Gregory's ServerJust finished the high-voltage controller firmware in 500 lines of C. It's mostly just housekeeping tasks. The actual interesting part of the circuit board is the analog circuitry under control, which I finally can start testing tomorrow... The low-voltage controller also needs a firmware, but for now I can simulate that using a stub program on the computer... #electronics
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 I just figured out the failure mode of the malfunctioned flyback chip. The flyback controller turns the transistor on, wait for inductor current to ramp up, and turns off the transistor only after the peak current comparator trips. If the power supply drops out after an overload, the original peak current target is never reached, now the transistor is permanently on! It's not that the gate drive malfunctioned and stuck high, it's actually working exactly as designed. #electronics I wonder if my fatal analog design problem can be "fixed in software" and suddenly had an idea... Instead of turning the flyback converter on continuously, I changed the firmware to run it in small 1-millisecond bursts per every 10 ms, allowing the 10 V isolated power supply to recover and recharge between pulses. Unbelievable, this hack worked! It successfully charged the capacitor bank to 1000 V... ⚡ #electronics Voltage regulation at 500 V is even better than my expectation. 2% within theoretical 536 V set point. #electronics Setting up the fail-safe crowbar and watchdog timer early on before debugging other parts of the high-voltage circuit turned out to be really worthwhile. Now I love how I can physically discharge the dangerous capacitor by just pressing Control-C in my Unix terminal emulator. #electronics Trap for young players... If you're disabling another interrupt within an interrupt, that disabled interrupt source may have already set its flag just before you've disabled it, causing a TACTOU situation. When the ISR returns, that supposedly-disabled IRQ is immediately invoked. I just spent an hour trying to debug a strange problem because of this race condition... :woozy_baa: #electronics Success. High-voltage supply and regulation are now fully functional. My fatally flawed analog power supply problem has been fixed in software. The burst-ON hack turned out to not "just a software hack" but kind of an optimal solution given the circuit components constraints. If I need a real hardware solution I'd just implement exactly the same control with RC timers and AND gates instead of an interrupt service routine. #electronics More progress on the high-voltage impulse generator. The huge capacitors, resistors and inductors for pulse shaping are now installed for testing. They mostly work. But this test uncovered a serious problem - each time the circuit fires, I can hear an arcing noise, the oscilloscope also shows a weird glitch, likely caused by arcing. The generator is supposed to destroy the device-under-test, not itself! Now my suspect is the isolated DC-DC module. Just ordered a better one for another try... #electronics Problem solved. The "arcing" sound during the impulse current discharge is NOT a fault. It's actually a common phenomenon in all high-current pulse circuits. The "snap" noise was caused by sudden physical deformations of the circuit board itself, due to Lorentz force from the intense current. #electronics Success! My first circuit board prototype of the IEC 61000-4-5 Combination Wave Generator (aka Lightning Surge Generator, aka Impulse Generator) is working after a month of development. This PCB has greatly improved safety and usability compared to the original perfboard. #electronics |
Now the firmware is finished, it's time to test the analog electronics. The circuit board is starting to look nice after the high-voltage supply and all surrounding analog components are soldered. Unfortunately I immediately found the entire board must be scrapped and revised (it's v0.00 for a reason...). The old prototype used a charge pump voltage doubler to bias the gate drive, which was deleted in the new design, and it was a fatal mistake.
The new high voltage's power supply is derived from an 1:2 isolation transformer and it already doubles the voltage, which was why I deleted the charge pump doubler. But it's an unregulated supply implemented by a quick and dirty push-pull driver chip. When the high-voltage is turned on, the voltage drops back to 5 V, causes the flyback chip to malfunction and stops switching. Worse, its gate drive actually get stuck on, turning the MOSFET into a permanent short circuit / dummy load. The fail-safe logic I programmed into the firmware saved everything from burning up by switching the power off after a timeout. #electronics
Now the firmware is finished, it's time to test the analog electronics. The circuit board is starting to look nice after the high-voltage supply and all surrounding analog components are soldered. Unfortunately I immediately found the entire board must be scrapped and revised (it's v0.00 for a reason...). The old prototype used a charge pump voltage doubler to bias the gate drive, which was deleted in the new design, and it was a fatal mistake.
The new high voltage's power supply is derived from...