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I need something faster than a 555 timer.

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Steve Conner

Sun May 26 2013, 10:30AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

BTW, nice find on the silicon carbide MOSFETs Ash. I still maintain they're not much good for an ignition application, but they'd be great for a 750V PFC front end for a Tesla coil.

Steve Ward

Mon May 27 2013, 08:03AM

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

What are you driving at 1MHz? If its some home-brew HV transformer you might be disappointed to find that the self-resonance of the HV winding is hundreds of kHz or something substantially lower than 1MHz. This means you wont get nearly the output voltage you are hoping for. You might as well be driving a tesla coil, a proper transformer will have too much self capacitance for 1MHz and enough kV to spark through air.

Ash Small

Mon May 27 2013, 12:12PM

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

Steve Ward wrote ...

What are you driving at 1MHz? If its some home-brew HV transformer you might be disappointed to find that the self-resonance of the HV winding is hundreds of kHz or something substantially lower than 1MHz. This means you wont get nearly the output voltage you are hoping for. You might as well be driving a tesla coil, a proper transformer will have too much self capacitance for 1MHz and enough kV to spark through air.

Yes, I realise the issues with self resonance, and the spark plug has ~10pF capacitance. This will be driven in flyback mode, and I assume I'll have to rectify the output. I need a high frequency due to the volt-seconds, if I'm to keep the windings to a minimum in order to keep self-capacitance, etc. to a minimum. I'm not expecting to get clearly defined sparks @1MHz, more of a continual current flow, as the inductance/capacitance 'smoothes' them. I need to read up some more on CCM and DCM, and on duty cycles.

The idea is that voltage builds up in the capacitance of the plug until discharge occurs, and then repeats but at lower voltage once breakdown has occured. With the capacitance directly accross the plug, current flow during discharge should be at a maximum, as stray inductance will be at a minimum. Also, voltage will only rise to the point where breakdown occurs, thus keeping current flow at a maximum.

I do realise that it's a trade off between volt-seconds and self resonance. If the frequency is high enough, windings can be kept to a minimum.

While I realise things will get hot, I'm relying on the fact that it will only have a duty cycle of a few %, due to the intermittent operation, so there should be room to push the limits a bit.

Thanks to everyone who has made suggestions above. I'm still reading through them all.

EDIT: I've not read enough to be certain yet, but if, for example, the MOSFET is on for, say, 100nS, and then the secondary has, say 900nS for current to flow before the MOSFET switches on again, rather than say, a 50% duty cycle, this would result in a virtually continuous flow of current in the secondary. The inductance will oppose any change in current flow, so a diode 'may' not be necessary, as the current won't have time to change direction, due to the self-resonance.

I'm still reading up on duty cycle, volt-seconds, and continuous mode and dis-continuous mode flyback, though, it's quite interesting.

(All the figures here are just to give an outline of the idea, none have been finalised, or anything, but are of the approximate order)

Steve Conner

Tue May 28 2013, 10:10AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Sounds like a job for "Tesla coil on plug"

Ash Small

Fri May 31 2013, 01:21PM

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

What 'scope probes would I need for this? I have a 30MHz 'scope, which I think might just be up to the job, but I will need some better probes than I have?

I don't need extreme accuracy, I just need to be able to see what's going on. I think the best way to approach this is by getting the primary and switching side working, and then experiment with secondaries (gradually increasing turns/inductance/self capacitance, and seeing what happens).

I should at least learn something from it

Shrad

Fri May 31 2013, 01:56PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

just roll a tiny wire around the ignition wire and scope this small wire with a standard probe

Ash Small

Fri May 31 2013, 02:22PM

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

Shrad wrote ...

just roll a tiny wire around the ignition wire and scope this small wire with a standard probe

Yes, I wasplanning to 'scope the secondary in this way, but the fastest probe I have is 100kHz. What should I be looking for as far as suitable probes go?

Patrick

Sat Jun 01 2013, 02:50PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Steve Conner wrote ...

Jan Martis made a nice flyback driver based on the UC3842 (or was it the UC3845?) The great advantage of this chip is its current-mode operation that limits the current through the switching device irrespective of the load. It might not go to 1MHz, but it should be fast enough.

first, yes, the UCC38XX are great ICs, and the whoel related families of high power switch mode ICs. TI and unitrode really makes our modern transistors function at there best. (i just pitty engineers of previous decades)

Second, even with a slow 400khz max cycle freq, the ICs and gate drivers have super dooper fast rise and fall times, so fast that 1/T = f math isnt really useful to describe the state change. just better to specify by time in uS or nS or whatever.

third, currnet mode smps' are enormously useful, yet few implement them, preffering to half-ass it with voltage mode - time interval averaging control and feedback.

Steve Conner

Sat Jun 01 2013, 09:19PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Ash, I've never seen a 100kHz scope probe in my life, where did you find one?

Wrapping a wire around the ignition lead creates a capacitor. You could connect it to the scope via a piece of coax, to create a capacitive divider.

Ash Small

Sun Jun 02 2013, 11:46AM

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

Steve Conner wrote ...

Ash, I've never seen a 100kHz scope probe in my life, where did you find one?

Wrapping a wire around the ignition lead creates a capacitor. You could connect it to the scope via a piece of coax, to create a capacitive divider.

I think it's 100kHz. I have two, both from Maplin. One is faster than the other. They were both cheap, but at the time I was only planning on building 25kHz circuits. I'll need something faster to 'scope a 100nS pulse to the primary, for example, and to 'scope the output.

I suppose I want a 100MHz probe, although the modules in the 'scope are only 30MHz, I think.

As I said, I don't require great accuracy, I just want a general idea of what's going on.

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