Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2001-12-26
2002-08-20
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S056040, C363S098000, C363S132000
Reexamination Certificate
active
06438004
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to a power conversion apparatus. More specifically, the present invention relates to a power conversion apparatus having a control circuit capable of achieving soft-switching of a switching element.
BACKGROUND ART
In a power conversion apparatus such as a converter or an inverter, various circuits having a soft-switching function are currently under development in order to reduce a switching loss in each switching element and increase a switching frequency.
For example, U.S. Pat. No. 5,047,913 discloses a converter in which the voltage of a DC (Direct Current) power supply is split into two one-half voltages, by a pair of series-connected capacitors, and the junction between the capacitors is connected to the junction between a pair of series-connected main switches, through a circuit including a series connection of a bidirectional switch and an inductor. A load is connected to the junction between the main switches. A diode is connected in parallel with each of the main switches to allow each of the diodes to become reverse biased with respect to the DC power supply. A snubber capacitor is connected in parallel with each of the main switches. In the power conversion apparatus described in this US Patent, it is intended to obtain a specific condition for achieving soft-switching of all switches by making up an auxiliary resonant commutation circuit with the circuit of the bidirectional switch and the inductor makes up and performing resonant operation through the auxiliary resonant commutation circuit.
In the above circuit, while such soft-switching is achieved when each of the main switches are turned on, a turn-off loss is caused by turning off each of the main switches. Specifically, in this circuit, for commutating from one of the main switches to the other main switch, the bidirectional switch is first turned on in the state when a load current is refluxed to the diode connected in parallel with the one main switch, so as to generate a resonant state. Then, when the current of the inductor increases up to a sufficient extent for commutation, the above one main switch is turned off. However, the control taught in this US Patent is inevitably involved with turn-off loss of the main switches. The necessity for detecting of a resonant current required for commutation also forces a complicated control.
Furthermore, in this prior art circuit, for preventing the accumulated energy in the snubber capacitor connected to each of the main switches from being consumed as short-circuit loss in the main switches under light load, the auxiliary switch is turned on before switching the main switches to commutate from one of the main switches to the other main switch. In this control, upon turning on the auxiliary switch, the inductor current starts passing through the one main switch along with the load current. When the current goes up to a certain threshold, the one main switch is turned off to charge or discharge the energy in each of the snubber capacitors. After the completion of commutation, the other main switch is turned on. Thus, a turn-on at zero current is achieved in the other main switch, and the energy of the snubber capacitor does not become a loss. However, the control taught in this US Patent is undesirably involved with complexity in control due to the switching control according to detecting the inductor current required for commutation.
Moreover, in the control taught by the US Patent, for commutating with passing the load current through the diode connected in parallel to the other main switch in the state when the load current passes through the other main switch, the commutation in large load current is achieved based on the load current without activating the auxiliary resonant commutation circuit. In small load current, the commutation is achieved based on the sum of the resonant current and the load current with activating the auxiliary resonant commutation circuit. This control process undesirably involves a ripple voltage caused by operating the power conversion apparatus as an inverter. Specifically, when the power conversion apparatus is operated as an inverter according to this control process, the current of the auxiliary resonant commutation circuit generates a ripple having the same cycle as that of the output voltage of the inverter at the midpoint of the potential of the capacitor connected in series with the DC power supply. If it is attempted to suppress this ripple voltage within the allowable range of voltage variation, it will be required to employ capacitors having larger capacity than those of conventional circuits, resulting in larger size components.
Japan Patent Laid-Open Publication No. Hei 07-115775 discloses an inverter in which one ends of auxiliary switches is connected respectively to a first split point having a first potential and a second split point having a second potential, the other ends of the auxiliary switches being connected with each other, and the junction between the auxiliary switches being connected with the junction between a pair of main switches through a resonant inductor. A snubber capacitor is connected in parallel with each of the main switches. A diode is connected in parallel with each of the main switches and in the reverse bias direction with respect to a DC power supply. In the circuit for a power conversion apparatus disclosed in this Patent Laid-Open Publication, an auxiliary resonant commutation circuit is formed of the auxiliary switches, the resonant inductor, and the snubber capacitors each connected in parallel with the main switches so as to achieve soft-switching based on resonant current passing through the formed resonant circuit.
The power conversion apparatus described in this Patent Laid-Open Publication employs a battery to obtain the first and second potentials. This undesirably makes the circuit larger in size. If a capacitor is used to provide smaller size apparatus, a ripple voltage having the same cycle as that of the output voltage of the inverter will be generated between the first and second potentials. This causes the same problem as that of the circuit described in the above US Patent occurs.
DISCLOSURE OF THE INVENTION
In view of the aforementioned problem in conventional power conversion apparatuses intended for achieving soft-switching, it is therefore a primary object of the present invention to provide an improved power conversion apparatus comprising a control circuit for generating a switching signal at the timing allowing soft-switching to be achieved, and free from any occurrence of ripple.
In order to achieve this object, a power conversion apparatus according to the present invention includes at least a pair of main switches composed of serial-connected first and second main switches, wherein one of the ends of the first main switch is connected with the positive side of a DC power supply, and one of the ends of the second main switch is connected to the negative side of the DC power supply. The power conversion apparatus further includes a diode connected in parallel with each of the main switches so as to become reverse biased with respect to the DC power supply, a main-switch snubber capacitor connected in parallel with each of the main switches, a load connected with the junction between the pair of main switches, and a control circuit for forming a switching signal for controlling the switching operation of the main switches by using a load voltage and/or a load current as an input thereof, wherein the main switches are controllably switched according the switching signal from the control circuit so as to generate an output. Based on the above construction, the power conversion apparatus of the present invention comprises a first auxiliary resonant circuit including serial-connected first and second auxiliary switches and a resonant inductor connected in series with the second auxiliary switch, wherein the first auxiliary resonant circuit is connected with each of the positive side of the DC power supply and the
Itoh Kazuyuki
Okita Yoshihisa
Tanaka Katsuaki
Cohen & Pontani, Lieberman & Pavane
TDK Corporation
Vu Bao Q.
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