Here is what I have: high power pulses (I~25A=constant, V~50-80V, t =300us, rep. rate=100Hz; the pulses come in bursts, long term average power is low). Input voltage is 10V. Flyback converter is working fine recharging energy storing capacitor at the moment. It's hand held instrument, I doubt I can fit much more than 100uF capacitor, converter output is 200V now. I was asked to increase power by factor of 4-6 (2x current, 2x frequency...). ~250W power bursts, that is. I am looking for the solution right now. Any suggestions how to recharge the high voltage capacitor from 10V power rail quickly (professional photoflashes/strobes may be doing something like it) will be appreciated. Thank you! Michael
The pulse energy is in the order of 1J now, it needs to go up to 2J+. I have 100uF cap that discharges from ~210V to ~160V with every pulse. I do not think I can increase capacitance (size) as I have to use photoflash/strobe capacitors (high current).
Then your only options are to replenish from the supply at much higher power or draw it down some more and boost via a MHz-switcher. Either would most likely consume more space and BOM budget than a larger cap. IOW you are between a rock and a hard spot.
You'd have to roll your own.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
This is a misconception drawn from the characteristics of simple saturation-limited blocking oscillators (self-oscillating flyback with complete energy transfer).
The audible frequency varies as reset intervals decrease due to the increasing storage voltage. It is a byproduct, not a design 'feature'.
At your pulsed power level, their design life is 1000 discharges, or
100 hours, with 6 minutes of of actual operation per full charge.
The 'flash' capacitor you are using, at 100Hz discharge frequency, will reach it's 5000 x discharge design life in one hour, or 10 battery recharge cycles.
constant?
Who or what has determined that the load current must be constant? Surely this is a simpler energy requirement that could be passively characterized, within acceptible performance-related limits?
You might consider separating partsthat must be actually in-hand, from parts that provide, process or store the energy. Product endurance and operating life may be a more important issue, as I can envisage few consumer products that would require or could safely employ the enegy characteristics that you describe.
I use two United Chemi-Con photoflash (PH series) caps. 10mm diameter,
27mm long. I am not sure I can use anything bigger. The existing board might take couple more of these, but I haven't taken into account other parts size increase... :o(
Ok, that looks pretty tapped out. You might have to defer other part size increases, migrate the circuitry to 0201 parts and so on.
Might want to look at chips like the LT3485 series for charging but check the availability situation first since this stuff is prone to go unobtanium at times:
formatting link
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
The caps are rated 5000 pulses into Xenon lamp (>>> Who or what has determined that the load current must be constant? I need to control something. Voltage is out of question (it's plasma). What is the other option?
I admit that I have no detailed application info on these parts, beside the general application guidelines published by UCC/NCC.
The 'special design' of photoflash capacitors is not one specifically addressing discharge current levels - it only addresses the electrolytic capacitor's tendency to internally reform (producing reduced capacitance) under the influence of DC current, in the absence of extreme pulses.
Any reliability evaluation performed assuming one pulse per second is probably useless under pulse repetition rates that are two orders of magnitude higher. A thermometer might produce more meaningful data.
With access to the actual circuitry, you are more able to determine the nature of part reliability issues than anyone else, once you are aware of them. You must not assume, however. If there is some urgency at producing a completed product, you will find things go much more smoothly if you use data for parts that have already been assessed for an intended application.
The voltage control being free, pulse forming, non-dissipative or recoverable energy transfer is popular, but usually at higher voltages where suitable switches are hard to come by.
The PWM output control method, which you suggest is intended or is already being used, could deliver the intended increased energy without increasing the current, simply by running longer (>300uSec) during the intended discharge interval, if the energy is there to deliver. I assume this has been determined to be ineffective.
Direct energy transfer by a boost circuit is not out of the question, although the source voltage is low. An inductor could store .6J (80v x
25A x 300uSec)and deliver it to the load at 100Hz rep rates, without intermediate storage. A 480uH inductor requires 3mSec to develop 50A peak from an 8V source, and will fully discharge into an 80V clamp in
300uSec. This uses a single switch and rectifier, with peak current and rep rate being the controllable quantities, once hardware is set in stone.
While consolidating the parts into one humungous switch and choke may not seem like a forward step, the thermal and current issues with the components involved are well known and less subject to crippling life factors.
Reducung the magnetic storage component size might be possible if the source and load would tolerate a 'scrubbing' HF current without malfunction vs the present pulsating DC being employed.
While I also would not use flash caps here, we did as kids (in Germany). Being on a shoestring budget but wanting to build those honking kilowatt amps we took the 230V mains in, sans transformer. Rectifier cascade to
900VDC, bingo (don't tell TUEV I did this...). I secured a bargain of nice professional and large Siemens caps. The others found a sale of flash caps, much smaller and prettier. "Zweite Wahl", loosely translated "Fell off the dump truck". We beat the dickens out of those amps.
One fine day ... KABLAM! One of my Siemens caps decided to mutate into a rocket. None of the flash cap dudes had one blow, ever. That really miffed me. But still I would not use flash caps for this kind of continuous duty application.
However, if this is supplied from a regular battery I guess it would require a similar size cap at the battery voltage level.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
It's a pity that 'flash' characteristics aren't offered in different voltages and temperature grades, or that they aren't issued basic cap ratings, for comparison. But, hey, the manufacturers are the ones on the line when it comes to dead inventory and some do offer 'custom' formulations.
Local ripple decoupling need not be as hairy an issue. Those ARE some really nice batteries - with the discharge rates permitted. If they work out, they're obvious replacements for nicad-only apps. I 'hope' it turns out that lithium has no serious side effects, besides the current known pharmacological ones. (Those might even help some folks ;)
Energy is 1/2 C V^2, so you are delivering 0.925 J from .5 * 100 micro F
((210V)^2+-(160V)^2); if you discharged to zero volts you'd get full
2.2 J . You probably have to up the voltage, which is easier than upping the capacitance because voltage comes in squared (you just need 40% extra to double the energy, etc).
Take a look at the LT1246 for example. You can easily vary its frequency over a 3:1 range with no risk of splattered solder.
formatting link
P1158
You just run a resistor from pin-4 to the output of the switcher. The resistor looks, near enough, like a constant current source to the oscillator circuit so other than making the ramp more linear it doesn't bother the operation of the part.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.