Electric power conversion systems – Current conversion – With voltage multiplication means
Reexamination Certificate
2000-03-01
2001-11-13
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
With voltage multiplication means
C307S109000
Reexamination Certificate
active
06317343
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power supply system comprising capacitors that are switchably connected to decrease variations of an output voltage.
2. Description of the Related Art
An electric double-layer capacitor is physically charged and, hence, can be charged more quickly than chemical batteries, such as lead-acid batteries and nickel/cadmium batteries. However, a power supply system using electric double-layer capacitors produces a terminal voltage V that varies greatly given by equation Q=CV
2
/2. An electrical energy storage system making use of the energy capacitor system (ECS) using electric double-layer capacitors has attracted attention as a power supply for an electric vehicle or as a large-scale electrical energy storage system.
The ECS has been introduced in various literature (e.g.,
Electronic Technology of Japan,
1994, 12, pp. 1-3; T.EEE Japan, in Japanese, B, Vol. 115, No. 5, 1995, pp. 504-610) as an electric energy storage system consisting of capacitors, parallel monitors, and current pumps. These capacitors are connected in series to form a capacitor bank. The parallel monitors are connected across the respective capacitors of the bank. When the capacitors are charged to more than a value set for the parallel monitors, these monitors bypass the charging current or discharge the capacitors until the terminal voltage reaches the set voltage. In this way, the state of charge of the capacitors is controlled. The parallel monitors may also be connected across the capacitor banks.
These parallel monitors act to make uniform the maximum voltages, prevent reverse currents, detect the terminal voltages on completion of charging, and perform controlled operation even if the capacitors have nonuniform characteristics and varying amounts of residual charge. Therefore, the parallel monitors are quite important in making almost full use of the capability of the capacitor bank to store electrical energy.
On the other hand, in a power supply system using capacitors whose terminal voltages vary widely at a high rate as energy is drawn from the fully charged state, it is required to suppress variations in the output voltage due to variations in the terminal voltages of the capacitors.
Therefore, a power supply system for switching capacitors between series connection and parallel connection to reduce variations in the voltage has been proposed as disclosed in U.S. Pat. No. 5,734,205. FIGS.
11
(A),
11
(B), and
11
(C) show examples of the structure of a power supply system in which capacitors are switched between series connection and parallel connection. In this configuration, as the terminal voltage drops, the capacitors are switched from parallel connections to series connections.
A series-parallel switching circuit for capacitors C
1
and C
2
of this power supply system is shown in FIG.
11
(A). The state is switched to a state shown in FIG.
11
(B), where more stages are cascaded. In this way, the state is varied in a stepwise fashion according to the state of charge. Consequently, variations in voltage can be reduced further with increasing the number of stages.
Where the voltage variations are reduced by the method described above, a large number of stages are switched from parallel to series connection. As the number of stages increases, more switches are necessary. That is, as can be seen from FIG.
11
(A), three switches Sp
1
, Sp
2
, and Ss
1
are used in one stage and so three switches are required for each stage.
Furthermore, these switches are used in power applications. Hence, use of large-sized electromagnetic contactors or power semiconductor devices, such as giant transistors, IGBTS, GTOs, and thyristors, is necessary. Therefore, the number of components including driver circuits for the switches and radiators is increased. Also, a large space is necessary to mount them. As a result, the cost of the system is increased. Furthermore, the reliability of the switches poses problems.
In addition, when the connection is switched from parallel to series, if the voltages across the capacitors C
1
and C
2
are not uniform, a large crosscurrent flows between the capacitors C
1
and C
2
. To prevent this crosscurrent, protective circuits A
1
, A
2
, and corresponding switching elements Q
1
-Q
3
are necessary, as shown in FIG.
11
(C).
SUMMARY OF THE INVENTION
The present invention is intended to solve the foregoing problems.
It is an object of the present invention to provide a power supply system comprising capacitors that can be connected switchably via a small number of switches to reduce variations of an output voltage, the system being further characterized in that the switches can be controlled easily to reduce switching loss.
This object is achieved by a power supply system comprising capacitors that are connected switchably via a small number of switches to decrease variations of an output voltage. The power supply system has a first set of capacitors connected in series at their series junctions and a second set of capacitors connected in series at their series junctions. The capacitors of the first set are equal in number with the capacitors of the second set. A series connection switching means is connected with first terminals of the capacitors of these two sets such that the capacitors of the first set are connected in series with the capacitors of the second set. Input/output terminals are connected with second terminals of the capacitors of the two sets which are not connected with the series connection switching means. The first set of capacitors is connected with the series connection switching means at a first series junction. A first set of switching means connects the first series junction with the second terminals of the capacitors of the second set and with the series junctions of the capacitors of the second set. The second set of capacitors is connected with the series connection switching means at a second series junction. A second set of switching means connects the second series junction with the second terminals of the capacitors of the first set and with the series junctions of the capacitors of the first set. A control means controls the first set of switching means, the second set of switching means, and the series connection means to switch connection of the capacitors of the first and second sets among a first state in which the first set of capacitors is connected in series with the second set of capacitors, a second state in which the first set of capacitors is connected in parallel with the second set of capacitors, and a third state in which one or more capacitors of the first set are connected in parallel with one or more capacitors of the second set.
The present invention also provides a power supply system comprising capacitors that can be connected switchably to decrease variations of an output voltage, the system further including a capacitor power supply, a voltage compensation circuit connected in series with the capacitor power supply for making voltage compensation, and a control circuit. The capacitor power supply switches the connection of the capacitors among plural states according to the state of charge. The control circuit controls a compensation voltage at which the voltage compensation circuit provides voltage compensation according to the voltage developed across the capacitor power supply.
Other objects and features of the invention will appear in the course of the description thereof, which follows.
REFERENCES:
patent: 4667280 (1987-05-01), Takamura et al.
patent: 5734205 (1998-03-01), Okamura et al.
patent: 5886887 (1999-03-01), Jenq
Mogami Akinori
Okamura Michio
Yamagishi Masaaki
Berhane Adolf Deneke
Jeol Ltd.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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