Paul_KD7HB:
Since transformers are current driven devices
Well that's not true is it? Transformers reflect the output impedance scaled by
the turns-ratio squared, so they can be voltage devices (as here), or current devices
(as in a current transformer sensor).
Jiggy-Ninja:
Here is a high level description of the physics of a transformer. When magnetic flux changes in a coil, it induces a voltage. Conversely, when voltage is applied to a coil is causes a change in flux. Specifically, a continuous voltage will cause a continuous, linear increase in flux that will theoretically continue forever. In reality parasitic resistance and saturation will get in the way of that. The exact relationship of the voltage to that change is determined by the core geometry and material, as well as the number of turns (more turns = slower changing flux).
An ideal transformer is thought of as having no flux at all. The total currents in the primary and secondary
(measured in amp-turns) should cancel out and leave no flux. The voltage ratio and current ratio are the same as
the turns ratio - that's all you need to know about a ideal transformer.
A more realistic model is of a very high value inductor sharing the primary and secondary currents, such
that the high impedance of the inductance keeps the total current close to zero, so that the primary and
secondary amp-turns totals almost cancel each other. The residual current (which you can measure
in the primary with the secondary open-circuit) is called the magnetizing current.
That ramping flux is also applied to the secondary of the transformer, which induces a voltage across it. In a mirror of what happened to the primary coil, the voltage across the secondary is determined by how much the flux is changing. Since they have the same core geometry, the what's left to determine the voltage is the turns (more turns = more voltage).
Changing the transformer's frequency will not change the rate of flux change,
Indeed, but it does affect the amplitude of the flux - the lower the frequency the further from
ideal the transformer becomes, and eventually the core can saturate too, which is very bad news.
More counter-intuitively pulling more current from the secondary doesn't change the flux, eventually
you end up with thermal dissipation issues, not saturation - though in practice imperfections like
leakage inductance and unequal distribution of coil windings can spoil this idealized view.
For a power transformer its important never to operate the primary at a higher voltage or
lower frequency than its designed for.
since that is determined by the value of the voltage and not how quickly the voltage changes. Changing duty cycle can only decrease the output voltage, since straying off of a 50% duty cycle gives it more time at one level to drift into saturation. Since you're not changing the actual level of the primary voltage, it doesn't increase the output voltage.
You're at risk of killing yourself with mains level power here. I hope you are taking more precautions than you think are necessary.
Jiggy-Ninja:
Why (and how) are you getting custom-made transformers when you don't even know how they work?Rant much? People here can barely understand what you're trying to do, let alone if it's even sensible to do that.For example, it's obvious from this that you don't even know what rectification means. Capacitors don't rectify. That's what diodes do.What "theory"? Pulling words out of your butt is not a theory.You are wrong.
You're at risk of killing yourself with mains level power here. I hope you are taking more precautions than you think are necessary.
Ok I've recovered from my emotional outburst but honestly, don't do this. As a comfortable, shall we say, complacent veteran of a forum it's too easy to fall into this trap of arrogance. I have little patience for people antagonizing me after I've had an unsuccessful question/answer session with somebody else. Your feigned concern for my well-being seems more a last-minute redemption for your taking a dump on my head, picking on my semmantics and insulting me. There are gaps in my knowledge but I'm not as dumb as you'd have others believe, just because you can cherry-pick through my rushed posts. You can make the argument that I don't explain much about what I'm trying to do but there's a reason for that I explained earlier on. Some... not all... but some cases will go like this:
OP asks a pointed question
Expert says need more info on what you're doing
OP provides info
Expert says well, you shouldn't be doing it that way. Do it this way.
Thread gets derailed, question not answered.
So maybe I make a bad gamble some of the time withholding info but I'm no stranger to forum psychology and I would rather not waste others time and my own indulging their ego about my project objectives when I just want to ask a question about some generic principle in physics let's say. Selfish? Perhaps. But I've found other more efficient ways to help others with my time than answering questions I'm too slow to jump on because I'm not on the forums much. So I've got low karma but I make a video or a tutorial once in a while to help people who know less than me (there are a few of those around, I dare say). In fact it's because of the generous help I've received here from several regulars, that I've been able to put together projects that I have later showcased to show others what is possible with Arduinos and how I did it.
