Flyback transformer power capability

[Uspring]

Secondary has no way of knowing which combination of amper turns is producing the flux it sees. It can be literally any, say 16 amps 16 turns or 32 amps 8 turns or 64 amps 4 turns etc...all these and many many others will produce exactly the same flux. Obviously changing the number of turns changes inductive reactance and thus different voltages will be needed to "push" current through primary at particular frequency...

Yes.

Secondary has no way, well, it clearly HAS a way, but no way we can determine, of knowing which among theoretically infinite combinations it is. We may get into the A-field and Magnetic vector potential but this does not explain the phenomena either. It implies some kind of INTELLIGENCE underlying this usually taken-for-granted phenomena.

You seem to imply, that the secondary knows, which of the many possible primary configurations produces the flux. How, e.g., does a secondary voltage measurement tell you details about the primary circuit? Except, of course, the total flux time derivative it generates in the secondary.

[nix85]

Yes.

You seem to imply, that the secondary knows, which of the many possible primary configurations produces the flux. How, e.g., does a secondary voltage measurement tell you details about the primary circuit? Except, of course, the total flux time derivative it generates in the secondary.

I don't only imply, obviously turns ratio "law" is respected and output voltage is predictable.

Secondary voltage does not really tell us anything about the primary circuit.

ALL it sees is certain flux change, what is generating it, as far as we can tell, it has absolutely no way of "knowing".

[Uspring]

I don't only imply, obviously turns ratio "law" is respected and output voltage is predictable.

That is, because

a) secondary voltage is proportional to the flux rate of change and proportional to the secondary turn count
and
b) the flux rate of change is proportional to the primary voltage and inversely proportional to the primary turn count.

Together this leads to the turns ratio law. The only thing, that the secondary "knows" about, is from a) . But we have the turns ratio law anyway.

[nix85]

That is, because

No it's not.

a) secondary voltage is proportional to the flux rate of change

I been saying from first post on this subject that voltage across the primary is proportional to rate of change of flux, this is Faraday's law we all know

V=--N*dΦ/d

Question is, however, is it violated in the secondary. I'll address this below.

and proportional to the secondary turn count

Obviously, turns ratio is respected, as said before.

and
b) the flux rate of change is proportional to the primary voltage and inversely proportional to the primary turn count.

Rate of change of flux is proportional to frequency of primary voltage and consequently current and of course number of turns. God knows why you wrote inversely proportional to primary turns, as if more turns means less flux. More turns means more inductive reactance and thus more voltage is needed to push the same current at the same frequency, but that is another thing.

Together this leads to the turns ratio law.

No it does not.

The only thing, that the secondary "knows" about, is from a) . But we have the turns ratio law anyway.

Secondary (as far as we can tell) knows nothing but flux in the core. It has no idea if there is a primary coil there at all, not to mention how many turns it has, what is the voltage and current across/through it, the flux it sees may as well be produced by spinning a permanent magnet inside a gap in the core. And if there is a primary there, again, many different combinations of turns, voltage and current can produce the same flux, secondary has no idea which combination it is, and yet it knows.

Back to the Faraday's "law".

V=--N*dΦ/d

According to the "law" particular rate of change of flux will always produce the same voltage across N number of turns.

But we know that secondary is respecting the turns ratio, even if change of flux is the same, it will generate different voltage and current clearly violating the "law".

If Faraday's "law" was valid for the secondary, it would mean there is only one combination of voltage, current and number of turns that can produce particular rate of change of flux. But that is not the case.

[Uspring]

Both of my statements above are versions of Faradays law. Explicitly:

a) secondary voltage is proportional to the flux rate of change and proportional to the secondary turn count
i.e.:
Vsec ~ Nsec*dΦ/dt

b) the flux rate of change is proportional to the primary voltage and inversely proportional to the primary turn count
i.e.:
dΦ/dt ~ Vpri/Npri
Note that multiplying this equation by Npri on both sides will lead to the familiar version of Faradays law. In this case for the primary coil.

Rate of change of flux is proportional to frequency of primary voltage and consequently current and of course number of turns. God knows why you wrote inversely proportional to primary turns, as if more turns means less flux.

