DRSSTC,what type of feed back is the best / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

DRSSTC,what type of feed back is the best

previous 1 2 3

Move Thread

LAN_403

HV Enthusiast

Tue Apr 25 2006, 07:17PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Steve Ward wrote ...

Using a current transformer on the primary is so simple, it seems rediculous to do it any other way. Though, i must say that watching the CEsat voltage would be nice to check for shoot-through conditions. Still seems difficult to implement, though...

Its not too difficult - its pretty straightforward, but does require a lot of extra components including some logic, comparators, and isolated power supply. Way overkill for small IGBT bricks, but for expensive bricks such as CM600's etc..., its generally standard practice. I believe the Powerex dedicated drivers for the CM series IGBTs include the desat protection already in them.

Avalanche

Thu May 04 2006, 09:06PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Coming back to the primary VS secondary feedback, could someone please explain why on smaller DRSSTCs, the feedback is generally only taken from the secondary?

I've been doing my research and this seems to be the case. I agree that primary feedback seems to be the way to go, but I cannot understand why it 'would not work' on a smaller coil. I would have thought the current would be higher in the primary anyway.

If I was to build a small / medium sized fullbridge DRSSTC, with say a secondary diameter of 4" and a winding length of 12", would primary feedback still be the way to go? What about with a coil half that size?

Another quick question - am I correct in thinking that the output from a primary feedback CT would be nearer a 50% duty cycle than the output from a secondary feedback CT?

Thanks!

HV Enthusiast

Thu May 04 2006, 11:31PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

There seems to be some confusion here. Firstly, secondary feedback does work and can be made to work extremely well, especially on smaller coils.

I think people use secondary feedback on the smaller coils because its easier than primary feedback, and/or maybe its because its beginners who usually start with smaller coils and secondary feedback tends to be easier.

You are correct in that primary feedback (with current limiting) is the preferred way to go, but there is no reason secondary feedback will not work for you. Our demonstration coils which have hundreds plus hours of demo time on them were all originally secondary feedback and worked extremely well. They have since been upgraded to primary feedback, but i never had any problems with secondary feedback.

Secondary feedback also may be preferred for smaller coils since with secondary feedback, you do not get primary current notching and in fact, no matter how you tune the primary circuit, you will always excite the lower frequency pole with secondary feedback. I have shown this with both experimental results (spectrum analyzer) and PSPICE simulations. Because the primary does not notch with secondary feedback, primary current will ramp up continuously and provide the biggest bang for the buck so to speak.

For smaller coils, if you use primary feedback, then you do want to detune the primary (lower than natural) to the point where only the lower frequency pole is excited during operation. This again, will cause primary notching to disapear and primary current will ramp up continuosly over your pulse burst.

You are not correct. The output waveform (duty, pulsewidth) will be for the most part identical between primary CT and secondary CT. The only difference would be phasing, and magnitude. Both current bursts would occur at the duty cycle that the modulator (interrupter) is operating at.

Steve Ward

Thu May 04 2006, 11:34PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

could someone please explain why on smaller DRSSTCs, the feedback is generally only taken from the secondary?

Probably just coincidence. What coils are you basing this thought from? It happens that my smallest DRSSTC is still secondary feedback, but this is simply because it works and i didnt want to modify it. I might go and change it to primary feedback actually...

If I was to build a small / medium sized fullbridge DRSSTC, with say a secondary diameter of 4" and a winding length of 12", would primary feedback still be the way to go? What about with a coil half that size?

The real concern is the switching speed of the IGBTs (and driving circuit). When you start going into higher frequencies, its difficult to get ZCS with simple circuits (maybe PLL, but i chose to stay clear of PLL). Making a really small DRSSTC can probably be harder than a resonably sized one operating at 150khz or so.

Another quick question - am I correct in thinking that the output from a primary feedback CT would be nearer a 50% duty cycle than the output from a secondary feedback CT?

They should both give pretty much 50% duty cycle.

previous 1 2 3

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob