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E-4

THEORY OF OPERATION

E-4

 

FIGURE E-4 ---SWITCH CIRCUITS & TRANSFORMER

 

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LINE

 

SWITCH

1

 

2

 

3

3A

 

FAN

AUXILIARY

TRANSFORMER

 

POWER SWITCH SECTIONS 20KHZ

LEFT SWITCH BOARD

FET MODULES

 

 

CAP

 

 

 

FET MODULES

CURRENT

1ø OR 3ø DETETCTION (H5)

 

 

TRANSFORMER

 

 

 

 

PRE-CHARGE

 

 

INPUT

DRIVER

 

 

RECTIFIER

BOARD

 

CONTROL

 

CR1

 

 

 

BOARD

AC1

 

PROTECTION

PULSE

 

AC2

BOARD

Y-Y FEEDBACK

TRAIN

AC3

 

 

CR2

 

 

 

 

 

A-LEAD

 

 

 

24VDC

PRE-CHARGE

 

 

 

 

 

 

 

RIGHT SWITCH BOARD

 

18VAC

2ND STEP PWM

FET MODULES

 

 

 

 

 

 

 

POWERBOARD

CAP

 

24VAC

 

 

 

 

 

 

 

FET MODULES

 

<1 VDC

MAIN TRANSFORMER

TOP

BOTTOM

TOP

CHOKE RECTIFIER HEATSINK

1 DIODE

5 DIODES

5 DIODES

1 DIODE

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TO WIREFEEDER

 

 

 

 

CHOKE

LOCAL

METER

MODE POT

POT

VOLTAGE FEEDBACK

REMOTE

 

 

 

 

 

1ST STEP PWM VOLTAGE

 

CURRENT FEEDBACK-PROTECTION

15VDC-CONTROL BOARD FUNCTION VOLTAGE 24VAC-THERMOSTATS-GUN TRIGGERING

CHOKE

SHUNT

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SWITCH BOARDS

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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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NOTE: Unshaded areas of block logic diagram are the subject of discussion

V300-PRO

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Image 26
Lincoln Electric SVM105-B service manual Switch Boards, Current

SVM105-B specifications

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