Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
1999-05-13
2002-06-11
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S037000
Reexamination Certificate
active
06404658
ABSTRACT:
FIELD OF THE INVENTION
Embodiments of the present invention are directed generally to a method and an apparatus for converting a DC voltage to an AC voltage. More specifically, embodiments of the present invention are directed to methods and apparatus for converting DC voltages to AC voltages using resonant bridge inverter circuits in devices such as uninterruptible power supplies.
BACKGROUND OF THE INVENTION
The use of uninterruptible power supplies (UPSs) having battery back-up systems to provide regulated, uninterrupted power for sensitive and/or critical loads, such as computer systems, and other data processing systems is well known.
FIG. 1
shows a typical prior art UPS
10
used to provide regulated uninterrupted power. The UPS
10
includes an input filter/surge protector
12
, a transfer switch
14
, a controller
16
, a battery
18
, a battery charger
19
, an inverter
20
, and a DC-DC converter
23
. The UPS also includes an input
24
for coupling to an AC power source and an outlet
26
for coupling to a load.
The UPS
10
operates as follows. The filter/surge protector
12
receives input AC power from the AC power source through the input
24
, filters the input AC power and provides filtered AC power to the transfer switch and the battery charger. The transfer switch
14
receives the AC power from the filter/surge protector
12
and also receives AC power from the inverter
20
. The controller
16
determines whether the AC power available from the filter/surge protector is within predetermined tolerances, and if so, controls the transfer switch to provide the AC power from the filter/surge protector to the outlet
26
. If the AC power from the rectifier is not within the predetermined tolerances, which may occur because of “brown out,” “high line,” or “black out” conditions, or due to power surges, then the controller controls the transfer switch to provide the AC power from the inverter
20
. The DC-DC converter
23
is an optional component that converts the output of the battery to a voltage that is compatible with the inverter. Depending on the particular inverter and battery used the inverter may be operatively coupled to the battery either directly or through a DC-DC converter.
The inverter
20
of the prior art UPS
10
receives DC power from the DC-DC converter
23
, converts the DC voltage to AC voltage, and regulates the AC voltage to predetermined specifications. The inverter
20
provides the regulated AC voltage to the transfer switch. Depending on the capacity of the battery and the power requirements of the load, the UPS
10
can provide power to the load during brief power source “dropouts” or for extended power outages.
In typical medium power, low cost inverters, such as inverter
20
of UPS
10
, the waveform of the AC voltage has a rectangular shape rather than a sinusoidal shape. A typical prior art inverter circuit
100
is shown in
FIG. 2
coupled to a DC voltage source
18
a
and coupled to a typical load
126
comprising a load resistor
128
and a load capacitor
130
. The DC voltage source
18
a
may be a battery, or may include a battery
18
coupled to a DC-DC converter
23
and a capacitor
25
as shown in FIG.
2
A. Typical loads have a capacitive component due to the presence of an EMI filter in the load. The inverter circuit
100
includes four switches S
1
, S
2
, S
3
and S
4
. Each of the switches is implemented using power MOSFET devices which consist of a transistor
106
,
112
,
118
,
124
having an intrinsic diode
104
,
110
,
116
, and
122
. Each of the transistors
106
,
112
,
118
and
124
has a gate, respectively
107
,
109
,
111
and
113
. As understood by those skilled in the art, each of the switches S
1
-S
4
can be controlled using a control signal input to its gate.
FIG. 3
provides timing waveforms for the switches to generate an output AC voltage waveform Vout (also shown in
FIG. 3
) across the capacitor
130
and the resistor
128
.
A major drawback of the prior art inverter circuit
100
is that for loads having a capacitive component, a significant amount of power is dissipated as the load capacitance is charged and discharged during each half-cycle of the AC waveform. This power is absorbed by the switches S
1
, S
2
, S
3
, S
4
, which typically requires the switches to be mounted to relatively large heat sinks. The power dissipation issue becomes greater for high voltage systems, wherein the energy required to charge the load capacitance is greater. The dissipation of power in the switches dramatically reduces the efficiency of the inverter, and accordingly, reduces the run-time of the battery
18
in the UPS
10
. Temperature rise of the switches also becomes a large concern.
SUMMARY OF THE INVENTION
In embodiments of the present invention, the problems associated with power dissipation in an inverter due to capacitor charging are eliminated by providing a resonant circuit in the inverter to charge and discharge the capacitor.
In one general aspect, the present invention features an uninterruptible power supply for providing AC power to a load. The uninterruptible power supply includes an input to receive AC power from an AC power source, an output that provides AC power, a DC voltage source that provides DC power, the DC voltage source having an energy storage device, and an inverter operatively coupled to the DC voltage source to receive DC power and to provide AC power. The inverter includes first and second output nodes to provide AC power to the load, first and second input nodes to receive DC power from the DC voltage source, a resonant element having a first terminal and a second terminal, the second terminal being electrically coupled to the first output node, a first switch electrically coupled between the first terminal of the resonant element and the first input node, and a set of switches operatively coupled between the first and second output nodes and the first and second input nodes and controlled to generate AC power from the DC power. The uninterruptible power supply further includes a transfer switch constructed and arranged to select either the AC line or inverter power source as an output power source for the uninterruptible power supply.
The set of switches in the inverter can include a second switch electrically coupled between the second output node and the second input node, a third switch electrically coupled between the second output node and the first input node, a fourth switch electrically coupled between the first output node and the first input node, and a fifth switch electrically coupled between the first output node and the second input node. The inverter can further include a sixth switch electrically coupled between the first terminal of the resonant element and the second input node.
The resonant element in the inverter may include an inductor. Each of the switches may include a transistor. The energy storage device can include a battery. The transfer switch can be constructed and arranged to receive the AC power from the input and to receive the AC power from the inverter and to provide one of the AC power from the input and the AC power from the inverter to the load.
In another general aspect, the present invention features an uninterruptible power supply for providing AC power to a load having a capacitive element. The uninterruptible power supply includes an input to receive AC power from an AC power source, an output that provides AC power, a voltage source that provides DC power, the voltage source having an energy storage device, an inverter operatively coupled to the voltage source to receive DC power and having an output to provide AC power. The inverter includes means for charging the capacitive element by supplying electrical current from the inverter to the load through a resonant element, means for supplying load current from the inverter after the capacitive element has been charged to a predetermined voltage, means for discharging the capacitive element through the resonant element, and means for transferring energy from the resonant
American Power Conversion
Mintz Levin Cohn Ferris Glovsky and Popeo P.C.
Patel Rajnikant B.
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