U.S. patent application number 09/811974 was filed with the patent office on 2001-10-25 for method and apparatus for converting a dc voltage to an ac voltage.
Invention is credited to Curtis, Jeffrey, Landsman, Emanuel E., Reilly, David E..
Application Number | 20010033505 09/811974 |
Document ID | / |
Family ID | 23205147 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010033505 |
Kind Code |
A1 |
Reilly, David E. ; et
al. |
October 25, 2001 |
Method and apparatus for converting a DC voltage to an AC
voltage
Abstract
Embodiments of the present invention are directed to an
uninterruptible power supply for providing AC power to a load. In
embodiments of the present invention, 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, 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, wherein during a first time period, the first switch is
controlled to allow an electrical current path to connect the
resonant element to the capacitive element, an electrical current
of the path storing energy in the resonant element and charging the
capacitive element to a first voltage level, and during a second
time period, the first switch is controlled to block the current
path to cause the stored energy in the resonant element to further
charge the capacitive element to a second voltage level during the
second time period. The uninterruptible 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.
Inventors: |
Reilly, David E.; (Concord,
MA) ; Landsman, Emanuel E.; (Lexington, MA) ;
Curtis, Jeffrey; (Dunstable, MA) |
Correspondence
Address: |
A. Jason Mirabito, Esq.
Mintz, Levin, Cohn, Ferris,
Glovsky and Popeo, P.C.
One Financial Center
Boston
MA
02111
US
|
Family ID: |
23205147 |
Appl. No.: |
09/811974 |
Filed: |
March 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09811974 |
Mar 19, 2001 |
|
|
|
09311043 |
May 13, 1999 |
|
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Current U.S.
Class: |
363/125 |
Current CPC
Class: |
H02J 9/062 20130101;
Y02B 70/10 20130101; H02M 7/53832 20130101 |
Class at
Publication: |
363/125 |
International
Class: |
H02M 007/00 |
Claims
What is claimed is:
1. An uninterruptible power supply for providing AC power to a load
having a capacitive element, the uninterruptible power supply
comprising: 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; an
inverter operatively coupled to the DC voltage source to receive DC
power and to provide AC power, the inverter including: first and
second output nodes to provide AC power to the load having the
capacitive element; 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, wherein during a first time
period, the first switch is selected to enable an electrical
current path from the resonant element to the capacitive element,
an electrical current of the path storing energy in the resonant
element and charging the capacitive element to a first voltage
level, and during a second time period, the first switch is
selected to block the electrical current path to cause the stored
energy in the resonant element to further charge the capacitive
element to a second voltage level during the second time period; 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; and 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.
2. The uninterruptible power supply of claim 1, wherein the first
voltage level is a portion of a voltage source and the second
voltage level is substantially the voltage source.
3. The uninterruptible power supply of claim 1, wherein the set of
switches includes: 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.
4. The uninterruptible power supply of claim 3, wherein the
inverter further includes: a sixth switch electrically coupled
between the first terminal of the resonant element and the second
input node.
5. The uninterruptible power supply of claim 4, wherein the
resonant element includes an inductor.
6. The uninterruptible power supply of claim 5, wherein each of the
switches includes a transistor.
7. The uninterruptible power supply of claim 6, wherein the energy
storage device includes a battery.
8. The uninterruptible power supply of claim 7, wherein the
transfer switch is 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.
9. The uninterruptible power supply of claim 1, wherein the
resonant element includes an inductor.
10. The uninterruptible power supply of claim 1, wherein each of
the switches includes a transistor.
11. The uninterruptible power supply of claim 1, wherein the energy
storage device includes a battery.
12. The uninterruptible power supply of claim 1, wherein the
transfer switch is 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 output of the inverter to the load.
13. An uninterruptible power supply for providing AC power to a
load having a capacitive element, the uninterruptible power supply
comprising: 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
including: means for charging the capacitive element to a first
voltage level by creating an electrical current path from the
inverter to the load through a resonant element, wherein the
resonant element stores energy from an electrical current of the
path; means for blocking the electrical current path after the
capacitive element has been charged to the first voltage level to
cause energy from the resonant element to be transferred to the
capacitive element to further charge the capacitive element to a
second voltage level; and a transfer switch constructed and
arranged to select one of the AC power source and the voltage
source as an output power source for the uninterruptible power
supply.
