Author Topic: Question about Flip-Flop wiring for DRSSTC  (Read 287 times)

Offline bockett

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Question about Flip-Flop wiring for DRSSTC
« on: September 11, 2022, 09:19:06 AM »
Hello! I've been researching SSTCs and DRSSTCs for a while now and I think I understand the theory pretty well by now, but I am pretty confused by the wiring of the flip-flop for zero current switching. I've looked over the schematics for the OneTesla TS and Steve Ward's DRSSTC .5, and I have a couple questions about the flip-flop:
  • Shouldn't the primary feedback be inverted before being fed into the clock input of the flip-flop? If the point is to only disable the inverter when the current is zero, and the flip-flop copies the data to its output on the rising edge of the clock, then wouldn't a non-inverted feedback cause the flip-flop to only switch when there is primary current?
  • Is there any reason both designs have the interrupter hooked up to the clear pin of the flip flop and use the inverted output? Wouldn't it be the same to use the normal output and have the interrupter preset the flip-flop (and also pull the data input low)? Does it have anything to do with the default value of the flip-flop or anything like that?
  • Lastly, is connecting the interrupter to the inverter input through resistors necessary? I know that it's supposed to kick-start the coil, but I read somewhere (I think it was another post here or on 4hv.org, but I can't find the exact link) that the UCC driver will send a small pulse on the output when it's enabled. If this is true (I didn't notice anything in the datasheet about it), then would it still be necessary to connect the interrupter to the inverter input, or would it be a good idea to connect it regardless?
I'm still learning so I apologize if I explained my questions badly or if they're dumb questions, but I've been trying to research them for a while and haven't found anything, so I would really appreciate any help. Thank you!
« Last Edit: September 11, 2022, 06:14:21 PM by bockett »

Offline davekni

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Re: Question about Flip-Flop wiring for DRSSTC
« Reply #1 on: September 11, 2022, 07:35:52 PM »
Quote
I'm still learning so I apologize if I explained my questions badly or if they're dumb questions, but I've been trying to research them for a while and haven't found anything, so I would really appreciate any help. Thank you!
These are great questions.  Wonderful to see you working to understand driver circuitry rather than just blindly building!

Quote
Shouldn't the primary feedback be inverted before being fed into the clock input of the flip-flop? If the point is to only disable the inverter when the current is zero, and the flip-flop copies the data to its output on the rising edge of the clock, then wouldn't a non-inverted feedback cause the flip-flop to only switch when there is primary current?
Primary current crosses zero twice per cycle, from negative to positive half-cycle and again from positive to negative half-cycle.  Either polarity of clock will work fine.  Roughly matches primary current at either rising or falling zero-crossing points.

Quote
Is there any reason both designs have the interrupter hooked up to the clear pin of the flip flop and use the inverted output? Wouldn't it be the same to use the normal output and have the interrupter preset the flip-flop (and also pull the data input low)? Does it have anything to do with the default value of the flip-flop or anything like that?
Yes, your version would work fine too.  I'd guess that ECB layout happened to work out cleaner using clear and Q-n for the design that was done first, presumably Steve's.  The other was likely copied from that.

Quote
Lastly, is connecting the interrupter to the inverter input through resistors necessary? I know that it's supposed to kick-start the coil, but I read somewhere (I think it was another post here or on 4hv.org, but I can't find the exact link) that the UCC driver will send a small pulse on the output when it's enabled. If this is true (I didn't notice anything in the datasheet about it), then would it still be necessary to connect the interrupter to the inverter input, or would it be a good idea to connect it regardless?
In Steve's version a resistor is needed to drop 9V from interrupter 555 chip to 5V of HC14 chip.  Even with resistor, that design feeds a small current through the input protection diode inside HC14.  For the other version with 5V optical receiver for interrupter, I don't see any need for the resistor.  The 10k resistor from interrupter to feedback input ensures at least one input transition to start oscillation.  (My preferred version to start oscillation is a resistor across the feedback HC14, pins 1 and 2 in Steve's version.)

