Excessive load capacitor detection circuit for UPS

Details Excessive load capacitor detection circuit for UPS Excessive load capacitor detection circuit for UPS

2001-03-19

2003-10-07

Sircus, Brian (Department: 2836)

Electrical transmission or interconnection systems

Plural supply circuits or sources

Substitute or emergency source

C363S098000, C324S679000

Reexamination Certificate

active

06630751

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 detecting excessive capacitance in a load when converting DC voltages to AC voltages using inverter circuits in devices such as uninterruptible power supplies (UPS).
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 input AC power to the UPS 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 for various inverter circuits 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. Part of this power is absorbed by the inverter circuit switches, which generates heat and causes temperature rises in those switches. To dissipate the heat, the switches are mounted on relatively large heat sinks. According to a known method, to better manage the heat dissipation, the inverter circuit is designed around a safe operating maximum capacitive load. However, in the event that a capacitive load greater than the specified load is applied to the inverter circuit, the heat generated by the switches may be greater than the heat dissipated. As a result, excessive heat causes components in the inverter circuit and in particular the switches to get hotter and hotter and eventually, the switches fail. Accordingly, a method and apparatus is required to overcome the shortcomings of above and other shortcomings.
SUMMARY OF THE INVENTION
One aspect of the invention is directed to an uninterruptible power supply for providing AC power to a load having a first 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 DC voltage source that provides DC power, the DC voltage source having an energy storage device, and an inverter operatively coupled to file 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 having the first capacitive element, first and second input nodes to receive DC power from the DC voltage source, a circuit operatively coupled to the first output node of the inverter, the circuit being configured to compare a value representative of load capacitance of the first capacitive element with a reference value to determine excessive load capacitance, 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 power supply further includes a transfer switch constructed and arranged to select one of the AC power source and the DC voltage source as an output power source for the uninterruptible power supply.
A second aspect of the invention is directed to an uninterruptible power supply for providing AC power to a load having a first 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 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 having the first capacitive element, first and second input nodes to receive DC power from the DC voltage source, means for comparing a value representative of load capacitance of the first capacitive element with a reference value to determine excessive load capacitance, 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 power supply further includes a transfer switch constructed and arranged to select one of the AC power source and the DC voltage source as an output power source for the uninterruptible power supply.


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