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Registered Member #1200
Joined: Mon Dec 31 2007, 01:43PM
Location: Kaohsiung, Taiwan
Posts: 27
I am trying to turn my minisstc into a musical sstc by way of supply voltage modulation. My concept is to add a MOSFET to the negative rail of the halfbridge like Steve W did for a smooth turn on transition in his very first sstc . link below http://www.stevehv.4hv.org/SSTC1/FBSSTCsch3.gif I am going to switch it at 100khz or so by 555 timer , which the signal is frequency modulated by the tone into pin5. Did you guys think this will work?
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
What is the frequency of your secondary coil? The problem that I see is that if you modulate the power supply like that, your coil may have a hard time "keeping up" with its resonant frequency. I think you would be better off doing amplitude modulation with a series mosfet as opposed to PWM with a 555 and mosfet.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Ive been meaning to try building a buck converter front end for a SSTC power supply. If done right (that means using feedback for fast control, no open loop crap) it should be able to modulate fairly quickly, but its hard to say what performance it would have. Ive got a bunch of class-D audio amp prototypes around here that would be suitable to power a small SSTC directly from (they have some balls ) so i might give it a shot when ive got some "spare time".
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Steve, just wondering, since you are a big geek on this, why is a closed loop class E amp so much better than open loop? Is it simply linearity?
Even if I use a precise triangle wave source and comparator (n my own) for modulation...
Is it because inductance is not linear? What if I used air cored inductor?
First, i assume you mean class-D? In any case, this goes for *any* amplifier (or regulated output device) not just class D: Your amp will never be perfectly linear and your power supply infinitely "stiff". These 2 things in particular will distort the output, and without feedback there is no compensation for it. People who build audio amps (with the exception of instrument amps which are often built to distort the signal) with no feedback are just idiots as far as my opinion goes. And people who think feedback is some sort of evil thing that taints your signal integrity are also idiots.
In any case, if you want your output to accurately track your reference input (audio signal, for example), you NEED to use some sort of feedback because it linearizes the system. Proportional feedback is OK, but there are other better methods out there too, i particularly like using a fast integrator in my feedback loop (for my class D amp) to give extremely tight tracking of my input signal (which reduces THD). The same principals apply to power supply design too, but in most power supplies the bandwidth doesnt need to be terribly high, and THD isnt usually a concern, so the feedback might be tailored differently for improving steady state tracking and rejecting transient tracking (so your power supply doesnt freak out when you switch your load on and off).
Your precision triangle generator is fine and all (still, its really tricky to get one thats *that* good) but you will still have 0 PSRR (power supply rejection ratio) which means that as your power supply rails sag, so does your output voltage, which is distortion. Feedback would compensate the PW control to overcome the sagging supply rails (until it clips of course) and gives you lots more PSRR. You can of course take feedback off of the supply rails too (as opposed to just the output) and improve the PSRR further, but i didn't find that necessary.
Finally, i wanted to mention that it can be difficult to encompass any sort of second order output filter in the feedback loop (which can be used to account for a non-linear inductor!), simply because it places 2 poles at marginally stable points. Self-oscillating SSTCs take advantage of this for exciting their resonance, using POSITIVE feedback of course! So if your feedback does not include the output filter, then you must strive to use very good components here that will not distort the output. IF your feedback loop does encompass some higher order low-pass filter (1st order RC filters are not much trouble) then you need to be extra careful that you don't excite its resonance!
This is all "basic" control theory stuff, and there does seem to be a fair amount of good information on the web about it. I, of course, find the topic quite interesting. If you have some more questions about it, feel free to ask me.
Registered Member #1200
Joined: Mon Dec 31 2007, 01:43PM
Location: Kaohsiung, Taiwan
Posts: 27
@EDY19, EastVoltResearch Well, I will try to make a AM modulator for the supply voltage since I can't access any modulation transformer.
I think a extra MOSFET will do the job. I am going to fed the carrier and modulating signal into the extra MOSFET to see if it will work.
Steve Ward wrote ...
Ive been meaning to try building a buck converter front end for a SSTC power supply. If done right (that means using feedback for fast control, no open loop crap) it should be able to modulate fairly quickly, but its hard to say what performance it would have. Ive got a bunch of class-D audio amp prototypes around here that would be suitable to power a small SSTC directly from (they have some balls ) so i might give it a shot when ive got some "spare time".
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
wow Steve. you are far away from me. I'm too tired lately, not noticing anything that gets through spell check... should post less here Class D surely, not E.
Your precision triangle generator is fine and all (still, its really tricky to get one thats *that* good) but you will still have 0 PSRR (power supply rejection ratio) which means that as your power supply rails sag, so does your output voltage, which is distortion. Feedback would compensate the PW control to overcome the sagging supply rails (until it clips of course) and gives you lots more PSRR. You can of course take feedback off of the supply rails too (as opposed to just the output) and improve the PSRR further, but i didn't find that necessary.
You are absolutely right, feedback directly increases ripple rejection and voltage sag. What does the power supply feedback do? How do you use two feedback signals simultaneously?
