hi,
That is correct.
BTW: Consider what happens when the +10V input is switched back to 0V.??
What do you think.?
E

 

Ok I get you.
Picture either a simple flyback or forward converter, during the period that the "pulse is HIGH", the switching transistor is turned ON, and current will flow in the primary winding.
In the case of flyback converter; an opposite polarity voltage will be induced in the secondary winding, which reverses bias the diode.
In the case of a forward converter; a same polarity voltage will be induced simultaneously in the secondary winding.
My main difficulty is, why are these voltages induced in the secondary winding when the input pulse is still ON(no change in current).

That's a good question.
The reason is because the CURRENT is still changing even when the voltage is not.

The simplest case is when the voltage is first applied, the current starts to RAMP up. A ramp is a continuously changing quantity that starts out low and keeps getting higher and higher. Because it keeps ge tting higher and higher, it is changing all the time. That means the flux is changing all the time, and that means a current in induced in the output winding.
As the voltage of the primary drops (turns off) the current starts to RAMP down. It is still constantly changing because the current is continuously falling now. The flux is still changing so there is still a current induced in the secondary. The voltage of the secondary will change polarity during that time.

When a voltage is connected across an inductor (the primary winding) the current ramps up (in the ideal case). When the voltage is removed, the current ramps down. This in turn means the flux ramps up and then down. The current ramps can produce a current in any secondary winding if one is present.

The main point is that even though the voltage is constant for a time, the current is not constant. Note however that the voltage can not stay constant forever because the current would ramp up too high and saturate the primary winding which would cause the current to reach some maximum value limited only by the winding resistance, and then the flux would stop changing also and thus the output voltage on the secondary would drop to zero. This often leads to the drive transistor burning out because of over current.

So to understand this you have to look at what both the voltage and the current are doing over time.
First the voltage rises fast and that causes the current to rise slowly as a ramp and that means the flux ramps up slowly and that means the flux in the secondary ramps up and that causes a current to be induced in the winding. Since the current in the secondary winding is a ramp, the output voltage may appear constant (in the ideal case).
If the primary voltage stays 'high' too long, the core saturates and that is an error condition so normally the circuit designer makes sure the voltage pulse is not too wide. That is why there is a minimum frequency and maximum voltage associated with any transformer design.

If you know algebra, the transformer equation looks like this:
B=E*10^8/(4.44*F*A*N)

and so we can see the flux density B depending on E the voltage, F the frequency, A the area of the core, and N the number of turns on the primary. The maximum flux density B is something we can not exceed or the core material saturates and the transformer becomes a hunk of metal with wires wrapped around it doing nothing useful anymore and the primary winding looks almost like a short circuit.

 

I have been reading about the concept of a flyback transformers and converters, which has led to various misunderstandings concerning the basic operation of a transformer.
Firstly, dc current don't induce voltage in the secondary windings of any transformer. But in a flyback converter, I read that when the switch is ON, current will build up in the primary winding, and a diode that was added in the secondary winding will prevent current flowing in the secondary winding, until the switch is open or OFF.
Same with a RCC converter.When the transistor turns ON, current will be induced in the auxillary winding.How does this happen?
So can a dc or a short constant current induce voltage in another winding of a transformer?

Consider that explanation: "as the current builds up" The voltage is induced in a second winding because of the change in the magnetic field. When the field stops changing the voltage is no longer developed. That is the principle of all transformers. The very first flyback transformers were part of the horizontal deflection circuit for CRTs in televisions with magnetic deflection picture tubes. The very rapid moving the beam back to start the next sweep across was the flyback interval, where the rapidly changing magnetic field was used to also produce the high voltage. So for over 50 years TV sets had flyback transformers, also known as horizontal output transformers.

 

That's a good question.
The reason is because the CURRENT is still changing even when the voltage is not.

The simplest case is when the voltage is first applied, the current starts to RAMP up. A ramp is a continuously changing quantity that starts out low and keeps getting higher and higher. Because it keeps ge tting higher and higher, it is changing all the time. That means the flux is changing all the time, and that means a current in induced in the output winding.
As the voltage of the primary drops (turns off) the current starts to RAMP down. It is still constantly changing because the current is continuously falling now. The flux is still changing so there is still a current induced in the secondary. The voltage of the secondary will change polarity during that time.

When a voltage is connected across an inductor (the primary winding) the current ramps up (in the ideal case). When the voltage is removed, the current ramps down. This in turn means the flux ramps up and then down. The current ramps can produce a current in any secondary winding if one is present.

