Theory of operation, V300-pro, Switch boards – Lincoln Electric INVERTEC V300-PRO SVM105-B User Manual

Page 26

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V300-PRO

THEORY OF OPERATION

SWITCH BOARDS

The Switch Boards contain the the Field Effect
Transistors (FETs) which, when switched ON, supply
power to the primary windings of the main transformer.
Each Switch Board powers a separate, oppositely
wound primary winding. The opposite direction of cur-
rent flow in those windings and a slight offset in of the
FET switching produces a square wave AC signal in
the secondary of the transformer.

The DC current of the primaries is clamped back to the
respective capacitors through diodes on the board
when the FETs turn off. This protects against inductive
voltage spikes due to the inductance of the windings
and also helps maintain capacitor balance.

The boards are fired during a 50 microsecond interval
with respect to a Pulse Width Modulated (PWM) signal
from the Control Board through the Driver Board. This
creates a constant 20Khz output in the secondary.

Along with ease of control, the 20Khz operating fre-
quency allows for a much smaller and lighter trans-
former

Signals from the Current Transformer insure that one
switch circuit is turned off before the other is gated on.

Field Effect Transistor operation and Pulse Width
Modulation are discussed in more detail later in this
section.

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