of the task is
compromised.
As such, terminal devices typically have built-in backup
power circuits providing temporary reserve power to the
essential circuits in the device when main power is lost. The
temporary power enables the devices to complete at least the
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critical processes before all power is lost. Conventional
methods for supplying temporary power use circuits having large
voltage supplies and capacitance devices located at the power
input side of the terminal device. These circuits require
expensive voltage supplies and capacitors.
SUMMARY OF THE INVENTION
The present invention provides a backup power circuit for a
terminal device, such as a POS terminal, EFT or DCE. Generally,
the terminal device has at least a main functional circuit
operating at a first voltage, a second functional circuit
operating at a second higher voltage and a power circuit which
provides at least the two voltages. The main functional circuit
contains the main control electronics for the terminal. The
second functional circuit provides the display electronics for
the terminal. The power circuit transforms an input voltage
from an input power source to provide the two voltages for the
functional circuits.
The backup power circuit is connected to the second
voltage and has an energy storage device to store energy from
the second voltage and a discharge circuit connected to the
power circuit. The backup power circuit stores energy from the
second voltage when the input voltage source is producing enough
voltage for the main functional circuit to operate; the backup
power circuit discharges the stored energy to the power circuit
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through the discharge circuit when the main functional circuit
is not being provided with sufficient voltage to operate.
It is an aspect of the invention to provide a backup power
circuit supplying temporary power to a terminal device. It is a
further aspect of the invention to detect when the main power to
the device is lost and then to provide a backup power
automatically.
It is a further aspect of the invention to provide a backup
power in an economical manner.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are shown in the
drawings, wherein:
FIG. 1 is a circuit diagram showing a power supply
circuit of a terminal device with the backup power circuit
feeding into the power circuit of the device;
FIG. 2 is a circuit diagram showing a power supply
circuit of a terminal device with a backup power circuit feeding
into the main electronics circuit of the device;
FIG. 3 shows a voltage vs. time graph of a terminal
device with a backup power circuit operating in a loss-of-power
situation; and
FIG. 4 shows a voltage vs. time graph of a terminal
device with a backup power circuit operating in a "brown-out"
situation.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure 1, a feedback electrical circuit for a
terminal device embodying the invention is shown. Power circuit
1 operates in the following manner. A DC power adapter
(typically 9 Volts DC at 650 mA) provides an input DC voltage at
node 7. Transformer 2 receives the input voltage at node 3.
Two output voltage taps are provided; the first produces 5
volts at node 19 and the second produces 40 volts at node 5.
The voltage at node 19 is conditioned by LC circuit 15 to
produce the 5 volt supply at node 4. The voltage is regulated
by the feedback path comprising connection 28, diode 26 and
transistor 27, which control the switching frequency of
transistor 29. The 5 volt supply provides power for the main
functional circuit 16 of the terminal device, including
microprocessor 17; the 40 volt supply provides the power for the
second functional circuit of the terminal device, including
display electronics 18. AC voltage tap 14 provides the display
filament in display electronics 18 with power. Backup power
circuit 6 provides the backup voltage for the circuit, consuming
less than 10 watts of power.
It can be appreciated that the terminal device and the
circuits therein can be designed to operate at other voltages.
Typically, the input voltage is between 9 and 12 volts at 500 to
1000 mA; the typical voltage required for the electronics
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connected to node 4 is between 3 to 5 volts; the voltage
required for the driver of the display is between 30 and 40
volts, and the voltage for the display filament is 4.5 volts
rms.
In the circuit of Figure 1, backup power circuit 6 takes
power from the 40 volt supply at node 5. By taking power from
the highest available voltage in the circuit, the storage power
of the backup power circuit is maximized. When backup power
circuit 6 is activated, the backup output voltage is fed back to
node 3, the input of transformer 2.
The main components of backup power circuit 6 are capacitor
8, diode 9 and transistor 10. Capacitor 8 stores energy from
the 40 volt power supply and provides the backup energy for
electrical circuit 1; in this embodiment, it has a nominal
capacitance of 4400 ~F. Transistor 10 acts as a current source
that provides the discharge of energy stored in the capacitor.
Diode 9 provides the voltage regulation of the back up power at
the emitter of transistor 10. It can be appreciated that
different component value can be utilized for capacitor 8, diode
9 and transistor 10 to effect different voltage, transient
response, timing or storage characteristics of the backup power.
It can further be appreciated that the output from backup
circuit 6 may be fed to an intermediary circuit which is, in
turn, connected to node 3.
