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V300-I
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.
E-4 E-4
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NOTE: Unshaded areas of block logic diagram are the subject of discussion
FIGURE E-4 ---SWITCH CIRCUITS & TRANSFORMER
1
ø OR 3ø DETETCTION (H5)
LINE
SWITCH
INPUT
RECTIFIER
AC1
AC2
AC3
A-LEAD
AUXILIARY
TRANSFORMER
TO
WIREFEEDER
18VAC
24VAC
POWERBOARD
2
4VDC
PRE-CHARGE
2ND STEP PWM
1ST STEP PWM VOLTAGE
15VDC-CONTROL BOARD FUNCTION VOLTAGE
24VAC-THERMOSTATS-GUN TRIGGERING
LOCAL
REMOTE
METER
MODE
POT
POT
<1 VDC
FAN
1
2
3
CONTROL
BOARD
Y-Y FEEDBACK
3A
POWER SWITCH
SECTIONS
20KHZ
CHOKE
RECTIFIER
HEATSINK
1 DIODE
5
DIODES
5 DIODES
1
DIODE
CHOKE
CHOKE
SHUNT
CURRENT FEEDBACK-PROTECTION
VOLTAGE FEEDBACK
PULSE
TRAIN
DRIVER
BOARD
PRE-CHARGE
CR1
CR2
PROTECTION
BOARD
TOP
T
OP
CURRENT
TRANSFORMER
LEFT SWITCH BOARD
FET MODULES
CAP
FET MODULES
MAIN
T
RANSFORMER
RIGHT SWITCH BOARD
FET MODULES
CAP
FET MODULES
BOTTOM