Electrical transmission or interconnection systems – Plural supply circuits or sources – Substitute or emergency source
Reexamination Certificate
1999-05-10
2001-09-04
Fleming, Fritz (Department: 2836)
Electrical transmission or interconnection systems
Plural supply circuits or sources
Substitute or emergency source
C307S066000
Reexamination Certificate
active
06285091
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to voltage source switching for electronic devices and, more particularly, to an improved voltage source switching circuit for use in electronic devices.
2. Description of the Related Art
Electronic devices, such as portable computing devices, have long been powered by alternative power sources. Typically, an electronic device can receive power from a main power source and an auxiliary power source. By way of example, in the case of a portable computing device, the main power source is AC power supplied by an AC outlet, and the auxiliary power source is DC power supplied by a battery. The auxiliary power source, e.g., the battery, is often employed as an alternative an/or backup power supply when the main power source is unavailable. The auxiliary power source, e.g., the battery, may also be used when the electronic device is turned off to maintain certain circuits such as memory cells that require small amount of power to maintain their data.
Conventionally, switching circuits have been used to facilitate switching between different power sources.
FIG. 1
depicts a conventional line switching circuit
102
for use in a portable computing device. The conventional line switching circuit
102
facilitates switching between a main power source
104
and a battery
108
. The main power source
104
is a primary power source and the battery
108
is an auxiliary power source. The main power source
104
can be connected to an AC power line
106
, typically via an AC power outlet. The battery
108
functions as a backup or alternative source of power. A charge circuit
114
is often provided to charge the battery
108
using power from the AC power source
104
.
The conventional line switching circuit
102
operates to couple either the main power source
104
or the battery
108
to an output power line
110
. The output power line
110
is often used to provide power internal to the electronic device or to a peripheral circuit board
112
. When the AC power line
106
is active and the electronic device is operating, it is desirable to supply power to the output power line
110
from the main power source
104
. In other situations such as unavailability of AC power from the AC Power line
106
or when the electronic device is powered off, it may be desirable to supply power to the output power line
110
from the battery
108
.
In such situations, to facilitate the switching between the main power supply
104
and the battery
108
, the conventional line switching circuit
102
uses two diodes
114
and
116
. The diode
114
is connected between the battery
108
and the output power line
110
. The diode
116
is connected between the main power source
104
and the output power line
110
. The switching provided by the diodes
114
and
116
is such that the power supply coupled to the output power line is either the main power supply
104
or the battery
108
depending on whichever offers the greater voltage level. For example, when the voltage at the main power source
104
is available and has a value of about 5.2 Volts and the battery
108
has a voltage of about 5 Volts, the conventional line switching circuit
102
would couple the main power source
104
to the output power line
110
because the main power source
104
has the higher voltage value. To elaborate, in this situation, with diodes
114
and
116
having identical characteristics, diode not switch-on since it would be biased below its turn-on voltage. In another situation, when the main power source
104
is not available, the voltage level for main power source
104
is dropped to about zero (0) Volts (or for some reason drops below the voltage of the battery
108
), the diode
114
would switch-on and couple the battery
108
to the output power line
110
(and the diode
116
would be switched off).
One major disadvantage with the conventional line switching circuit
102
is that there is a significant voltage drop across the diodes
114
and
116
. That is, voltage level for output power line
110
is significantly lower than the voltage level at the selected power source (i.e., either main power supply
104
or battery
108
) due to the voltage drops across the associated diodes
114
and
116
. Typically, the voltage drop is about 0.6 Volts or higher when silicon diodes are used. By way of example, if the main power supply
104
has a voltage level about 5.0 Volts, then the voltage on the output power line
110
would be about 4.4 Volts or lower when the main power supply
104
is utilized. Similarly, when the battery
108
is utilized, the battery
108
has a voltage level about 5.0 Volts because of the voltage drop for the diode
114
the output power line
110
would be about 4.4 Volts or less. To reduce voltage drops across the diodes
114
and
116
, schottky diodes can be used. Schottky diodes have a lower voltage drop (about 0.4 volts) than silicon diodes (about 0.6 volts), however, schottky diodes are more expensive components than silicon diodes. It should also be noted that germanium diodes may offer a lower voltage drop than schottky diodes (about 0.2 Volts), however, they have higher reverse current leakage which makes them undesirable for power switching applications.
The effect of the voltage drop across the diodes
114
and
116
is to reduce the voltage appearing on the output power line
110
. Typically, the output power line
1
10
is used to supply power to the peripheral circuit board
112
. However, the peripheral circuit board
112
is manufactured with certain tolerances and the reduced voltage supplied to the peripheral circuit board
112
as a result of the voltage drops can lead to breach of tolerances and thus unstable conditions. While use of schottky diodes are of some hell), even using schottky diodes to implement a convention line switching circuit cannot satisfactorily address the problems associated with the undesired voltage drops, which are becoming more severe as operating voltages drop (e.g., from 5 Volts to 3 Volts) primarily for power conservation reasons. For example, if the tolerance level for the peripheral circuit board
112
is 10% and the main power supply
104
is on the order of 5 Volts, the peripheral circuit board
112
can function properly as long as the voltages provided to the peripheral circuit board
112
do not fluctuate more than 10%. (i.e., accepted range would be 4.5 to 5.5 volts). In this situation, even if the tolerance level for the main power supply
104
itself is 5% (i.e., power supply
104
is guaranteed to be output voltages between 4.75 volts to 5.25 Volts), there can only be a voltage drop up to 0.25 Volts across diodes
114
and
116
in order to remain within tolerance (e.g., 4.75−4.50=0.25). Recall, even using schottky diodes, the conventional line switching circuit
102
could yield a voltage drop of about 0.4 Volts. Therefore, the voltage levels provided to the peripheral circuit board
112
could fall below accepted levels.
To address the problem of voltage drop levels associated with diodes, a switching regulator could be utilized to boost voltages output from the power supplies to compensate for the voltage drop due to the diodes. However, using a switching regulator, especially for boosting DC voltages is an inefficient, costly, and complicated solution.
In the view of the forgoing, there is a need for the voltage source switching circuit capable of outputting voltages at a reduced voltage drop level.
SUMMARY OF THE INVENTION
Broadly speaking, the invention relates to a voltage source switching circuit capable of switching between different voltage sources with reduced voltage drop levels. A selected one of the different voltage sources is supplied to a peripheral circuit or internal circuitry.
The invention can be implemented in numerous ways, including as a device, an apparatus and a method. Several embodiments of the invention are discussed below.
As a voltage source switch for outputting an output voltage corresponding to o
Chan Anthony David
Russell Anthony Gerard
Beyer Weaver & Thomas LLP
California Micro Devices Corporation
Fleming Fritz
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