If you want to control the rectifier bridge then you will need to know when the mains crosses zero and then whether it is going + or - .
When it goes + you apply a delay and trigger the thyristor in the + side,when it goes - you apply a delay then trigger the - side . Again the longer the delay the less the voltage and power.
My previous reply deals mainly with AC connection,but it the same control in software except that you fire a different thyristor for the + or - half of the waveform.