Proportional feedback is OK, but there are other better methods out there too, i particularly like using a fast integrator in my feedback loop (for my class D amp) to give extremely tight tracking of my input signal (which reduces THD).
I don't really understand, what does integrator in feedback loop do? I did sometimes see amplifiers in feedback loops but didn't really understand what they do.
Your knowledge is way too much to me, I'd need to be tutored which I can't demand from you. I hope you'll get your site unlocked in some time and that all these insane projects will appear on it.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
There are many ways to modulate the spark output of a TC driven from full-bridge inverters if analogue audio output is the goal. A few are listed below:
1. High-level modulation of the supply rail using either a linear pass regulator (poor efficiency) or a switching buck-regulator (good efficiency.) This causes the RF voltage output from the inverter to be directly amplitude modulated in response to supply variations. This is inherently very linear and forms the basis of many modern AM broadcast transmitters.
2. PWM of the drive signals to the inverter devices. (High efficiency, but inherently non-linear.) This is easy to implement in the driver at low power. It causes amplitude modulation of the fundamental frequency component from the inverter. Since the TC only responds to this frequency it draws a modulated load current. (Note: Also causes phase modulation and complex modulations of all other harmonics too, so not commonly used in radio transmitters!)
3. Frequency Modulation of the drive signal to the inverter. (Very high efficiency and easy to implement, but the TC's response is non-linear.) The output from the inverter deviates in frequency but still remain at full amplitude. Modulation of the spark is achieved because of "slope-detection" in the resonant TC. The Tesla Coil effectively converts the FM modulation from the inverter to AM, and modulates it's own current draw. (Slope detection is sometimes employed to allow an FM radio station to be copied on an "AM only" receiver.)
4. Phase-shift-control. Both legs of an H-bridge are driven with signals at a fixed frequency but with an adjustable phase-shift between them - The load is tied between them as normal. (Very high efficiency, but inherently non-linear.) When both bridge legs switch in-phase the load sees no voltage. When both bridge legs switch in anti-phase the load sees maximum RF voltage and maximum current flows. Phase shifts between 0 and 180' give smooth modulation of output power. (Commonly used in switch-mode power supplies because ZVS, and fixed duty ratio are among its benefits.)
5. Outphasing. In a similar way to above the RF outputs from several amplifiers can be combined in such a way that you get varying degrees of constructive or destructive interference at the load. All you have to do is drive both amplifiers appropriately to make sure this interaction takes place. (Was used to achieve amplitude modulation in early RCA broadcast transmitters, but is not very linear. Google "Ampliphase" for more.)
6. Pulse-position-modulation. A single RF source is gated on and off in a way that either "pumps up" the amplitude of the resonating RF signal or allows it to ring down depending on whether it is currently above or below the desired amplitude. Linearity and response time depend on the control stratergy. (Not commonly used in radio transmitters because the repeated abrupt 0 to 100% switching of power potentially causes lots of interference.)
7. Digital switching of multiple RF power sources. The outputs from an array of RF power sources are combined at the load. Individual amplifiers are switched on and off in sequence to achieve the required amplitude out of the combiner. If the output amplitudes of the individual sources are chosen carefully it is possible to achieve many discrete amplitude levels at the load for only a small number of sources. (Fault tolerance, load sharing and redundancy make this method very popular for high-power broadcast transmitters.)
Although methods 1 & 7 jump out as being inherently linear techniques, the non-linearity of the spark or corona as a load still causes distortion and largely negates the advantage of these methods. When you change the amplitude of the RF drive voltage, the corona changes size, so its load impedance changes! So it now doesn't draw the current you expected it to draw at the new drive level - hence distortion. This effect doesn't happen with a radio transmitter where the load is independent of power level. As Steve W said negative feedback can be help... but I would consider forward linearisation (pre-distortion) first because adding excessive negative feedback around a very non-linear system can cause problems.
One thing at least is clear, in this day and age with high-speed microcontrollers, cheap DSPs, and fast power-electronics there are a lot of avenues for consideration when designing an audio-modulated SSTC.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Your knowledge is way too much to me, I'd need to be tutored which I can't demand from you. I hope you'll get your site unlocked in some time and that all these insane projects will appear on it.
Marko, my main intention was perhaps more to inspire you to start researching these topics. If in fact you do, and have questions, id be happy to help. As to all these "insane" projects, there arent many to post about. The amplifier project will be made public domain after i complete a report on it at the end of this semester (its a university project).
As Steve W said negative feedback can be help... but I would consider forward linearisation (pre-distortion) first because adding excessive negative feedback around a very non-linear system can cause problems.
Certainly pre-distort the signal if its possible! I think that might be the only real way to do things, unless you can find a direct corallation between say base current (or rather, the derivative of its average/RMS value) and sound pressure level (just an example). I was merely suggesting feedback to regulate the output of any supply level modulation, so that its one less thing to be totally messed up and its easy to measure a voltage directly!
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Modulate the power supply
) so i might give it a shot when ive got some "spare time". 
