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Ash Small

Wed Jan 29 2014, 11:54PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

That first link is amazing, Peter. Firstly, it confirms that what I wanted to do in the first place is possible (100 nanosecond primary switching times) and secondly, it confirms that I'm on the right track (multiple cores with parallel one turn primaries, and secondaries in series).

It also explains pretty much how to achieve it. I'm thinking of using a stack of powdered iron toroidal cores, and maybe having something like a ten turn secondary, and running in 'flyback mode', incorporating diodes into the secondary, but otherwise it looks like a very similar design. I don't think I'll need as many MOSFETS per primary, though.

Ash Small

Wed Feb 05 2014, 04:38PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Well, this is where I am at the moment. Based on the 100nS pulsed supply linked to by Peter, here:

I've come up with this:

I need to start filling in some numbers, etc., but assuming I use the same high speed MOSFET's and MOSFET Drivers as used in the above link, (MOSFET IXZR08N120, MOSFET gate driver IXDD614SI), and run it in 'flyback mode', and maybe use powdered iron toroidal cores, I think I can get up to 1000 'pulses' in a millisecond, which equates to ~3 degrees at 5000RPM, so I think I may be 'in the ballpark'.

]mosfet_datasheet.pdf[/file]

]mosfet_driver.pdf[/file]

I'll post again when I've done a bit of maths

EDIT: Well, at ~Â£40 each for those MOSFET's from Farnell, I'll be looking at alternatives, I think.

Sulaiman

Wed Feb 05 2014, 06:48PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

You could make the pulse transformers using 'binocular'/'balun'/'pig-nose' ferrite
for high turns ratio use single turn primary tube, with secondary turns wound inside the primary tube for close coupling.
e.g. my HF PA (still characterizing i/p o/p etc.) over 50W over 50 MHz so far so definitely fast enough for your use and easier to construct low leakage transformer.these are 2861006802 but I also have larger in 43 and 61 material in case a re-work is reqd...still playing.

For your purposes I'd go for 2861010002 and slip a ptfe tube inside the primary tube/pipe then thread the secondary turns through that.
You can closely stack the cores with little interaction.
The cores behave pretty much as two isolated sleeve cores,
which are also often used and have even larger central hole diameters.
Whichever transformer style you use, insulating the pri:sec won't be trivial.
info here

Look at construction of transformer (1:2 turns ratio in this case) on left.

P.S. I've multiple purchased MRF185 from an eBay uk seller,
(good to deal with, no connection to me)
they are used but at GBP10 good for up to 85W output, d.c. to 1GHz and experimenting/sacrificing.

1391625879 162 FT153912 Txrxii Pa Mfr185 Board

Ash Small

Thu Feb 06 2014, 04:28PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for the suggestions, Sulaiman. What I had in mind was using powdered iron toroidal cores, with a distributed air gap, as I'm planning on running this in flyback mode. I'd need gapped cores if I use ferrite, plus the permeability of the powdered iron cores is significantly higher than ferrite, I believe. I understand there may be fewer magnetic losses with ferrite though.

One of the 'mistakes' in the schematic, because I was unsure how to represent it, and what is there gives a good idea of what I'm thinking, but I was planning on stacking the toroidal cores with their individual primaries, then just passing ten turns of copper cored ignition lead through the stack, as this would take up less space (how would you represent this in a schematic?).

I'm looking at various MOSFET/voltage/primary switch on time combinations at the moment.

EDIT: Very nice bit of work there, Sulaiman.

Sulaiman

Thu Feb 06 2014, 06:15PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

my mistake, I assumed that to get the required dv/dt the transformers would be in feed-forward / pulse type operation.
( massive leading edge current / almost instantaneous spark)
If flyback then you are back to limited secondary current / low dv/dt

Ash Small

Thu Feb 06 2014, 08:11PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sulaiman wrote ...

my mistake, I assumed that to get the required dv/dt the transformers would be in feed-forward / pulse type operation.
( massive leading edge current / almost instantaneous spark)
If flyback then you are back to limited secondary current / low dv/dt

My understanding is that 'feed-forward' is voltage dependant (ie voltage is proportional to turns ratio), and 'flyback mode' is current dependant (ie voltage can rise much higher than turns ratio alone).

I 'assumed' flyback mode would be the preferred option here. Would 'feed-forward' work ok with the diodes on the secondaries?

My plan is to get, say, 1000 pulses in a millisecond (figures not finalised yet), which will charge the capacitor until voltage is sufficient for breakdown to occurr, with breakdown occuring at a lower voltage during subsequent pulses. The figure I'm working on for maximum breakdown voltage is <=50kV. With a turns ratio of 50:1 (or is it 1:50?), my understanding is that with 'feed-forward' I'd require a primary voltage of 1000V, whereas this is not the case for flyback mode.

Sulaiman

Thu Feb 06 2014, 09:56PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

rf (plasma) ignition seems to be the direction that you're moving,
plenty documentation on that.

based on playing with flybacks, ignition coils etc.,
I think that you are heading for a whole load of insulation problems
at your intended voltage, dv/dt and prf.

Ash Small

Thu Feb 06 2014, 10:16PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sulaiman wrote ...

based on playing with flybacks, ignition coils etc.,
I think that you are heading for a whole load of insulation problems
at your intended voltage, dv/dt and prf.

I imagine there may be a few problems, but I'm sure they can be overcome. The biggest problem at the moment seems to be my reluctance to spend Â£1,600 on MOSFET's that I'm bound to blow during 'development'

Sulaiman

Fri Feb 07 2014, 10:27AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

with your experience I'm sure you will not blow up too many transistors,
I'd START with the transformer design as that is obviously the weakest link

lash-up an ignition coil or flyback Xfmr and experiment with dry insulating windings etc. don't want to spend $$$$$ on transistors for a project that gets shelved due to problems other than the transistors, which will be superceeded by better ones in a month or two anyway ... last choice.

why are any secondary diodes required whether feed-forward/pulse or flyback ?

I imagine that for efficiency and for low electrical stresses
if you want say 1000 pps for ignition
design the sum of transformer secondary inductances to resonate with the sum of secondary capacitances to resonate at 500 Hz ... etc.
then flyback would be a good option.

Just to state the obvious
... emi/rfi radiation from such an ignition system will be significant

Ash Small

Fri Feb 07 2014, 12:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sulaiman wrote ...

why are any secondary diodes required whether feed-forward/pulse or flyback ?

Because the capacitor needs DC to charge up to the point that breakdown occurs. This will vary due to varying conditions within the combustion chamber.

The plan is to build one MOSFET/ trigger/capacitorcoil unit and see what happens, I suppose, but I'll probably use cheaper MOSFET's for initial testing.

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