What I meant to say is, that for a given primary voltage, an increase in turn count will lead to less flux. That is because the current will drop with rising turn count, due to the larger primary inductance. In equations:

Ipri ~ Vpri/Lpri ~ Vpri/Npri^2   due to the inductance being proportional to the square of turn number
Φ ~ Ipri*Npri ~ Vpri/Npri      using Ipri from above

Again, Npri appears in the denominator here as it must in order not to violate Faradays law.

More turns means more inductive reactance and thus more voltage is needed to push the same current at the same frequency, but that is another thing.

The turns ratio law relates primary voltage, secondary voltage and turns ratio. So the effect of more voltage needed to push the same current cannot be disregarded.

[klugesmith]

b) the flux rate of change is proportional to the primary voltage and inversely proportional to the primary turn count.

Rate of change of flux is proportional to frequency of primary voltage and consequently current and of course number of turns. God knows why you wrote inversely proportional to primary turns, as if more turns means less flux. More turns means more inductive reactance and thus more voltage is needed to push the same current at the same frequency, but that is another thing.

Should we stop feeding the troll?    Uspring is right: for given primary voltage, more turns means less volts per turn, so less flux, and less voltage in secondary.  Also, rate of change of flux is proportional to primary voltage and _not_ proportional to frequency.
We could forget about the "inductance"view and say: Core flux will be whatever it needs to be, so the induced voltage in primary winding matches the applied voltage.

We could further avoid cause-and-effect arguments by using the coupled inductor model.
L_pri = 1 henry.  L_sec = 100 henries.  K (L1,L2) = 0.99.  When an AC voltage is applied to L_pri, L_sec (even if unintelligent) has a voltage greater by a factor of about 10.  It's just the solution of circuit equations, not much fancier than a resistive voltage divider.

[nix85]

Both of my statements above are versions of Faradays law...

Ok, the way you formulated b) was not obvious that you were just stating the Faraday's law rearranged.

What I meant to say is, that for a given primary voltage, an increase in turn count will lead to less flux. That is because the current will drop with rising turn count, due to the larger primary inductance.....

Well, i been saying exactly the same thing. Higher inductance > higher inductive reactance > smaller current/flux, that is, unless voltage is increased to compensate for additional impedance. To remind of few basic formulas

Inductive reactance

XL= 2πfL

Alternating current flowing through inductor is applied voltage / inductive reactance

I= V/XL

​In equations:

Ipri ~ Vpri/Lpri ~ Vpri/Npri^2   due to the inductance being proportional to the square of turn number
Φ ~ Ipri*Npri ~ Vpri/Npri      using Ipri from above

Again, Npri appears in the denominator here as it must in order not to violate Faradays law.

I know very well that inductance is proportional to turns squared just like it is inversely proportional to coil length, as well as Faraday's law and turns ratio law, these are basics.

The turns ratio law relates primary voltage, secondary voltage and turns ratio. So the effect of more voltage needed to push the same current cannot be disregarded.

We all know what law of turns ratio relates, again, turns ratio is respected. As for more voltage to push.. i brought that up first, i'm sure not disregarding it but that is not the main point.

Again main point is many different volt amp turn combinations can produce the same flux change and secondary has no way of differentiating.

Unless you maybe want to claim there is only one volt amp turn combination that can produce any particular change of flux.

Should we stop feeding the troll?

I may call you a fool, but i'm gonna remain polite. I am not a troll.

Uspring is right: for given primary voltage, more turns means less volts per turn, so less flux, and less voltage in secondary.

No one is denying that more turns on the primary means smaller voltage in the secondary, again, turns ratio is respected, obviously.

Also, rate of change of flux is proportional to primary voltage and _not_ proportional to frequency.

In fact, i brought that up first when i mentioned need for higher voltage to "push" same amount of current through an inductor at higher frequency, basically the same thing in other words...so, i'll call semantics on this one.

We could forget about the "inductance"view and say: Core flux will be whatever it needs to be, so the induced voltage in primary winding matches the applied voltage.

