Transformer turns ratio #toroid #transformer

Kevin Polston

I wondered what physical/electrical difference there is increasing the number of windings in a transformer and what the advantages/disadvantages in doing so?

For instance:  Let's say that a circuit specifies a transformer with a turns ratio of 2:7.  Now, I could choose 2:7 or 4:14 or 6:21 etc.   Is there a reason to choose the lowest ratio or something higher?

Now, I’m speculating but I guess that the inductance of each coil will increase with each turns 'multiple' which should increase the efficiency (?);  but on the other hand,  the resistance will increase a bit too and the capacitance then maybe causing unexpected resonances, phase shifts and other headaches (?)  In practice do electrical engineers worry about such matters or try to forget them?

The reason I’m asking this question is that I have a suspicion I mis-wound a toroid on a particular project - then I started to think far too much about what I thought was a simple subject...

Kevin M0XYM

Timothy Fidler

K : The Last sentence is highly ambiguous a toroid could be an iron powder toroid or a ferrite one.  A miswind on a resonant circuit obviously has big implications.. basically nothing works.  I think you mean a transformer that has been configured as a ferrite toroid.

As for the rest.  An RF transformer needs to present about  4-5 x  the output impedance of the driver to the driver as its primary inductance.. ie the transformer as an unterminated device wrt the secondary side.  This ensures that there is minimal shunting of the transformer 's  reflected load (as seen at the primary ) by its own inductance.  Eg a transformer with a 2.25 Z ratio will present say a 70R pure resistive load to the primary as 31.1 ohms. We'd like the  jwL of the transformer's primary  to be say 150j or so that the basic transformer action is not compromised by the shunting effect.  

Obviously if we have a given AL  nH/Tsquared for a transformer we can calculated what the equiv turns should be to achieve this. But now it gets interesting. The AL applies only for turns hard up against the core (I am thinking binocular type cores here which are the only suitable type for high power applications - for power transfer).  As you  want PTFE insulated wire for maximum security ( a short could kill $ 150 USD of RF Power mos just like that ) you are going to have the wires building away from the core web so the L is going to be somewhat less than calculated. 

As the turns go up the resistance will indeed go up and this is a consideration.  The resistance is going up as  N approximately - if you look at the geometry of the core windings the outer windings will have a slightly longer path than the inner ones.  L is going up nominally as L squared so as you load in more turns the R to L ratio of a given wire primary gets better. What works against you is that you need to use smaller diameter wire to fit the windings in.

There's another factor to consider.. you may well be trying to create a broadband  transformer. Therefore  , given a final constant load, the antenna (nominally resistive and sitting in the mm 60- 80 ohm range for a  dipole and lower for a Monopole.  is  being transferred back to the driving generator through a core of varying  AL (over frequency) with a higher proportion of  Rs (effective series resistance caused by the core , as opposed to that caused by skin effect in the copper)... so what may be an optimum configuration at 4Mhz may be very suboptimal a 14Mhz.  If you are designing for a single frequency or small range as for example a monoband SSB Tcvr the whole thing starts to get a bit  simpler ; otherwise it is starting to get even messier.   Type 43 material  may be optimal for a core at 4 Mhz esp. as it  will have higher AL for given dimensions and will therefore drive down the number of turns required at that frequency . But at 14 Mhz it is very lossy c/w Ferronics type K material or NXP type 4C65.

If you look at  a  PDF  file from the Fair Rite catalogue I posted under Ferrites in the Files section you will find some graphic comparison of the two major Ni-Zn ferrite materials.  IT makes it clear that the concept of a single AL value at HF  for a given core geometry and material is a nonsense.

I think I better stop here - there is science to it but it has to be validated experimentally so it gets into the  black art area.

Really high power cores , say over 80 W Rms RF are generally all of the PTFE wire in a copper tube type.  that's because at the 100W level power you can only make a class b amp work if you have the drain to drain impedance of your  MOSFETs way down.  Given that you want to have the primary with  a Z  AT FREQUENCY say 4 times the  D-D impedance you will often find that the only clean solution is a copper tube core. That then gives you next to no options about putting a second turn through the core , even if you want to.  If you want more Web cross section surrounded by turns the only way to do it is to gang up balun cores (or ferrite tubes  if you have the separate tube construction format).  

There is more that can be covered but I think that's enough.  In general unless you have the training and time and extensive test equipment including a VNA and  HF oscilloscope it is  a very bad idea to mess with the RF transformer winding details specified in a build book because the implications might surprise you and be quite expensive too. If you are of the experimental type build one exactly to spec an that's the golden build . then by all means if you can afford it buy a second version and  chop and change core materials and windings in an attempt to optimise  the circuit for you application. Eventually of course the PC B will say goodbye as the traces lift off from too many resolder attempts. Hopefully you are at the top of the curve before that happens - otherwise it's CHing ching  at Hans' cash register again  :-)

Regards, TEF 

Timothy Fidler

"L is going up nominally as L squared so as you load in more turns "

ah yes ...nominally as n squared I should say . 

- My story is I wrote that original version just to see you had not gone to sleep reading the spiel.