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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
ben123324 wrote ...
just a question for my benefit- what happens if you don't have a perfect fall time? the arc is sustained during the cycle where the exhaust is being vented? That does no harm though right?
Well, I'm still in the early stages of what might be described as a 'feasibility study', I'm reading up on CCM flyback operation. The 'spark' will last for <1mS, but during that millisecond the MOSFET will switch <1000 times @ <1MHz (the figures are only a guide at the moment). The reason being that at faster switching speeds I can have more (much more) volts per turn, so therefore, much less turns.
It will get hot, but as it's only running for 1mS every 20-50mS or so (less than 5% duty cycle) I'm hoping it won't be too much of an issue.
During that millisecond that it is operating, due to the capacitance on the output side (10pF in the plug, plus stray capacitance, plus maybe added capacitance if required), the output voltage will rise until breakdown, then current will continue to flow at a lower voltage (that's the theory anyway).
With the MOSFET switching at those speeds, shorter switching time means lower switching losses, plus the faster the turn off time, the higher the voltage peak on the output.
So I really want to start with something that switches as fast as possible, to increase efficiency.
Tjhe whole idea relies on the small duty cycle of a few % allowing me to push everything further than would be possible at higher duty cycles, but I still want to keep losses to a minimum.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Thanks. I can see (I think) that with the 'newer' 555 chips I'll need a MOSFET totem pole to drive the main MOSFET, as they appear to be lower power units (I think).
I'm still reading up on CCM flybacks, and I'm learning I may have made a few incorrect assumptions, but I'll get there in the end.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Just supposing I was to use a MOSFET with a turn on delay of 8.8nS, a fall time of 21nS, a turn off delay of 38nS and a rise time of 34nS, like, for example, this CREE SiC Z-FET for ~£20 (I realise this one is only rated for ~50A pulsed drain current, but I could probably push it a bit with less than 5% duty cycle, and others are available), How should I drive it at these fequencies? GDT?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
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.
IGBTs can be overloaded much more for short periods than MOSFETs. The MOSFET's finite Rds(on) limits the current, but the IGBT's minority carriers kick in and just keep delivering more and more current. Effectively the Rds(on) gets smaller the more current you draw. That's why we all use IGBTs in our DRSSTCs.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
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.
IGBTs can be overloaded much more for short periods than MOSFETs. The MOSFET's finite Rds(on) limits the current, but the IGBT's minority carriers kick in and just keep delivering more and more current. Effectively the Rds(on) gets smaller the more current you draw. That's why we all use IGBTs in our DRSSTCs.
Or you could buy an ignition coil...
I'll look at Jan's page later. Thanks.
What concerns me regarding IGBT's is is firstly, the 'tail off' at switch off, which is not ideal for flyback topology, and secondly, they don't like high frequencies.
I thought it might be worth looking at SiC Z-FET's here, as they might offer advantages over standard MOSFET's, and they are starting to become affordable.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
If you're driving open loop, you could also use one of those Function Generator ICs. I think they can go up to a few MHz at least. Like the Maxim MAX035 or whatever the number was. Exar also has a few function generator and timing ICs that might go into the MHz region.
Registered Member #1460
Joined: Thu May 01 2008, 12:45AM
Location:
Posts: 19
The fastest I could get my CMOS based 555 timer from TI (can't remember the exact PN) was right around 2MHz.
I think the dedicated frequency generator ICs are getting hard to find. I played with an XR2206 years ago and can't find them anymore.
If you are comfortable with microcontrollers and can shield them adequately, you could use a DDS chip from Analog Devices with a PIC or something and have incredibly accurate frequency control and ridiculous range. Even their smallest 10-pin devices will do up to 12.5MHz in something like 0.1Hz steps. Check out the AD9833 if this interests you.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
If I can make another suggestion, you could build a Colpitts oscillator and rather than having a crystal, use a variable inductor to form a circuit equivalent to the typical crystal.
Here's a modified ltspice document I adapted to fit your needs:
It was previously using just a crystal to simulate an rf amplifier I’ve been working on.
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I need something faster than a 555 timer.
