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Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
I feel inspired by Christopher Robins 2kva DRSSTC and I want to make sparks. I have looked at a number of DRSSTC scematics that use differant forms of feedback. antena, primary and secondary return. Now all forms work but what is the most efective. I will probly use primary or secondary feedback. In ither case does the turns ratio in the cupling trnsformer have to be a high ratio to get ocilation started. Set me straight. Thanks N.B.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I am not an expert in drsstc's but... Primary feedback is 'the best' because you are guaranteed that what you are looking at is the actual current; as opposed to secondary/antenna which are usually related to the primary current, but could be a little off.
By 'the turns ratio in the coupling transformer' I assume you are talking about the device that you are measuring the current of the primary/secondary with? Then it is not so much that you want it to be a high ratio, but that you want to have the ratio that will give you the desired current (and thus voltage across the shunt resistor) out. This ratio would depend on the size of the coil...
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
teravolt wrote ...
I feel inspired by Christopher Robins 2kva DRSSTC and I want to make sparks. I have looked at a number of DRSSTC scematics that use differant forms of feedback. antena, primary and secondary return. Now all forms work but what is the most efective. I will probly use primary or secondary feedback. In ither case does the turns ratio in the cupling trnsformer have to be a high ratio to get ocilation started. Set me straight. Thanks N.B.
The best feedback method for a DRSSTC at this time is primary feedback. Secondary base current feedback works well, but unfortunately, the current in the secondary coil is not completely represenative of the current in the primary coil, and because of this, it is difficult if not impossible to achieve perfect zero current switching by using secondary base current. Not to say it doesn't work though. I have several demonstration DRSSTCs using secondary base current that have proven to be quite reliable and practically bombproof.
Primary current feedback is quite easy to implement, and does not require complex hand built transformers either. Magnetek offers high frequency current transformers which are good to at least 500A and only cost about $6.00 each. They are available at These work well for both the current feedback and current sampling circuits for relatively low power DRSSTCs (up to 500A peak current)
Beyond 500A operation, you'll want to wind your own transformers.
Another thing you'll want to implement is the use of synchronized shut-down logic in your system if you are using active current limiting. What this does is allow the system to shut-down at a zero crossing in your switching devices rather than arbitrarily shutting down during a overcurrent condition.
I posted an improved synchronized driver circuit awhile back that we have been using and it both synchronizes shutdown as well as latches shutdown until a new modulator (interrupter) pulse is received. I believe Steve Ward's design using a one-shot delay circuit to keep shutdown latch, but this new driver uses additional logic to absolutely keep the shutdown latched until the next mod pulse once the fault signal is removed.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I believe Steve Ward's design using a one-shot delay circuit to keep shutdown latch, but this new driver uses additional logic to absolutely keep the shutdown latched until the next mod pulse once the fault signal is removed.
The one-shot (555) is just an LED driver really... i use it to put out about a 4mS pulse so that you can easily see the LED light up when it triggers. It also serves as a bit of a buffer to make sure that the "shut down" pulse is strong, and long enough to ensure that the flip flop actually detect it (this probably isnt really a problem). Otherwise, i think the circuits basically work the same, either should work just fine.
I of course vote for primary feedback. Constructing the CTs is very easy and inexpensive... look at my website.
Registered Member #358
Joined: Sat Apr 01 2006, 06:13AM
Location: UCSB
Posts: 28
I guess it's pretty much been covered, but I'll share my thoughts anyway...
Secondary feedback (secondary base current, or antenna) typically doesnt ZCS the first few cycles, but after that it tends to do pretty well. I haven't looked at the simulation of this recently, but IIRC, one polarity would cause it to switch too soon for a bit, and the other polarity would switch it too late for a bit.
Primary feedback will always switch slightly after the primary current zero crossing, so you don't have to worry about shutting off during current flow.
The problem with shutting off before current crosses 0 is that the inductance in the rails can put a big voltage spike on the IGBTs, and possibly blow them.
