Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
1999-06-23
2001-07-03
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S069000, C363S126000
Reexamination Certificate
active
06256213
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to direct current (DC) power supplies and, more specifically, to DC power supplies wherein novel output boost and input buck regulation means are used to convert alternating current (AC) power to regulated DC power in an aerospace environment. The present invention has particular use with power regulation applications in the field of aerospace technology.
BACKGROUND OF THE INVENTION
Years ago aircraft electrical systems operated on low voltage DC power, for example, 6, 12 or 28 volts. However, the burgeoning use of electrical instrumentation such as radios increased the amount of power required on the aircraft. Oftentimes the added power demands could be managed by the low voltage DC power systems in spite of the modest capabilities of distribution systems. Soon, however, electric motors and other high power demand equipment began to appear on aircraft thereby making the use of low voltage DC power and the concomitant distribution system impractical. For example, a modest amount of power such as 6 kilowatts (KW) required large, heavy and inflexible cables to carry currents in excess of 200 amperes. To address these new power needs, AC aircraft power systems distributing power at 115 volts AC, three phase, 400 hertz (Hz) began to emerge. These AC systems distributed and switched power easily. Furthermore, the use of 400 Hz allowed electromagnetic devices such as motors and transformers to remain of modest size and weight.
While most instruments operate on 115 VAC and 400 Hz, during emergencies and when no engines are running, power is supplied by batteries. DC power is therefore needed to keep these batteries charged and also to supply power to DC devices on the aircraft, e.g., engine starters. Currently the most widespread means of this conversion is the transformer rectifier unit (TRU).
The purpose of a TRU is to convert AC power to DC. For the purposes of the description herein, AC power is taken to be 115 volts, line to neutral, three phase 400 Hz. DC power is taken to be 28 volts DC. These values are typical in aerospace applications and as such may a depending on the desired end use application. Such a TRU
100
is shown in
FIG. 1
(
FIG. 1
) and represents a configuration that has been well known in the art for decades. The three phase AC input is connected to a transformer primary
101
that may be wound in either a star or a delta configuration. There are two secondary windings, one star
102
and one delta
103
which are connected to six pairs of rectifiers
104
to provide 28 volts. Rectifying the six phases from the star and delta windings leads to a small ripple voltage that is 100 (1-cos 15)% of the output which corresponds to 0.95 volts peak to peak for a 28 volt system.
This TRU configuration has become a standard throughout the aerospace industry for several reasons. First, the 400 Hz AC frequency of aircraft systems affords the use of a small, light transformer. Second, the system is inherently very simple. Third, the semiconductor diodes used as rectifiers are rugged and can withstand very high overloads for sufficient time to clear circuit breakers or start aircraft power units. Fourth, due to the combination of the star and delta wound secondaries the output ripple is inherently low, thereby obviating the need for large filter capacitors. Fifth, the input power factor (a measure of the phase angle difference between input current and input voltage and also a measure of currents at harmonics of the input frequency) is close to unity with low input harmonic distortion. Last, the efficiency of the system is high. However, a drawback of this TRU configuration is that the output voltage is not regulated. It varies with the variations of input voltage and the output load, as increasing current causes increasing voltage drops across the transformer windings.
Regulating a TRU is no easy task if its beneficial features are to be retained. Electronic power supplies that have a unity power factor AC input and a regulated DC output are commonly available, but do not have the high efficiency of the TRU—90% and higher—nor do they provide high output currents that are ten times the rated output and greater.
One system that is presently in use has SCR (silicon controlled rectifier) switches in the transformer primary circuit in order to regulate the transformer input voltage. This particular system manufactured by Leland Electrosystems Inc. (Moore et al. U.S. Pat. No. 5,541,830) provides good regulation and will supply the required high overload currents but is inefficient, has poor input power factor and high input current harmonics. Known AC/DC power converters have utilized TRU's comprising a transformer having a primary winding which receives input AC power and one or more secondary windings which are coupled to one or more rectifier circuits. The rectified transformer output is thereafter filtered and regulated to produce DC output power. None however, embody the regulation means taught presently.
One type of regulated AC/DC power converter is disclosed in Terry et al. U.S. Pat. No. 2,642,558. This power converter includes a transformer having a primary winding coupled to an AC source and multiple secondary windings, one of which is coupled to a rectifier bridge for providing approximately half the desired output voltage at full load. A pair of additional secondary windings are coupled to the anodes of a pair of gas discharge tubes, the cathodes of which are coupled together to an output filter of the device. The gas discharge tubes are fired at a variable point in each half cycle of the AC waveform to in turn supply the proper additive voltage to the output of the full wave bridge so that the output voltage is maintained at a regulated value.
Yenisey U.S. Pat. No. 3,270,270 discloses a regulated power supply wherein an input AC power source is coupled to a first transformer primary winding. The AC power source is further coupled through a controlled switch to a second primary winding of the transformer. A pair of secondary windings of the transformer are coupled through rectifiers to output terminals of the device. A main portion of the output power is provided through the first primary winding while intermittent or variable power modulated by the controllable power switch is provided through the second primary winding and combined with the main output power to regulate the same. In a further embodiment of the power supply, intermittent power is provided through each of a pair of primary windings, one of which adds intermittent power to the unregulated power and the second of which subtracts intermittent power from the unregulated power.
Another regulated AC/DC power supply is disclosed in Higuchi et al. U.S. Pat. No. 4,232,363. Higuchi et al. teach a closed loop regulated AC-DC power supply comprising a power transformer having two secondary windings and a compensating transformer having two compensating windings individually in a series circuit with the principle power supply secondary windings. The other secondary windings on the power transformer supply exciting windings for compensating variations in output potential by control of the compensating transformer circuitry which has an input connected across the load and an output for varying the AC supply to the exciting windings inversely of variations in load potential. A differential amplifying circuit is used for determining the load potential error and driving a transistor effectively to insert a DC component in inverse feedback relationship into the exciting windings of the compensating transformer.
Glennon et al. U.S. Pat. No. 4,739,466 teaches a regulated AC/DC converter including a transformer having a primary winding which receives AC input power and a secondary winding, a rectifier coupled to the secondary winding for rectifying the output of the same, a switching circuit coupled to the rectifier for developing trim voltage which is combined with the rectified output of the secondary winding to develop voltage-regulated DC output powe
Avionic Instruments Inc.
Laxton Gary L.
Ward & Olivo
Wong Peter S.
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