Theory Of Operation; Input Switch And Buck / Boost Board - Lincoln Electric Tomahawk 1500 Service Manual

LINCOLN ELECTRIC Tomahawk® 1500 Service Manual

Theory of Operation

Figure E.1 – Input switch and buck / boost board

Input Switch and Buck / Boost Board

The single or three phase input power is applied to the Tomahawk 1500 through the main input switch
located on the back panel of the machine. When the switch is closed the AC input power is applied to the
rectifier board where it is rectified to a DC voltage. This rectified input voltage is applied to the buck /
boost board.
The buck / boost board houses a soft-start circuit consisting of a 100 ohm resistor and a DC relay. Initially
the DC relay is not activated and the incoming DC voltage is applied to the DC link capacitors via the 100
ohm resistor. The 100 ohm resistor functions as a current limiting device allowing the DC link capacitors to
charge slowly. Upon receiving a soft-start command from the input board, the DC relay closes and the 100
ohm resistor is by-passed by the relay contacts and the full input potential is applied to the DC link
capacitors. The DC link capacitors also function as voltage clamps for the buck / boost circuit.
The buck / boost circuit consists of a buck converter followed by a boost converter. The boost switch is
active when the input voltage is at 230 VAC input or less. Under this condition the buck switch is held on
the entire time. The buck switch is active when the input voltage is at 325 VAC or more. Under this
condition the boost switch is not active for most of the time. The buck / boost circuit operates at 20 KHz.
The buck / boost circuit's output is a 400 VDC regulated bus that is filtered and applied to the inverter
board.
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