Notices by niconiconi (niconiconi@cybre.space), page 3

Galvanic isolation turned out to be the correct decision. Just had an electrical breakdown. The SCR was completely destroyed, feeding high voltage back to the gate, then the gate drive transistor arced, high voltage kept going back further to destroy the optocoupler. ⚡? The sacrificial optocoupler did its job to stop damages from propagating further... #electronics

What killed my SCR? :blobcatthinking2: Either an arc over caused by low creepage & clearance, or max di/dt violation. Need to try again tomorrow... #electronics

Problem solved, a n00b mistake. The SCR was incorrectly wired as a high-side switch, not a low-side one. As the load was switched on, the gate and cathode would rise to 200 volts, an exploding gate drive transistor was guaranteed... ⚡ ? Replaced it with a high-voltage one as a temporary fix, now it works fine. :blobfacepalm: #electronics

The next long-term goal is converting this "Improvised Electronic Device" to a proper impulse generator that can produce a proper 8/20 µs surge per IEC 61000-4-5. The 4000 V / 2000 A Class 4 test is difficult to implement, but the basic 500 V / 250 A surge should be easy. #electronics

Just found the full design equations for the 8/20 µs surge generator in [1]. The author was nice enough even to have all component values precalculated for you. Looks easy if you can find a suitable HV switch. I initially worried about core saturation, the surge is 100 to 1000 amps, but the L is just 10 µH, a practical value for winding an air-core coil by hand.

[1] Elementary and Ideal Equivalent Circuit Model of the 1,2/50 – 8/20 μs Combination Wave Generator, Carlo F. M. Carobbi https://ieeexplore.ieee.org/document/6714698 #electronics

Back to switch selection... At millisecond-scale, the limitation of SCRs in an non-repetitive, oneshot pulse is thermal, as the pulse gets shorter, the allowable surge current goes up significantly, a SCR only rated for 60 A steady-state can switch even 2000 A. But at microsecond-scale, di/dt becomes the new problem, and that makes the allowed surge current to go down significantly... Apparently the responsible mechanism is localized hotspots in the silicon before it fully turns on.

Most SCRs have di/dt (max) = 50 - 100 A/μs. So for a 8/20 µs surge, the current peak must be less than 400-800 A. With a standard 2 Ω effective ESR, surge voltage is restricted to 800-1600 V. Let's see if an IGBT does this better... #electronics

IGBTs are not really much better it seems... Common discrete devices only exist below 2000 volts, roughly the same range. #electronics

Pulse capacitors also become hugely expensive at 2000 volts... 2000 volts really looks like a barrier in power electronics. Beyond this point components suddenly become either hugely expensive or come with inconvenient packages... But if you stay at 1500 or 1700 V, it's much easier... #electronics

Now I need 1500-volt, 6 µF pulse capacitors, what are my options.... :blobcatthinking2: It turned out the choices are pretty limited, basically three: Kemet R75 & R76 series, Vishay MKP385 series, and Cornell-Dubilier 940C & 941C series (this one seems to have a cult following by Tesla coil builders). 941C is currently the cheapest, 1.5 µF for just $11. But I need FOUR and only THREE are in stock until 2023. :blobfacepalm: #electronics

High-voltage flyback transformers look like a dying breed these days. All the experimenters seem to get them either from old CRTs or random eBay surplus sources. But my project needs to be replicable from standard parts on the product catalog of a big vendor, so random components are unacceptable. CCFL lightning transformers seemed to be a good source in the past decade, but most are discontinued in this LED age. I guess I'll just stop wasting time and use a pre-made SIP DCDC module instead. #electronics

The datasheet says the pulse resistors I'm using to build a 8/20 µs surge generator are themselves tested by a 8/20 µs surge generator by the vendor. Now the cycle is complete. :blobcatgiggle: #electronics

You know pulse resistors are specialized when a manufacturer in China sells them at a higher price than Vishay in the US... And the official website shows zero stock. #electronics

I'd just like to interject for a moment. What you’re referring to as a "8/20 μs surge", is in fact, a 1.2/50-8/20 μs surge, or as I've recently taken to calling it, the output from a 1.2/50 µs combination wave generator. The 8/20 μs waveform is not a surge unto itself, but rather just the generator's short-circuit current output as defined by IEC 61000-4-5. It's normally generated in combination with the 1.2/50 µs open-circuit voltage waveform. All the so-called "8/20 μs wave" are really the "1.2/50-8/20 μs" combinational wave... #electronics

Huge pulse capacitor is huge. Cornell-Dubilier 941C series #electronics

0.1 Ω pulse resistors arrived. Time to do more test on the original quick-and-dirty HV pulser before building a proper one: 31.2 A short circuit, 200 V open-circuit, so the effective ESR is 6.4 Ω, a bit on the high side. The di/dt is high, 10-90 rise time is just 400.0 ns, it's 65 A/µs - I'm already violating the absolute maximum of the SCR. #electronics

Saying I'm building a "8/20 μs impulse surge protector tester" sounds boring, I think I'll start calling it a "0.3 megawatt pulser" instead. I hope I can rightfully call it a "megawatt pulser" when I break the 2000-volt barrier one day... :blobcatgiggle: #electronics

Just found a workable solution to switch 4000 volts and 2000 amps. Time to go old-school!!!

Except there are three problems. The tube has no trigger electrode, only free running is possible. Each tube costs over $200. Also, each tube is filled with 5.44 MBq of radioactive tritium ☢️ ! #electronics

Just finished the construction of my new 8/20 μs impulse generator. Its output surge should be in compliance with IEC 61000-4-5 now, but I haven't checked it yet. Need to get some sleep and continue the test tomorrow. Working with lethal voltage at midnight is definitely not a good idea... #electronics

My impulse generator's current output failed to meet the standard waveform requirement. The rise time is 50% too short. I think I didn't wind that inductor correctly. Just ordered a simple LCR meter to continue the experiment (trimming an inductor with a oscilloscope or VNA is just too awkward). #electronics

I found the solution for switching 2000 volts - two thyristors in series should be able to handle it. I was worriying about off-state leakage current due to voltage balancing resistors in the original capacitor discharging circuit. But this problem disappears in the standard 8/20 μs impulse generator, it already has two 20 Ω resistive loads, this diverts leakage mostly to the ground instead of the load, making the series switch connection feasible again.

Capacitor size and cost are still the problem. Using non-pulse rated capacitors is perhaps an acceptable workaround if I'm willing to replace them once in a while. A lifetime of 10,000 shots is practically infinite for my purpose. #electronics