14. The uninterruptible power supply of claim 13, wherein the
energy storage device includes a battery.
15. The uninterruptible power supply of claim 14, wherein the
resonant element includes an inductor.
16. The uninterruptible power supply of claim 15, wherein the
transfer switch is constructed and arranged to receive the AC power
from the input and to receive the AC power from the output of the
inverter and to provide one of the AC power from the input and the
AC power from the output of the inverter to the load.
17. The uninterruptible power supply of claim 13, wherein the
resonant element includes an inductor.
18. The uninterruptible power supply of claim 13, wherein the
transfer switch is constructed and arranged to receive the AC power
from the input and to receive the AC power from the output of the
inverter and to provide one of the AC power from the input and the
AC power from the output of the inverter to the load.
19. A method of supplying an uninterruptible AC voltage to a load
having a capacitive element using an uninterruptible power supply
having a DC voltage source with an energy storage device, the
method comprising steps of: charging the capacitive element to a
first voltage level by supplying electrical current from the DC
voltage source to the load through a resonant element in the
uninterruptible power supply, storing energy in the resonant
element from the electrical current; blocking the electrical
current from the DC voltage source to the load through the resonant
element after the capacitive element has been charged to the first
voltage level; and transferring the stored energy from the resonant
element to the capacitive element to further charge the capacitive
element to a second voltage level.
20. The method of claim 19, further comprising steps of: supplying
load current from the DC voltage source to the load after the
capacitive element has been charged to the second voltage level;
blocking the load current from the DC voltage to the load after a
predetermined period; discharging the capacitive element through
the resonant element; and transferring energy from the resonant
element to the energy storage device in the DC voltage source.
21. The method of claim 19, further comprising steps of: receiving
an AC voltage from an AC power source; selecting one of the AC
power source and the DC voltage source as an output power source
for the uninterruptible power supply.
22. The method of claim 19, wherein the resonant element includes
an inductor.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation in part of application
Ser. No. 09/311,043 titled "Method and Apparatus for Converting a
DC Voltage to an AC Voltage," filed on May 13, 1999, which is
incorporated herein by reference.
[0002] This application is related to an application titled
"Excessive Load Capacitor Detection Circuit for UPS," filed on Mar.
19, 2001, which is incorporated herein by reference.
FIELD OF THE INVENTION
[0003] 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
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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 18a and coupled to a typical load 126 comprising a load
resistor 128 and a load capacitor 130. The DC voltage source 18a
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. 2A. Typical loads
have a capacitive component due to the presence of an EMI filter in
the load. The inverter circuit 100 includes four switches S1, S2,
S3 and S4. 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 S1-S4 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.
[0008] 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 S1, S2, S3, S4, which typically requires
the switches to be mounted to relatively large heat sinks. The
issue of power dissipation becomes greater for high voltage
systems, in which 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.
The rise in temperature of the switches also becomes a large
concern.
SUMMARY OF THE INVENTION
[0009] In one 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 DC voltage source that provides DC
power, the DC voltage source having an energy storage device, an
inverter operatively coupled to the DC voltage source to receive DC
power and to provide AC power, the inverter including first and
second output nodes to provide AC power to the load having the
capacitive element, 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, wherein during a first time
period, the first switch is controlled to allow an electrical
current path to connect the resonant element to the capacitive
element, an electrical current of the path storing energy in the
resonant element and charging the capacitive element to a first
voltage level, and during a second time period, the first switch is
controlled to block the current path to cause the stored energy in
the resonant element to further charge the capacitive element to a
second voltage level during the second time period, 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, and 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.
[0010] Other features may include one or more of the following: the
first voltage level is a portion of a voltage source and the second
voltage level is substantially the voltage source; the set of
switches includes 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 further includes a sixth switch
electrically coupled between the first terminal of the resonant
element and the second input node; the resonant element includes an
inductor; each of the switches includes a transistor; the energy
storage device includes a battery; and the transfer switch is
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.
[0011] In another general aspect, 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
including means for charging the capacitive element to a first
voltage level by creating an electrical current path from the
inverter to the load through a resonant element, wherein the
resonant element stores energy from an electrical current of the
path, means for blocking the electrical current path after the
capacitive element has been charged to the first voltage level to
cause energy from the resonant element to be transferred to the
capacitive element to further charge the capacitive element to a
second voltage level, and a transfer switch constructed and
arranged to select one of the AC power source and the voltage
source as an output power source for the uninterruptible power
supply.
