EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: electronz on March 16, 2016, 06:28:31 am
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I'm trying to design a circuit to charge a bank of four 315V/330uf capacitors (hooked up in parallel). I'm already using these caps as part of a larger project and have been charging them with a cannibalized disposable camera flash cap charging circuit. I'd like to get away from the camera flash circuit because the type of transformer it uses seems very difficult to source.
The new design is based around a charge pump, with its MOSFET being driven by a 50/50 duty cycle DC pulse from the output of a 555 timer (with a MOSFET driver in between to assure the MOSFET switching is clean and quick). The cap-charging power comes from a 9V battery, and the circuit gets switched on by via a transistor connected to an Arduino +3.3V digital output pin.
I am relatively new to circuit design, and at best, don't fully understand what I'm talking about some of the time, so I'd like to get some opinions on whether I'm headed in the right direction with this design. I haven't done much math for this yet, just general design, so specific component values are vague at this point. I've attached a schematic of my current design if anyone feels like taking a crack at it :box: Thanks!
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Those are just regular caps I think. So what voltage are you trying to charge them up to?
Sent from my horrible mobile....
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They probably are just regular caps. The running joke at my office has been "Don't fry yourself with those supercaps!", and I think the name just stuck.
I'm charging them to between about 200V-250V. Don't remember off hand how much voltage is enough for the application, but I think 200V should do it.
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I've only had a quick look at your circuit, so could easily have missed things, or be wrong.
(1)...There does NOT appear to be anything LIMITING the maximum charge voltage, applied to the capacitor bank (C1 .. C4). This could damage them and/or lead to too high a voltage, causing other problems.
E.g. Limit it to 250V. By making it switch off when it reaches >= 250V.
(2)...The 1N4004 (MIGHT BE FINE, depends on required maximum output voltage) are only just about high enough voltage (400V). 1N4007's (1000V, typically same price) might be better (but other diode specs can be worse). 1N4000's are a bit slow for SMPSs as well, but it might be ok, especially at lower frequencies. But a 1N4004, is NOT the end of the world. Just very little safety margin. Especially at >=400V
(3)...How does your Arduino connect to the power supply (9V) ?
Because it looks "upside down" to me. (D3, Q2, R1).
I.e. It needs about (8.3V - diode drop) or lower to switch the transistor on, from the 3.3V output!
(And/or the 1N4004 diode would BLOCK the voltage as well!)
(4)...You could do with a bleeder/discharge resistor, across the final output. It needs to be high enough so that it does NOT load the output too much. Yet low enough so that the capacitors get discharged, when switched off, fairly quickly. For safety reasons. E.g. Getting a nasty surprise, after opening it, and accidentally touching the capacitors, while switched off.
They can keep a charge for quite a while (probably minutes (guess, depends on quality of capacitors) or longer, if open circuit).
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If you want to charge the capacitor bank in the shortest possible time in the most efficient way a self oscillating flyback converter is the best design.
That's a bold statement. The reason I claim this is that as you charge the capacitor bank the output inductor is resetting into a varying voltage and the time it takes for the inductor to empty itself depends on the voltage (I=Cdv/dt). With a fixed frequency design the inductor can't empty itself before the next ON pulse from the controller. The result is that the inductor operates in continuous conduction mode until the voltage across the caps climbs enough for the inductor to be able to empty itself. This is undesirable.
The solution is a self resonant converter that only switches on the MOSFET when the inductor has emptied itself completely. A feedback winding on the transformer senses when the inductor current drops to zero and a new cycle can begin. This is the basis of most photo flash charge pumps.
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You really should use an isolated mosfet driver for this design, otherwise a shorted mosfet will fry the entire circuit.
Have a look at flyback transformer circuits if you want to build this properly.
Basic idea is transformer driven by mosfet, with and optoisolator to return a signal when the output voltage gets too high.
Have a look at what Linear Technologies have available, http://www.linear.com/parametric/Flyback,_Forward_and_Isolated_Controllers (http://www.linear.com/parametric/Flyback,_Forward_and_Isolated_Controllers)
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Take a look over : http://cds.linear.com/docs/en/application-note/AN118fb.pdf (http://cds.linear.com/docs/en/application-note/AN118fb.pdf)
I'm also working on something similar - getting HV for a small CRT tube ( 1.2kV )
The transistors do not matter too much if you do not care about efficiency - just take care not to blow them / overload the circuit.
It's somewhat self-limiting, it will not instantly blow up if you short the output.
The transformer is a standard CCFL one - you can get them from ebay, etc.
For my project I've bought a new transformer from farnell.
