Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-03-16
2001-05-22
Patel, Rajnikant S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S057000, C318S722000
Reexamination Certificate
active
06236576
ABSTRACT:
FIELD OF THE ART
The invention relates to a resonance inverter as defined in the introductory portion of claim
1
, and to a method of maintaining an oscillation in a resonance circuit of a resonance inverter as defined in the introductory portion of claim
6
.
U.S. Pat. No. 5,111,374 discloses a resonance inverter of the short-circuit type. The resonance inverter described in the patent specification comprises a voltage source or a DC bus with an associated resonance circuit, which can in turn feed or be fed by an associated switch circuit consisting of a plurality of power switches each having antiparallel-coupled diodes.
However, the above-mentioned circuit has the drawback that maintenance of a resonance oscillation in the resonance part of the circuit is complicated and inexpedient. The resonant oscillation is thus initialized and maintained by short-circuiting the resonance capacitor. First, the power switches are subjected to stress, which is particularly pronounced when an oscillation is initiated, which, other things being equal, influences the service life of the inverter. Second, the application range is restricted with respect to the achievable maximum voltage, and, third, the short-circuiting method used requires comprehensive current control of the oscillation state in the resonance circuit.
Another problem which occurs in this connection is to determine when the switches of the resonance inverter are to be turned on and off, as it is not given that a given threshold reference, e.g. 0 V across the switches, is achieved at all in the oscillation course concerned. Thus, it is extremely difficult and cumbersome to synchronize the switch control with the oscillation of the resonance circuit.
SUMMARY OF THE INVENTION
When, as stated in claim
1
, the resonance inverter additionally comprises a pulse generating circuit having at least one control inductance which is inductively coupled to at least one of the inductances of the resonance circuit, it is ensured that energy may be fed to the resonance circuit from a pulse generating circuit via an inductive coupling. As a result of this, the resonance circuit may be initiated and controlled very specifically to a desired oscillation course, no matter whether start of an oscillation in the resonance circuit is desired or this is to be maintained in a simple manner.
The invention thus enables the resonance circuit to be controlled in a simple manner, thereby obviating component stress, frequent power downs and consequent restart of the circuit.
It should moreover be noted that undesired transients may be damped in an optimum manner or be avoided completely, as sudden voltage jumps because of e.g. short-circuit initiation do not occur.
It should also be noted that a control circuit for controlling the oscillation state of the resonance circuit may be simplified considerably, as an oscillation in the resonance circuit may be made self-regulating so to speak.
Thus, according to the invention it is possible to avoid the situation that oscillations in the resonance circuit cease completely because the oscillation does not get down to 0 V in its lower oscillation state.
It should moreover be noted that the very simple control of the resonance circuit provides a very predictable and uniform reproducible output of the resonance circuit, and the control algorithm for the switch circuit may therefore be simplified. One of the reasons of this is that zero crossings on the output of the resonance circuit provide a very simple basis for detection and control.
Additionally, the invention allows the control circuit to be insulated galvanically from the resonance circuit itself in the inverter.
When the pulse generating circuit is coupled inductively to the resonance circuit, it is additionally ensured that the physical properties of the resonance circuit may control the pulse generation, e.g. by allowing the generation of pulses to be dependent on the sign of the voltage of the inductance in the oscillation circuit.
Thus, according to the invention it is possible to achieve a very high efficiency, as the resonance circuit may optimally be applied such that the switches in the switch circuit have minimum oscillation losses, as on-off switching may be performed at optimally 0 V, just as a minimum dv/dt for each switch may be obtained.
The invention thus makes it possible to compensate for ohmic losses in the resonance circuit by inductively feeding energy to the resonance circuit.
According to the invention it is also possible to carry out a “soft” start of the resonance oscillation of the circuit, so that the desired resonance oscillation is achieved after a reasonable period of time without unpleasant transients or overshoot, just as it is possible to maintain the oscillation with a minimum feed of energy.
It should moreover be noted that, in principle, the resonance inverter may be arranged to be bidirectinal.
Power electronic components may e.g. be formed by bipolar transistors, GTO's, IGTB's or the like.
When, as stated in claim
2
, the power electronic components of the resonance inverter are antiparallel-coupled to at least one diode, a particularly advantageous embodiment of the invention is achieved, as the antiparallel-coupled diodes advantageously impart bidirectional properties to the system.
When, as stated in claim
3
, the circuit additionally comprises a switch circuit having at least one power electronic component, said switch circuit being electrically connected to the resonance circuit of the resonance inverter so that the resonance circuit is connected to at least two switch circuits, said resonance circuit also comprising at least one DC collecting capacitance, an AC-DC-AC converter is achieved, capable of bidirectionally connecting two AC networks or generators via the DC collecting capacitance in a manner known per se.
Thus, according to the invention it is possible to achieve a transient-free or approximately transient-free and dynamic coupling between two AC networks with minimum power losses.
If a bidirectional connection between each AC side is desired, it will be appreciated that the individual switches in the switch circuits must be bidirectional.
When, as stated in claim
4
, the frequency generating circuit comprises means for detecting the sign of the voltage on either a control inductance and/or at least one of the inductances of the resonance circuit, said frequency generating circuit comprising means for generating current pulses in at least one control inductance so that current and voltage in the control inductance or inductances are completely or partly in in-phase, it is possible to initiate, control and maintain oscillations in the resonance circuit in a simple manner, as the natural oscillation property of the resonance circuit and especially the actual oscillation state of the resonance circuit determine the transient via the inductive coupling between the inductance of the resonance circuit and the inductance of the pulse generating circuit.
The decisive thing is thus that energy is transferred from the pulse generating circuit to the resonance circuit in at least a part of the oscillation period.
When the pulse generating circuit emits current in dependence on the phase of the voltage across one of the above-mentioned inductances, a combined double feedforward and feedback between the pulse generating circuit and the resonance circuit is thus obtained.
It should additionally be noted that, according to the present embodiment, it is extremely simple to construct the means for detecting the sign of the voltage on the control inductance, since this may be made in a simple manner by means of a single comparator which is coupled to the windings of the control inductance.
It should moreover be noted that the present embodiment is extremely simple and elegant when it is constructed exclusively galvanically insulated from the resonance circuit, since this may be made in low power electronics which is loaded only by low voltage.
In certain circuits, it will be preferred according to the inventi
APC Danmark A/S
Dykema Gossett PLLC
Patel Rajnikant S.
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