Super High Frequency TC - Page 1
Forum section: electronics / projects

author: IanBerry
author_rank: Contributor
timestamp: May 27, 2017, 11:02:27 pm
content: Hello, I've been designing a project for awhile to kind of push the limits of a Tesla coil. My goal is to drive a coil at somewhere around 100MHz. Though, I'm a little hesitant to construct the circuit because I'm worried that things won't function normally at such a high frequency. All of the components are rated for that kind of frequency, I'm just still unsure. Like in the bootstrapped H-bridge, I kind of pulled the capacitor value right out of my rear end since I'm not very familiar with them and I don't know what capacitor values work with what frequencies. As well as using 2n904/2n3906 as my MOSFET driver/bootstrap driver. Also, I'm worried about voltage spikes and I was thinking with the parasitic diodes of the MOSFETs and a large input capacitor those spikes could be absorbed. I also might have to put a capacitor in series with the Tesla coil primary to counteract some of the inductive reactance at such a high frequency. Just in general, I was hoping someone could look over the circuit plan and point out any flaws or give some suggestions. I attached a picture of my drawn out circuit. (Please excuse the cursive.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2017, 12:01:23 am
content: You'll find that the parasitics you can kind-of-sort-of ignore at low frequencies (say ~100kHz) are dominant by 10MHz, give or take depending on transistor type.RF transistors help out, but you have to drive them quite strongly (notice the input power at 175MHz), which makes that difficult, let alone H-bridges.Tim

author: IanBerry
author_rank: Contributor
timestamp: May 28, 2017, 12:41:28 am
content: Hmm, okay. If the frequency is way too high and I end up driving it at maybe 20 to 50 MHz, are inductive spikes going to immediately kill the MOSFETs or did I assume correctly with the parasitic diode and a large input capacitor? Also, do the two BJTs in that configuration make a viable MOSFET driver? Did I configure my bootstrap correctly?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2017, 10:21:40 am
content: Inductances don't just magically make voltage spikes, the voltage only goes as high as you allow it to. A MOSFET H-bridge is clamped by the body diodes, so there is no spike from load inductance at all.What you will have trouble with, is the complex network formed between junction capacitance, stray inductance, supply bypass, output lead lengths, and anything else nearby. When one transistor turns on or off, and the voltage swings, the dV/dt charges and discharges capacitances, which draws current through stray inductances. As combinations of transistors turn on and off, you get a couple different resonant networks, and if they are mismatched to your switching speed and load, you get large current and voltage spikes within the bridge itself.At frequencies this high, you don't have the luxury of "minimize inductance". It is not physically possible to build a circuit this fast, that also switches slow enough to avoid exciting those parasitics.And yes, that means parasitics (mainly stray inductance) are proportional to length. Lead length, trace length, distance between devices: all of this matters!So you must instead optimize parasitics. Make sqrt(stray L / junction C) <= (load V / load I). And yes, that means, if you can't reduce stray L enough (because you're limited by layout size -- those TO-220s have a whopping 7.5nH ESL from drain to source, mind!), you need to increase C, even though that makes peak current and switching speed worse.BJTs driven lazily by a couple resistors won't suffice at 400kHz -- here's a driver I built for 2MHz:The logic signal comes in on twisted pair on the right, goes into a differential amplifier, then a common-emitter amplifier with CCS load, then a BJT follower, then a MOSFET inverter (complementary common source), then another BJT follower. Rise/fall time is only 12/8 nanoseconds:The overshoot and ringing is intentional, as the MOSFETs produce a lot of shoot-through current in this configuration. The waveform is quite adequate to drive MOSFETs with 50nC gate charge at 2MHz. The drive power is almost a watt with that load!Notice the construction: copper pours over ground plane (the PCB bottom side is solid ground, with vias coming to the top side every so often). The shielding effect is necessary to deliver this much current, this quickly. You have the added challenge of needing one driver to float on top of an RF output, which means you need enormous CMRR at its logic input. It may not even be feasible to use a "flying" gate driver at all, because of the parasitic load of all that extra metal.I would not use bootstrap power here.Tim