[0012] Other features may include one or more of the following: the
energy storage device includes a battery; the resonant element
includes an inductor; and the transfer switch is constructed and
arranged to receive the AC power from the input and to receive the
AC power from the output of the inverter and to provide one of the
AC power from the input and the AC power from the output of the
inverter to the load.
[0013] In another general aspect, the present invention features a
method of supplying an uninterruptible AC voltage to a load having
a capacitive element using an uninterruptible power supply having a
DC voltage source with an energy storage device. The method
comprising steps of charging the capacitive element to a first
voltage level by supplying electrical current from the DC voltage
source to the load through a resonant element in the
uninterruptible power supply, storing energy in the resonant
element from the electrical current, blocking the electrical
current from the DC voltage source to the load through the resonant
element after the capacitive element has been charged to the first
voltage level, and transferring the stored energy from the resonant
element to the capacitive element to further charge the capacitive
element to a second voltage level.
[0014] Other features may include one or more of: supplying load
current from the DC voltage source to the load after the capacitive
element has been charged to the second voltage level, blocking the
load current from the DC voltage to the load after a predetermined
period, discharging the capacitive element through the resonant
element, and transferring energy from the resonant element to the
energy storage device in the DC voltage source; receiving an AC
voltage from an AC power source, selecting one of the AC power
source and the DC voltage source as an output power source for the
uninterruptible power supply; and wherein the resonant element
includes an inductor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] For a better understanding of the present invention,
reference is made to the drawings which are incorporated herein by
reference and in which:
[0016] FIG. 1 is a block diagram of a typical uninterruptible power
supply;
[0017] FIG. 2 shows a schematic diagram of a typical prior art
inverter circuit;
[0018] FIG. 2A shows a block diagram of a voltage source used with
the inverter circuit of FIG. 2.
[0019] FIG. 3 shows timing waveforms for the inverter circuit shown
in FIG. 2;
[0020] FIG. 4 shows a schematic diagram of an inverter circuit in
accordance with one embodiment of the present invention;
[0021] FIG. 5 shows timing waveforms for the inverter circuit shown
in FIG. 4;
[0022] FIG. 6 illustrates a current path through the inverter of
FIG. 4 during a charging mode of the inverter corresponding to a
starting point of the positive half cycle of the output voltage
waveform;
[0023] FIG. 7 illustrates a current path through the inverter of
FIG. 4 during a positive half cycle of the output voltage
waveform;
[0024] FIG. 8 illustrates a current path through the inverter of
FIG. 4 during a discharging mode of the inverter at the end of the
positive half cycle of the output voltage waveform;
[0025] FIG. 9 illustrates a current path through the inverter
during an energy recovery mode of the inverter; and
[0026] FIG. 10 illustrates alternative timing waveforms for the
inverter circuit in FIG. 4.
DETAILED DESCRIPTION
[0027] One embodiment of an inverter 200 in accordance with the
present invention will now be described with reference to FIG. 4
which shows a schematic diagram of the inverter 200 coupled to the
voltage source 18a and the load 126. The inverter 200 includes
MOSFET switches S1, S2, S3 and S4 of the prior art inverter 100 and
includes two additional MOSFET switches S5 and S6 and an inductor
140. In one embodiment, the switches S5 and S6 are similar to
switches S1-S4 and include a transistor 134, 138 having an
intrinsic diode 132, 136. Each of the transistors 134 and 138 has a
gate 115 and 117 that is used to control the state of the
transistor.
[0028] In one embodiment that provides an output of 120 VAC, 400
VA, 25 amps peak current to the load from an input to the inverter
of approximately 170 VDC, the switches S1-S6 are implemented using
part no. IRF640 available from International Rectifier of E1
Segundo, Calif. For 220 VAC applications, the switches may be
implemented using part no. IRF730 also available from International
Rectifier. The inductor 140, in the 120 VAC embodiment, may be
implemented using a 1.8 mH inductor having a very high Bsat value
to be able to withstand high peak currents without saturating. In
one embodiment, the inductor may be made from an EI lamination
structure of M-19, 18.5 mil steel having a large air gap between
the E and I laminations. Other values of inductors may be used with
embodiments of the present invention depending upon the peak switch
current and physical size of the inductor desired. In selecting an
inductor for use, the transition time, or time required to charge
or discharge the load capacitance, should also be considered to
prevent the transition time from becoming either too short or too
long. If the transition time is too long, then the pulse width of
the output waveform may become too long. If the transition time is
too short, the peak switch currents become greater.
