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enter image description hereWe are a group of student who are working on a project, where we want to develop an alternative type of power supply.

First some quick information about our project: We want to sent 12V DC from a battery source to a 12 volt load, through electromagnetic induction. Electromagnetic induction will flow through a ferrite core (Transformer with primary and secondary coils). You can compare this principle to that of an electric toothbrush. In order to do this, we need to convert DC to AC, and back to DC again when the power has passed to the secondary coil. Buttom line is; we need to build or buy an isolated DC-DC converter, which we can use with our own transformer.

Our demands to the power source: - It should support a load of around 300W - It should be as compact as possible. - It should be as cheap as possible. - Because of the 12V, and 300W, is needs to be able to handle current around 25 amperages.

As far as we know, we could use a forward converter, and implement our own transformer. A source told us to use a 2 transistor push-pull forward converter.

Our question is: Would a forward converter be sufficient for this project, and can anyone give us a reference to a circuit diagram that we can use/buy to build this?

We would really appreciate any answer we could get.

Janus Rathke
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  • Just to clearify. The transformer is not used to step-up og step-down the voltage. It is just used as a way to transfer the power. – Janus Rathke Dec 12 '17 at 13:14
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    Have you an idea of some kind of wireless transfer? Otherwise it's difficult to understand the usefulness of the system, because you already have 12VDC. Maybe you should add some unconditional mechanical requirements, too to make the reason of the project understandable. BTW. I have found a toothbrush which needs no electricity. –  Dec 12 '17 at 13:14
  • The whole system is a bit more complicated. this power transfer system is just a part of the concept. This transformer is only used as a way to connect the load to power. Just like your electric toothbrush, when you need to charge it. – Janus Rathke Dec 12 '17 at 13:17
  • Do you want to attempt something like connectionless inductive chargers of smartphones, but handling 300W? –  Dec 12 '17 at 13:21
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    You still haven't explained what it is that is unique about your transformer. Other than that, you're just describing a standard COTS isolated DC-DC converter. – Dave Tweed Dec 12 '17 at 13:23
  • @user287001: Yes something like that. It does not need to be wireless. – Janus Rathke Dec 12 '17 at 13:24
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    But an electric toothbrush IS wireless. Make up your mind! – Dave Tweed Dec 12 '17 at 13:26
  • The transformer is in the shape of a cylinder. The idea is that you connect the secondary coil to the cylinder, and you have power. We just need a circuit diagram that can convert 12V DC to AC, so it can pass through the transformer cylinder, and then convert the AC to DC power again. – Janus Rathke Dec 12 '17 at 13:28
  • @Dave Tweed - if you research on how the electric toothbrush transfers power from the adapter to the brush, it would make sense. - http://www.planet-science.com/categories/under-11s/our-world/2011/07/how-do-electric-toothbrushes-charge-through-plastic.aspx – Janus Rathke Dec 12 '17 at 13:30
  • Do you mean [something like this](https://electronics.stackexchange.com/a/318149/11683)? – Dave Tweed Dec 12 '17 at 13:32
  • Dave Tweed: Not really. That looks like a way to create motion with electricity, like a DC motor. The concept is just a way to transfer power from a battery to a load. I would rather not reveal the whole business idea that would ruin everything. Please look at the drawing i just uploaded at my post. This is just a very simplified block diagram, but the point is that power goes through the ferrite core (transformer) and to the secondary coil. – Janus Rathke Dec 12 '17 at 13:45
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    The "transformer" shown in your sketch simply WILL NOT work. You'll need to bend the ferrite core round to make a complete closed loop (for some configurations, with an air gap much less than 1mm between the two ends) - such as the core of a toroidal transformer. Two semicircles, with two tiny air gaps, is also viable. Designing such a magnetic circuit is not trivial and absolutely key to success especially at the 300W level. –  Dec 12 '17 at 13:46
