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Wireless powering SSTC

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Steve Ward

Mon Nov 26 2007, 06:33AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Steve: don't you have a DC block cap on your GDT anyway (it would saturate first on uneven duty cycle).

Oh, i meant DC blocking cap on the SSTC primary this whole time. Of course the GDT needs one, and when it has one it certainly can work with uneven duty cycle. As mentioned, the waveform just slides up or down to keep the time integral of the voltage = 0. Imagine if you have 25% on and 75% off. You need to make the + area = the - area (that is V*time = area), so the waveform can just shift up by 3/4 so that the narrow 25% pulse is larger positive voltage. The DC offset (generated by non-symmetric input) shows up as level shifting because of the DC blocking capacitor just gets charged up. Likewise, the output of the GDT also has this level shifting since it must maintain zero average voltage (time integral of V(t) = 0). Make sense?

BTW, ive used this very property to regulate the output of a SMPS i designed. By only using half-wave rectification of the output, i could swing the output voltage by maybe 35% just by shifting the duty cycle (ratio?) around.

The point i was trying to really get across is you cant really depend on anything else to eliminate the need for a DC blocking cap at the output of the bridge. And while the DC blocking cap there may not be necessary, it certainly can be helpful when things arent working as expected.

Marko

Mon Nov 26 2007, 10:16AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Yeah... uneven duty cycle can get through the GDT block cap as long as V/s (areas in + and -) are the same, so it's still AC...

I never really thought about what happens if you have DC on SSTC primary...

I wonder if I can stick 2x2 series-parallel of those PC PS caps for 1uf total..

as I don't have much of good high current caps here. Or CDE942's (300nF).

Tom540

Mon Nov 26 2007, 09:45PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

Speaking Of primary magnetizing current. How do you calculate that? I'm a tad rusty on my math.

Steve Ward

Mon Nov 26 2007, 11:40PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

I wonder if I can stick 2x2 series-parallel of those PC PS caps for 1uf total.. as I don't have much of good high current caps here. Or CDE942's (300nF).

Id parallel as many PC PS caps as you can find. If your cap isnt up to the task, then you just keep run times short... the caps will just heat up, not suddenly explode or something.

Marko

Thu Nov 29 2007, 04:41PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Speaking Of primary magnetizing current. How do you calculate that? I'm a tad rusty on my math.

You first determine inductance of your primary - I don't see anything wrong with doing it as if there was no secondary present.

You determine your inductive resistance and simply divide the supply voltage by it.

You can use this

for inductance and this

for your reactance.

Marko

Tom540

Thu Nov 29 2007, 05:03PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

uhh oh yeah. Too bad I broke my bridge LCR meter. Guess I'll have to do it the hard way.

Marko

Sat Dec 08 2007, 10:28PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I've tried out this circuit today...

Breadboarded, smaller secondary, and a halfbridge of IRFP450's powered from smoothed 325V.
I used 2 UCC's as drivers with scheme I posted.

For startup I used a simple switch to dump a 1nF cap into input of one UCC.

When I press the button coil seems to give off a short burst, one big streamer at least 10cm long and branchy, but doesn't oscillate. (that one at the pic may be even more).

Quick fiddling didn't reveal why.

I tried increasing the CT resistors to 10k ohm - bursts lasted a bit longer and streamers were larger.

Maybe I just don't have enough turns on my CT? I'm using 15+15 right now, still quite crowded on the small core. I wonder will the current get too low then?

The primary I'm using on the coil is about 20 turns, calculated to keep magnetizing current at about 3 amps - I wanted to be safe with it but don't know if I overdone it.

Update: the circuit officially works now. It has apparently been matter only of input voltage - running from 230V isolation transformer gave some nice 7cm CW streamers.

Pics after some replies.

uzzors2k

Sun Dec 09 2007, 11:18AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Looking good Marko, and CW yet!

Marko

Sun Dec 09 2007, 02:47PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I'm not happy yet though - I don't believe this circuit will work for pulsed voltage input as it is.

And in order to keep it's properties I would have to use some complex logic I can't bother with. I'll probably have to use either PLL or oscillator trigger - I can make NE 555 to turn the coil on and then be disabled, but this means that coil will turn on simply if I apply mains voltage too.

Hmm...

Zum Beispiel

Sun Dec 09 2007, 04:03PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

I'm not sure if I'm qualified to be posting here, but here's my 2 cents:

First I'd try making the feedback transformer with larger amount of turns, maybe 50+50? It could be just that it isn't reaching the threshold voltage?

Secondly, I recommend using a 555 to kick it to oscillation. I haven't had any trouble doing this and it seems reliable.

Also, when you use an interrupter, the gate driver should send out a small pulse every time it's enabled at the beginning of each interrupter cycle, kicking it into oscillation. Atleast I think it does that.

Anyway, keep up the good work

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