Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-01-31
2001-02-20
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S098000, C363S132000
Reexamination Certificate
active
06191957
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrical power converters and, more particularly, to an electric power converter that can operate with an input voltage varying over a wide range.
BACKGROUND OF THE INVENTION
Converter circuits, such as DC-to-DC converters, are often used in electronic systems of the type, such as avionics systems and the like, where an electronic-regulated power supply is required to operate even though energized with an input voltage which varies over a very wide input voltage range. One such regulated power supply is commonly known as a boost converter. In general, a boost converter circuit operates to boost the input voltage to generate a higher output voltage. A conventional boost converter circuit
10
is depicted in
FIG. 1
(Prior Art), wherein a DC input voltage Vin is applied at an input terminal
10
a
with reference to a common terminal
10
c.
An output voltage Vout is developed at an output terminal
10
b
with reference to common terminal
10
c
(e.g. ground potential), and thus appears across a capacitor
18
. An inductor
12
has a first terminal
12
a
coupled to input terminal
10
a
and a second terminal
12
b
coupled to both the anode of a rectifier diode
14
and a drain element of a switching device
16
. As one skilled in the art understands, switch
16
(which is coupled between the output side of the boost inductor
12
and ground terminal
10
c
) is switched on and off responsive to the switching device gate electrode drive signal, which has a duty cycle (i.e. ratio of ‘on’ or ‘off’ portion to an entire on-off cycle) D, which is never greater than
1
. In each switching cycle or duty cycle D, energy is stored in the inductor
12
when the switch is closed or conducting (ON period) and released to output terminal
10
b
via diode rectifier
14
when the switch is opened or non-conductive (OFF period). Thus, energy is stored in inductor
12
such that energy output from the inductor upon discharge is added to the input voltage Vin to produce an output voltage Vout that is greater than the input.
FIGS. 2A and 3A
illustrate conventional enhancements to the basic boost configuration shown in FIG.
1
.
FIG. 2A
shows a converter
10
′ with a conventional transformer
20
forming a push-pull transformer-coupled boost converter operated in a boost mode (greater than 50% duty cycle). The duty cycles DQ
1
and DQ
2
associated with the switching devices
16
-
1
and
16
-
2
for this circuit are shown in FIG.
2
B.
FIG. 3A
shows a conventional full-bridge transformer-coupled boost converter
10
″ operated in boost mode (greater than 50% duty cycle) with duty cycles DQ
1
and DQ
2
associated with the respective switching devices Q
1
, Q
3
and Q
2
, Q
4
driven by the switching waveforms as shown in FIG.
3
B. Each of the converters shown produces an output voltage according to the equation Vo=N*Vin/(1-D) where D is the duty cycle of the circuit and N is the secondary winding-to-primary winding turns ratio of the transformer
20
(N=1 if no transformer, as in converter
10
of FIG.
1
).
From the foregoing, one can ascertain that, in any of the circuits depicted in these Figures, the output voltage has a range between Vin and an extremely large value. That is, the output voltage cannot be less than the product of the input voltage and the turns ratio. Since the boost circuit only stores energy in excess of the input voltage, such a circuit is inherently higher efficiency than a circuit that must store the entire output energy, such as a conventional flyback or buck-boost converter system. However, the inability to control the output voltage to a value less than the input voltage can produce significant problems, even when normal operation requires an output voltage greater than the voltage at the input. For instance, at startup, the output voltage is zero while the input voltage, when applied, is usually non-zero. This can lead to a very large current applied to raise the output voltage from zero to the input voltage. In addition, an abnormal condition such as a fault or short circuit at the output may also produce a condition where the output voltage may be less than the input voltage. Under both of these conditions, a boost converter is uncontrolled and the currents produced are not controllable. To permit operation under these conditions, it is customary to add a second switch in series with the boost inductor, and a flyback diode, so as to operate the boost converter as a buck-mode converter. This, however, results in energy loss associated with the additional switch, even when that switch is not in use. In addition, in applications where a rectified alternating-current (AC) waveform, such as a rectified sine wave, is used as the input source, it may be desirable to operate at a voltage that is less than the peak voltage of the input. Conventional transformer-isolated boost converter circuits, such as those depicted in Prior Art
FIGS. 2A and 3A
, include additional switches that operate to open connections between the input and the output terminals in order to steer the transformer flux as well as control large currents caused by the above-described conditions. Opening of these switches, however, has the undesirable effect of interrupting the current flowing in the boost inductor. Since the energy stored in the boost inductor no longer has a path through which to flow, it will discharge through whatever element it can, thereby destroying the device. Thus, for conventional boost converters, operation in a buck mode (where the switches are off for a given time interval) is not permissible. Adding an additional winding to the boost inductor as disclosed in commonly assigned U.S. Pat. No. 5,654,881, entitled “Extended Range DC—DC Power Converter Circuit” issued Aug. 5, 1997 to Albrecht et al, the subject matter of which is herein incorporated by reference, allows the flux in the inductor to be continuous and produce a buck operating range where the output can be less than the input. However, use of additional windings and associated circuitry to provide an extended range converter proves to be quite costly in most applications. Furthermore, the voltage on the switches when the inductor is discharged may be less than optimal. Still further, it is known that boost converters suffer from parasitic losses such as loss due to leakage inductance, resulting in undesirable energy loss and circuit inefficiency. Accordingly, a power converter which overcomes these problems and which obviates the need for additional windings to operate over an extended range of voltages, is highly desired.
SUMMARY OF THE INVENTION
In accordance with the invention, a power converter comprises: a power transformer having a primary winding and a secondary winding with a secondary winding flux coupled to the primary winding; input terminals for receiving an input voltage; and output terminals for providing an output voltage. A power switching arrangement comprising two pairs of switching devices arranged in a bridge configuration has a corresponding duty cycle which is selectable so as to cause the power converter to manifest different input-output transfer characteristics corresponding to buck and boost modes of operation. An inductor having a single winding is coupled between the input terminal and the primary winding through the switching devices. Secondary winding rectification and filtering provides the power supply output. A reset operating circuit coupled to the output terminal of the single winding inductor and connected to the input terminals provides a current path for discharging the inductor during a predetermined time interval corresponding to that portion of the duty cycle when all of the switching devices are off, to enable operation of the power supply in a buck mode of operation.
A buck-boost converter can comprise a power transformer having a primary winding and a secondary winding with a secondary winding flux coupled to the primary winding, input terminals for receiving an input voltage, output term
Bae Systems Controls Inc.
Berhane Adolf Deneke
Krauss Geoffrey H.
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