Transformer physics question

Suppose I wanted a transformer but I screwed up the math and bought one that is sqrt(2) lower than I need it. Suppose we throw "do it right" out the window for the sake of argument and I want to force this transformer to put out more than it's supposed to.

Given that the transformer is 1:10 and rated for 20kHz and that initially I was going to feed it a 50% duty voltage to get a 10x step up but in fact it's only going to be 5x once I average it over a huge capacitor (pk-pk will be 10x but the DC equivalent will only be 5x), can I increase the output voltage by either increasing frequency or duty cycle? Naively I thought duty cycle would do the trick BUT, at some point, the coil is mostly charged and only being discharged over a very short interval at high duty cycles (at 90% it's only off 1/10th of the time so can't fully discharge due to time constant). So can duty even increase the voltage at all or is anything other than 50/50 hurting the output? I imagine 50/50 gives you maximum flux but also the transformer's time constant might come into play. Maybe you can get a higher duty if the time constant is very low compared to the switching frequency?

Aside from duty, any other tricks to "overclock" the transformer or otherwise get it to put out more voltage through some sort of trickery, even if it comes at the expense of efficiency? Also this is all in the context that the secondary side is being loaded and not open circuit.

You can’t do that I’m afraid the voltage is translated by the turns ratio of the transformer - that’s fixed .

You could amplify your resulting DC signal ( I take it you are using a bridge rectifier and capacitor on the output ) or make the downstream parts work at the lower voltage - all depends on what you are trying to achieve.

If you out a higher voltage in, you’ll get a higher voltage out .

Please explain EXACTLY what you are trying to do.

Your whole discussion is very confusing.
You write of a transformer you bought that you are using at 20kHz, so it is a ferrite core transformer with very low capacity windings. Ok. And you are feeding it with a square wave and using a the transformer to boost the voltage of the square wave. Since transformers are current driven devices, how much current can you supply to the primary of the transformer?
Then you write of DC voltages. Are you rectifying the output of the transformer? Seems to be the case. How are you rectifying it? What circuit? Have you looked at voltage multiplying rectifier circuits?

A schematic of your project would certainly help to clarify the project.
Paul

Sorry let me be a bit clearer.

So I feed a square wave into the primary side at 24V and 50% duty. It can take about 10A on that side. It stands to reason (and I've verified) that the voltage is amplified 10x so I get a 240V pulse out the secondary side but since it's a 50% duty pulse, when you rectify it, you get half that. So I'm not saying I want more than the 240 the ratio is supposed to give me. I'm saying I want more than the 120 I get after rectifying it. The rectification is nothing but a capacitor in my case. Just a big-ass capacitor and that's it. I'm going to use an H-Bridge on the primary side to chop my 20kHz. I want to stay away from adding stuff to the circuit to replace the function of the transformer ( like DC boost/buck converters and the like ) and just rely on the transformer. My whole premise is that the core is designed not fully saturated so I know it can be saturated further. This extra "power" ought to be able to be expressed as voltage in theory, should it not? I'm looking for a way to extract the most voltage I can out of the transformer in a rectified form and am asking about which parameters might be altered to achieve that. Duty cycle would seem obvious but if you look at the extreme cases, 1% duty is essentially off so how can there even be an alternating magnetic field in the steel and 99% is essentially on so again, there's no real field activity to make the transformer work. I therefore assumed that there would be some cutoff point where changing duty actually makes output drop off.

Gahhhrrrlic:
Sorry let me be a bit clearer.

So I feed a square wave into the primary side at 24V and 50% duty. It can take about 10A on that side. It stands to reason (and I've verified) that the voltage is amplified 10x so I get a 240V pulse out the secondary side but since it's a 50% duty pulse, when you rectify it, you get half that. So I'm not saying I want more than the 240 the ratio is supposed to give me. I'm saying I want more than the 120 I get after rectifying it. The rectification is nothing but a capacitor in my case. Just a big-ass capacitor and that's it. I'm going to use an H-Bridge on the primary side to chop my 20kHz. I want to stay away from adding stuff to the circuit to replace the function of the transformer ( like DC boost/buck converters and the like ) and just rely on the transformer. My whole premise is that the core is designed not fully saturated so I know it can be saturated further. This extra "power" ought to be able to be expressed as voltage in theory, should it not? I'm looking for a way to extract the most voltage I can out of the transformer in a rectified form.