Depending on the layout and other variables, the spike may or may not actually damage the IGBTs. My first DRSSTC was actually driven open loop, and I could sweep through the full range of tuning at full power without blowing it, and the layout wasn't even that nice. My smaller DRSSTC had a pretty good layout, but switching even relatively little current would blow it (probably has to do with shorter turn off time)
Secondary feedback can be made to work, and it can even be set up so that the switching only occurs after the zero crossing, but the only way to *guarantee* that you aren't going to switch too soon and blow it up is primary current feedback.
Over current protection is virtually a necessity with primary feedback, since ground arcs/detuning can cause primary current to rise unchecked. With secondary feedback, these conditions reduce primary current, so over current protection isn't as necessary.
As far as actually implementing it, the only thing to look for is that its synched with the 0 crossings (if it isn't, the whole point of primary feed back is moot). Steve Ward's design works well, and is well documented.
I posted an improved synchronized driver circuit awhile back that we have been using and it both synchronizes shutdown as well as latches shutdown until a new modulator (interrupter) pulse is received. I believe Steve Ward's design using a one-shot delay circuit to keep shutdown latch, but this new driver uses additional logic to absolutely keep the shutdown latched until the next mod pulse once the fault signal is removed.
I used a simple RC and inverter with some hysteresis so that it wouldn't re-enable immediately. Since the ON time is more than an order of magnitude smaller than the OFF time at even the highest break rate, it is easy to choose a delay that will not re-enable during the burst.
The question is however, is that really an 'improvement'? Didn't Conner purposely use a short delay to effectively 'PWM' the output? It just depends on if you want to be niggardly with the power use, or try to pump some extra energy in to make the ground arc brighter, or the streamer thicker.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Christopher Robin wrote ...
My little DRSSTC (Nicked named Little Red) uses secondary feedback and I can push it harder than my Primary feedback coils in pulse mode on the interrupter. I just can not blow it up (I have tried) I did lose one IGBT due to a primary hit but now push sparks up and away from primary. As long as I keep the duration to 125us, the coil will run for 25 mins and any bit rate and pulse mode I want. Never attempted to build a big DRSSTC with secondary feedback as may create a black hole and suck me up, he he. Here is a little animated gif of Little Red gone wild @
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
wrote ...
The problem with shutting off before current crosses 0 is that the inductance in the rails can put a big voltage spike on the IGBTs, and possibly blow them.
Hard switching is a problem as well if synchronized turn-off isn't used. If you the current limit is set to high, and a particular operational condition is presented continuously (i.e. sustained ground strike to close target), you will current limit during each pulse burst and if the turn off isn't synchronized, hard switching will occur causing a significant increase in heat dissipation. This is even more pronounced considering turn off times of IGBTs are typically greater than turn-off times and there will be a short period in which the device is in its linear region with high current still passing from collector to emitter. I have actually done the thermal calculations for a condition such as this and heat dissipation increased three-fold without synchronized shut-down. Of course, this is the extreme case, as most (except for me! ) won't operate their DRSSTC in such a manner where the current limit will continually engage on a pulse-by-pulse basis. Plus the synchronized turn-off eliminates this anyway.
As far as improvements in driver design goes, the only improvement is if one uses more faults than overcurrent alone to drive the shutdown synchronization. In these cases, you may have a fault which can occur arbitrarily during any period both during the ON burst and OFF period, so there may be benefit to sychronize the drive with the external modulation pulse as well. The benefits of course would vary depending on entire system design and how you wish to run the system.
BTW, interesting term you used there, i.e. Niggardly. Although the definition of the word itself has no negative connotations, i'd be real careful about who i'd use that around, especially in the world we live today.
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DRSSTC,what type of feed back is the best
) won't operate their DRSSTC in such a manner where the current limit will continually engage on a pulse-by-pulse basis. Plus the synchronized turn-off eliminates this anyway.