Weird results from air-core transformer

[Chancy99]

I'm seeing some strange results from an air-core transformer - basically two coils of 30ga wire. Perhaps someone can shed some light ?

Primary coil is 170 turns of 30ga wire that was wound on a 2" dowel. The secondary is 100 turns the same way. The two coils are sitting on the desk, the secondary on top of the primary with about a 1/4" gap.

The primary is driven by an IRF540 - the gate goes via 200ohm resistor to a micro generating a 15kHz 50% duty cycle PWM signal. The Source goes to GND. The coil is across the Drain and VCC. The secondary goes through a bridge rectifier.

I'm seeing about 23Vdc across the secondary. I was expecting around 8Vdc or so. 100/170 * 14 right ? Hrmm. The 23Vdc is what works out from 170/100 * 14, so it's as if the primary and secondary are backwards.

But if I reverse them, put the 100 turns as primary and the 170 turns as secondary, it puts out around 90Vdc.

The other oddity is that I can increase the 14V up to around 30V, and the output only rises from 23V to about 27V or so.

Dean.

 

[dhwilliams]

I'm not 100% sure but don't transformer calculations only work for sine waves? You are applying a pulse so the main effect will be one of the back-emf generated when the voltage drops (if I had a memory I would quote the formula for the voltage generated, it's something to do with the rate of change of voltage and your pulse has a steep falling edge, so lots of volts!)

 

[cbarn24050]

Hi, a few things here. 1 you dont say what the drive voltage is. 2 you cant get a dc across the secondary. 3 use a scope to measure your voltages, you are probably getting ringing which is being rectified.

 

[Madcow]

Sounds like you built a flyback converter.
What is your load?
You may be rectifing the ringing depending on your load.
If its ringing alot try a r-c snubber across primary and
mayby one across the secondary.
Check your primary and secondary wave forms you should
see a corrilation with your turns ratio.
You will probably see more voltage on the primary then
what you expect.

 

[Lewish]

200 ohms is an awfully big gate resistor. You may not be turning the FET on to saturation. What is the threshold voltage of the FET you are using? What is the output voltage being supplied to the gate?

You really want a gate resistor of about 33 ohms, and you want a gate drive voltage of not less than 10 volts.

 

[brennaj]

Don't forget the coefficient of coupling between the coils in your calculations. And as stated in other posts - look at the output with an oscilloscope. Make sure you have sufficient primary inductance for the desired waveform.

 

[Chancy99]

Cool, some responses pretty quick These forums are great.

dhwilliams - I think you may be right. Have to dig up my old books, as I was just going on the memory of transformer ratios. I was only scoping the gate drive to make sure it was a 5V 50% signal, which it was.

cbarn - drive voltage across the primary is 14Vdc, switched by the IRF540 mosfet. The secondary feeds a typical bridge rectifier to spit out ugly peaky DC. I'll put a couple of caps across it to smooth it out.

Madcow - hrm - no load at the moment, just the multimeter. I was looking to see that the output voltage was within expectations. Never built a flyback converter before, at least not deliberatly. The meter shows 14V across the primary.

Lewish - the IRF540 barely gets warm at all. If it wasn't turning on completely, wouldn't it heat up ? Gate threshold voltage is 2-4V, being driven by an output pin on the AVR micro at 5V. The 200ohm resistor is to make sure that the current doesn't exceed the pin max - I actually thought it should be higher ...

http://www.fairchildsemi.com/ds/IR/IRF540.pdf

Dean.

 

[Lewish]

Take a look at the "Gate Charge vs Gate-to-Source Voltage" curve on the datasheet. It shows that it takes about 5.5 volts to fully turn this part on. So, I think you are definitely not getting it turned on. Since it is not turning on more than just a very small amount, NO, it will not be getting warm.
When I need to use a FET as a switch driven by a microcontroller, I use a sensitive-gate FET which has a threshold voltage of 0.9 to 1.2 volts. Even then, I seem to have trouble with some devices not fully turning on. The lower your Source to Drain voltage, the harder the FETs will be to turn on.

 

[Chancy99]

Lewish - Hmmm I see what you mean. OK, I'll use a 2n3906 or similar to switch the 14V into the IRF540 gate. That way we'll get the full 14V into the gate which should turn it on fully.

From microcontroller pin through resistor (1k or so?) to Base of 2n3906. Collector to +14V, Emitter through resistor (what value ?) to IRF540 Gate. +14V through primary coil to Drain, and Source to GND.

That should do it I think ?

Dean.

 

[Lewish]

A 1k to 2.7kohm resistor would be good for the base. Emitter resistor needs to be sized to limit the current thru the 2n3906 to the allowable DC current limit of the 2n3906.

That should do it for you.

 

[itsmeRICK]

It looks like the transformer's primary and secodary are looslely coupled and there is some resonance in the secondary. The loosle coupling would explain why the output voltage doesn't track the input (VCC) voltage changes, and the resonance coupled with the loose coupling results in the secondary ringing up to a higher frequency.

At 15 kHz, you really should be using a ferrite core or equivalent.


Dick Cappels

 

[Chancy99]

I'll futz with it tonight more if I get time. Got a sick daughter though ...

This is for transferring power to a rotating circuit, so no way really to use a core. The rotating part needs about 400mA at 4V min available to be regulated down to 3.3V.

Would the coupling improve with a higher or lower frequency ?