[0029] The operation of the inverter 200 to provide AC power to the
load will now be described with reference to FIGS. 5-9. FIG. 5
provides a timing diagram of the operation of the switches S1-S6 of
the inverter 200 and also provides the output voltage waveform
across the load 126. In the timing diagram of FIG. 5, for each of
the switches S1-S6, when the corresponding waveform is in the high
state, the switch is turned on (conducting state) and when the
corresponding waveform is in the low state the switch is turned off
(non-conducting state).
[0030] In the inverter 200, the switches are shown as being
implemented using NMOS devices. As known by those skilled in the
art, for an NMOS device, a control signal having a high state is
supplied to the gate of the device to turn the device on
(conducting), while a control signal having a low state is supplied
to the gate to turn the device off (non-conducting). Accordingly,
the timing diagram of each of the switches also represents the
state of the control signal provided to the gate of the
corresponding transistor. In embodiments of the present invention,
the control signals may be provided from, for example, controller
16 of the UPS of FIG. 1 when the inverter is used in a UPS.
Alternatively, the control signals may be supplied using timing
logic circuits residing within the inverter itself as is known in
the art.
[0031] During a first time period from t0 to t1 in FIG. 5, switches
S4 and S5 are turned on and switches S1, S2, S3 and S6 are turned
off creating a current path through the inverter 200 in the
direction of arrows 150 as shown in FIG. 6. Only the components of
the inverter 200 in the current path created during the first time
period are shown in FIG. 6. As shown in FIG. 6, with switches S4
and S5 turned on, the inductor 140 and the load 126 are connected
in series across the voltage source 18a. During the first period,
the output voltage across the load Vout rises in a resonant manner
from zero volts to the voltage of the voltage source 18a. The
output voltage Vout is prevented from rising beyond the voltage of
the voltage source by the diode 104 (FIG. 7) of switch S1. The
diode 104 will conduct current to limit the output voltage Vout to
the voltage of the voltage source.
[0032] Once the output voltage Vout reaches the voltage of the
voltage source (or shortly thereafter), at time t1, switch S1 is
turned on and switch S5 is turned off. Switches S1 and S4 remain on
for a second period from time t1 to time t2, during which time, the
load is coupled across the voltage source 18a. FIG. 7 shows the
current path through the inverter during the second time period. As
shown in Fig, 7, load current during the second period follows
arrows 154. Also during the second time period, the energy that was
stored in the inductor during the first time period causes the
voltage across the inductor to reverse and energy in the inductor
is released to a storage device in the voltage source, such as a
battery or a capacitor, through a current that follows a path along
arrow 156 through diode 104 of switch 1 and diode 136 of switch 6.
In addition, depending upon the load impedance, current from the
energy stored in the inductor may also follow a path through the
load.
[0033] During a third time period from time t2 to time t3, the
voltage across the load is returned to zero. At time t2, switches
S1 and S4 are turned off to disconnect the load from the voltage
source and switch S6 is turned on to place the inductor effectively
across the load as shown in FIG. 8. During the third time period,
energy stored in the load capacitor 130 is transferred to the
inductor 140, and the voltage across the load decreases to zero.
The output voltage Vout is prevented from going negative by diode
110 (FIG. 9) of switch S2. The diode 110 will conduct current to
limit the output voltage to zero.
[0034] At time t3 switch S6 is turned off, and all switches remain
off during a fourth time period from t3 until t4. The current path
through the inverter 200 during the fourth time period follows
arrows 160 shown in FIG. 9. During the fourth time period, the
energy in the inductor 140 freewheels into the voltage source 18a
through diodes 110 and 132 of S2 and S5, and the voltage across the
load typically remains at zero. The time from t3 until t4 is
normally chosen to be long enough to permit all of the inductor
energy to be transferred to the voltage source 18a.
[0035] During a fifth time period from t4 to t5, switches S1 and S3
are turned on to maintain a low impedance across the load to
prevent any external energy from charging the output to a non-zero
voltage. This is referred to as the "clamp" period. At time t5, all
switches are again turned off and remain off for a sixth time
period until time t6.