We could further avoid cause-and-effect arguments by using the coupled inductor model.
L_pri = 1 henry.  L_sec = 100 henries.  K (L1,L2) = 0.99.  When an AC voltage is applied to L_pri, L_sec (even if unintelligent) has a voltage greater by a factor of about 10.  It's just the solution of circuit equations, not much fancier than a resistive voltage divider.

You are also missing the main point....many different volt amp turn combinations can produce the same flux change and secondary has no way of differentiating which one it is.

[nix85]

I may also remind of Farraday's law for moving conductor

where V is voltage B magnetic field L length of a conductor/coil and v relative velocity of a magnet and a coil

V = BLv

If we were to draw a parallel, we may say that there are many combinations of these 3 values that will generate the same voltage. It is not a perfect analogy cause situation is very different, but somewhat portrays the point.

[klugesmith]

OK, I apologize for saying the T word.  And we don't want readers to start fetching popcorn.

>>You are also missing the main point....many different volt amp turn combinations can produce the same flux change and secondary has no way of differentiating which one it is.

Agreed. So what? Who says a secondary coil does differentiate, or needs to differentiate, between primary cases that have same outcome in secondary circuit?   I am not seeing an unexplained mystery.

Secondary "sends" signal to primary, just like primary "sends" signal to secondary, by having ampere turns that magnetize the shared core.  Under normal loading conditions the core flux from secondary amp-turns is about the same magnitude as that from primary amp-turns, with opposite sign.

[nix85]

What do you mean so what. If there are multiple variables on the primary side that can result in same flux change and flux change is all that secondary sees....do i need to draw it that secondary has no way of telling which combination is generating the input, yet, it behaves as if it does.

You must either disagree with me that multiple combinations of volt amp turns can produce the same change of flux in the primary or if you agree, you must deny the validity of Faraday's law, you can NOT have it both ways.

Faraday's law demands that for certain rate of change of flux same number of turns will always result in same voltage across those turns.

Also, i know well "Under normal loading conditions the core flux from secondary amp-turns is about the same magnitude as that from primary amp-turns, with opposite sign." and that max-load flux is slightly lesser than no-load flux etc.

[nix85]

Here are few more basic formulas

flux density = amper x turns x core permeability x core area / m² (T)

F = ILxB force on a conductor in a magnetic field - laplace
as load increases, current in the conductor must increase to balance the forces: I = F/BL

v = L(di/dt) BACKEMF from an inductor

t = L/R inductor time constant, after ~5t (transient time) current reaches 99.5%

etc

[klugesmith]

>>You must either disagree with me that multiple combinations of volt amp turns can produce the same change of flux in the primary or if you agree, you must deny the validity of Faraday's law, you can NOT have it both ways.

I agree with the first part, but don't see Faraday's law breaking down.   Would you mind reviewing the following example, designed with "flyback-like" size and frequency, and point out what I'm missing?

The core has effective length 10 cm, area 1 cm^2, permeability 796.   To make coil inductances 1 microhenry per turn squared.
Let's operate at 15.9 kHz, for radian frequency of 10^5.

Our first primary coil has 10 turns (L = 100 uH), intended to operate at 10 V (peak) with magnetizing current of 1 A (peak).  MMF is 10 amp-turns, H is 100 A/m, Bmax is 0.1 T, peak flux is 10 uWb, dF/dt (peak) = 1 Wb/s, inducing 1 volt/turn.  The unit "webers per second" is dimensionally identical to the volt unit.

Our alternate primary coil has 5 turns (L=25 uH), to operate at 5 V (peak) with magnetizing current of 2 A (peak).   MMF is 10 amp-turns, ..., dF/dt is 1 Wb/s as before.

Our secondary has 40 turns (L = 1600 uH), and will have peak voltage of 40 V no matter which primary coil is used.   Each turn of the secondary goes around 1 Wb/s of flux change rate, so Faraday says 1 volt per turn.

If a secondary load draws 1A, that would increase the primary current by 4 A (original coil) or 8 A (alternate coil).   Net MMF (ampere turns) and net dF/dt are about the same as the unloaded case, no matter which primary coil is used.

What suggests to you that Faraday's law isn't valid here?  If the example is missing a pathological case, what can we vary to expose the failure?

