On Tue, 12 Aug 2003, Peter L. Peres wrote:

> > Duane, would you mind in developing more about this subject? How about a
> > good "example" of CCFT inverter, and some explanation about the
> > workings?
>
> I'm not Duane. Most cheap inverters use something called the Royer circuit
> which is a symmetrical oscillator using two bipolar transistors and a
> transformer. There are also chips that can drive this cricuit, also with
> FETs, but this seems to be used rarely (it costs more money). It has
> relatively low efficiency, since it drives the transformer into
> saturation.
>
> Most not cheap inverters (as in laptop, palmtop, efficient, dimmable) use
> an advanced version of the same circuit that uses a specialised chip to
> drive the transformer at resonance but without saturating it. You have to
> read chip datasheets to understand what they do. These can be 95+%
> efficient and work extremely well, often outlasting the lamp. They use
> almost exclusively MOSFETs for switching.
>
> > These things seems to be simple, but I always found hard to make these
> > beasties work. Why that capcacitor on the end of the circuit, on the
> > voltage output for the lamp??? :o)
>
> They are not simple, they *look* simple. There are lots of critical items
> in the circuit, like the transformer core and the lamp, and small
> capacitors that need to sustain several amperes of current and hundreds
> of volts, and the unloaded Q of the transformer which must be high enough
> to produce the lamp ignition voltage but low enough to prevent blowing the
> transistors to kingdom come in case the lamp is cold and does not light
> immediately and more like this.
>
> The capacitor in series with the lamp makes sure that the lamp does not
> have dc through it. The lamp is in fact a mercury vapor rectifier that
> just so happens to emit light. It tends to amplify any asymmetry in the
> electrodes by generating dc. The capacitor prevents that. This is
> especially true with dimmed FL lamps which have relatively cold
> electrodes in operation. The problem is common to all mercury vapor lamps
> (including common fluorescent tubes).
>
  Peter, I'm not very sure you have right here. The circuit does not work
at resonance this is sure. A series resonance  (R-plasma, L-transformer
coil, C-series capacitor) which is current resonance will not help at all.
If would be a parallel resonance then yes, I would be agree. But is not.
The capacitors values are usualy 2x33pF/2KV. There are also usually two
bipolars as you say and one NMOS. I have doubts the DC created by tube
rectifying will bother the circuit. It's posible just to saturate the
coil as reversed supply ...? Changing the 33pF with 100pF does not
change too much the current through load. I think this is just a XC
limiter and nothing more. More than that, I've exeperiment with short
circuiting one of the capacitors (one stage, for one tube) and there was
no difference in current load comparing with the standard one. Also a
saturated coil becomes very quickly hot, but it wasn't hot there.

best regards,
Vasile

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