Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2003-06-13
2004-09-28
Berhane, Adolf (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C323S247000
Reexamination Certificate
active
06798675
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the transfer of electrical energy from an energy source to a capacitive store and, more particularly, involves energy trapping and adaptive clocking of the energy transfer cycle in conjunction with a resonant circuit.
BACKGROUND
The charging of a capacitive energy store requires the transfer of energy from an energy source. Energy sources such as generators, batteries, fuel cells, and solar cells are typically voltage sources. The capacitive store initially appears as a short circuit when connected to a voltage source that has a voltage higher than that across the capacitive store. Consequently, the flow of current must be controlled.
The simplest control means is a series resistor as shown in FIG.
1
. Voltage source
1
having a voltage V
dc
charges capacitor
2
, having a capacitance C, in series with resistor
3
, having a resistance R. This circuit limits the peak current to a value of V
dc
/R, and results in a relatively long charging time to achieve 99% V
dc
, i. e., approximately 3RC seconds. The charging efficiency is only 50%; that is, resistor
3
dissipates the same amount of energy that is transferred to capacitor
2
, or C/2V
dc
2
. In some low-energy applications, resistive charging is the best engineering choice. However, in high-energy applications, the relatively long charging time or the 50% efficiency is unacceptable.
The charging time and efficiency are improved by resonant charging. This accomplished by replacing resistor
3
with an inductor
4
, having an inductance L, as shown in FIG.
2
. The theoretical efficiency of resonant charging approaches 100% and is typically greater than 95% in practice. The charging time is given by &pgr;(LC)
1/2
seconds, with the peak current being limited to V/(LC)
1/2
. Diode
5
is used in the circuit because the capacitor
6
charges to almost twice the voltage of d.c. voltage source
7
, V
dc
, and it is necessary to prevent the charge transferred to capacitor
6
from flowing back into voltage source
7
.
The peak energy storage rating of inductor
4
is one-fourth the energy rating of capacitor
6
. The specific energy of a capacitor is on the order of 2000 J/kg, and that of an inductor is typically much less, on the order of 50 J/kg. Therefore, inductor
4
is typically on the order of 40 times larger than capacitor
6
.
In moderate low-energy applications, such as pulsed radar transmitters, resonant charging is a good engineering choice. However, when the capacitive stored energy is greater than a few kJ, a better alternative for the charging apparatus is a switching-type capacitive charging power conditioner, or “SCCPC.” The SCCPC operates from a d.c. source and provides fast and efficient charging of the capacitive store. In most applications, it also replaces the large d.c. power supply required for the input power by operating from a directly rectified a.c. power line. The SCCPC can also operate from any other suitable d.c. source, such as a battery.
A transformer is an important part of an SCCPC because it accommodates the difference between the voltage source and the load voltages, and isolates the voltage source from the load. Transformers must operate with bipolar voltages that contain no d.c. components. In general, transformers are inversely related in size and cost to the frequency of operation, which is motivation for operating the SCCPC at high frequency. There are two basic configurations of the SCCPC, the center-tapped transformer configuration shown in FIG.
3
and the “H” bridge switch configuration shown in FIG.
4
.
The principle of operation is the same for both SCCPC configurations. A small amount of energy is measured out by the primary capacitor, switched through the transformer, then rectified and deposited into the load capacitor. This process is repeated at a high frequency until the load capacitor is fully charged and in a manner such that the transformer is subjected to only a bipolar voltage.
More particularly, center-tapped configuration
8
of the prior art is schematically illustrated in FIG.
3
. Center-tapped configuration
8
operates by alternately charging small capacitors
9
and
10
by means of switches
11
and
12
, from voltage source
13
, through the primary winding of transformer
14
. Transformer
14
usually steps up the voltage by a factor of N, i. e., N is typically greater than 1, where N is the turns ratio of a transformers secondary and primary windings; but in some cases the voltage may be stepped down, i. e., N may be less than 1. The secondary current of transformer
14
passes through bridge rectifier
15
and then into load capacitor
16
. This process is repeated at a high frequency such that over a period, load capacitor
16
is charged to the desired voltage. The switches
11
and
12
are operated in an alternating sequence such that the voltage applied to transformer
14
is bipolar and has no d.c. component.
H-bridge circuit
17
of the prior art is schematically shown in FIG.
4
. H-bridge circuit
17
has only one small primary capacitor
18
, which is charged through the primary coil of transformer
19
. The H-bridge switches
20
,
21
,
24
and
26
are sequentially operated to alternately apply a bipolar voltage through capacitor
18
to transformer
19
. Specifically, in the first energy transfer cycle, the switch pair
20
and
26
are turned on, while switch pair
21
and
24
remain in the off state. This connects the positive side of voltage source
27
through small primary capacitor
18
to the top of the primary coil of transformer
19
.
After this energy transfer cycle is completed, the next energy transfer cycle begins with switch pair
21
and
24
being turned on while switch pair
20
and
26
are switched to the off state. This connects the positive side of voltage source
27
through primary capacitor
18
to the bottom side of the primary coil of transformer
19
, thus providing the reverse polarity and ensuring that the bipolar signal received by transformer
19
has no d.c. component. This sequence of operating two the switch pairs is repeated until the required amount of energy is transferred through bridge rectifier
28
to load capacitor
29
.
The basic energy transfer process and the functions of the switches during a single switching event of the same polarity can be better explained using simplified equivalent circuit
30
shown in FIG.
5
. Circuit
30
illustrates the operation of both center-tapped circuit
8
of FIG.
3
and H-bridge circuit
17
of FIG.
4
.
Transformer
14
in circuit
8
of FIG.
3
and transformer
19
in circuit
17
of
FIG. 4
, are replaced in
FIG. 5
by equivalent leakage inductor
31
. The equivalent inductance of inductor
31
can be obtained by calculation familiar to those skilled in the electrical art, using the transformer turns ratio N. Likewise, load capacitor
16
in circuit
8
and load capacitor
29
in circuit
17
are represented by equivalent load capacitor
32
. The capacitance of capacitor
32
can be calculated using equations and methods well known to those reasonably skilled in the electrical art. The voltage across load capacitor
32
divided by the voltage of source
33
is defined as the charge ratio &agr;. Forward switch
34
is a silicon controlled rectifier, or “SCR,” with parallel back diode
35
. However, any suitable switch may be used, such as an isolated gate bipolar transistor, or “IGBT,” or monolithic oxide silicon field effect transistor, or “MOSFET.”
The operation of the switch cycle begins when forward switch
34
closes, i. e., is turned on. A resonant current flows from voltage source
33
through switch
34
, through capacitor
36
, through inductor
31
, through the bridge rectifier formed by diodes
37
,
38
,
39
and
40
, and into load capacitor
32
. Being in a resonant circuit, the voltage across capacitor
36
will increase and ultimately exceed the voltage of the voltage source
33
. When this occurs, forward switch
34
is turned off, and the current through capacitor
36
reverses and flows ba
Berhane Adolf
Skorich James M.
The United States of America as represented by the Secretary of
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