Steve Ward wrote ...

I made a digital PLL.

There are a few key points about it:

1) the user has to define a starting frequency "Fstart". The driver switches to feedback/pll operation after "N" cycles (set in 0.5 cycle increments), typically i use 0.5-1 cycles for a "transient" type DR, and 4-8 cycles for SSTC/QCW where you need to have a well defined starting frequency/mode). However, it works acceptably with pretty big errors... so its not overly burdening.

2) The user must define the desired phase-lead time "Tlead", which is referenced to the output signals from the PSoC (Conner already explained this perfectly). This lead time is defined as the whole delay time of the system (that includes the feedback CT delay, the processors delay, and IGBT delay).

So how's it work?

First, start driving at "Fstart", during this time, measure the period between zero crossings in the current feedback signal. After "N" cycles at "Fstart", PLL operation can begin. The PLL works as follows:

A one-shot timer is started upon each zero-current-detection of the feedback signal (that is, 2 events per full cycle). The one-shot is set to time out "just before" the next expected zero crossing. When the one-shot times out, the output to the gate driver flips state (as if the feedback signal told it to). If you measured the previous half-cycle's period "Thp", you can predict the next one as Tnext_switch = Thp - Tlead.

If that makes sense, then you understand enough to build your own digital PLL as i did. However... there was a problem with stability if the half-cycle periods were inconsistent, so i added a digital filter that averages the previous 4 half-cycle periods and uses that result for the predictive one-shot timer (knocking it down to 2 half-cycles ought to work too, just havent tried it yet). Conner would probably point out here that there is some dynamic performance loss from this, but thanks to the fact that the one-shot is reset with the feedback input, you can never get too far out of phase with the desired switching command even if the system frequency is changing rather quickly. Oh yeah, and just in case the prediction is too late, the zero-cross detect will command a gate drive transition, making the driver no worse than my old universal driver without phase lead.

Ive used this same driver on a wide variety of tesla coils and its performance is on par with the inductor prediktor and about 1000 times better than the 4046 PLL crap i used to play with. However, getting the whole scheme to work reliably was a considerable challenge and took months of debugging/tweaking to get it to a robust state. Having an FPGA with essentially limitless hardware would have made the task easier, most of my time was spent coaxing the design to fit within my hardware budget on the PSoC. Also, 64MHz clock leaves a bit to be desired at higher TC frequencies.

Steve Ward wrote ...

MMC: 11 strings of 20 series 27nF 1600VDC MMKP type, total 14.85nF at 13kVAC (RMS).

Intra wrote ...


Hi Steve, could 942C20P15K-F caps be replacement of MMKP for MMC?

loneoceans wrote ...


The 942 CDE caps have excellent RMS capability and work great. However a similar capacitor bank would require about 90 of those caps (30 x 3) which will be pretty expensive though...

Intra wrote ...


Thank you Gao!
According to datasheet Irms of 942C20P15K-F is 13.5A and AC is 500V.
Did I made right calculations about if 3pcs 942C20P15K-F in parallel give 20.25kVAC RMS?

The reason why I ask, 104pcs of 942C20P15K-F I already have.

Mads Barnkob wrote ...


You could use the MMC calculator here to give some quick estimates. 500V is however a pretty conservative rating for these capacitors. But on the other hand they are being punished more in a QCW than in a DRSSTC. Just change the voltage rating in the calculator and the rest of its specifications is inthere.

You simply just need that many to get down in capacitance, these are not suitable to the job, too expensive an option in my opinion.

Mads Barnkob wrote ...

10 strings of 30 in series of these:

Gives you 15,6nF at 13,2kVAC, uses 300 capacitors that would cost you a total of 36 Euro, however the RMS current handling is lower than the 942C. But it might be total overkill with the 942Cs.

The downside is also you have to do much more soldering with the cheap method :)

Steve Ward wrote ...

Intra, almost got it, but you didnt get the extra IGBT/diode boost circuit correct. It follows a standard boost converter scheme, i think you can fix your schematic .