Hypertherm MAX42, Service Manual

Page: 34 / 100

THEORY OF OPERATION
Page 2-4 MAX42 Service Manual
• Modulator Board PCB12
This circuit board compares the operator set current level from the control board
PCB14 to the output signal from the shunt (R1, R2, and R3) on I/O board PCB1.
It controls the output of the inverter by varying the pulse width of the switching
waveform applied to the gates of the FET packs in the inverter power section.
Power Circuits
• In-rush Protection Circuit
The in-rush protection circuit consists of relays K1 and K2 and resistors R1 and
R2 located on PCB2. If the power on/off circuit breaker S1 is set to 1 (ON) at the
instant the AC line voltage is high, a large pulse of current will surge through the
input rectifier diodes (PCB3) and possibly destroy them. To protect the input
rectifier diodes, the two 5-ohm resistors, R1 and R2, are used to limit the current
during in-rush (approximately 0.020 second). After in-rush, relays K1 and K2
energize to bypass the resistors.
• Primary Rectifier Circuit
The input rectifier circuit, PCB3, converts the alternating line current (AC) to
direct current (DC). The DC current output is 120 pulses per second at 60 Hz or
100 pulses per second at 50 Hz. The DC output of PCB3 is input to the capacitor
bank printed circuit boards PCB4 and PCB5.
• Capacitor Bank Circuit
The capacitor bank circuit, which consists of PCB4 and PCB5, acts as a filter to
smooth out the ripple of the DC pulse output of PCB3. When the DC pulse
output of PCB3 begins to fall, the capacitors discharge thereby maintaining a
constant DC pulse level output for input to the switching circuit (FET packs) for
conversion into high frequency AC current.
• Switching Circuit
The switching circuit, which consists of PCB6 (FET packs 1 and 2) and PCB7
(FET packs 3 and 4), converts the DC voltage/current from the capacitor bank
into 20 Khz AC signal for the output transformers T3 and T4. The feedback loop,
under the control of the modulator board PCB12, transmits the gate drive signals
(switching waveform) to gate drive transformer for input to the gates of the FETs
to vary the pulse width.
• Output Transformer Circuit
The output transformer circuit, T3 and T4, serves two purposes: (1) to convert
the output voltage to approximately 300 volts and (2) to isolate the user and
welding circuitry from the potentially dangerous line voltages. The transformers
operate synchronously with each transformer passing half the power. In the
208-240 VAC power unit, the transformers are connected in parallel.