Marx generator - Page 1
Forum section: electronics / projects

author: Regected
author_rank: Newbie
timestamp: December 28, 2014, 10:19:54 pm
content: I've been working on a solid state Marx generator for a long while now. Nothing huge. I'm not trying to make a million volts or anything like that. This is intended to be a high energy, repetitive, and variable pulse generator. This will be driving an inductive load at a frequency up to 4Mhz at 5-50% duty cycle.The big issue has been how to drive the switch (MOSFET/IGBT/SCR) with each one floating at variable voltages as referenced to ground. I was considering driving the switch with a transformer on each device, but this showed to be problematic in timing. Variations in production of the transformers caused each device to switch at a slightly different time. To overcome this would require pre-selecting of both transformers and switch devices, but that would be a large expense in both time and money.It dawned on me a bit ago. Why do the stages need to be directly connected? They don't need to be. They just need to charge in parallel and discharge in series. So, why not charge each stage independently, then discharge them through a set of series connected transformers. This would keep the triggering and control circuitry reasonable while allowing for the high energy pulses I require. It would introduce losses in the output via the transformers, but nothing unreasonable. Transients will be an issue, as will resetting remnant magnetic fields in the cores, but this are small problems.I threw together a quick drawing of two stages. This is in no way a final schematic, but only a way of conveying my idea. Any suggestions, critiques or other feedback?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 28, 2014, 10:33:16 pm
content: That works, but after just a few transformers, your capacitance piles up prohibitively, so it doesn't work for very long.Just what are you trying to do? 4MHz at 1MV? Surely you're trying to build a Tesla coil the hard way and you don't realize it.Tim

author: Regected
author_rank: Newbie
timestamp: December 29, 2014, 01:38:18 am
content: I actually have a couple projects in mind for this, and one of them is a Tesla coil driver O0Coilcraft makes a flat spiral, high current transformer series that is designed to minimize capacitance. I don't remember the exact application they are designed for, but I have a couple samples on my workstation. They are good for up to 20 amps at 1Mhz. I know they make some higher frequency ones. The capacitance is in the single digit pico range.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2014, 02:15:24 am
content: Sounds like a planar transformer... but how exactly are you going to get 1MV of isolation with that?Anything with 1MV isolation will naturally have low isolation capacitance, but incredibly high leakage inductance. So, you're most likely going to be building a Tesla coil anyway (a resonant transformer). And even then, what little capacitance is there, is magnified by the square of the turns ratio -- it adds up ridiculously quickly.If you build this with transmission line transformers, you might be able to achieve fast risetimes (which seems to be the intent), but with lumped windings, forget about it. I'd be impressed if one of these can be constructed with a 4MHz rep rate; a few kHz would be more reasonable.There are configurations for high isolation, like stacking a series of transformers, so each sees a fraction of the total voltage; obviously to keep voltage up through the stack, you need smaller and smaller transformers, and ratios slightly over 1:1. Or if it's sine wave, maybe some capacitance to cancel the leakage inductance.. but that's kind of getting back into TC territory.Tim

author: Regected
author_rank: Newbie
timestamp: December 29, 2014, 02:50:40 am
content: I'm not trying to get to 1MV. I don't have a voltage yet, but it will probably be in the 10KV to 50KV. My big goal at this step is to confirm the usability of the architecture at this frequency range. The pulses will be no more than 50 joules. This will ramp up to greater pulse energies later, but not now. No need to build the Pentagon when a cabin in the woods will suffice.

author: Regected
author_rank: Newbie
timestamp: January 08, 2015, 01:45:41 pm
content: Well, after some number crunching, I think the design concept of this device will need to change a bit. The output inductor will not be able to handle a decade range. Getting the power output at 50ns makes the inductor reactance too low at 500ns. Think I'll reel this back to a 50-150ns pulse width. Time between pulses will maintain no less than 500ns.I've tested several transformer designs. The phase shift has remained fairly constant at 88.5 to 89.5 degrees between 500kHz and 13MHz(my test equipment's max), so the capacitance should not be a problem.I was thinking about using SQ701 MOSFETs as I have a large number of these on hand already. They should handle the pulse power at an acceptable slew rate. They are dual drain common source dual MOSFETs. I contacted the manufacturer about running both sides of each device in parallel, and was told that each half is matched within +-0.1%, so no load leveling resistors required, but I think I may add a couple milli-ohm resistors in just to cover the possible difference.