Field Effect Transistor (Fet) Operation - Lincoln Electric INVERTEC POWER WAVE 450 SVM112-B Service Manual

E-11

FIELD EFFECT TRANSISTOR (FET) OPERATION

SOURCE
TERMINAL
SOURCE (N)
SOURCE (N)
An FET is a type of transistor. FETs are semi-
conductors well suited for high-frequency
switching.
Drawing A above shows an FET in a passive
mode. There is no gate signal (zero volts) from
the source and, therefore, no current flow. The
drain terminal of the FET may be connected to
a voltage supply. But since there is no con-
duction, the circuit will not supply current to
downstream components connected to the
source. The circuit is turned off like a light
switch in the OFF position.
THEORY OF OPERATION
FIGURE E.10 – FIELD EFFECT TRANSISTOR OPERATION
GATE
TERMINAL
(0 VOLTS)
SUBSTRATE (P)
A. PASSIVE
GATE
TERMINAL
(+ 6 VOLTS)
B. ACTIVE
POWER WAVE 450
DRAIN
TERMINAL
N CHANNEL
DRAIN (N)
ELECTRONS
Drawing B above shows the FET in an active
mode. When the gate signal, a positive DC
voltage relative to the source, is applied to the
gate terminal of the FET, it can't conduct cur-
rent. A voltage supply connected to the drain
terminal will allow the FET to conduct and
henceforth supply current to downstream
components. Current will flow through the
conducting FET to downstream components
as long as the gate signal is present. This is
similar to turning on a light switch.
E-11
DRAIN (N)