Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-02-10
2002-01-29
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
Reexamination Certificate
active
06342791
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-033195, filed Feb. 10, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a diode defect detecting device of a DC load supply apparatus used in, e.g., an inverter type resistance welding machine.
FIG. 1
shows an example of a controller of a conventional inverter type resistance welding machine.
An inverter circuit in which IGBT switching elements
2
,
3
,
4
, and
5
are bridge-connected is connected in parallel with a DC power supply
1
, thereby converting a direct current into an alternate current. This alternate current is supplied to the primary side of a transformer
7
.
A load resistance
11
is connected to the secondary side of the transformer
7
. An alternate current on this secondary side is rectified into a direct current by a rectifying circuit in which a plurality of diodes
8
and
9
are connected in a center tap manner. This direct current is supplied to the load resistance
11
.
A current sensor
6
senses the primary current of the transformer
7
. A current detector
15
detects this current after converting it into a direct current. An overcurrent detector
16
detects an inverter output overcurrent from the output from the current sensor
6
.
The diodes
8
and
9
convert the secondary alternate current of the transformer
7
into a direct current and supplies the current to the load resistance
11
via a wiring inductance
10
. This wiring inductance
10
is not particularly formed; it represents the inductance present in wiring.
An output current reference value from a current reference unit (current reference circuit)
12
is input to an amplifier
14
when a start switch
13
is turned on. This amplifier
14
amplifies any deviation of the output from the current detector
15
from the output from the current reference unit
12
. This amplified deviation and a triangular-wave frequency output from a triangular-wave generator
17
are input to a PWM circuit
18
. On the basis of these outputs from the amplifier
14
and the triangular-wave generator
17
, the PWM circuit
18
outputs a PWM signal. This PWM signal is applied to the gates of the switching elements
2
to
5
via a polarity changing circuit
19
in order to switch the polarity of the inverter output.
As described above, when the diodes
8
and
9
are normal the current control is performed such that the peak value of the primary current of the transformer
7
corresponds to the current reference.
FIG. 2
is a timing chart for explaining an operation when only the diode
8
is shorted in FIG.
1
. At time t
1
, the start switch
13
is turned on to apply an inverter output voltage V
1
having pulse widths t
1
and t
2
to the primary side of the transformer
7
.
Consequently, on the secondary side of the transformer
7
a transformer primary current I
1
rises in a circuit of diode
8
(shorted)→wiring inductance
10
→load resistance
11
. When the inverter output voltage V
1
is shut off at time t
2
, the transformer primary current I
1
becomes zero.
Next, the inverter output voltage V
1
is inverted at time t
3
. Since the transformer secondary side is shorted by a circuit of diode
9
→diode
8
(shorted), the transformer primary current I
1
rapidly rises. At time t
4
at which this transformer primary current I
1
reaches the detection level of the overcurrent detector
16
, the overcurrent detector
16
detects an overcurrent, and the inverter stops operating.
If the pulse width of the inverter output voltage is smaller than the interval between t
3
and t
4
, the inverter sometimes keeps operating because the overcurrent level is not reached.
The conditions are affected by the magnitude of the current reference or the response (generally, PI control is used) speed of the amplifier
14
.
As described above, when the diodes
8
and
9
are short-circuited in the controller of the conventional inverter type resistance welding machine, depending on the conditions the overcurrent detector
16
can detect an overcurrent or the overcurrent detection level is not reached. This makes the detection unstable. At the same time, the inverter operates at a current slightly lower than the overcurrent detection level. This applies stress on the switching elements
2
to
5
constructing the inverter and may degrade the reliability of the inverter. Furthermore, where the diodes are shorted, a normal current can not flow into a welding object, thus a problem arises that defective items flows to following process.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a diode defect detecting device capable of reliably detecting defects of diodes without applying the stress to switching elements of an inverter, thereby improving reliability of the device and detecting a defect of the diode at an earliest stage.
To achieve this object, according to the first aspect of the present invention, there is provided a diode defect detecting device used in a controller in which an inverter converts a direct current into an alternate current and supplies the alternate current to a primary side of a transformer, and a rectifying circuit having a plurality of diodes connected to a secondary side of the transformer rectifies an alternate current induced on the secondary side into a direct current and supplies the direct current to a load, comprising:
a current detector for detecting a primary current of the transformer;
a comparator for comparing the detection current detected by the current detector with a current reference and outputting a reset signal if the detection current is larger than the current reference;
an oscillator for generating a clock signal;
a flip-flop circuit for receiving a set signal on the basis of a front and an end edge of the clock signal generated by the oscillator and receiving the output reset signal from the comparator;
a polarity changing circuit for outputting a polarity changing signal for changing polarities of the plurality of diodes on the basis of the output clock signal from the flip-flop circuit; and
a determination circuit for calculating, in order to detect malfunctions of the plurality of diodes, an impedance change on the secondary side viewed from the primary side of the transformer on the basis of a pulse width of each polarity of the output clock signal from the flip-flop circuit.
According to the second aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the determination circuit calculates the impedance change by using a modulation factor of the pulse width of each polarity and a peak value of the primary current detected by the current detector, thereby detecting a diode malfunction.
According to the third aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the determination circuit calculates the impedance change by using a modulation factor of the pulse width of each polarity and the current reference, thereby detecting a diode malfunction.
According to the fourth aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the determination circuit detects a diode malfunction when the modulation factor of the pulse width of each polarity becomes equal to or smaller than a predetermined value.
According to the fifth aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the determination circuit obtains a pulse width of each polarity during a few cycles after the inverter is started.
According to the sixth aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the determination circuit detects a diode malfunction if a value obtained by dividing a pulse width of one polarity by a pulse width of the othe
Fujino Suteo
Ichikawa Kinya
Izume Takatomo
Kurata Koichiro
Okado Chihiro
Kerveros James C
Nas-Toa Co., Ltd.
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