flyback and transformer concept

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
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?
 

ericgibbs

Joined Jan 29, 2010
10,054
hi and,
The transformer windings are inductors, if you apply a DC step voltage across the primary of a transformer it will induce a voltage across the secondary winding.
Consider the rate of rise of current in that primary inductor and the magnetic field it creates around the primary winding, which is magnetically coupled to the secondary winding.
This is the basic operation of an older vehicles ignition system, where the primary coil current is switched On then Off, which produces the High Tension voltage for the spark plugs.
Look up Lenz's Law.
E
Check this link.
https://www.coilcraft.com/edu/flyback_transformer.cfm
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
hi and,
The transformer windings are inductors, if you apply a DC step voltage across the primary of a transformer it will induce a voltage across the secondary winding.
Consider the rate of rise of current in that primary inductor and the magnetic field it creates around the primary winding, which is magnetically coupled to the secondary winding.
This is the basic operation of an older vehicles ignition system, where the primary coil current is switched On then Off, which produces the High Tension voltage for the spark plugs.
Look up Lenz's Law.
E
Check this link.
https://www.coilcraft.com/edu/flyback_transformer.cfm
But generally transformers don't operate on steady current...
Please what do you mean by "a DC step voltage".
 

AlbertHall

Joined Jun 4, 2014
9,875
But generally transformers don't operate on steady current...
Correct.

Please what do you mean by "a DC step voltage".
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.
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
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.
Oh thanks so much, am relieved.The more I encounter challenges, the more I discover.
Ok so is that one of the major reasons ferrite core indutors differs from the normal iron core transformer? or will a similar process also occur
in a normal conventional iron core transformer?
Please educate me more.
 

crutschow

Joined Mar 14, 2008
24,980
is that one of the major reasons ferrite core indutors differs from the normal iron core transformer? or will a similar process also occur
in a normal conventional iron core transformer?
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.
 

MrAl

Joined Jun 17, 2014
7,594
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?
The basic operation for a converter usually uses some kind of square wave or pulse. The wave can pass through the transformer pretty good if the transformer is designed right.
So in short, you start with a true steady DC input voltage, chop that up into pulses, then use a transformer to step it up or even step it down. The output is a square or pulsing wave so you use diodes to rectify it just as you would with AC but the output is smoother to being with.

The main idea with any transformer is there has to be a CHANGE in the input and that produces a change on the output. A perfectly steady DC is not able to get through the transformer because the transformer depends on a changing flux to induce the voltage in the secondary winding, and you cant get that change in flux without a change in input voltage. It can change in many ways though does not have to be a sine wave unless the transformer was designed with that in mind.
 

dendad

Joined Feb 20, 2016
3,535
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.
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
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.
Ok thanks but the main point of my question is that even though the magnitude of the current is not changed yet, (square wave pulse not yet low) a voltage will still be induced in the secondary winding that (according to the flyback or forward converter operation) will reverse bias the output diode.
Why is this voltage induced when the pulse is not yet low?
 

ericgibbs

Joined Jan 29, 2010
10,054
hi andi,
Have you considered that if you apply a voltage across an inductive/coil the current thru the coil will increase exponentially, this changing current creates a changing magnetic field around the inductor.
If there is another inductor close to the first inductor, the 'changing' magnetic field will induce a current in the second inductor, [ a load or path is assumed in the second inductors circuit.]
E
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
Ok I g
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.
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).
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
hi andi,
Have you considered that if you apply a voltage across an inductive/coil the current thru the coil will increase exponentially, this changing current creates a changing magnetic field around the inductor.
If there is another inductor close to the first inductor, the 'changing' magnetic field will induce a current in the second inductor, [ a load or path is assumed in the second inductors circuit.]
E
That is to say that if you apply DC voltage to a transformers primary winding, a voltage can be induced in the secondary.Even if the input voltage is not pulsating?
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
The basic operation for a converter usually uses some kind of square wave or pulse. The wave can pass through the transformer pretty good if the transformer is designed right.
So in short, you start with a true steady DC input voltage, chop that up into pulses, then use a transformer to step it up or even step it down. The output is a square or pulsing wave so you use diodes to rectify it just as you would with AC but the output is smoother to being with.

The main idea with any transformer is there has to be a CHANGE in the input and that produces a change on the output. A perfectly steady DC is not able to get through the transformer because the transformer depends on a changing flux to induce the voltage in the secondary winding, and you cant get that change in flux without a change in input voltage. It can change in many ways though does not have to be a sine wave unless the transformer was designed with that in mind.
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).
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
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.
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).
 

ericgibbs

Joined Jan 29, 2010
10,054
That is to say that if you apply DC voltage to a transformers primary winding, a voltage can be induced in the secondary.Even if the input voltage is not pulsating?
Hi,
Applying a fixed DC voltage on the primary of a transformer will give one pulse of current on the secondary.
E
 

dendad

Joined Feb 20, 2016
3,535
And afterwards it will stop, that is the current will stop inducing to the secondary
Yes. Once the current stabalizes to a fixed value, the secondary voltage falls to zero.
Power supplies give more or less constant outputs because the charge stored in the capacitors.

Have a try with this...
Wind a lot of fine wire on a cotton reel. Insert a steel bolt in it.
Attatch a red and green LED in parallel, with reversed polarity to the coil.
Now, if you clunk a strong magnet to the bolt, one LED will flash. Then go out.
Next, quickly take the magnet off, and the other LED will flash.

If you repeat with the magnet reversed, the LED colours will reverse.
This is a toy I made for the electronics class I taught for a bit.
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
Yes. Once the current stabalizes to a fixed value, the secondary voltage falls to zero.
Power supplies give more or less constant outputs because the charge stored in the capacitors.

Have a try with this...
Wind a lot of fine wire on a cotton reel. Insert a steel bolt in it.
Attatch a red and green LED in parallel, with reversed polarity to the coil.
Now, if you clunk a strong magnet to the bolt, one LED will flash. Then go out.
Next, quickly take the magnet off, and the other LED will flash.

If you repeat with the magnet reversed, the LED colours will reverse.
This is a toy I made for the electronics class I taught for a bit.
Thanks for sharing your experience,
I really appreciate it.
I'll try it.
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
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
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
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?
 
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