Weird gas discharge lamp ballast - curious about analog circuit design - Page 1
Forum section: electronics / projects

author: george.r
author_rank: Contributor
timestamp: February 16, 2023, 09:22:08 am
content: So what was supposed to be a nice stay in Canada is being "stuck in an airbnb feeling sick and not doing much". Well, on a trip between that and the pharmacy, I couldn't stop myself from poking at a black box with wires that I found alongside some other discarded garbage. Said enclosure contained a small 24V power supply with some other circuitry on the side, plus an indicator LED, the subject of this post, and what I discovered to be the remains of a special UV-C bulb. The bulb is obviously gas discharge and therefore not running straight off the 24V, and the rest of it sure looked like an electronic inverter/driver/ballast (call it whatever you prefer) - transistor on a heatsink, spot for a (removed) transformer, cutouts on the PCB to increase dielectric rigidity, widely spaced output connector pins, yeah, that's a HV PSU.Anyway, googling the part numbers on the PCB yielded a match for one of them (PXZ120I2 leads me tohttps://www.ushio.com/files/specifications/care222-filtered-far-uv-c-excimer-lamp-module.pdf).Being quite bored, and not feeling in okay enough shape to visit the museums I'd like to, I set out about reverse engineering this just for the fun of it, first because I mostly a digital guy and know very little about analog design, and second because this seems very weird and overly complex for just being a dumb lamp power supply. Besides the missing transformer, the bulb has been gutted - I only have the backside of the yellow casing visible in Ushio's PDF of the thing, and some aluminum heatsink looking things to which the wires go bolted via ring terminals. Even if not, I don't have any equipment on me that could do HV probing.I've drawn what I think is an accurate schematic and learned almost nothing from it. I can discern a few things, but not the overall "what does what and why". There's plenty of empty footprints which I've not included in the schematics, nor do I have a way to measure the caps. Also, I wonder why they use such weird component values, and double them up as necessary - would the BoM line item difference matter this much? Why use two double comparators instead of a single quad?In short, why would anyone design an HV power supply like this, and how did they get to this? Seems like a very clumsy design and layout.

author: jonpaul
author_rank: Super Contributor
timestamp: February 16, 2023, 03:14:34 pm
content: Ushio, and other Japanese lamp OEM sold millions of custom HF fluorescent, discharge or UV lamps for copiers, disinfectants, water treatment.These ballasts are specifically for the discharge particular lamp.I worked for a Japanese OEM in 1990s on such electronic ballasts.Nowadays advanced LEDS have replaced the custom gas discharge lamps.Jon

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 16, 2023, 04:47:37 pm
content: Quote from: jonpaul on February 16, 2023, 03:14:34 pmThis supply is for a 222 nm krypton chloride excimer lamps, LEDs have not reached these wavelengths at competitive cost or efficiency yet.Looking at the circuit, it seems to be a flyback topology (possibly resonant). IC100A appears to be wired as an integrator + schmitt trigger oscillator. Its output is buffereed by IC100B, and wire-ORed with IC101B which serves as overtemperature protection. Optical feedback controls the oscillator to maintain a given output. The parts around CN101 are used to sense lamp current and give an indication that the circuit is running. There are still some aspects of the circuit that seem strange, the collectors of Q107 and Q109 are not connected to anything in the schematic, can you verify this with the board?

author: george.r
author_rank: Contributor
timestamp: February 16, 2023, 08:52:01 pm
content: Yow - my bad, Q107 and Q109's collectors are tied to IC100 and IC101:B's output, pin 7 on both packages (already wired together there) - thus also wire-OR'ing that part of the circuit into the push-pull pair that switches the big driver MOSFET.Yeah, that much I have figured about the general architecture. Thanks for reassuring me on IC100A being more than just an oscillator. I I am still so very puzzled why do it this way instead of...just use a monolithic IC made for this sort of thing? There's gotta be a few ways out there that would reduce design complexity and probably cost too. Though these things seem pretty specialized (given the 222nm wavelength of these excimer lamps), so maybe they're not too worried about cost? I am trying to find a 'gotcha' as to why it was made like this, or if the lamp would have needed to be driven a certain way instead of just the common "feed it a big spike to strike a plasma, sustain it on high-freq HV", that kinda thing.