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Based on the recent discussion about SSTCs in this forum I want to add some remarks about the choice of secondary tank parameters. These determine significantly, what kind of load appears to the driver feeding the primary inductance. Below is a diagram showing real and imaginary part of the primary resistance as a function of frequency. The blue curve shows the real (resistive) part and the red one the imaginary (inductive) part.
Plot of primary Z (in ohms) for a light arc load (secondary Q about 30).
I've used a Lsec of 0.3H, Ctop 0f 28pF, coupling k=0.25 similar to what StormInABottle proposed to build. Near the secondary resonance frequency of 55kHz, the red curve, i.e. the inductive part goes to 0, which means, that zero current switching is achieved. This lessens the burden on the transistors. If the load is mostly inductive, i.e. where the red curve is much higher than the blue one, the transistors will mainly drive an inductance. This means, that most of the energy in the primary will be swapped back and forth between the bus caps and the primary and not contribute to the power transfer to the secondary.
Under heavy loads, e.g. a 7kW arc, secondary Q drops to about 5. The corresponding diagram looks like this.
Plot of primary Z (in ohms) for a heavy arc load (secondary Q about 5).
Here the primary always looks very inductive, so the transistors will be mostly loaded by energy transfers between bus caps and primary inductance. A change of primary inductance doesn't help here, as both curves, the red and the blue, are proportional to Lpri. The only way out is an increase of secondary Q, i.e. a lower secondary inductance. A rough guideline is that k^2 * Qsec > 2. In this case zero current switching is possible and transistor load minimized. For a k of 0.25, Q would have to be about 30. Probably a somewhat lower Q is tolerable, but a 0.3H secondary looks too high. An estimate for Qsec can be obtained from an arc resistance load of about 500k at 7kW and 80kHz. At higher powers the load resistance will drop. Probably it is advantageous to add some adjustable phase shift frequency circuitry between feedback pickup and PLL to steer the coil to its optimal frequency.
Thank you for the link. Burnett considers secondary base fed coils. In that case zero current switching can always be be obtained by driving it at its resonance. The big drawback is, that you need rather high voltages at the base to get any current into it unless you use a very small secondary inductance of maybe a few 10s uH. With usual top loads of a few 10s pF, you'd be running at impossibly high frequencies. Too much for power transistors.
That is why real transformers with primary coils are necessary. Primary inductances don't consume any energy unless they "feel" the effect of the magnetic field of the secondary coil. For this a large coupling is required _and_ a high secondary Q even under arc load, since a high Q means lots of secondary voltage and therefore a lot of secondary current and thus a strong magnetic field of the secondary.
I believe, SSTCs can be quite easily misdesigned, if the effect of secondary Q is not taken into account. Such coils will have the inverters switch much useless inductive current. In DRSSTCs this all is a non issue, since the inductive part of primary current is cancelled out by the series cap.
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Hi Uspring, Strange that I have observed the opposite behavior than your graphs seem to suggest. Driving the primary coil exactly at the secondarys resonant frequency results in inductive (turn-off) current equal to driving the primary without secondary in place. This is the same as in your graph. However, if I increase the frequency, the turn-off current seems to increase (transitions move further before zero), so you would say the primary looks more inductive. On the other hand, by detuning the primary slightly lower (eg. by increasing the feedback delay), the transitions move closer to zero current.
Hi Uspring, Strange that I have observed the opposite behavior than your graphs seem to suggest. Driving the primary coil exactly at the secondarys resonant frequency results in inductive (turn-off) current equal to driving the primary without secondary in place. This is the same as in your graph. However, if I increase the frequency, the turn-off current seems to increase (transitions move further before zero), so you would say the primary looks more inductive. On the other hand, by detuning the primary slightly lower (eg. by increasing the feedback delay), the transitions move closer to zero current.
See that is super interesting because that's what I figured should happen and why I was a little confused by his graphs... I wonder if the OP could show a schematic? You said you are using transistors?
Hi DrDC, my graphs might be a bit misleading regarding the voltage to current phase. This phase depends on the ratio of inductive to resistive resistance, not on any of the curves alone. I've made a diagram for your coil using 0.16H secondary L and a Ctop to get your observed secondary res at 80kHz. I've also used an arc load of 500k, which comes from 60kV (RMS) top voltage and 7kW power consumption.
The blue and red curves are the real and imaginary parts of primary resistance, the green one is the ratio imaginary/real. The phase depends on the green curve. You might have made your measurements a bit above resonance, which would explain your results.
@twist2b: The schematic is just that of a primary coil cuopled to a secondary tank wit an arc load resistance parallel to Ctop.
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