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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
You can almost use any value you like for the capacitor decoupling the DC bus of an SSTC running off the mains. The factors that influence the choice are as follows:
Large capacitor gives low ripple on DC bus voltage, Large capacitor achieves RF envelope closer to CW, Large cap gives rise to short brush like corona that hisses instead of crackles or buzzes, Large cap gives poor mains power factor, therefore uprate rectifier, fuse, variac, wiring etc. Large cap gives good suppression of conducted emissions from SSTC back into mains wiring, Large cap facilitates good over-voltage clamping action from free-wheel diodes, Large cap stiffens the high-frequency impedance of the mains supply, Large cap can typically support lots of RF current so runs cool, Large cap is expensive and bulky,
Conversely...
Small capacitor gives high ripple on DC bus voltage, Small capacitor achieves short burst RF envelope, Small cap gives rise to longer sparks that crackle and buzz for the same average power, Small cap gives good mains power factor, therefore lower losses in rectifier, fuse, variac, wiring etc. Small cap gives poor suppression of conducted emissions from SSTC back into mains wiring, Small cap gives poor over-voltage clamping action from free-wheel diodes, and may ring at harmonic of switching freq! Small cap does little to stiffen a mains supply that looks increasingly inductive at high frequencies. eg variac! Small cap usually has limited current handling and may get hot due to large AC component from bridge. Small cap is cheap and compact
For what it's worth, in a SMPS application hold-up time (during temporary loss of mains cycles) is usually the limiting criterion. You don't want your PC rebooting if just one cycle of the mains goes AWOL.
Hopefully that gives you some idea of the tradeoffs when sizing the bus capacitor
Registered Member #2315
Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161
Thanks Mads, will do a reading later on...
Thanks Richie, those infos are MORE than enough.. cool! almost(if not all) of the possible ANSWERS I *may* ask in the future
ohhh another question since this IS about capacitor decoupling also.. I have read from some sources andcame with something about *some* (or all?) lytics having self resonance and WILL ring the DC bus and kill fets... I do not completely understood this(cuz I havent read much info either)
can you shed light on this one too?
EDIT: yeah I agree with choosing the right decoupling(and filter caps)
OT: with my current SSTC I have tried with mains input = fullwave rectified Filtered (100uF/400v + 47uF decoupling) and WAS using interruptor for DR(very narrow pulse width) ..wtith BPS set to >150 and PW set to 200uS, I get THIN branching arcs to air (And IS noisy too)
reconfiguring the same SSTC to using halfwave rectified UNfiltered (decoupled by 1.88uF mylar) , and using "standard" SSTC interruptor w/c has wider "pulses", I get same? arc length BUT are swordlike in appearance .. and more of a Buzzz/humm...
now I have to choose w/c is MORE energy efficient of the two setup.... #1> halfwave mains rectified UNfiltered && wider int pulses #2> full wave mains rectified filtered && much shorter (200uS) pulses
w/c is best? for my IRFP450 of course
though I would surely have problems with #2 if filter cap will be greater than 500uF (might blow breakers) I dont have variac... or I could make something similar to Steve Conner's "SS" variac (again : need money : buy parts)
I find the arcs for setup #2 quiet "horrifying" .. noisyyyyyyyyYYYY too BbbrrrooAAAARRRR!!
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decoupling CAP for halfwave rectified mains??
.. noisyyyyyyyyYYYY too BbbrrrooAAAARRRR!! 