Pulsing PSU - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: November 28, 2019, 03:05:47 pm
content: Hi,I am looking for schematic or any information or pointers to designing a simple PSU for pulsing applications (for general experimentation and testing of various parts - mostly coils) and with the following specs:- charging an energy storing capacitor (for repetitive fast pulse discharge!) which can be anything between 100nF to 1uF and a variable voltage between 100 and 400V (variable so we can control the energy delivered)- either isolating the capacitor when it needs to be discharged into the test system/part or be such that the charging part of the system does not get damaged by discharging the cap into what sometimes would be almost a short circuit (sometimes the test coil is few ohms or less)- the frequency of discharge can be as high as 60HzWhat are the best types of caps for pulse applications and fast discharge (apart from the classic low ESR electrolitics)?We are open also to suggestions to use some off the shelf sub-systems providing they are relatively low cost. Otherwise we can design the full PCB in house.Any suggestions, pointers or even better partial or full schematics would be very much appreciatedMany thanksRik

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 28, 2019, 04:18:27 pm
content: Can it be powered from the mains, and if so, does it have to be isolated? The most straightforward and robust solution I can think of is a bridge rectifier on the output of a variac with an incandescent bulb on the output as a current limiter. If isolation is needed, add an isolation transformer.If you want to power it from a low voltage DC source, look up DC/DC converters for xenon strobes.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 28, 2019, 05:17:17 pm
content: Thank you Mike, much appreciated!

author: Marco
author_rank: Super Contributor
timestamp: November 28, 2019, 07:34:33 pm
content: That's 4.8 Watt max, don't sweat the capacitor ripple current in the PSU.Simplest solution would be taking a high voltage bench power supply and just a chunky resistor current limiter for charging and a DC SSR to disconnect it (they're slow, but it's only 60 Hz). With a 1K resistor it will still charge in a couple ms or less, which presumably is more than fast enough.

author: james_s
author_rank: Super Contributor
timestamp: November 29, 2019, 01:52:42 am
content: https://uzzors2k.4hv.org/index.php?page=capcharger2

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2019, 03:54:48 am
content: Any old flyback will do, here's one for something like 100-800V and 50W:The schematic is uninteresting, a UC3843 flyback circuit. Check the datasheet / appnote for typical use.- The two transistors are in parallel, just because I didn't have a single part rated for the purpose. (Or, actually, they don't really get hot, I probably could've used just the one, with that big heatsink on there...)- Drain voltage rate snubber, FR302 || 47R + 4.7nF to GND.- Stack of caps on the primary side is just for bypass, 4 x 220uF 50V.- 1.5KE15A clamps controller supply, just in case of overvoltage/inrush/surge. Not needed, just nice to have in some cases.- Transformer is wound with foil primary, secondary is dual, 28AWG. Interleaved PSPSP. Gapped EE33 core (Ae = 100mm^2). Diagram:https://www.seventransistorlabs.com/Images/DCDC_800V_FoilWindup.jpg- Secondaries are independent, half-wave rectified each, and stacked in series. Effectively it's a dual 400V supply. One side is grounded, hence the output is up to +800V.- 2 x 0.68uF film caps and 2 x 10uF 450V electrolytics bypass the output. UF4007 rectifiers. 100uH choke to 2 x 0.47uF film caps provides additional filtering.- 200k bleeder keeps output safer to handle when deenergized, and sense divider is 1M--20k trimpot--3k to GND.Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: November 29, 2019, 09:18:13 am
content: Thank T3sl4co1l, James and Marco for the replies/infos!

author: ocset
author_rank: Super Contributor
timestamp: December 06, 2019, 06:37:05 pm
content: attached please find ltspice sims etc of that kind of thing

author: ricko_uk
author_rank: Super Contributor
timestamp: December 13, 2019, 08:14:23 pm
content: Thank you very much Treez!!! For the schematic and simulation!!!

author: mariush
author_rank: Super Contributor
timestamp: December 13, 2019, 08:59:39 pm
content: You could use a transformer to provide the insulation and boost the voltage to around 420vThen you could use a regulator like LR8 from Microchip to adjust voltage between 13.2v .. 438v (input voltage - 12v) :http://www.farnell.com/datasheets/2138212.pdfYou could do like linear power supplies and have multiple taps on the primary transformer to reduce dissipation on the regulator.The regulator above can only do 20mA though, not sure that's enough to charge the capacitors fast enough for your needs.

author: David Hess
author_rank: Super Contributor
timestamp: December 14, 2019, 02:38:01 am
content: Quote from: ricko_uk on November 28, 2019, 03:05:47 pmGiven your voltage and capacitance requirements, film capacitors are the most suitable. Almost any type will do however the larger metal film rather than metalized film have a lower ESR. Special high ripple current ones which use spade or screw terminals are available. Many lower value leaded parts in parallel will reduce ESR and ESL but a careful layout is necessary for good current sharing.QuoteIn the past I would have tried using an SCR for the discharge switch however an IGBT is probably be a better choice now. The initial discharge profile is very much like turn off into an inductive load which high power thyristors and transistors of all types are often characterized for so finding a suitable part from datasheets should be easy.For best performance, pay attention to construction with heavy conductors to minimize resistance and inductance. Keep loop areas at a minimum. I have built things like this which were more like plumbing projects.If the peak current is too high, then some inductance can be added to control it. In the extreme case, pulse circuits like these sometimes use a lumped transmission line for the storage element to lower the peak current and extend the discharge profile into a fixed length pulse.

author: ricko_uk
author_rank: Super Contributor
timestamp: December 17, 2019, 08:59:19 pm
content: Thank you David and Mariush!