So, you are charging a capacitor on 1/2 of the cycle and discharging it on the other half and wondering about the net results?
What are you attempting to create?
Paul

The capacitor is just to get DC. It'd be like starting with a square wave going through a simple wire and connecting a cap to it. You just get half the pk-pk value of the square wave with negligible ripple. I'm trying to make AC in the long run but the voltage is too low for the RMS to be what I want it to be so I need higher rectified DC on the secondary side.

Gahhhrrrlic:
The capacitor is just to get DC. It'd be like starting with a square wave going through a simple wire and connecting a cap to it. You just get half the pk-pk value of the square wave with negligible ripple. I'm trying to make AC in the long run but the voltage is too low for the RMS to be what I want it to be so I need higher rectified DC on the secondary side.

The you obviously need a diode in series with the capacitor to stop the discharge of the capacitor!!!!
Paul

Well that'd keep the cap from discharging but I need to put a load on the secondary eventually so floating the voltage up to the peak is a cheat isn't it?

I mean say I put a 100 ohm resistor across the secondary. That's going to pull the voltage down to a steady state of 1/2 the pk voltage, is it not? I need to do something the changes the "area under the curve" so that no matter what you do to load the circuit, it will settle at a higher value. The answer must be on the primary side, I think.

Gahhhrrrlic:
Well that'd keep the cap from discharging but I need to put a load on the secondary eventually so floating the voltage up to the peak is a cheat isn't it?

I mean say I put a 100 ohm resistor across the secondary. That's going to pull the voltage down to a steady state of 1/2 the pk voltage, is it not? I need to do something the changes the "area under the curve" so that no matter what you do to load the circuit, it will settle at a higher value. The answer must be on the primary side, I think.

Who can say! You have never told us what you are attempting and how an Arduino is involved.
Paul

Lol. I'm just asking about transformers in the general electronics topic. If I go on and on about Arduinos and other discrete circuitry, the focus will get entirely derailed. The question is precisely about transformer physics and what is possible with a fixed piece of hardware by varying the electrical input parameters. I could be controlling it with a Raspberry Pi or flipping a knife switch with my hand and it would make no difference.

Gahhhrrrlic:
Lol. I'm just asking about transformers in the general electronics topic. If I go on and on about Arduinos and other discrete circuitry, the focus will get entirely derailed. The question is precisely about transformer physics and what is possible with a fixed piece of hardware by varying the electrical input parameters. I could be controlling it with a Raspberry Pi or flipping a knife switch with my hand and it would make no difference.

Ok, care to share a data sheet on this transformer you are using? I hope you are using an oscilloscope to view the in put and output wave forms. They will not be as you have described them.
Paul

Ferrite 3C97
0.41T
280mm^2
f = 20kHz
Np = 1.75
Ns = 17.55

Ip = 16.17A (18 AWG / 16 AWG)
Is = 1.47A (26 AWG)

I have already scoped it using 6V and it's about 10x (60-66V out) with a sort of sawtooth'ish waveform at 20k.

Gahhhrrrlic:
Ferrite 3C97
0.41T
280mm^2
f = 20kHz
Np = 1.75
Ns = 17.55

Ip = 16.17A (18 AWG / 16 AWG)
Is = 1.47A (26 AWG)

I have already scoped it using 6V and it's about 10x (60-66V out) with a sort of sawtooth'ish waveform at 20k.

Then your transformer is not up to transferring a square wave at 20k. Do you understand why?
Paul

No. I had this custom made by a transformer maker. Unless he screwed up.

In any case, I’m not particularly concerned with whether or not the transformer was properly spec’d. This is something I can correct later on (let’s call it Rev 2.0). For now the laws of magnetic fields apply equally to all transformers whether they are properly designed or ripped from a random TV power supply. The question is how to take a fixed quantity such as the hardware above, and stimulate it enough to yield a higher average voltage on the secondary side. As mentioned, I can speculate that the 2 main parameters I have control over, frequency and duty, may accomplish this but I’d rather somebody with practical experience confirm or deny this line of reasoning.

Changing the frequency changes the flux.

Changing the duty cycle changes the flux.

With the same turns ratio, to get more voltage out, you’ve got to put more voltage in.

The first reply gave you the correct answer to your question: No.

