Resonant inverter circuit

Details Resonant inverter circuit Resonant inverter circuit

2002-04-05

2003-11-04

Patel, Rajnikant B. (Department: 2838)

Electric power conversion systems

Current conversion

Using semiconductor-type converter

C363S098000

Reexamination Certificate

active

06643157

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inverter circuit for driving a load such as a motor, and relates particularly to a resonant inverter circuit comprising a snubber capacitor for performing soft switching.
2. Description of the Related Art
Examples of conventional inverter circuits for driving a load such as a motor include the technology disclosed in U.S. Pat. No. 5,710,698, U.S. Pat. No. 5,642,273 and U.S. Pat. No. 5,047,913. For example, as shown in
FIG. 8
, in a soft switching inverter according to conventional technology, a motor
1
comprising a three phase induction motor or a DC brushless motor or the like is connected to the soft switching inverter as a load, and comprises, for example, an inverter using IGBT (Insulated Gate Bipolar Transistor) Q
1
to Q
6
as switching elements.
In the inverter, the IGBT Q
1
to Q
6
are connected to both sides of a DC power source
3
in a three phase bridge structure comprising a U phase, a V phase and a W phase. Free wheeling diodes (FWD) D
1
to D
6
are connected between a collector terminal and an emitter terminal of each IGBT for the purpose of circulating the regenerative energy produced by the inductive load of the motor
1
and the electric current energy stored by the inductive load. Furthermore, snubber capacitors C
1
to C
6
for absorbing the surge voltage applied between the collector terminal and the emitter terminal of the IGBT during turn-on or turn-off are connected between the collector terminal and the emitter terminal of each IGBT.
In addition, the DC power source
3
and a smoothing capacitor C
9
are connected to the inverter. Mid-point voltage storage capacitors C
7
and C
8
for storing a mid-point voltage are connected in series to both sides of the smoothing capacitor C
9
. An inductance L
1
which resonates with the snubber capacitors C
1
and C
2
, and a bi-directional switching unit SU
1
for channeling the resonant current via the inductance L
1
are connected between the connection point of the mid-point voltage storage capacitors C
7
and C
8
and the connection point of the snubber capacitors C
1
and C
2
of the U phase. In a similar manner, an inductance L
2
which resonates with the snubber capacitors C
3
and C
4
, and a bi-directional switching unit SU
2
for channeling the resonant current via the inductance L
2
are connected between the connection point of the mid-point voltage storage capacitors C
7
and C
8
and the connection point of the snubber capacitors C
3
and C
4
of the V phase. In addition, an inductance L
3
which resonates with the snubber capacitors C
5
and C
6
, and a bi-directional switching unit SU
3
for channeling the resonant current via the inductance L
3
are connected between the connection point of the mid-point voltage storage capacitors C
7
and C
8
and the connection point of the snubber capacitors C
5
and C
6
of the W phase.
A configuration as shown above may also be called an auxiliary resonant commutated arm link type snubber inverter. In such a soft switching inverter, if for example the IGBT Q
1
is turned off, and the IGBT Q
2
is then turned on after a short delay, the charging current of the snubber capacitor C
1
and the discharging current of the snubber capacitor C
2
flow through the mid-point voltage storage capacitors C
7
and C
8
via the inductance L
1
. At the same time, if the IGBT Q
4
and Q
6
are turned off, and the IGBT Q
3
and Q
5
are then turned on after a short delay, the charging current of the snubber capacitors C
4
and C
6
and the discharging current of the snubber capacitors C
3
and C
5
are supplied from the mid-point voltage storage capacitors C
7
and C
8
via the inductance L
2
and L
3
.
By charging and discharging the snubber capacitor according to the resonant current of the snubber capacitor and the inductance in this manner, when the IGBT is turned off and the snubber capacitor is charged, because the rise in the voltage applied to the IGBT is delayed according to the time constant applied by the snubber capacitor, ZVS (Zero Voltage Switching) of the IGBT can be realized. Conversely, if the snubber capacitor is discharged before the IGBT is turned on, a free wheeling diode conducts and the voltage and current applied to the IGBT becomes zero, thereby realizing ZVS (Zero Voltage Switching) and ZCS (Zero Current Switching) of the IGBT. Consequently, the loss which occurs during turn-on and turn-off of the switching elements such as the IGBTs, can be reduced.
Furthermore,
FIG. 9
also shows a soft switching inverter according to conventional technology, which may also be called an auxiliary resonant AC link snubber inverter. In a similar manner as in the auxiliary resonant commutated arm link type snubber inverter shown in
FIG. 8
, a smoothing capacitor C
9
and the inverter are connected to both sides of a DC power source
3
. In the inverter, the IGBTs Q
1
to Q
6
, to which are connected free wheeling diodes D
1
to D
6
and snubber capacitors C
1
to C
6
respectively, are connected in a three phase bridge structure comprising a U phase, a V phase and a W phase. An inductance L
4
which resonates with the snubber capacitors C
1
and C
2
, and a bi-directional switching unit SU
4
for channeling the resonant current via the inductance L
4
are connected between the connection point of the snubber capacitors C
1
and C
2
of the U phase of the inverter and the connection point of the snubber capacitors C
3
and C
4
of the V phase of the inverter. Furthermore, an inductance L
5
which resonates with the snubber capacitors C
3
and C
4
, and a bi-directional switching unit SU
5
for channeling the resonant current via the inductance L
5
are connected between the connection point of the snubber capacitors C
3
and C
4
of the V phase of the inverter and the connection point of the snubber capacitors C
5
and C
6
of the W phase of the inverter. In addition, an inductance L
6
which resonates with the snubber capacitors C
5
and C
6
, and a bi-directional switching unit SU
6
for channeling the resonant current via the inductance L
6
are connected between the connection point of the snubber capacitors C
1
and C
2
of the U phase of the inverter and the connection point of the snubber capacitors C
5
and C
6
of the W phase of the inverter.
The only difference between the auxiliary resonant AC link snubber inverter shown in FIG.
9
and the auxiliary resonant commutated arm link type snubber inverter shown in
FIG. 8
is the path of the electric current for charging and discharging the snubber capacitors, and the principles involved in achieving ZVS and ZCS at each of the IGBT switching elements are the same.
In a soft switching inverter according to the above conventional technology, the electric current which flows through the IGBT (the switching elements) and the voltage applied to the IGBT can be controlled by forming a resonant circuit comprising the snubber capacitor and each inductance. Consequently, this is effective in reducing the loss which occurs in the switching elements during turn-on or turn-off.
However, because the core capacity required for the inductance is determined by the peak conducted current, as the controlled load current increases, the weight and capacity of the inductance also increases. Consequently, a problem arises in that a soft switching inverter according to conventional technology, which requires three inductances with an electric current which is at least as large as the load current, cannot be made smaller or lighter due to the increase in weight and capacity required for the inductances.
SUMMARY OF THE INVENTION
In consideration of the above circumstances, an object of the present invention is to provide a resonant inverter circuit that can be made lighter in weight and smaller in capacity.
In order to resolve the above problems, a resonant snubber inverter circuit according to the present invention comprises: six main switching elements (such as IGBT Q
1
to Q
6
of the embodiment) which either conduct or are cutoff by mean

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