Good topology for a capacitor charging power supply - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: April 07, 2019, 06:49:02 am
content: What's an efficient topology for charging capacitors.The charging voltage would be from 100 to 400V DC.Input voltage 12 to 15V DC.I have seen topologies that use hard inductor switching. but I'm leaning more towards Soft switching, ZVS / ZCS or resonant mode.I've read a few threads on EEVblog regarding these and especiallyhttps://www.eevblog.com/forum/beginners/what_s-inside-a-low-power-isolated-dc-dc-converter-brick/WhereT3sl4co1lmentions there are of 3 kinds: Free running, Flyback and resonant.

author: tautech
author_rank: Super Contributor
timestamp: April 07, 2019, 07:19:52 am
content: For what requirement ?Cap leakage tester ?

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: April 07, 2019, 07:24:23 am
content: Quote from: tautech on April 07, 2019, 07:19:52 amNot really, this is a general purpose power supply would charge the capacitors to a set voltage say between 100 to 400V, the charged capacitors can then be used for various pulse discharge circuits.Basically the aim of building this power supply is to efficiently charge a capacitor with a constant current output.

author: tautech
author_rank: Super Contributor
timestamp: April 07, 2019, 07:39:07 am
content: Quote from: ZeroResistance on April 07, 2019, 07:24:23 amPut the word Pulse in there and immediately you're into Pulse Grade cap territory. The discharge curve will determine what cap type is required.Pulse frequency ?

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: April 07, 2019, 07:47:24 am
content: Quote from: tautech on April 07, 2019, 07:39:07 amApologies I wasn't clear, the intention of this question was to build a constant current capacitor charger that can charge capacitors to various voltage levels like 100V to 400VDC.Basically I needed a efficient capacitor charger, i used to charge my capacitor with a voltage source and a current limiting resistor.The charged capacitors can the be sued for various other circuits / experiments. The capacitors would by polypropylene film/foil type .

author: tautech
author_rank: Super Contributor
timestamp: April 07, 2019, 07:51:54 am
content: Boost flyback with inbuilt current limiting if you were to KISS.I guess now to find one with the voltage rating you need.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: April 07, 2019, 08:13:32 am
content: Quote from: tautech on April 07, 2019, 07:51:54 amThats good enough I guess but that would be hard switched? wouldn't it?

author: tautech
author_rank: Super Contributor
timestamp: April 07, 2019, 08:21:09 am
content: Quote from: ZeroResistance on April 07, 2019, 08:13:32 amMaybe not at the higher voltages where you're out of the realms of low ESR.Anyways strategically placed inductors should soften things up some.

author: Zero999
author_rank: Super Contributor
timestamp: April 07, 2019, 09:14:48 am
content: Is it battery powered? If not then go old school, with an variac connected to a diode bridge. Increase the voltage slowly enough to avoid blowing the rectifier or connect an incandescent lamp or inductive fluorescent lamp ballast in series with the rectifier to limit the current.If it's a battery powered application and it doesn't have to be lightweight, then use the above, run off a cheap modified sinewave inverter.You may want to consider an isolation transformer, especially if it's connected directly to the mains and it's mandatory if it will have an oscilloscope or signal generator connected to the secondary.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: April 07, 2019, 09:30:44 am
content: Quote from: Zero999 on April 07, 2019, 09:14:48 amIs not battery powered, I was thinking on the lines of a modified Mazilli ZVS oscillator with some kind of current limiting, the current limiting part is what I'm currently stuck at?

author: Zero999
author_rank: Super Contributor
timestamp: April 07, 2019, 10:00:37 am
content: Quote from: ZeroResistance on April 07, 2019, 09:30:44 amThe currrent limiting needn't be complicated. All you need is a high enough impedance in series with the rectifier to limit the current to a safe level. You could add a separate inductor or design the transformer with a high leakage inductance to limit the current.

author: MagicSmoker
author_rank: Super Contributor
timestamp: April 07, 2019, 11:16:52 am
content: Quote from: ZeroResistance on April 07, 2019, 09:30:44 amNo such thing as a "Mazilli ZVS" - that was just some overly-arrogant person sticking their name on the well-known-for-several-decades "current-fed, parallel-resonant push-pull" converter.A topology which, btw, is singularly awful at charging up capacitors because it is not tolerant of short circuits (in fact, no parallel resonant converters are) and a capacitor looks like a short at the beginning of charging.In addition to the ultra-simple Variac + isolation transformer idea above, the self-oscillating flyback (aka "blocking oscillator") was used for decades to charge up the capacitor in camera flashes because it is almost ideal for the task, as long as the ultimate charging voltage isn't too high and the capacitance not too large. The useful operating range can be extended by using a current mode (PWM'ed) flyback with, optionally, secondary side current limiting (primary side current limiting - automatically provided with current-mode operation - only limits primary power, same as the blocking oscillator, but that can still result in dangerously high current in the secondary rectifier).For really big capacitors charged to really high voltages - or when you need charging rates of 1kJ/s or faster - the series resonant full bridge converter operated at a switching frequency of less than 1/2 the resonant frequency will automatically provide a constant current output and is immune to short circuits (but does not tolerate an open circuit, like any series resonant topology). Truly no offense is intended, but you have zero chance of getting such a circuit to work reliably in anything less than 1 year based on your current level of knowledge, but it would be a helluva learning experience to try.

author: soldar
author_rank: Super Contributor
timestamp: April 07, 2019, 11:23:46 am
content: Quote from: ZeroResistance on April 07, 2019, 07:24:23 amUnless I am missing something this seems extremely simple. By rectifying the 230V mains you can get up to 320 V. Using a voltage doubler up to twice that. So having a capacitor charged to any of those voltages is trivial. That is your starting out DC voltage.Now from there you want to charge a capacitor to a fraction of that voltage? Just use a resistor and a switch that cuts off when the required voltage is reached.What am I missing?