To the rest, it's been a while since my electronics lectures but one of the only things I remember was dphi/dt = V or change in flux over time = voltage. While I agree the turns dictate the ratio and that's fixed, I was really thinking about the loaded condition where the peak voltage will inevitably drop to RMS or average or whatever it drops to. In my mind it's like taking the square wave and pulling on the string until it flattens out somewhere. If voltage depends on change in flux and flux is a property of magnetic fields permeating a "conductive" medium such as a ferrite core, and the core is usually designed with some saturation tolerance, then there should be some headroom to further saturate, and while that wouldn't change the pk-pk voltage on the secondary side, maybe? it might affect where the RMS value sits when you load it down. I'm only pursuing this hypothesis because when I play with the values of f and D in TINA, I do get different voltages out when I drop the secondary over a fixed resistor. So maybe I created the circuit wrong and am getting bad simulation results, but that's why I'm doing a sanity check here to see if I'm totally full of it or not. Even if I'm wrong, it's good to know why changing the field strength or flux in a core would have 0 effect on the (loaded) voltage, from a physics point of view.
Mark/George: I apologize for poor behaviour. Thanks for the information. I'll try to grab a screenshot of my Tina schematic though please bear in mind, my "style" of working is to brute force things into being first and then refine later so don't be too surprised if there are foolish over-simplifications in there. It would be best to read between the lines instead.
Hi,
I've for the most part not read most of the above so I'll just tell you what I know about transformers.
From your description your transformer was designed to operate at a 20kHz square wave. It has a 1:5 turns ration.
Can we assume your transformer is AC coupled or in an H-Bridge?
Efficiently designed transformers ($$ and electrically) have enough "core volume" capacity to result in a flux is up near saturation at the end of each 1/2 the input signal (i.e. the first 1/2 cycle). i.e. consider that flux will no longer build in the core at a certain quantity
of volt-seconds. I don't recall your input voltage but lets say its 10 volts.
The core with 10V for 50 µSeconds enough flux will be generated to bring the core to maybe 90% of max. When you get above 100% max flux the flux is no longer changing so you primary turns into a resistor of the value of the primary wire resistance.
So flux cannot be increased very much before saturation of the core occurs.
Now increasing the frequency. This will generate ~half the flux of the above and will likely operate fine. However with the same 10Vin you will still only get the 1:5 voltage out. So increasing the frequency alone won't be much help.
The only way I know to get the increased output voltage is to double the input voltage and double the frequency. You will still have the same amount of volt-seconds as the original at 20kHz.
The transformer is 1:10 so you should be able to get 240 V out of 24V. That should be 240 V after rectification. Or maybe even 480 V with H-bridge on input and voltage doubling on output.
There are many possible transformer topologies. Probably you are doing something wrong. You should show your circuit.
The warning about lethal voltages is valid, be careful!
Pic A is being fed 24V pwm at 50% duty and 20kHz.
Pic B is being fed 24V pwm at a 70 on / 30 off duty and 10kHz.
Curious to know what you're referring to as voltage doubling? Is there a way to do that even if the primary side is all positive voltage?
Hi,
Why are you measuring Amps and not Volts across C1?
Why do you have a load of 100R
What is the resistor in series with the diode?
In your bottom trace you have 180A through 100R
P = I2 x R == 180 x 180 x 100 == 3,240,000W .... 3.2MW !!!!
Output volts = I x R == 180 x 100 == 18000V == 18kV !!!!
Assuming 100% efficiency
Input power = 3.24 MW
P = V x I
I = P / V == 3240000 /24 == 135,000A !!!!!!
How have you modelled your transformer? ? ?
Thanks.. Tom.. 
The image is a bit nondescript but it's not A for amps. It's just some letter. There are 3 traces on the same graph in different units. The top most curve is the volt meter on the far right, which IS measuring the cap, since both the cap and resistor are in parallel. The series resistor on the transformer is a guess of .1 ohm on my part to make it non-ideal and the 100ohm resistor is a test load. The thick green band is the primary side input pwm (24V) which you can't see properly at this scaling. The ammeter, the trace for which is too small to see, is 1.6 or something.
Hi,
So you have now got a circuit that gives you a higher voltage in DC form.
What are you aiming to use the DC voltage for?
What is the load and how much current and voltage will it need?
Have you actually used your transformer and put it in such a circuit?
Tom....
Just trying to chop mains voltage from it, but that's the easy part. The issue is not having enough peak voltage to start with. From the screenshots above, TINA seems to be telling me that I can increase the loaded voltage by means of frequency and duty cycle but most respondants here have said it's impossible, probably because they're thinking peak voltage while I've been emphasizing loaded voltage. The load, for the sake of argument can be any plug in device you're familiar with in the regime of say 200w or so, as long as it pulls down the voltage from what it's floating at, open circuit. I will naturally do a proper bench test of the transformer but before I make a big mess, I just wanted to determine whether or not I'd be using the time well or if I should just go buy another transformer.
