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Hi Andreas,
I don't know which one of xenon tube or avalanche switching a power MOSFET will last the longest. Both types of phenomenon happen very fast. One is a flash over through a gas while the other is a flash over through a semiconductor crystal. Some of the other figures of the patent application have the load across the inductor. When in parallel the load can be adapted to limit the flyback voltage to below the breakdown voltage of the MOSFET switch. It is also mentioned that it wasn't known which way works the best. The measurements mentioned in the patent application shows up to twice the input power being output in the resistor.
More information or experimentation is needed for any conclusion. Questions like does the MOSFET switch have to go into the breakdown region? Or are the readings valid? It is mentioned that the load resistor has a hollow core. If it's an inductive resistor it has inductance. If having inductance some of the power must be reactive power stored as a magnetic field in the inductive resistor. If this is the case the measurements aren't valid! A calorimeter measurement would settle this. Else a non-inductive load will have to be used for determining the output power. This can be done by splitting up the inductive part and the resistive part of the load by using more idealized components and then measuring on the more ideal components. Alternatively an oscilloscope with mathematical functions rigged up as a power meter will show the active and reactive parts of the power. Or a simple two channel scope will be able to show the phase relationship between the current and voltage. The curves have to be scaled copies of each other for pure active power. It not there is also reactive power.
Regards
Ole
---In [email protected], <freeen2012@...> wrote :
Hi Ole,
On 21 Nov 2018, at 15:55, onielsen@... mailto:onielsen@... [EVGRAY] <[email protected] mailto:[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# 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 https://www.vishay.com/docs/91254/91254.pdf) 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 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