Pulse Width Modulation - Lincoln Electric INVERTEC V300-PRO SVM105-B Service Manual

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