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Inverter Welding DC-DC Converter: Transformer Saturation

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Full Member level 5
Aug 11, 2004
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I'm building a prototype 37V-350A inverter welding machine. Currently i'm using Full bridge topology and hard switching (with 2 PWM control signals, one for LEG A, One for LEG B). Rectified 3 phase AC is connected as main input voltage. Switching frequency is 31250 Hz. (32 us period). my Main controller is an ARM processor that produce PWM signal and i'm using a driver circuit to drive 1200V-75A IGBTs. my main transformer has a special Core ,which according to the manufacturer is specially designed for welding machines upto 500A.
(transformer turn ration : N1=21 N2=3) i'm using a special HIGH Voltage-LOW ESR Capacitor in series with main transformer for Possible DC-Blocking.

The machine works fine and without any problem up 190A OUTPUT. when i started to use the machine at higher output currents ,the Over current protection triggered. After Investigating the CT ( current Transformer) outputs i noticed some spikes on one of the PWM Pulses which means that main transformer has been saturated. I tried the test several times and each times it randomly saturated on one the PWM Signals only.( in some tests on signal for LEG A, in some tests on SIGNAL for LEG B but this happen above 190A)

After investigating PWM signals , i see both legs are ok and pwm signal are matched and there is no unbalance between PWM pulses(resolution = 10ns).Also there is enough dead time between both PWM Signals ( PWM duty cycle is barely over 50% Duty Cycle!).

Currently i don't know this this happens above 190A.Any ideas?

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