Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2003-02-18
2004-02-03
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S098000, C363S056020, C363S056030
Reexamination Certificate
active
06687136
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power supply used for performing arc-machining operations such as arc welding, arc cutting and plasma arc-machining. In particular, it relates to an arch-machining power supply that can reduce switching loss incurred when direct current is converted to high-frequency alternating current by a switching device.
2. Description of the Related Art
FIG. 1
of the accompanying drawings is a circuit diagram illustrating a conventional power supply used for performing the arc-machining of an object
2
with a torch
1
. The sign DR
1
refers to a primary rectifier for converting the output from the commercial power source (AC) to direct current. The sign Cl refers to a capacitor for smoothing the voltage of the converted direct current. The combination of the primary rectifier DR
1
and the smoothing capacitor Cl provides the DC power source of the illustrated system.
The conventional power supply system includes an inverter provided by the bridge connection of first to fourth switching elements TR
1
~TR
4
. The first and the fourth elements TR
1
, TR
4
make a first switching pair, while the second and the third elements TR
2
, TR
3
make a second switching pair. For conversion of the direct current to the high-frequency alternating current, the first and the second switching pairs are alternately turned on and off in accordance with the first to fourth driving signals Tr
1
~Tr
4
supplied from a switch driver, or inverter driver SD.
When the switching elements TR
1
~TR
4
are changed from the on-state to the off-state, a high voltage (surge) of the polarity reverse to that of the elements TR
1
~TR
4
may occur. To protect the elements TR
1
~TR
4
from the surge, four diodes DR
3
~DR
6
are connected in parallel to bypass the elements TR
1
~TR
4
. A main transformer INT, connected to the inverter, is provided for changing the primary voltage to a secondary voltage suitable for arc-machining. The secondary coil of the transformer INT is connected to a secondary rectifier DR
2
that converts the AC output of the transformer INT to direct voltage for the arc-machining. This voltage is supplied via a direct current reactor DCL.
An output current detector ID outputs an output current detection signal Id. A comparison operator ER compares this detection signal Id with an output current setup signal Ir, and produces a comparison signal Er=Ir−Id. An output controller SC performs PWM (pulse width modulation) control, in which the frequency of the pulse remains the same, while the width of the pulse is varied. Specifically, based on the comparison signal Er, the output controller SC controls the pulse width of a first output control signal Sc
1
(see Sc
1
in
FIG. 2
) and that of a second output control signal Sc
2
(see Sc
2
in the same figure).
The switch driver SD outputs first and fourth driving signals Tr
1
, Tr
4
, both of which are identical, based on the first output control signal Sc
1
, and also outputs second and third driving signals Tr
2
, Tr
3
, both of which are identical, based on the second output control signal Sc
2
.
FIG. 2
is a timing chart showing the relationships among the first output control signal Sc
1
, the second output control signal Sc
2
, the first driving signal Tr
1
(which is the same as the fourth driving signal Tr
4
), the second driving signal Tr
2
(which is the same as the third driving signal Tr
3
), the superposed collector-emitter voltage V
1
(solid lines) & collector current Ic
1
(broken lines) of the first switching element TR
1
, and the superposed collector-emitter voltage V
2
(solid lines) & collector current Ic
2
(broken lines) of the second switching element TR
2
.
The workings of the first and the second switching elements TR
1
, TR
2
will now be described. It should be noted that the third and the fourth switching elements TR
3
and TR
4
behave in the same manner as the first and the second switching elements, and therefore they will not be discussed below.
First, the startup switch TS shown in
FIG. 1
outputs a startup signal Ts to the output controller SC. Upon receiving the signal, the controller SC outputs the first output control signal Sc
1
and the second output control signal Sc
2
shifted half cycle relative to the first output control signal Sc
1
. As shown in
FIG. 2
, the first and the second output control signals Sc
1
, Sc
2
have pulse durations T
1
and T
2
, respectively, that are determined by the comparison signal Er (=Ir−Id).
In general, the switching elements will take a relatively long time to change from the on-state to the off-state than from the off-state to the on-state. Due to this, without taking any countermeasures, the turn-on states of the first and the second switching pairs would overlap, whereby “arm short-circuiting” occurs. To prevent this, there is an appropriate pause T
7
(see
FIG. 2
) between the on-state of the drive signal Tr
1
and the on-state of the drive signal Tr
2
.
At t=t
1
, the switch driver SD outputs the first drive signal Tr
1
and the fourth drive signal Tr
4
. Upon receiving this, the first and the fourth elements TR
1
, TR
4
change from the off-state to the on-state. At this time, a switching loss (called “turn-on loss” below) occurs, as represented by the region Ln
1
in FIG.
2
.
At t=t
2
(FIG.
2
), the switch driver SD, in synchronism with the first output control signal Sc
1
, turns off the first and the fourth drive signals Tr
1
, Tr
4
. Accordingly, the first and the fourth elements TR
1
, TR
4
change from the on-state to the off-state, which results in the switching loss, or “turn-off loss”, as shown by the region Lf
1
. In addition to this, saturation loss (not shown) will occur when the first and the fourth elements TR
1
, TR
4
are operating in the saturation region during the on-period T
3
.
When the above-mentioned pause T
7
expires, the first and the fourth elements TR
1
, TR
4
, for example, change from the on-state to the off-state, while the second and the third elements TR
2
, TR
3
have already been in the off-state. Thus, a surge voltage will occur across the emitter and the collector of the first and the fourth elements TR
1
, TR
4
. The surge voltage is conducted through the bypassing diodes DR
3
~DR
6
, to be absorbed by the smoothing capacitor C
1
.
The turn-off loss will now be described. During the transition period from the on-state to the off-state, the first and the fourth elements TR
1
, TR
4
are unsaturated. At this time, the collector current Ic
1
of the first element TR
1
(and that of the fourth element TR
4
) reduces than when the element is saturated, while the collector-emitter voltage V
1
of the element TR
1
(and that of the element TR
4
) increase. The turn-off loss is determined by the product of the collector current Ic
1
and the collector-emitter voltage V
1
(see the region Lf
1
in FIG.
2
). If IGBTs (Insulated Gate Bipolar Transistors) are used for the first and the fourth elements TR
1
, TR
4
, the collector current Ic will become zero rather slowly after the collector-emitter voltage V
1
arises. As a result, the turn-off loss becomes greater.
The turn-on loss will now be described. During the transition from the off-state to the on-state, the first and the fourth elements TR
1
, TR
4
(or TR
2
, TR
3
) become saturated. Due to this, the collector-emitter voltage V
1
of the first element (and that of the fourth element as well) decreases than in the off-state, while the collector current Ic
1
of the first element (and that of the fourth element) increases. The product of the collector current Ic
1
and the collector-emitter voltage V
1
produces the turn-on loss (see the region Ln
1
shown in FIG.
2
). The turn-on loss is very small in comparison with the turn-off loss, and its effect is negligible.
Next, the saturation loss will be described. When the first and the fourth elements TR
1
, TR
4
are saturated in the on-state, the collector current Ic of the first element TR
1
is a rated curr
Doi Toshimitsu
Manabe Haruhiko
Morimoto Keiki
Daihen Corporation
Merchant & Gould P.C.
Vu Bao Q.
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