# Flyback dot convention in transformer

#### mishra87

Joined Jan 17, 2016
1,039
Hey Guys,
My concept is shaking with transformer dot convection. i want to show you the case study of two circuit.

As per my understanding SW is on voltage develop at dot end is positive and when SW is off voltage polarity reversed.
case 1: SW=ON, Voltage at dot end of L1 is + so secondary side voltage at dot end is + and D2 side is negative hence D2 is reverse bias.
SW=OFF, Voltage at dot end of L1 is - so voltage secondary side at dot end is - hence D2 is Forward bias.

Case 2: In this circuit i did the same but this does not work.
When SW is ON diode gets forward bias which is against the theory of flyback and vice versa when SW is off.

I know both are correct design but trying to figure out with voltage across transformer winding approach.
Regards,M

Joined Feb 20, 2016
4,476
The dot indicates the start of the winding as far as I understand. It is not +ve or -ve.
If you swap both windings, the circuit should work the same. When the FET is on, in the second circuit, the dot is not +ve but -ve as it is connected to the gnd just the same as the top circuit.

#### ronsimpson

Joined Oct 7, 2019
3,037
Part of what is confusing about a flyback transformer is that it functions very differently than a power line transformer.

With a flyback transformer, power is put on the transformer when the transistor is on. The power is stored and comes back out when the transistor is off.

#### Ian0

Joined Aug 7, 2020
9,826
Hey Guys,
My concept is shaking with transformer dot convection. i want to show you the case study of two circuit.

As per my understanding SW is on voltage develop at dot end is positive and when SW is off voltage polarity reversed.
case 1: SW=ON, Voltage at dot end of L1 is + so secondary side voltage at dot end is + and D2 side is negative hence D2 is reverse bias.
SW=OFF, Voltage at dot end of L1 is - so voltage secondary side at dot end is - hence D2 is Forward bias.
View attachment 313662

Case 2: In this circuit i did the same but this does not work.
When SW is ON diode gets forward bias which is against the theory of flyback and vice versa when SW is off.

View attachment 313664
I know both are correct design but trying to figure out with voltage across transformer winding approach.
Regards,M
A flyback converter does not have a transformer. It has an inductor with two windings.
A transformer does not store energy, but an inductor does.
The dot does not represent positive or negative. It just shows the relative phase of the two windings.
Your first diagram is correct. When the MOSFET is off its drain voltage is higher than the supply voltage, so the dot end of the winding is the more positive.

#### crutschow

Joined Mar 14, 2008
34,455
A flyback converter does not have a transformer. It has an inductor with two windings.
Yes, but the inductor acts like a transformer in that it takes power from the input winding and transfers it to the output winding.

#### mishra87

Joined Jan 17, 2016
1,039
Thanks to call for answer.For me still some physics is missing.
If we want negative output voltage from flyback so that is really confusing how to put diode and transformer start winding.
There should be some physics.

#### ronsimpson

Joined Oct 7, 2019
3,037
There are many ways to get a negative voltage. Here is one that makes positive and negative.

#### crutschow

Joined Mar 14, 2008
34,455
There should be some physics.
What do you mean by "physics" in this discussion?

Joined Feb 20, 2016
4,476
Negative ouput?
Try your original circuit and just hook the C1 V+ to gnd and use the C1 V- as the supply feed.

#### Ian0

Joined Aug 7, 2020
9,826
Yes, but the inductor acts like a transformer in that it takes power from the input winding and transfers it to the output winding.
I would disagree, as no power is output when the input winding in energised, and power is only output when the input winding is effectively disconnected.
In the model of a perfect transformer, there is no mechanism for the storage of energy. The core has infinite permeability

#### ronsimpson

Joined Oct 7, 2019
3,037
In the model of a perfect transformer,
All other transformers have inductance and permeability and resistance and capacitance and a resonant frequency and ......

Does it matter? if you see a transformer as a thing or you see it as two or more coupled inductors.

#### crutschow

Joined Mar 14, 2008
34,455
In the model of a perfect transformer, there is no mechanism for the storage of energy. The core has infinite permeability
Infinite permeability means infinite inductance.
The ideal model of a transformer in LTspice has inductance.
All practical transformers have inductance that can store energy so can be used in a flyback converter circuit.
The name for the magnetics with more than one winding used in a flyback circuit is commonly called a flyback transformer.
Why do you want to make a distinction in name that is not normally used?

#### Ian0

Joined Aug 7, 2020
9,826
Why do you want to make a distinction in name that is not normally used?
because it is fundamental to understanding the difference between the component used in a flyback converter and the component used in a forward converter circuit, which ostensibly look the same; and the different way that the two circuits operate.
In a transformer the output voltage is proportional to the input voltage. In a flyback converter, it isn't.

There are plenty of instances in electronics of things being called by a name which is not perfectly accurate.

#### ronsimpson

Joined Oct 7, 2019
3,037
In a transformer the output voltage is proportional to the input voltage. In a flyback converter, it isn't.
Just to be a 'donkey' about it: In a flyback converter, the voltage across the primary and secondary are proportional to each other.
Example. During the output time, the secondary voltage X turn ratio = primary voltage.
During the input time, the primary voltage X turn ratio = secondary voltage.

I understand that the entire power supply, input to output, is not directly related to turns ration.

#### Ian0

Joined Aug 7, 2020
9,826
Just to be a 'donkey' about it: In a flyback converter, the voltage across the primary and secondary are proportional to each other.
Example. During the output time, the secondary voltage X turn ratio = primary voltage.
During the input time, the primary voltage X turn ratio = secondary voltage.

I understand that the entire power supply, input to output, is not directly related to turns ration.
Agreed. Whilst the switch is on, the voltage on the secondary is proportional to minus the primary voltage, and when the switch if off the primary voltage is the supply voltage plus a voltage proportional to the output voltage, but those are only significant is choosing the voltage rating of the rectifier and snubber. In terms of the entire converter, they don't make the output voltage proportional to the input voltage.

#### crutschow

Joined Mar 14, 2008
34,455
In a transformer the output voltage is proportional to the input voltage. In a flyback converter, it isn't.
Not the converter output, but the output voltage of the flyback transformer is indeed proportional to the turns-ratio times the flyback voltage at the input.