20_kHz_ARC_WELDING_INVERTER

Published:2009/7/8 23:33:00 Author:May | From:SeekIC

The Class A series resonant inverter portrayed is well-known and respected for its high efftciency, low cost, and small size, provided that operating frequency is greater than about 3 kHz. The disadvantages are, at least in high power versions, the difftculty in effecting smooth RFI-free output voltage modulationwithout significant added complexity, and a natural tendency to run away under no-load (highQ) conditions.The 20-kHz control circuit (see Fig. 2) overcomes these shortcomings by feeding back into the asymmetrical thyristor trigger pulse generators (see Fig. 3) signals that simultaneously shut the inverter down, when its output voltage exceeds a preset threshold, then time-ratio modulates the output. This feedback is accomplished with full galvanic isolation between input and output thanks to an H11L opto-Schmitt coupler. The fundamental 20-kHz gate firing pulses are generated by a PUT relaxation oscillator Q1. The pulses are then amplified by transistors Q2 and Q3. The 20-kHz sinusoidal load current flowing in the pri-mary of the output transformer is then detected by current transformer CT1, with op amp A1 converting the sine wave into a square wave, whose transitions coincide with the load current zero points.Consequently, each time the output current changes, phase A1 also changes state and, via transistor Q4, either connects the thyristor gate to a -8 Vdc supply for minimum gate assisted turn-off time and highest reapplied dVldt capability or disables this supply to prepare the thyristor for subsequent firing.Modulation intelligence is coupled into this same H11L through two additional PUTs, Q6 and Q7, Q6 oscillates at a fixed 1.25 kHz, which establishes the modulation frequency. The duty cycle is determined by a second oscillator, Q7, whose conduction state, on or off, establishes or removes current from the H11L diode. With afundamental inverter frequency of 20 kHz and a modulation frequency of 1.25 kHz, the resultant time ratio-controlled power output is given by:where PM= 100% continuous output power. Minimum power is one cycle of 20 kHz (50 μs) in the 1.25-kHz modulation frame (800 μs), that is, 6.25% PM.



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