[0036] Beginning at time t6, and continuing until time t9 the
negative half cycle of the AC waveform is created. The negative
half cycle is created in substantially the same manner as the
positive half cycle described above in connection with FIGS. 5-9,
except that switch S3 is substituted for switch S4, switch S6 is
substituted for S5 and switch S2 is substituted for S1. The
positive and negative half cycles then continue to be generated in
an alternating manner to create an AC output voltage waveform.
[0037] In the embodiments described above, and in particular with
reference to FIGS. 5-7, switch S5 is left on until the load voltage
Vout reaches the voltage of the voltage source 18a. At time t1,
switch S5 is turned off and as shown in FIG. 7, the energy stored
in the inductor 140 freewheels into the voltage source 18a.
However, some of the inductor's stored energy also freewheels into
the load and bus capacitance resulting in some power loss. In
another embodiment of the invention, which will now be described,
an alternative timing sequence minimize this power loss.
Furthermore, another benefit of the alternative timing sequence is
that lower peak and rms current flows through the resonant circuit.
Thus, the inductor stores less energy and therefore a lower Bsat
value may be used along with a smaller inductor. The alternative
timing sequence will now be described with reference to FIG.
10.
[0038] With reference to FIG. 10, during a first time period from
t'0 to t'1, switches S4 and S5 are turned on and switches S1, S2,
S3 and S6 are turned off creating a current path through the
inverter 200 in the direction of arrows 150 similar to that shown
in FIG. 6. With switches S4 and S5 turned on, the inductor 140 and
the load 126 are connected in series across the voltage source 18a.
During the first time period, the load voltage Vout rises in a
resonant manner from zero volts to a portion of the voltage of the
voltage source 18a, preferably, approximately half of the voltage
of the voltage source 18a. At time t'1, switch S5 turns off
blocking the current path from the voltage source 18a to the
inductor 140. During the second time period from t'1 to t'2, the
inductor 140 freewheels through reverse diode 136 and the energy
stored in the inductor continues to charge the capacitor and
increase the load voltage Vout to the voltage of the source voltage
18a. Accordingly, the power loss due to the inductor's stored
energy being freewheeled into the bus capacitance is minimized.
According to one embodiment, the controller 16 controls appropriate
switches such that freewheeling or "swing" time is made
approximately equal to the inductor charge time. For example, if
the inductor charge time is 100 us the inductor freewheeling time
is set at about 100 us. The output voltage Vout is prevented from
rising beyond the voltage of the voltage source by the diode 104
(FIG. 7) of switch S1.
[0039] Once the load voltage Vout reaches the voltage of the source
voltage (or shortly thereafter), at time t'2, switch S1 turns on
and switches S1 and S4 remain on for a third time period from t'2
to t'3, during which time, the load is coupled across the source
voltage 18a similar to that shown in FIG. 7. At time t'3, switch S1
turns off to disconnect the load from the voltage source 18a and
switch S6 turns on to place the inductor effectively across the
load similar to that shown in FIG. 8. During a fourth time period
from t'3 to t'4, some of the energy stored in the load capacitor
130 is transferred to the inductor 140 and the voltage across the
load decreases to approximately half the voltage source 18a, at
which time t'4, the switch S6 is turned off. During the fifth time
period from t'4 to t'5, with the switch S6 turned off, the inductor
140 freewheels through reverse diode 132 and its stored energy is
returned to the voltage source 18a in a manner similar to that
shown in FIG. 9 and finishes discharging the load capacitor to zero
volts. The output voltage Vout is prevented from going negative by
diode 110 (FIG. 9) of switch S2. The diode 110 will conduct current
to limit the output voltage to zero.
[0040] During a sixth time period from t'5 to t'6, switch S2 turns
on and switches S2 and S4 maintain a low impedance across the load
to prevent any external energy from charging the output to a
non-zero voltage. This is referred to as the "clamp" period. At
time t'6, all switches are turned off.
[0041] Beginning at time t'6 and continuing until time t'12, the
negative half cycle of the AC waveform is created. The negative
half cycle is created in substantially the same manner as the
positive half cycle described above in connection with FIGS. to ,
except that switch S3 is substituted for switch S4, switch S6 is
substituted for S5 and switch S2 is substituted for S1. The
positive and negative half cycles then continue to be generated in
an alternating manner to create an AC output voltage waveform.
