ESAB EPP-400 Manuel D'instructions page 107

sectIon 4 operatIon
4.1 Block Diagram circuit Description (con't.)
The EPP-400 Block Diagram (after Subsection 6.4.4) shows the main functional elements of the power source. T1, the Main
Transformer, provides isolation from the primary power line as well as the proper voltage for the 310V DC Bus. The Bus Rec-
tifiers convert the three phase output of T1 to the 310V bus voltage. A capacitor bank provides filtering and energy storage
that supplies power to the high speed electronic switches. The switches are IGBT's (Insulated Gate Bipolar Transistors). The
310V bus provides power for both the Left (Master) Chopper and the Right (Slave) Chopper.
Each chopper contains IGBT's, Free Wheeling Diodes, a Hall Sensor, a Filter Inductor, and Blocking Diodes. The IGBT's are
the electronic switches that, in the EPP-400, turn on and off 10,000 times per second. They provide the pulses of power
filtered by the inductor. The Free Wheeling Diodes provide the path for current to flow when the IGBT's are off. The Hall
Sensor is a current transducer that monitors the output current and provides the feedback signal for the control circuit.
The Blocking Diodes provide two functions. First, they prevent the 425V DC from the Boost Starting Circuit from feeding
back to the IGBT's and the 310V Bus. Second, they provide isolation of the two choppers from one another. This permits
independent operation of each chopper without the other chopper functioning.
The Control Circuit contains regulating servos for both choppers. It also contains a third servo that monitors the total
output current signal fed back from the Precision Shunt. This third servo adjusts the two chopper servos to maintain an
accurately controlled output current commanded by the Vref signal.
The Vref circuitry is galvanically isolated from the rest of the power source. The isolation prevents problems that can arise
from "ground" loops.
Each chopper, the Left Master, and the Right Slave, contain their own PWM / Gate Drive PC Boards mounted directly on the
IGBT's. This circuitry provides the on / off PWM (Pulse Width Modulation) signals to drive the IGBT's. The Left (Master) PWM
provides a synchronized clock signal to its own Gate Drive circuitry as well as to the Right (Slave) Gate Drive circuitry. It is
through this synchronized signal that the IGBT's from the two sides switch alternately reducing output ripple.
The EPP-400 contains a Boost Supply for providing approximately 425V DC for arc starting. After the cutting arc is estab-
lished, the Boost Supply is turned off with a contact on the Pilot Arc Contactor (K4).
A Biased Snubber reduces the voltage transients created during cutting arc termination. It also reduces the transient volt-
ages from a parallel power source thus preventing damage to the power source.
The Pilot Arc Circuit consists of the necessary components for establishing a pilot arc. This circuit disengages when the
cutting arc is established.
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