Anyone familiar with a high voltage charge pump designed as a step down power supply? The voltage is between 175 & 200. Numerous high voltage versions exist using inductors, (Onsemi has one that is "self supplied" which is attractive) but not a one that works with just a capacitor. Are there any out there I've missed?
Capacitor step-up supplies are called Cockcroft-Walton generators and a similar form are Marx generators. An inverse version of the circuit can do step-down. We encountered a fellow, Steve Cerwin, who promoted that form. Crazy, we thought, but went ahead anyway, and wrote it up for the AoE x-Chapters, section 9x.15.5, page 439. Capacitors are charged in series, but discharged in parallel. We show a circuit that does 120 Vac to 24 Vdc, using six capacitors, and 14 diodes, awk! The output current is stepped up, of course. But, just because you can do it, that doesn't mean you should.
Win - In this case, the existing power supply is between 175 & 200 volts. The needed supply is 12 volts. Similar to a nonisolated dc supply from a 115 volt ac line I suspect there is such a device on the market, but proper search terms are required.
Of course there is, but it'll most likely be transformer based (an inductor will have too poor a duty cycle). No capacitive circuits, such as I mentioned will be found. (There is low-power AC-to-DC, using a single cap, which we cover in some detail, but that's another story).
What you're forgetting is that most AC offline to low-V DC converters start with a bridge rectifier.** In the case of 120Vac, they're internally working from about 170Vdc on their bulk capacitor. Many work up to 230Vac, which more than covers your range. They convert down to all types of DC voltages. But they're happy with DC as an input. So you simply hook up your DC source to one of these "AC supplies", and hey, Bob's your uncle.
** Inspect to be sure there's a input bridge rectifier
The level-shifting required to make switches for the whole voltage range is one problem, and the high inrush currents when capacitors are connected (like, only limited by the switch resistance) is another. Microscopic switches can get hot FAST. Inductance solves the inrush problem, but once you go with power inductors, the capacitors aren't required.
I'm imagining a wooden drum with brass shim contacts, a bank of paper caps and a small motor. Probably feeding a CLC filter, and thence to a post-WW1 radio.
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380V to 95V (4:1). Add a couple of more stages, and your ratio is solved. T his is not easy stuff though, plenty of easy solutions are available. Pick any:
The advantage is at low power. I built a 6 stage capacitive divider 48V to 5V at max 10 microamperes input (on-hook POTS 48V). Microamp inductor based buck converters do not appear to be easy.
There was the Harris HV2405 in the late 1980s but that was AC input - it charged the capacitor on the leading portion of the ac line waveform and disconnected when it exceeded the output voltage. Won't work from a DC supply.
Did you specify if the HV input is AC or DC and how much current is demanded at 12V?
Input varies between 150 & 200 volts dc. Current is at the 12 volt output is
60 ma. There will probably be a 6 volt negative output also but that will cause little added circuitry. Simply stated, 2 npn's, 1 pnp, 6 resisters &
2 1nf capacitors comprise the device. That''s doable but cumbersome in pcb real estate and design time. An integrated package would be a neater fix.
in my experience the only types of charge pumps which aren't dreadful are CW multipliers to make high voltages at very small currents, and the types that come in IC packages that can use techniques like fractional conversion to improve their efficiency/regulation and can use internal switches that have had their parameters tuned to suit the chip application.
Every other time I thought I'd use a roll-your-own charge pump for something I've ended up going inductor-based because naive charge pumps made with logic or discretes suck harder IRL than the by-the-book calculations would lead you to believe
Using one capacitor and one diode to boost e.g. 3.3V logic output square wave to ~4.something to reliably drive a white or blue LED at a few mA is OK too but it's not really high-investment charge pump design here
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