Dean.

 

[itsmeRICK]

The loose coupling results in high leakage inductance, so as the frequency goes up, the regulation problem will get worse. If you aren't transferring very much power, a pair of back-to-back zeners across the secondary would help stabilize the voltage.

On apporach to transfering power to something on a rotating shaft that a fellow I know said he use once was to take a a pot core and put it in the middle of the rotating shaft. One half of the pot core rotated on the shaft, and the other half stayed fixed. There was an air gap between the two halves. I suspect that made the project a mechanical problem, but at least they were able to transfer plenty of power this way.

Good luck.

Dick

 

[Chancy99]

Heh, mechanicals there must have been "interesting". I've been looking around for a slip-ring too, and have found a few, but they're all too big, and bloody expensive.

My basic requirements are pretty simple. Given a source of around 12-14V, I need to get around 4-5V at around 400mA across to a rotating circuit. The basic air-core transformer looks like a reasonable way to do it, and experiments above show that it will transfer energy pretty well.

Actually, very well. I was able to hold the secondary coil above the primary by about 6" and still see around 5V no load on it by the multimeter. I have a little field strength meter (uncalibrated) that pegs the scale at around 12-18" from the coils, while a typical monitor pegs it when it's about 10" in front of the screen. So there's plenty of energy there ...

Had some time to scope it briefly last night - very briefly. The +14V point on the primary coil appears to be seeing a 100V (wow!) spike on it. More like an exaggerated square pulse really.

So, to smooth this out, and get rid of those 100V spikes the primary should have a smoother waveform applied to it than the general square wave ... I can see a couple of ways to do this, none perfect.

The mosfet wants to be a switch. If it's not fully on or off, it heats up, right ? So that's probably not the best switching device to use, should probably look for a power transistor.

Apply some capacitance across the gate/base connection to GND, to smooth out that square wave from the micro PWM output. Apply some more capacitance across the primary coil to smooth out the resultant waveform being sent to the secondary. But how much ?

For grins I stuck a 470uF cap across the primary before I had to run back upstairs. Didn't see any appreciable change in the waveform.

So what would be the best way to do this ?

Thanks -

Dean.

 

[Lewish]

OK, a couple of thoughts. Don't worry about the voltage being a square wave. If you look at the current you will find that it is a triangle wave. Power Factor Correction circuits work by chopping the incoming AC voltage into very small slices that are in effect square waves.

Keep the MOSFET. It is the best switching device for this application. Just make sure you saturate it so that it doesn't over heat. But, that probably won't happen as the part you listed is good for 28 Amps continuous DC.

The circuit typology you have described is a "flyback converter". That means it is going to need a snubber circuit on the primary side, or else a lot of you power will go into waste voltage. That is the 100 volts you are seeing.

Take a look at the Linear Tech app. notes on flyback converters for a good tutorial on what you are doing.

 

[itsmeRICK]

I think you will do well to take a look at some applications notes on switchmode power supply transformer design since this is what you are trying to design. None of the app notes I've seen deal with air cores, but the basic principles are the same - reduce leakage fields by increasing coupling thereby improving regulation. With air you will not have to worry about saturating the core, though!

Switching from an FET to a bioplar transistor will not solve the basic problem of poor coupling in the transformer. That you have high and likely variable leakage inductances suggests that an FET is a better choice since it will not avelanche.

To transfer power efficently you need to get the best coupling between the primary and secondary you can, and eliminate losses wherever you can - fast switching times (another reason to stay with an FET), minimize gate (or base) drive, minimize rectifier losses. Yes, a snubber is an excellent idea, by the way.

Take a look at resonant topologies. Maybe you can make the leakage inductance work in your favor by using it to tune the secondary, then you can achieve regulation by varying the switching frequency -but you'll need to invent a feedback scheme that can bridge the gap.

This looks like a challenging project. Good luck with it.

 

[Chancy99]

Heh. This always seems to happen. Something simple on the surface always reveals hiddens depths and layers ...

It sounds like the principle of using a PNP to switch the gate drive to the mosfet is the right approach. Guarantees proper saturation, and only needs the extra pnp and base resistor.

In case anyone is interested, I found an excellent description of flyback converters here :

http://ece-

http://www.colorado.edu/~ecen4517/course_material/Exp6/flyback.pdf

General idea seems sound - get at least 4-5V usable at 400mA across the gap. The rotating part has it's own regulation (LDO linear regulator - LM1086-3.3), so the actual values aren't critical.

In talking to colleagues at work, it's been suggested to tune the RC circuit that the primary coil and caps make to resonate at the drive frequency (15KHz) to increase the coupling efficiency and reduce the drive requirements.

Dean.

 

[Lewish]

Be careful making the circuit resonant as your flyback primary voltage will GREATLY increase. Make sure your snubber can handle that voltage level.

 

[Chancy99]

Yup, definitely have to be careful. Actually, I don't really think that should be a criteria, since efficiency isn't really drastically necessary (apart from the purist ideal).

This is wall-wart powered, and so long as it gets that 400mA at 4-5V across, well, that's fine. Ugliness in *how* it does that while aesthetically nasty, doesn't really affect things in the long run.

Now, to choose the right components and design for the snubber ...

Dean.

 

[Madcow]

Try this link for your snubber design.
It helped me.

http://www.ridleyengineering.com/snubber.htm