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Backup circuit 6 operates in two modes: a charging mode
and a discharging mode. In the charging mode, circuit 1 is
operating under sufficient power supply conditions. Diode 11
rectifies the voltage received from node 5 and prevents any
reverse flow of energy through it. Resistor 12 limits the
current flowing to the backup circuit. Capacitor 8 stores the
voltage passed through resistor 12. The discharge path for the
energy stored in capacitor 8 is connected to the input tap of
transformer 2 at node 3. However, in order to capacitor 8 to
maintain its charge, it is necessary to break the discharge path
circuit. As such, transistor 10 cannot be conducting. In this
operating mode, there are 5 volts at node 4 and 40 volts at node
5, approximately 7.8 volts at node 3 and 7.2 volts at the
cathode of diode 9. As such, the base-emitter junction of
transistor 10 is reversed biased.
When there is a power failure, the voltage at node 3 drops
from the originally supplied 9 volts. Output power at node 3
can disappear within 1 to 3 ms of the loss of the input power
source. The backup circuit detects the loss of power and
switches to discharging mode.
In discharging mode, the discharge circuit path connected
to capacitor 8 must be completed. This occurs when the base-
emitter junction of transistor 10 becomes forward biased. In
circuit 10, the voltage at node 3 must drop to approximately 6.6
volts to forward bias transistor 10. Afterwards, capacitor 8
discharges through transistor 10 to node 3. In turn, the
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voltage produced by backup circuit 6 provides output voltage to
nodes 4 and 5, which, in turn, power main electronics 16 and
display electronics 18.
As capacitor 8 discharges, its stored voltage drops from
its initial 40 volt charge. The voltage is conditioned by zener
diode 9 and transistor 10 to provide node 3 with a regulated
voltage of 6.6 volts. The capacitance of capacitor 8 has been
selected to provide approximately 200 ms of backup power. It
can be appreciated that modifications may be made to backup
circuit 6 to vary the amount of backup power provided. If
shorter backup times are sufficient, then smaller and less
expensive capacitors can be used. If longer backup times are
required, larger capacitors may be used.
As the amount of backup power is limited, when a failure of
the main power supply occurs, main electronics 17 must recognize
the power failure and perform emergency power-loss shutdown
routines within at least the 200 ms backup power window. Such
routines may include completing pending transactions and
communications and saving system state to non-volatile memory or
reset the system.
Reset circuit 13 detects when the voltage stored in
capacitor 8 to falls below a predetermined value. Afterwards,
after a preset time window, a reset signal is activated, to
reset microprocessor 17. In this circuit, the reset signal is
generated 250 ms after the initial loss of power.
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When power is restored, the 9 volt power supply once again
produces sufficient voltage to provide the required voltages at
nodes 4 and 5. As the voltage rises above 6.6 volts at node 3,
transistor 10 will become reversed biased, thereby breaking the
discharge path circuit between backup circuit 6 and node 3. At
that time, the voltage at node 5 can recharge capacitor 8.
Figure 3 shows a voltage-time graph of voltages at selected
nodes in the circuit in Figure 1 when main power is lost. Graph
20 represents the voltage supplied by a DC power supply. Graph
21 represents the voltage stored in capacitor 8. Graph 22
represents the voltage at node 4; graph 23 represents the reset
signal. At time 24, the circuit has been operating for at least
50 ms; Graph 22 shows 5 volts; graph 21 shows approximately 36
volts. In this example, immediately before time 24, power from
the voltage supply is lost. Around time 24, graph 20 shows a
decline in the voltage provided by the voltage supply. The
decline in the voltage activates backup circuit 6, thereby
causing capacitor 8 to provide backup power to the circuit.
Graph 21 shows that after time 24, the voltage in capacitor 8
decays. Note that the power supplied by backup circuit 6 allows
transformer to maintain 5 volts at node 4 for more than 200 ms,
indicated by the relatively stable signal shown in graph 22.
Referring to graph 25, 250 ms after the loss of power from the
voltage supply, the reset signal is activated.
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Figure 4 shows another voltage-time graph of voltages at
nodes in the circuit shown in Figure 1 in a power "brown-out"
condition. The nodes represented in Figure 3 are also
represented in Figure 4; reference numbers in Figures 4 share
those with Figure 3, but with an "a" suffix. In Figure 4 at
time 24a, the circuit has been operating for at least 50 ms;
Graph 22a shows 5 volts on node 4; graph 21a shows capacitor 8
charged to approximately 36 volts. Just before time 24a, power
from the voltage supply is lost.
Here, a "brown-out" lasts for 143 ms. Full power is lost
just before point 24a and power is restored at point 25a.