The main point is that even though the voltage is constant for a time, the current is not constant. Note however that the voltage can not stay constant forever because the current would ramp up too high and saturate the primary winding which would cause the current to reach some maximum value limited only by the winding resistance, and then the flux would stop changing also and thus the output voltage on the secondary would drop to zero. This often leads to the drive transistor burning out because of over current.

So to understand this you have to look at what both the voltage and the current are doing over time.
First the voltage rises fast and that causes the current to rise slowly as a ramp and that means the flux ramps up slowly and that means the flux in the secondary ramps up and that causes a current to be induced in the winding. Since the current in the secondary winding is a ramp, the output voltage may appear constant (in the ideal case).
If the primary voltage stays 'high' too long, the core saturates and that is an error condition so normally the circuit designer makes sure the voltage pulse is not too wide. That is why there is a minimum frequency and maximum voltage associated with any transformer design.

If you know algebra, the transformer equation looks like this:
B=E*10^8/(4.44*F*A*N)

and so we can see the flux density B depending on E the voltage, F the frequency, A the area of the core, and N the number of turns on the primary. The maximum flux density B is something we can not exceed or the core material saturates and the transformer becomes a hunk of metal with wires wrapped around it doing nothing useful anymore and the primary winding looks almost like a short circuit.

This is cool.
Thank you so much. At first I thought it was a kinda pointless question(I shouldn't bother to know why it occurs so. I should just accept the fact that voltage can be induced by a DC even though it contradictory). But now I've satisfied my curiosity and I appreciate your help. I won't hesitate to ask questions anymore.

 

This is cool.
Thank you so much. At first I thought it was a kinda pointless question(I shouldn't bother to know why it occurs so. I should just accept the fact that voltage can be induced by a DC even though it contradictory). But now I've satisfied my curiosity and I appreciate your help. I won't hesitate to ask questions anymore.

It was a very reasonable question asked in a reasonable manner.

 

Ok I notice that as the switch is closed, current in the primary winding will quickly step up to the input voltage level (10v), and at the same time, the same magnitude of voltage is induced to the secondary winding but will steadily decrease to zero.
Am I right?

Not quite.
Technically, the primary winding voltage will instantly step to the input voltage level, and the current will increase at a rate as determined by the inductance, until it is limited by the primary resistance (I = V/R).
The output voltage will step to a value of 10V times the turns ratio until the magnetic core saturates, at which point the output voltage will rapidly drop to zero.

 

Faraday's law is your best friend to understand this concept.

 

This is cool.
Thank you so much. At first I thought it was a kinda pointless question(I shouldn't bother to know why it occurs so. I should just accept the fact that voltage can be induced by a DC even though it contradictory). But now I've satisfied my curiosity and I appreciate your help. I won't hesitate to ask questions anymore.

I am sure we all asked this same question at one time or another that is how we found out.

 

It occurs with both types of core.
The main reason for using ferrite is that it has lower eddy current losses when operated at a high frequency (which is typical of switch-mode power supplies).

In flyback operation, a dc voltage is applied to the transformer primary, causing the current to rise based on the primary L di/dt, creating stored energy in the inductance.
When the primary current is suddenly stopped (by a switch) the primary voltage will rapidly rise due to the inductance and the stored magnetic energy.
This generates a voltage across the secondary as determined by the turns ratio from transformer action, where it either generates a spark, as in an ignition system, or is rectified to a DC voltage, as in a power supply.

I donno if this is acceptable, but i will like you to view my next thread on self oscillating flyback converters. I posted it just yesterday.
https://forum.allaboutcircuits.com/...switching-power-supplies.170463/#post-1520872

 

That's a good question.
The reason is because the CURRENT is still changing even when the voltage is not.

The simplest case is when the voltage is first applied, the current starts to RAMP up. A ramp is a continuously changing quantity that starts out low and keeps getting higher and higher. Because it keeps ge tting higher and higher, it is changing all the time. That means the flux is changing all the time, and that means a current in induced in the output winding.
As the voltage of the primary drops (turns off) the current starts to RAMP down. It is still constantly changing because the current is continuously falling now. The flux is still changing so there is still a current induced in the secondary. The voltage of the secondary will change polarity during that time.

When a voltage is connected across an inductor (the primary winding) the current ramps up (in the ideal case). When the voltage is removed, the current ramps down. This in turn means the flux ramps up and then down. The current ramps can produce a current in any secondary winding if one is present.

The main point is that even though the voltage is constant for a time, the current is not constant. Note however that the voltage can not stay constant forever because the current would ramp up too high and saturate the primary winding which would cause the current to reach some maximum value limited only by the winding resistance, and then the flux would stop changing also and thus the output voltage on the secondary would drop to zero. This often leads to the drive transistor burning out because of over current.