I recommend including D13-D16 of Steve's circuit.  UCC driver chips don't handle GDT inductance well otherwise, as they are not designed to handle current attempting to pull outputs beyond supply rails.  I'd also include gate diodes and resistors from Steve's.

The TVS diodes across IGBT collector-emitter leads sometimes cause more problems than they solve.  You may want to leave those off depending on avalanche breakdown energy capability of the IGBT parts you use.  Some IGBTs have higher avalanche energy capability than the TVS diodes, so are more reliable without the external diodes.

Have fun building!  Please feel free to ask more questions as they come up.
« Last Edit: September 11, 2022, 08:12:48 PM by davekni »
David Knierim

Offline bockett

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Re: Question about Flip-Flop wiring for DRSSTC
« Reply #2 on: September 11, 2022, 09:24:04 PM »
Thank you so much! This is incredibly helpful. If you don't mind, I have just a few more clarifying questions:
Quote
Primary current crosses zero twice per cycle, from negative to positive half-cycle and again from positive to negative half-cycle.  Either polarity of clock will work fine.  Roughly matches primary current at either rising or falling zero-crossing points.
Doesn't the primary feedback get clamped to +5V and ground (Or at least very close to ground)? How would the flip-flop be able to detect the negative to positive zero crossing?
Quote
My preferred version to start oscillation is a resistor across the feedback HC14, pins 1 and 2 in Steve's version.
Would you be able to explain this a little more? I kind of understand how it could start oscillation, but wouldn't it mess with the actual feedback once the coil has started oscillating on its own?
Thank you and have a nice day!

Offline davekni

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Re: Question about Flip-Flop wiring for DRSSTC
« Reply #3 on: September 11, 2022, 10:15:59 PM »
Quote
Doesn't the primary feedback get clamped to +5V and ground (Or at least very close to ground)? How would the flip-flop be able to detect the negative to positive zero crossing?
Refer to Steve's circuit.  Just noticed that the other one is missing the input coupling capacitor C1.  During operation, the right side of C1 is centered around +2.5V, clamped to 0-5V.  The left side of C1 is centered around 0V.  The rising edge from current transformer (CT for short) is the one detected by FF.  The falling edge does not clock the FF.  The circuit would work just as well if the FF clock input were inverted, clocking on falling edge (positive-to-negative transition of CT output).  As is, FF clocks on negative-to-positive transition of CT output.

Quote
Would you be able to explain this a little more? I kind of understand how it could start oscillation, but wouldn't it mess with the actual feedback once the coil has started oscillating on its own?
In the existing circuit, CT signal must be strong enough to overcome 10k load to interrupter input.  So it can also overcome a 10k load to HC14 pin 2.  The advantage of pin1-2 resistor is that it keeps C1 charged close to 2.5V between interrupter pulses.  Less feedback voltage is required to start oscillation.  On the other hand, it is likely best to start with a conventional design before trying my tweaks.  (Other forum members have tried this self-oscillation mod on antenna-feedback SSTCs and on UD2.7 DRSSTCs successfully.  I don't know if anyone has tried self-oscillation on CT feedback SSTC before.)

If you plan to continue studying analog electronics, I suggest learning circuit simulation.  My favorite free simulator is LTSpice.  There are other free programs as well as free online simulators.  There is a robust LTSpice users group.  Most questions about LTSpice can be found with an internet search (if built-in help isn't sufficient).  With simulation you can try different ideas without the work of physical construction or the pain of failed components.

Quote
Thank you and have a nice day!
You are welcome.  Have fun learning and experimenting!
David Knierim

Offline bockett

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Re: Question about Flip-Flop wiring for DRSSTC
« Reply #4 on: September 11, 2022, 10:28:43 PM »
Got it, thank you so much for all the help!

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Re: Question about Flip-Flop wiring for DRSSTC
« Reply #4 on: September 11, 2022, 10:28:43 PM »

 


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