Multi-mode power converters incorporating balancer circuits...

Details Multi-mode power converters incorporating balancer circuits... Multi-mode power converters incorporating balancer circuits...

2001-01-08

2001-11-06

Vu, Bao Q. (Department: 2838)

Electric power conversion systems

Current conversion

Including an a.c.-d.c.-a.c. converter

C363S058000, C363S132000, C307S066000

Reexamination Certificate

active

06314007

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to electrical power devices and methods of operation thereof, and more particularly, to power conversion devices and methods of operation thereof.
BACKGROUND OF THE INVENTION
Uninterruptible power supplies (UPSs) are power conversion devices that are commonly used to provide conditioned, reliable power for computer networks, telecommunications networks, medical equipment and the like. UPSs are widely used with computers and similar computing devices, including but not limited to personal computers, workstations, mini computers, network servers, disk arrays and mainframe computers, to insure that valuable data is not lost and that the device can continue to operate notwithstanding temporary loss of an AC utility source. UPSs typically provide power to such electronic equipment from a secondary source, such as a battery, in the event that a primary alternating current (AC) utility source drops out (blackout) or fails to provide a proper voltage (brownout).
Conventional UPSs may be classified into categories. Referring to
FIG. 1
, a typical off-line UPS disconnects a load from a primary AC source
10
when the primary AC source fails or is operating in a degraded manner, allowing the load to be served from a secondary source such as a battery. The AC power source
10
is connected in series with a switch S
1
, producing an AC voltage across a load
20
when the switch S
1
is closed. Energy storage is typically provided in the form of a storage capacitor C
S
. The secondary power source, here a battery B, is connected to the load
20
via a low voltage converter
30
and a transformer T. When the AC power source
10
fails, the switch S
1
is opened, causing the load to draw power from the battery B. The low voltage converter
30
typically is an inverter that produces a quasi-square wave or sine wave voltage on a first winding L
1
of the transformer T from a DC voltage produced by the battery B. The first winding L
1
is coupled to a second winding L
2
of the transformer T connected across the load
20
. When the AC power source is operational, i.e., when the switch S
1
is closed, the battery B may be charged using the low-voltage converter
30
or a separate battery charger circuit (not shown).
A line interactive (LIA) UPS topology is illustrated in FIG.
2
. Here, the transformer T has a third winding L
3
that may be connected in series with the load
20
using switches S
2
, S
3
to “buck” or “boost” the voltage applied to the load
20
. As with the offline UPS topology of
FIG. 1
, when the AC power source
10
fails, the switch S
1
can be opened to allow the load
20
to run off the battery B.
As illustrated in
FIG. 3
, a typical on-line UPS includes a rectifier
40
that receives an AC voltage from an AC power source
10
, producing a DC voltage across a storage capacitor C
S
at an intermediate node
45
. An inverter
50
is connected between the intermediate node
45
, and is operative to produce an AC voltage across a load
20
from the DC voltage. As shown, a battery B is connected to the intermediate node
45
via a DC/DC converter
60
, supplying auxiliary power. Alternatively, the DC/DC converter can be eliminated and a high-voltage battery (not shown) connected directly to the intermediate node
45
.
Each of these topologies may have disadvantages. For example, typical conventional on-line and LIA UPSs for 60 Hz applications use 60 Hz magnetic components (e.g., transformers and inductors) that are sized for such frequencies, and thus may be large, heavy and expensive. LIA UPSs often exhibit step voltage changes that can affect the performance of the load. Conventional off-line, LIA and on-line UPSs often use large storage capacitors, which tend to be bulky and expensive, in order to maintain an acceptable output voltage under heavy loading conditions. Moreover, because conventional UPSs are typically designed to operate in only one of the above-described off-line, LIA or on-line modes, sellers of UPSs may be required to maintain large inventories including several different types of UPSs in order to meet a variety of different customer applications.
SUMMARY OF THE INVENTION
In light of the foregoing, it is an object of the present invention to provide improved power converters and methods of operating power converters for use in devices such as uninterruptible power supplies (UPSs).
It is another object of the present invention to provide power converters that can be operated in a number of different modes.
It is yet another object of the present invention to provide power converters that can utilize smaller magnetic components and storage capacitors.
These and other objects, features and advantages may be provided according to the present invention by power converters and methods of operation thereof in which a rectifier circuit produces first and second voltages (e.g., ±DC voltages) on first and second voltage busses from an AC input voltage produced by an AC power source, an inverter circuit produces an AC output voltage from the first and second voltages, and a balancer circuit controls the relative magnitudes of the first and second voltages responsive to respective first and second rates at which the balancer circuit couples the first and second voltage busses to a neutral bus of the AC power source. Preferably, the rectifier circuit includes first and second switches that selectively couple the first and second voltage busses to a phase bus of the AC power source through a first inductance, the inverter circuit includes third and fourth switches that selectively couple the first and second voltage busses to a load through a second inductance, and the balancer circuit includes fifth and sixth switches that selectively couple the first and second voltage bussed to the neutral bus through a third inductance, such as an inductor or transformer winding.
The use of circuit topologies as described herein can provide several advantages. The balancer circuit enables energy transfer between first and second storage capacitors connected between the neutral bus and the first and second voltage busses, respectively, thus allowing the storage capacitors to be smaller than the storage capacitors typically used in conventional power converters with comparable power ratings. The switches in the rectifier, inductor and balancer can be controlled such that the power converter can be operated in a number of different power transfer modes. A secondary power source, such as a battery, may also be coupled to the power converter via a winding of a transformer that also serves as an inductance for the balancer circuit. In one embodiment, this coupling may be achieved through a combination battery converter/battery charger circuit that can also charge the battery when the converter is running off an AC power source. According to another aspect of the present invention, switches in the balancer circuit can be operated at varying duty cycles in positive and negative half-cycles of the AC input voltage, which can allow the power converter to be operated in a more efficient manner.
In particular, according to one embodiment of the present invention, a power converter includes first and second voltage busses and a neutral bus. A first switching circuit, e.g., a rectifier circuit, is operative to selectively couple an input node thereof to the first and second voltage busses. A balancer circuit is operative to selectively couple the neutral bus to the first and second voltage busses such that relative magnitudes of respective first and second voltages on the first and second voltage busses are controlled responsive to respective first and second rates at which the balancer circuit couples the first and second voltage busses to the neutral bus. A second switching circuit, e.g., an inverter circuit, is operative to selectively couple the first and second voltage busses to a load at an output node thereof.
The balancer circuit preferably includes first and second switches operative to selectively couple respective ones of the

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