For the prototype I've used a transformer from a scrapped monitor - it used a different circuit so it did not have the feedback winding - i just had to squeeze 3 turns over the primary winding. It worked once I got the phasing right and it's quite stable.
You can also use some off-the-shelf CCFL inverters from ebay - you can find them cheap and you can add voltage limiting ~ easy
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MK14:
1) I planned to set and limit the charge voltage via the inductor output voltage, or just time the charge cycle so that it won’t have time to charge beyond a certain voltage. Maybe adding a switch-off mechanism at 250V or so would be safer, though.
2) I’ll look into the 1N4007’s. If there aren’t any significant disadvantages, it would be nice to overbuild this aspect if the cost difference is negligible.
3) The Arduino itself doesn’t connect to the 9V power supply - it’s just switching on the 9V supplied cap charging circuit. The D3, Q2, R1 circuit looks upside down because it would be if the 9V supply were headed to the Arduino. Rather the Arduino is sending a +3.3V signal (digital OUTPUT pin HIGH) through the diode and resistor to open the transistor and turn on the charging circuit.
4) Yeah, I agree a bleeder resistor is a great idea. I want to keep the caps charged up until their charge is needed, but could probably just leave the charging circuit on until that point?
Richard Head, station240, & ealex:
Thanks for the improved circuit design ideas! I’m all for revising this thing to as optimal as possible. I’ll look into the resources you all sent over and post back with any questions.
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Just skimmed your post, and couldn't hold back the need to rant :/
The trouble with using a 555 timer is, the on time should be set depending on your inductance, and power supply. Whilst inductance tends to be within 10% tolerance, you'll also have to account for power supply variations. Or you can sense the inductor current with a current sense resistor on the source of your MOSFET. If you're going with constant on-time and off time, you'll want the on-time short enough to limit the peak current in the inductor (work out worst case, highest supply voltage, with lowest inductance), and make the off-time long, because on start up the time it takes for the inductors current to fall to 0 can be quite long (high Vin, low Vout, when caps are 0v). As the cap voltage rises, this time will get smaller and smaller. It will limit charge time but prevent current in the inductor steadily increasing to the point of saturation - putting extra stress on your switch and your power supply.
Also, as others have pointed out, any boost/flyback circuit is essentially a current source, it will continue to dump energy into the caps, raising their voltage until something gives. That could be the caps, or the MOSFET (the drain jumps up to the voltage on the caps + Vf of the diode). This is why flyback transformers (coupled inductors) are useful because you can keep the voltage spikes relatively low on the switch, whilst charging to a higher voltage. A single inductor boost could probably do 200V from 12v, although it would require a high voltage MOSFET, which then raises its Rds limiting efficiency. But a flyback of 1:6 to 1:12 will allow you to use a much better FET.
So a comparator with a reference and tapping the output with a voltage divider is required at the very minimum. This can feed the reset line of the 555, shutting it off once your set voltage is reached. Adding hysteresis means if left 'on', the caps voltage will start to drop (leaking through the voltage divider) and once lower than say 10% of your set voltage, it'll charge it up again.
In an effort to make a relatively cheap and cheerful cap charger for various 'got photoflash caps and want to do unwise things with them' I designed a 555 based flyback charger. It had current limiting and shut off when it reached the set voltage. Wasn't pretty, or particularly efficient, but I'll post if you're interested. I used it as a more powerful charger than those neat simple camera flash units. I did however wind my own flyback for it.. (100uH, 1:10 ratio, Isat ~2A). I suspect a single inductor boost along with a diode multiplier could be used as well.
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Buriedcode, I appreciate your rant :) I'm still green to EE (I've more experience with DIY/technician related stuff), but am doing my best to keep up with these concepts and suggestions. I'm very interested to see your 555 based flyback charger design if you don't mind sharing.
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You're most welcome. I'll have to dig it out of an old harddrive (its *somewhere* around here..)...
In the meantime, whilst you probably just want to build, an excellent article that explains boost converters, and flybacks (just an isolated boost converter really) is here:
http://www.dos4ever.com/flyback/flyback.html (http://www.dos4ever.com/flyback/flyback.html)
It explains it beautifully, without dumbing things down, or boring one to tears with the real nitty gritty. Its practical and was the inspiration for my (somewhat shitty) converters.
Edit: The above, and most details about boost converters usually assume one is making a DC supply, so they want low ripple, and feedback to keep the output smooth and stable. For capacitor charging, we don't need that. We *do* need feedback to stop the caps over charging, but thats just a threshold. You won't have to worry about output ripple or trying to get the converter to work at a certain frequency. It's just storing packets of energy in an inductor, then dumping that into the cap bank, probably several thousand times a second.