Thyristor controlled DC Power Supply.

[ronsimpson]

I speak analog, digital, software and some English.
You say hunting (digital) and I say oscillating (analog). If I have oscillating or ringing I add phase reversal at that frequency. My friends that speak DSP and think time (not frequency) have a word that I cannot remember; sorry I cannot say what they do in this application. I will try in digital. Error amplifiers I make treat errors at the “ringing” frequency (time period) different than errors at a slower frequency. I see I need to get on a different computer and draw pictures.

Change subject!

Current feedback. I have built too many 35,000 volt supplies. They have a potential overshoot problem.
First I worked on the voltage feed back loop and got it as close as possible. This loop has delay and phase delay. 35,000kv=full load, 35,010kv= no load, with overshoot.
Next plot load current verses duty cycle or phase angle (in your case). Example 1A=50% duty cycle and 0.1 amp = 5% duty cycle. Then add a feed back loop with out delay (very fast). If the load is switching from 1 amp, 0.1 amp, 1 amp the duty cycle switches 50%, 5%, 50% in one cycle each. My current loops are not built for accuracy! Maybe the 50% should be 55%. In this case the load starts at 0.1 amps and 5%. The load jumps to 1 amp and the duty cycle jumps to 50% in one cycle, but it should have gone to 55%. The voltage loop sees the small error and slowly adds the extra 5%. When the load drops to 0.1 the very next cycle will be back at 5% with out the error amplifier time to respond. In this example the error amplifier effects are reduced by 10:1.

I have millions of “current mode” power supplies in production where the error amplifiers see only 2% of the load variations. I need to think how to get your supply in truly current mode. Too much for my old head today! I think this is too complex to talk about this way.

[sougata]

Dear Ron,

Since the system is thyristor control the closed loop control responds every AC cycle. The thyristors used are not GTOs so need to wait for the coming cycle. A sliding control method does give good results. Depending on the current consumption the system switches to constant voltage or constant current (foldback type) mode. The line voltage is also not constant. It it were a HF IGBT based system then PID control works great. As for sliding (call it a servo) control the system increases/decreases the firing angle to approach minimum error (deadband inserted). I have seen motor controllers which respond really quick (less than 6 cycles) to a motor start stop.