Everything since that post has been about you refusing to accept the physical laws of electromagnetism. What’s next? Are you going to ask how to get 100 watts out of your transformer with only 50 watts in?

The horse is dead. Stop beating it.

Actually the reality is quite the opposite. I ask a direct question (even the title says transformer physics) and get a page of irrelevant, tangential questions about arduinos and discrete circuitry, which, is why I don't tend to give great quantities of detail so as not to invite wasted time and effort. And at what point did I deny the laws of physics? By your own admission, frequency and duty "change" flux, something I theorized earlier in my posts. Change it how? You mean increase it? As in further saturation of the steel? Does that not produce more power, which is a commodity flexibly expressed as either voltage or current? My question was about when such tweaks cease to produce a positive return, assuming that the time constant of the transformer becomes a bottleneck at extreme duties, be they small or large, or when the frequency is so high it doesn't allow the magnetic field to grow and collapse fully. You would seem to be contradicting yourself if your short answer is, no and your long answer is, both parameters change the flux. I happen to believe that changing these parameters does increase voltage. TINA seems to agree with me as well. Forgive me for not putting blind faith in computer models however, and seeking confirmation here. Rather than chiming in on page 2 to tell me to STFU, you should go back a page and see how I was repeating myself over and over to try to end the conversation rather than perpetuate it. But then maybe my interpersonal skills are no match for the convoluted way in which people interpret questions these days.

Gahhhrrrlic:
No. I had this custom made by a transformer maker. Unless he screwed up.

Why (and how) are you getting custom-made transformers when you don't even know how they work?

Gahhhrrrlic:
Actually the reality is quite the opposite. I ask a direct question (even the title says transformer physics) and get a page of irrelevant, tangential questions about arduinos and discrete circuitry, which, is why I don't tend to give great quantities of detail so as not to invite wasted time and effort. And at what point did I deny the laws of physics? By your own admission, frequency and duty "change" flux, something I theorized earlier in my posts. Change it how? You mean increase it? As in further saturation of the steel? Does that not produce more power, which is a commodity flexibly expressed as either voltage or current? My question was about when such tweaks cease to produce a positive return, assuming that the time constant of the transformer becomes a bottleneck at extreme duties, be they small or large, or when the frequency is so high it doesn't allow the magnetic field to grow and collapse fully. You would seem to be contradicting yourself if your short answer is, no and your long answer is, both parameters change the flux. I happen to believe that changing these parameters does increase voltage. TINA seems to agree with me as well. Forgive me for not putting blind faith in computer models however, and seeking confirmation here. Rather than chiming in on page 2 to tell me to STFU, you should go back a page and see how I was repeating myself over and over to try to end the conversation rather than perpetuate it. But then maybe my interpersonal skills are no match for the convoluted way in which people interpret questions these days.

Rant much? People here can barely understand what you're trying to do, let alone if it's even sensible to do that.

The rectification is nothing but a capacitor in my case. Just a big-ass capacitor and that's it.

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.

My whole premise is that the core is designed not fully saturated so I know it can be saturated further. This extra "power" ought to be able to be expressed as voltage in theory, should it not?

What "theory"? Pulling words out of your butt is not a theory. Why do you think the core isn't being fully saturated? More likely it's designed to be right on the edge of or a little over the saturation point, because if there's room that means a bigger core which takes more material to build. You might not have much room to go up.

A transformer's output voltage is COMEPLETELY determined by the primary voltage multiplied by the turns ratio. That is an absolute limit. If you want more voltage out of the secondary, the ONLY thing you can do is increase the voltage applied to the primary. Changing frequency will not increase your output voltage. Changing the duty cycle will not increase your output voltage. This was pointed out to you in literally the first response.

I happen to believe that changing these parameters does increase voltage.

"What you believe" means nothing. You are wrong.

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).

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, 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.

Hi,

The rectification is nothing but a capacitor in my case. Just a big-ass capacitor and that's it.

What makes you think just a capacitor alone will convert your 240AC to DC?

If you have a squarewave like in the diagram below.

If you connect like below with a suitable hi-speed diode and suitable voltage rated capacitor you will get near the DC voltage of 240Vdc
You have to have at least one diode to rectify to DC.
If your capacitor is a polarized type I am surprised you have not had it explode or overheat.


Do you have a scope?
Tom... :slight_smile:
PS. Can you please post a diagram of your transformer and connections as well a a picture of your transformer?