During the "brown-out", energy stored in capacitor 8 provides
backup power to the circuit. At point 21a, voltage at capacitor
8 is 36 volts. This decays to 22 volts by point 25a. V~hen
input power is restored at point 25a, capacitor 8 stops
discharging and recharges to 36 volts. Throughout the entire
timespan, graph 22a shows that node 4 always provides 5 volts to
the electronics circuits. Graph 23a indicates that the system
reset signal is not initiated.
Figure 2 shows another embodiment of the invention wherein
the backup circuit provides a fed forward voltage connected
directly to the main electronics power rail. Where appropriate,
reference numbers for similar components found in Figure 1 and
Figure 2 have been repeated. Such references in Figure 2 have
an additional suffix "a" attached.
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In Figure 2, power circuit 1a operates in the following
manner. A DC power adapter provides a DC voltage at node 7a.
Transformer 2a provides a 5 volt supply at node 4a and a 40 volt
supply at node 5a. The 5 volt supply provides power for main
electronics 16a, including microprocessor 17a; the 40 volt
supply provides the power for the display electronics 18a
associated with the terminal device. Backup power circuit 6a
provides the backup voltage for the circuit.
Backup power circuit 6a takes power from the 40 volt rail
at node 5a. V~hen backup power circuit 6a is activated, the
output voltage is fed forward to node 4a.
The main components of backup power circuit 6a are
capacitor 8a, diode 9a and transistor 10a. Capacitor 8a stores
energy from node 5a to provide backup energy for electrical
circuit 1a; in this embodiment, it has a nominal capacitance of
2200 ~.F. As the feed forward circuit is supplying power
directly to node 4a, the size of capacitor 8a is smaller than
capacitor 8 in Figure 1. Transistor 10a acts as a switch
controlling the discharge of voltage from capacitor 8a. Zener
diode 9a provides voltage regulation while transistor 10a acts
as a current source for backup circuit 6a. Again, different
component value can be utilized for capacitor 8a, diode 9a and
transistor 10 to effect different response characteristics. It
can further be appreciated that the output from backup circuit
6a may be fed to an intermediary circuit which is, in turn,
connected to node 4a.
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Again, backup circuit 6a operates in two modes: a charging
mode and a discharging mode. In the charging mode, the voltage
from node 5a charges capacitor 8a. Diode 11a rectifies the
voltage of node 5a and blocks discharge of any energy stored in
capacitor 8a to node 5a. Resistor 12a limits the current
flowing to the capacitor 8a.
In the charging mode, it is necessary to break the
discharge path circuit for the charge stored in capacitor 8a.
As such, transistor 10a cannot be conducting. In this example,
zener diode 9a clamps the voltage at the base of transistor 10a
to 6.0 volts. As long as the voltage supply is operating to
produce 5 volts at node 4a, the voltage at the emitter of
transistor l0a would be approximately 5.6 volts, due to diode
30. In this situation, the emitter base junction of transistor
10a remains reverse biased.
In discharging mode, the discharge path from capacitor 8a
must be completed. After main power has been lost, the voltage
at node 4a falls from 5 volts. Consequently, the voltage at the
emitter of transistor 10a begins to fall. However, the voltage
at the base of transistor 10a remains at 6 volts because of
zener 9a. Transistor 10a becomes forward biased when the
voltage at the emitter of transistor 10a falls below
approximately 5.4 volts. Transistor 31 and double diodes at 32
provide a voltage drop of 0.5 volts across resistor 33. This
regulated voltage drop provides a constant current source of
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approximately 2 mA to diode 9a. This provides improved voltage
regulation during the discharge of the energy stored in
capacitor 8. Thereafter, a discharge path is established from
capacitor 8a to node 4a. As such, capacitor 8a can then provide
a voltage to node 4a.
As capacitor 8a discharges, its stored voltage drops from
its initial 40 volt charge. The voltage is conditioned by zener
diode transistor 10a and diode 30 providing node 4a with a
regulated voltage of 4.8 volts. The capacitance of capacitor 8a
has been selected to provide approximately 200 ms of backup
power. It can be appreciated that modifications may be made to
backup circuit 6a to vary the amount of backup power provided.
When power is restored, the 9 volt power supply once again
produces sufficient voltage to provide the required voltages at
nodes 4a and 5a. At that point, transistor 10a becomes reverse
biased, thereby breaking the discharge path circuit. At that
time, capacitor 8a can recharge again.
Reset circuit 13a operates in a similar manner to reset
circuit 13.
Although various preferred embodiments of the present
invention have been described herein in detail, it can be
appreciated that the present invention is not restricted to what
is described above and shown in the drawings, but can be changed
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or modified in many different way within the scope of the
invention defined in the attached claims.