Hi,
What RMS voltage are you aiming for, 110Vac or 240Vac?
What you are doing is the the basics of some inverters, especially sinewave output type.
I assume once you have the peak voltage in DC form, you then feed that to a H-bridge fed with modulated PWM Duty cycle to get a sinewave output after a filter.
Tom...
Anything in the 110, 120'ish area is fine but as you can see from what the simulation is indicating, I'd be hard pressed to get the upper end of that range. So this is why I'm focusing on the transformer in particular. The rest of the circuit is no problem for me. I merely want to do a proof of concept on the existing hardware.
Do you have any comment on the duty/frequency effect in those screenshots? Is increasing the duty/lowering frequency to get more voltage, limited by the time constant of the transformer?
Just trying to chop mains voltage from it, but that's the easy part. The issue is not having enough peak voltage to start with. From the screenshots above, TINA seems to be telling me that I can increase the loaded voltage by means of frequency and duty cycle but most respondents here have said it's impossible, probably because they're thinking peak voltage while I've been emphasizing loaded voltage.
It appears you are switching a DC voltage going into the primary of the transformer. If this is true, I don't see any way to reset the flux in the core? Unless I'm mistaken, you are putting 24Volts on the primary for 50% of 20kHz. Then you are putting 0V on the primary for the remainder of the cycle. Have I missed something?
There is a similar circuit called an isolated flyback boost converter, however it used a coupled inductor (a special case of transformer) and the polarity is reversed from your design.
Does your analysis account for core saturation of the transformer?
If not the Loaded voltage can only be the primary voltage x turns ratio. At the simplest analysis the load is irrelevant.
I'm really not trying to be sarcastic but I don't see how your circuit shown in the analysis will work.
I believe you can get more voltage on the output by changing the rest of the circuit. Maybe driving the primary by a H-bridge to get higher peak-to-peak voltage or something like flyback. I have no practical experience with this so I cannot say if either would have reasonable efficiency with your transformer.
John, I'm not sure what you're trying to say. The circuit depicted in my earlier post has a 24V DC source being chopped into PWM with 50% duty. It's 24V half of each period and 0 for the other half so the voltage swings back and forth from 24 to 0. There should be enough time for full sat and de-sat of the core. The turns ratio is only relevant if there's no load. When you put a load, the transformer has to supply enough current to the secondary side to keep up with the rate of consumption, which is what my simulation indicates, if you check the voltage level each simulation stabilizes at (it is not 10x the input, as it would be for an open circuit).
Smajdalf, I will be using an H-Bridge so I'll salvage both + and - waveforms. I looked up voltage multiplication in transformers and saw some funky circuit with a bunch of diagonally arranged diodes and caps but before I get into convoluted scenarios like that, I'm really only interested in the effects of frequency and duty on the secondary loaded voltage output. I guess I should just try it out at this point. In 3 pages I've probably repeated this objective a dozen times and nobody has said a word about it except that it's irrelevant or impossible, which I just don't think is supported by evidence.
It's 24V half of each period and 0 for the other half so the voltage swings back and forth from 24 to 0. There should be enough time for full sat and de-sat of the core. The turns ratio is only relevant if there's no load. When you put a load, the transformer has to supply enough current to the secondary side to keep up with the rate of consumption, which is what my simulation indicates, if you check the voltage level each simulation stabilizes at (it is not 10x the input, as it would be for an open circuit).
Thoughts:
You don't want to saturate the core, when this happens the output voltage goes to zero and the input back emf goes to zero (meaning the 24V input is only seeing the DCR of the transformer primary.
Don't know how you specified your transformer, but running single sided this the flux in the core is never being fully reset so you would be operating in a minor hysteresis loop of the core resulting in an overly large core.
Transformers are voltage devices. If you put 24Volts into the primary, you will get 240 volts out minus the Primary dcr loss and secondary loss. And of course the diode drop. The turns ration is not meaningless.
3a) in the resistor before the diode the transformer winding equivalent resistance?
I don't understand your output graphs. You should see an output ripple current at 20kHz yet your chart shows amperage in the ms range with not discernable ripple.
Is C1 0.1F ?
The most power you can get at 50%