[0042] In one embodiment of the present invention, in an inverter
designed to generate 50 Hz voltage waveforms, the first time period
from t'0 to t'1 is approximately 100 microseconds, the second time
period from t'1 to t'2 is approximately 100 milliseconds, the third
time period from t'2 to t'3 is approximately 4.8 milliseconds, the
fourth time period from t'3 to t'4 is approximately 100
microseconds, the time period from time t'4 to t'5 is approximately
100 microseconds, and the time period from t'5 to t'6 is
approximately 4.8 milliseconds. In this embodiment, the negative
half cycle of the waveform is symmetric with the positive half
cycle, and accordingly, the rise time, fall time and duration of
the negative half cycle are approximately equal to those of the
positive half cycle.
[0043] In embodiments described above, during the clamp period from
t'5 to t'6 after a positive half cycle switches S2 and S4 are
turned on to clamp the output to a low impedance. During the clamp
period from t'11 to t'12 after a negative half cycle, switches S1
and S3 are turned on to clamp the output to a low impedance. In
another embodiment of the present invention, following a positive
half cycle, switches S1 and S3 are turned on to clamp and after a
negative half cycle, switches S2 and S4 are turned on to clamp.
This method is less desirable because circulating currents will
flow through inductor 140 during the clamp periods, resulting in
additional power losses. In a third embodiment during both clamp
periods, switches S1 and S3 are turned on to clamp. In a fourth
embodiment during both clamp periods, switches S2 and S4 are turned
on to clamp.
[0044] In embodiments of the present invention, the inverter 200,
is used in the manner described above, to create an output AC
voltage having the waveform shown in FIG. 10 from an input DC
voltage using a resonance circuit. The use of the resonance circuit
allows the load capacitance to be charged and discharged with only
a minimum power loss. The only power losses incurred in the
inverter 200 are due to characteristics of inverter components
including the ESR of the inductor and due to series resistance of
each of the switches when in the on state. Thus, inverters in
accordance with embodiments of the present invention, do not
require bulky heat sinks like inverters of the prior art, and are
more efficient than inverters of the prior art. The improved
efficiency of inverters in accordance with embodiments of the
present invention make them particularly desirable for use in
uninterruptible power supplies, wherein they can extend the
operating time of a UPS in battery mode, reduce the size and weight
of the UPS and reduce electromagnetic emissions from the UPS.
[0045] In embodiments of the present invention described above,
inverters are described as being used with uninterruptible power
supplies, for example, in place of the inverter 20 in the UPS 10 of
FIG. 1. As understood by those skilled in the art, inverters of the
present invention may also be used with other types of
uninterruptible power supplies. For example, the inverters may be
used with UPSs in which an input AC voltage is converted to a DC
voltage and one of the converted DC voltage and a DC voltage
provided from a battery-powered DC voltage source is provided to an
input of the inverter to create the AC output voltage of the UPS.
In addition, as understood by those skilled in the art, inverters
in accordance with embodiments of the present invention may also be
used in systems and devices other than uninterruptible power
supplies.
[0046] In the inverter 200 described above, MOSFET devices are used
as the switches S1-S6. As understood by those skilled in the art, a
number of other electrical or mechanical switches, such as IGBT's
with integral rectifiers, or bipolar transistors having a diode
across the C-E junction, may be used to provide the functionality
of the switches. Further, in embodiments of the present invention,
each of the switches S1-S6 need not be implemented using the same
type of switch.
[0047] In embodiments of the invention discussed above, an inductor
is used as a resonant element in inverter circuits. As understood
by one skilled in the art, other devices having a complex impedance
may be used in place of the inductor, however, it is desirable that
any such device be primarily inductive in nature.
[0048] In the embodiments of the present invention described above,
energy is returned from the inductor to the voltage source after
the load capacitance has been discharged. As understood by those
skilled in the art, the voltage source may include a battery that
receives the energy from the inductor, or the voltage source may
include a storage device other than a battery, such as a capacitor,
coupled in parallel across the voltage source that receives the
energy.
[0049] Having thus described at least one illustrative embodiment
of the invention, various alterations, modifications and
improvements will readily occur to those skilled in the art. Such
alterations, modifications and improvements are intended to be
within the scope and spirit of the invention. Accordingly, the
foregoing description is by way of example only and is not intended
as limiting. The invention's limit is defined only in the following
claims and the equivalents thereto.
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