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    I downvoted this because it's delusional to call this stuff a *business idea*. First you get the technical expertise **everyone** in your field has. Then you specialize in one field and eventually you come up with something no one with similar understanding as yours have implemented before. You are stuck at the first step, you don't have basic understanding of the field you want to work in. You can't do *business* in it. – Janka Dec 12 '17 at 13:54
  • Brian Drummond - Are you absolutely sure about this? I know that a core with circuit is more efficient, but we was hoping that the number of turns on the secondary coil, would compensate for this. – Janus Rathke Dec 12 '17 at 13:56
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    Actually it can well work. You only must be able to produce several tens of times reactive power, too. 300W loss is a light load for a resonant circuit which has say 30kVAr reactive power. Loose transformers work perfectly. I presume some difficulties with the electrical strength of materials and the vicious fields. Nikolai Tesla in his heydays dreamed of even more loose electricity transfer systems. Unfortunately the parasitic radiowaves spoiled his ideas. –  Dec 12 '17 at 13:59
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    Yes, unless you're happy with a few percent efficiency, which I don't recommend at 300W. The configuration you've drawn is commonly called a "ferrite rod antenna" .(30000W reactive power sounds about right to me. You haven't told us your chosen frequency, but at a few hundred kHz you'll probably hear it on another continent. There may be regulatory issues about that.) But don't believe me, when you get to designing the magnetic circuit, you can calculate the leakage reactance of both configurations and simulate the perfomance you get. –  Dec 12 '17 at 14:01
  • IIRC the toothbrush loaders have less than 10% efficiency. And they only charge with 10mA@1.2V == 12mW, with about 1var on the primary. – Janka Dec 12 '17 at 14:02
  • The reason to the cylinder is not because we are'nt familiar with the standard transformer core shapes. It is a part of the design. We managed to transfer 12 V from the primary to the secondary, of a ferrite rod, using a flyback converter and some dummy loads. We are Mechanic engineers, not electical engineers. That is why we ask you guys. We are open to sceptism, but please keep opinions to yourself. – Janus Rathke Dec 12 '17 at 14:22
  • Think instead of a rod two U-shaped ferrites. Fitting the ends you have a perfect closerd loop transformer core. You will do better if you get allied with a local electronics specialist. If he's an equal part of your group you do not worry of revealing too much essentials. A person with faces also has better possiblities to input to the project some useful opinions, for example an opinion which is called "Physics".. –  Dec 12 '17 at 14:30
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    *We are Mechanic engineers, not electrical engineers.* Hmm, that explains a lot. What makes you think you can design a wireless power transfer system without having the necessary background in electronics, magnetism etc.? *We are open to sceptism* and you will get it because your concept is easy to debunk as you did not see the fundamental flaws that it has. Flaws which you would have known if you had the required knowledge on the subject. – Bimpelrekkie Dec 12 '17 at 15:05

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The transformer, as you have drawn it, will have a very high leakage inductance. Presumably it's drawn this way to show some physical distance between input and output. This is commonly used for wireless power transfer.

To drive this high leakage topology efficiently, you should look at LLC resonant converters, for instance this sort of thing. Any other type of converter, not being designed to handle that problem, will be grossly inefficent.

Neil_UK
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  • Thank you for your comment! It's hard to see, but there is supposed to be a ferrite rod, linking the two coils together. So it is a Wireless Power transfer system, with a ferrite core inductor. – Janus Rathke Dec 12 '17 at 15:05
  • @JanusRathke You still don't get it do you? The magnetic field will "escape" at the end of the rod making for very high losses. Take a look at some transformers. Note how they **all** have some form of magnetic loop, the magnetic field must "go around" and stay trapped in the core. Your proposal does not have that. – Bimpelrekkie Dec 12 '17 at 15:08
  • Bimpelrekkie - yes i do get it! But if you are able to transfer power wireless with an IPT system, why should'nt a Ferrite Rod not increase the efficiency? Please read this report https://calhoun.nps.edu/bitstream/handle/10945/47323/15Sep_Rosenberry_Seth.pdf?sequence=1&isAllowed=y – Janus Rathke Dec 12 '17 at 15:13