Body
I bought a tube of Rosemary's mosfets that she is using with success in her experiments.Of course, I am am more interested in a practical, reliable system.Even a flakey method that depends on deviant aspects of a mosfet will energize a manufacturer to shape up, and produce what might be marketing magic. The Chinese are very good at giving customers what they want. Funny enough, Texas Instruments will do prototype manufacturing as well, and seem easy and friendly to work with. I also used a custom manufacturer in Buffalo, that was very helpful for the deadmau5 spherical video head I made. About 15% of prototypes are rejects, and you have to test each one yourself, after recieving them from the manufacturer.So with that in mind, let us not be pessimistic regarding producing a component that might have unexpected defects, that have a use, never considered of being the main feature for choosing it.Cheers Warren
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On Thu, 22 Nov 2018 at 5:06 AM, An Hö [email protected] [EVGRAY]<[email protected]> wrote:
Hi Ole,
On 21 Nov 2018, at 15:55, [email protected] [EVGRAY] <[email protected]> wrote:
I just looked at one of Rosemary Ainslie's patent applications (https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=WO&NR=03007657A2&KC=A2&FT=D&ND=3&date=20030123&DB=&locale=en_EP#).
This is a very basic switcher that has nothing to catch the flyback energy except for the avalanche breakdown of the switch. This is a very stressful way of using a transistor which is normally avoided.
Figure 2 of the above referred patent.
Normally when an avalanche (secondary breakdown) happens in a transistor it cannot shut off by itself and thus ends its life in smoke or in an explosion if the current isn't shut off externally.
The MOSFET Andreas mentioned (IRFPG50s) is repetitive avalanche rated and as such can withstand avalanches to some degree without destruction after the first electron avalanche through it.
Here is a document about avalanche in MOSFETs (Application Note AN-1005): https://www.vishay.com/docs/90160/an1005.pdf.
The missing snubber in the patent drawing can mean two things. Either, it was omitted (considered an important detail for a patent application), or it was intentionally left out. We usually describe an inductor with
UL = L dI/dt
but all know that this might not be the complete story. There very well might also be an extra term
… + k d^2I/dt^2
with k being so small that it can normally be neglected. But for very steep and accelerated current rises it might be significant. Could this cause an extra voltage spike so high that some current is flowing back into the power supply while energising the inductor? Hector suggested a xenon flash to band a coil. Why? Bacause it switches on much faster than a semiconductor. May be Ainslie intentionally or not even consciously drives the MOSFET into avalanche to get a very steep current rise that exhibits an usually overseen effect in the inductor!?
If so, let’s look a bit close to her circuit. The gate gets a potential to switch the MOSFET on (usual stuff). The gate signal is then dropped down to switch off. The fly-back spike has nowhere to got (thus pretty high potential) and drives the MOSFET into availing which causes it to switch on/off very quickly due to the very fast avalanches which triggers the usually overseen inductive effect described above. Does this make any sense. Can we really use avalanche rated MOSFETs for such endeavours or will they die pretty quickly? Should we rather look into xenon flashers to get the same effect but more reliably?
Regards,
Andreas
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