THEORY OF OPERATION
E-4 E-4
POWER MIG 350MP
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OUTPUT RECTIFIER, CHOPPER
ASSEMBLY AND FEEDBACK
The 60 VAC output from the main transformer sec-
ondary is connected to an output rectifier. The resul-
tant 80 VDC is applied to four parallel capacitors incor-
porated within the Chopper Assembly. These capaci-
tors function as filters and also as power supplies for
the insulated gate bipolar transistors or IGBT. See
IGBT Operation in this section. The IGBT acts as a
high-speed switch operating at 20KHZ. This device is
switched on and off by the Weld Control Board through
pulse width modulation circuitry. See Pulse Width
Modulation in this section. This "chopped" DC output
is applied through an output choke coil to the positive
output terminal and through a current transducer to the
negative output terminal. The choke functions as a
current filter. A free-wheeling diode is incorporated in
the Chopper Assembly to provide a current path for the
stored energy in the choke when the IGBT is turned off.
See Chopper Technology Fundamentals in this sec-
tion.
Output voltage and current feedback information is fed
to the Weld Control Board. This information is sensed
from the current transducer and voltage sense circuitry
on the output terminal circuits. If current or voltage
become abnormally high, the Weld Control Board will
shut off the IGBTs, thus disabling the machine's output.
NOTE: Unshaded areas of Block Logic Diagram are the subject of discussion.
FIGURE E.4 – OUTPUT RECTIFIER, CHOPPER ASSEMBLY AND FEEDBACK