Note 1: normally the secondary load current and corresponding change in primary current are in phase with the voltage, while the magnetizing currents have 90 degree phase lag wrt the voltage.

[nix85]

Our secondary has 40 turns (L = 1600 uH), and will have peak voltage of 40 V no matter which primary coil is used....

Now that you gave a practical example, i gotta say my bad. I knew this, i even said it from post one that voltage across the primary is proportional to it's inductance, that is, flux, so turns ratio is automatically respected.

Note 1: normally the secondary load current and corresponding change in primary current are in phase with the voltage, while the magnetizing currents have 90 degree phase lag wrt the voltage.

We all know when secondary is open IV through the primary are 90° outta phase, in reality little less due to various magnetizing losses. Loading the secondary with resistive load brings primary IV more into phase, like i in the first post, "as if resistor appears in parallel with the primary inductance". Ofc magnetizing IVs are always 90° outta phase and load currents are always in phase, well, at least if load is purely resistive.

If load is inductive then picture is not so clear, but extending what happens in the core to the load, we can assume this inductive load will also appear as an inductor and resistor in parallel and larger the work done larger the virtual resistor in parallel will appear again bringing IV in phase.

I assume we can extend the last paragraph to capacitive loads altho i never read about this anywhere, of course, electric field just like magnetic field can also be used to do work and as you all probably know there are various electrostatic motors, some newer ones of significant power (some even speculate about replacing magnetic ones). Anyway..

[Mads Barnkob]

Insults and massive double posting got this thread locked. Some posts will be deleted to keep the initial thread within site rules.

Read the rules before posting on HVF: https://highvoltageforum.net/index.php?topic=31.0



EDIT: Thread has been unlocked, for a second chance to stay on topic.

[nix85]

To recap on the subject, for "normal" (non flyback) transformer, i'd summarize it like this to cover all the angles.

When transformer is unloaded (secondary open) only magnetizing current flows through the primary which is tiny compared to load current and is always (almost) 90° outta phase with the driving voltage, almost but not 90° due to various losses, namely, eddy currents, hysteresis, magnetostriction and copper losses (P = I²R). Average flux in the core is maximum in no-load state and slightly smaller in the full-load state.

When secondary is loaded, counterflux developed by the secondary demagnetizes the core and this makes the voltage across the primary to drop since this voltage is directly proportional to rate of change of flux, we all know Faraday's (or should i say Henry's) law V=--N*dΦ/dt.

Voltage across an inductor can also be expressed as V = L(di/dt)

And current through an inductor I = (V-E)/Z where V is voltage of the source driving the primary and E is voltage drop across primary's inductance. Clearly, when secondary demagnetizes the core and flux through the primary drops, so does it's inductive reactance Z and voltage across it E, V remaining the same means current must rise and so it does trying to bring the flux back to the original value but it never fully manages to do so, so, as said before, max load flux is slightly less than no load flux.

To the circuit driving the primary, it appears as if a resistor appears in parallel with the inductance of the primary, bigger the load smaller the resistor appears, obviously.

As said above magnetizing current is always almost 90° out of phase with the driving voltage while the load current is always in-phase. At least when the load is purely resistive.

If load is inductive then picture is not so clear, but extending what happens with the resistive load, we can assume this inductive load will also appear as an inductor and resistor in parallel and larger the work done larger the virtual resistor in parallel will appear again bringing IV in phase.

I guess we can extend the last paragraph to capacitive loads too, of course, electric field just like magnetic field can also be used to do work and as you all probably know there are various electrostatic motors, some newer ones of significant power (some even speculate about replacing magnetic ones).

As for flyback, the only difference is, as said before in the thread, induction in the secondary happens with a delay (due to internal diodes blocking the current in one direction) when the primary flux collapses. Ignition coil uses the same principle, store, collapse, get 10x (or more) voltage in the primary and x turns ratio in the secondary.

Talking of ignition coil, the reason for cap across the switch (be it mechanical relay or a MOSFET) is to limit the peak voltage, cap appearing as a short for an instant and lower the resistance in the circuit of the collapsing flux, lower will be the peak, obviously, higher the resistance higher the peak and faster will it burn out.