Burst current

[shjukheter]

Hi,

           I am trying to make burst current mode for my welding machine. My wish is to produce burst current at maximum duty cycle. Let me clear with brief description.

The hardware is built by a electronic circuit with 4 transistors after the transformer to make the switching mechanism for positive and negative periods. A PI(proportional integral) controller was used inside to meet the user required current in a continuous operation. Currently, my Duty cycle is depend on the latest current value in a continuous operation. So, the actual current is not always same and thus duty cycle is changing accordingly.

          My plan is to make constant duty cycle regardless of the current with a burst current mode i.e..for example  In one cycle, the RMS current should be maximum(should be near to user value) for certain period and then blank for the remaining cycle period. This blank period time may not be constant and it should be like very few milliseconds and it should work for all the user entered current value. By the way the PI controller should be used to accomplish this task. As far I don't have any idea to go on this and therefore I am looking for any suggestions here. You can assume the output as like in below picture. I forgot to say that the weld time is controlled by user and it varies from 1 to 2seconds and the time period of a cycle is 2.5ms.

Any ideas would be appreciated.