author: ocset
author_rank: Super Contributor
timestamp: April 07, 2019, 03:37:40 pm
content: Your going to struggle to find a fully resonant (as opposed to a quasi resonant converter) converter to do the 12 to 300v charging.This is because most fully resonant converters like to have a pretty fixed vin and vout, and with a charger, the vout is changing constantly.I would use a dual cascaded boost converter.... for that...you could always make it pretty well BCM switching to reduce switching losses and reduce diode reverse recoveryIll send some stuff on it soon.(I had a similar situation as you when i worked at a place in South Wales)If you want my full free SMPS course, ill send it to you...just emailandymassey22@gmail.comand ill send you the google drive link....unless you know of a way i can somehow post the google drive link on here?

author: StillTrying
author_rank: Super Contributor
timestamp: April 07, 2019, 04:19:50 pm
content: Ebay topology.https://www.ebay.co.uk/itm/DC-DC-10-32V-to-45-390V-High-Voltage-Boost-Converter-Step-up-Booster-ModuleUULK/264224502351

author: Gyro
author_rank: Super Contributor
timestamp: April 07, 2019, 04:30:21 pm
content: Quote from: soldar on April 07, 2019, 11:23:46 amGiven that this is the Beginners section, a mains isolating transformer. However given the hazardous amount of energy being stored in the capacitor, an easy mistake to make.

author: soldar
author_rank: Super Contributor
timestamp: April 07, 2019, 04:42:12 pm
content: Quote from: Gyro on April 07, 2019, 04:30:21 pmYes, I guess I should have said so.On re-reading I see I also missed the essential spec where he wants to charge from a low voltage battery so what I was suggesting is out.Hereis a post of mine where I included a couple circuits from flash chargers.

author: David Hess
author_rank: Super Contributor
timestamp: April 07, 2019, 04:54:52 pm
content: Current mode converters are automagically current limiting although you may want to configure a specific current limit. Many switching battery charger ASICs include a separate current limit and might be amendable to using a flyback transformer reducing the voltage demands on the switch. I think a SEPIC controller can operate this way and I know some have separate current limiting.The more powerful configuration that I would use when charging from a low voltage DC source is a current limited buck converter driving a push-pull output transformer. Some off-line switching regulators work this way. Output current is continuous and the transformer operating on a current instead of a voltage is inherently balanced.

author: Buriedcode
author_rank: Super Contributor
timestamp: April 07, 2019, 06:37:24 pm
content: I have built similar things for medium voltage (200-400V) cap charging, and used flyback topology. It generally lends itself better to cap charging, as its pretty much a current source out of the transformer/coupled inductor. It can handle shorts on the output, and allows for isolation if you use primary side sensing. There are many dedicated devices to control it, and you can control the input current, and charge time by adjusting the peak current and off-time. It scales up nicely by I've only ever built one up to 50W.

author: Berni
author_rank: Super Contributor
timestamp: April 07, 2019, 07:43:20 pm
content: For capacitor charging flyback topology is usually the best because it works well over a large output voltage range and doesn't mind its output being shorted.A ZVS oscillator circuit can be used too, but current limiting it can be quite tricky. Softening of its output is likely best done by adding a inductor in series with the transformer secondary, otherwise it will try to draw a massive current if the output is shorted. The troubles of keeping the ZVS cirucit under control are only really worth it if you need a lot of power (over 50W) due to its incredibly low switching losses, or you want the circuit to produce a very clean, noise free output.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: April 07, 2019, 08:13:28 pm
content: Quote from: treez on April 07, 2019, 03:37:40 pmI guess there is a "Insert Hyperlink" buttion when you try to post a reply. You could use that.Or just directly post the link as plain text incase you don't want to go through the hassle of doing all that.Also thanks for offering your smps course would be delighted to hear more about that. Do you have any website with more details?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 07, 2019, 09:03:15 pm
content: Resonant is probably not in the cards for such a wide output voltage range. Well, actually Idunno, resonant converters can have nice output V-I characteristics. You wouldn't get the constant-power sort of response that a boost/flyback approaches, but this doesn't make much difference in the charging rate, just a few ten percent at the low end.What power level?Tim

author: ocset
author_rank: Super Contributor
timestamp: April 07, 2019, 10:05:50 pm
content: QuoteThis is the link from my google drivehttps://drive.google.com/open?id=1cmk81KxXA7-EliHHWr8ZimpEp7HCMIcYPlease tell me if it doesnt work.

author: james_s
author_rank: Super Contributor
timestamp: April 07, 2019, 10:08:24 pm
content: I've built several of the circuits this guy designed, and my own variations of them, they've worked well.http://uzzors2k.4hv.org/index.php?page=capcharger2

author: Berni
author_rank: Super Contributor
timestamp: April 08, 2019, 05:55:31 am
content: Quote from: james_s on April 07, 2019, 10:08:24 pmThat is also a good way to limit the output of a ZVS oscillator by winding the primary and secondary on opposing sides of the core. This causes extra leakage inductance that makes the output sag under heavy load rather than trying to push more and more current into it.

author: Circlotron
author_rank: Super Contributor
timestamp: April 08, 2019, 07:09:56 am
content: Simple and resonant? An inductor, diode and switch in series. Or just an SCR and inductor. The cap will charge in one half cycle of resonance of the LC combination. Whatever DC voltage is supplied to the input side, the cap will charge to double this voltage. The charging current is a half cycle of a sine wave. You may need a fairly large inductor though, say one out of a mercury vapour street lamp.