So to understand this you have to look at what both the voltage and the current are doing over time.
First the voltage rises fast and that causes the current to rise slowly as a ramp and that means the flux ramps up slowly and that means the flux in the secondary ramps up and that causes a current to be induced in the winding. Since the current in the secondary winding is a ramp, the output voltage may appear constant (in the ideal case).
If the primary voltage stays 'high' too long, the core saturates and that is an error condition so normally the circuit designer makes sure the voltage pulse is not too wide. That is why there is a minimum frequency and maximum voltage associated with any transformer design.

If you know algebra, the transformer equation looks like this:
B=E*10^8/(4.44*F*A*N)

and so we can see the flux density B depending on E the voltage, F the frequency, A the area of the core, and N the number of turns on the primary. The maximum flux density B is something we can not exceed or the core material saturates and the transformer becomes a hunk of metal with wires wrapped around it doing nothing useful anymore and the primary winding looks almost like a short circuit.

I really like your explanation, I donno if this is acceptable, but i will like you to view my next thread that uses the principle of your explanation. It's on self oscillating flyback converters. I posted it just yesterday.
https://forum.allaboutcircuits.com/...switching-power-supplies.170463/#post-1520872

 

hi,
This is a demo simulation showing what happens.
The 10v is switched on at 2mSec, note the secondary voltage and current.
E
View attachment 207733

will like you to view my next thread on self oscillating flyback converters. I posted it just yesterday.
https://forum.allaboutcircuits.com/...switching-power-supplies.170463/#post-1520872

 

Correct.


If you take a transformer primary and then connect a battery across it, the voltage across the primary suddenly steps up to the battery voltage. Because this is an inductor the current doesn't suddenly up but it rises exponentially with a time constant of L/R (inductance/resistance) until the current reaches its final value which id the supplied voltage divided by the winding DC resistance. All the time the current is rising a voltage will be induced in the secondary. When the battery is disconnected the current will fall and again voltage will be induced in the scondary until the falls to zero.

I will like you to view my next thread on self oscillating flyback converters. I posted it just yesterday.
https://forum.allaboutcircuits.com/...switching-power-supplies.170463/#post-1520872

 

Just remember as @MrAl mentions, it is the CHANGE of magnetic flux that produces the voltage. The current is the cause of the flux, but the magic in in the magnetic field.
When a DC is applied to the primary coil, the resulting building magnetism induces a voltage in the secondary. The rate of the magnetic field growth is limited by the speed of the current increase. In fact, the rising magnetic field induces an opposing voltage in the primary that slows the current increase down. When the magnetic field stabilizes, this opposing induced voltage drops to zero so the current now is just limited by the resistance. And the secondary output voltage is zero and there needs to be "relative motion" between the magnetic field and a conductor to produce the voltage. Steady state produces nothing. When the primary power is removed, assuming an open circuit, the magnetic field collapsed very rapidly so the secondary will produce a large voltage spike. A reverse diode across the primary supplies a path for the collapsing field's induced current to flow, slowing the collapsing field down and limiting the voltage. That is why a diode across a relay is good idea to limit the voltage spike. It does slow the relay release a bit too.

Please I will like you to view my next thread on self oscillating flyback converters. I posted it just yesterday.
https://forum.allaboutcircuits.com/...switching-power-supplies.170463/#post-1520872

 

I am sure we all asked this same question at one time or another that is how we found out.

Really, the voltage is produced by the change in the magnetic field. So really, it is not true DC producing the voltage, it is the change in the magnetic field caused by the change in current, which is what happens when a voltage is applied and the inductance does not let the current instantly change. So it is sort of complicated and a detailed mathmatic expression is sort of tedious.

 

Really, the voltage is produced by the change in the magnetic field. So really, it is not true DC producing the voltage, it is the change in the magnetic field caused by the change in current, which is what happens when a voltage is applied and the inductance does not let the current instantly change. So it is sort of complicated and a detailed mathmatic expression is sort of tedious.

Ok thanks as well

 

Really, the voltage is produced by the change in the magnetic field. So really, it is not true DC producing the voltage, it is the change in the magnetic field caused by the change in current, which is what happens when a voltage is applied and the inductance does not let the current instantly change. So it is sort of complicated and a detailed mathmatic expression is sort of tedious.

Yes, the full length viewpoint goes like this:
1. A constant (or changing) primary winding voltage causes a changing primary current.
2. The changing primary current produces a changing magnetic flux.
3. The changing magnetic flux produces a current in the secondary winding.
4. With the current out of the secondary a voltage appears across the secondary load.

That's the full view although we skipped what happens to the core and we are sequencing it as if there is a cause and effect delay when really there is not any delay ... it all happens at the same time.