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Some doubts in flyback converters

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SherlockBenedict

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What do you mean by isolating the ground? How will it protect the circuit? I couldn't understand that concept for a long time.

I am quite confused with the working of flyback converter. In the on state I understand that current flows through the inductor in the primary coil but no current should flow through the inductor of the secondary coil because the diode is reversed biased, right? Okay now when you open the switch then no current flows through the inductor in the primary coil and this means that infinite or huge voltage should appear across the primary coil and also initially zero current should flow through the inductor in the secondary coil (at t=0) but a huge voltage would also appear across the secondary coil at the same time. Somewhere I am wrong so please help me in understanding the concept behind flyback converter.

Also here's one sentence that I came across while reading about flyback converters in wikipedia-

"Also there is a need for a controlling rail which has to be loaded before load is applied to the uncontrolled rails, this is to allow the PWM to open up and supply enough energy to the transformer."

What do you mean by "rail" and "controlling rail"?

Thanks in advance.

- - - Updated - - -

I have one more important doubt. Transformer doesn't couple DC but how could this couple for flyback converter? Confusing :( please help me.

Waiting for your reply people
 

Dear Sherlock
Hi
It is pretty simple . at first , a clarifying about isolated ground : we have a transformer in series with our switch ( mosfet or IGBT or perhaps BJT ) , thus if you keep one of the wires of out put in your hands , there won't be any danger . but if you do the same action with your hands in primary , your hairs will become stubby !! and the reason is , the ground is connected to the earth . but after your transformer the ground doesn't connect to the earth ! so your legs can't create a loop up to ground !
And about windings of transformer , the vector of windings are different together , thus there will be 180 degree phase shift . hence , when the switch is on , the secondary has a minus voltage , and the diode in the secondary can't conduct .
so the energy will store in the core , during the time that key is on . so when key is off , there will be , an instantaneous pulse , due to the lenz law , that will keep itself , until the core be empty from stored energy . so the polarity of voltage in primary will be minus and in secondary will be positive , hence at this time the out put has current . you understand ?
Best Wishes
Goldsmith
 

Hello goldsmith,

Can you explain again, even if possible with a sketch, the conceptthat you tried to explain back then of "isolating ground"?

Best regards,
Charles
 

Dear Charles
Hi
Of course . but i should write my explanations on piece of paper . so , i think you should wait for 1/2 hour ( maximum ) until i complete writing .
Best Wishes
Goldsmith

- - - Updated - - -

Hi again , i came back !
See below , please :
View attachment Tranformer for charles.pdf
Good luck
Goldsmith
 

Dear Charles
Hi
Of course . but i should write my explanations on piece of paper . so , i think you should wait for 1/2 hour ( maximum ) until i complete writing .
Best Wishes
Goldsmith

- - - Updated - - -

Hi again , i came back !
See below , please :
View attachment 75314
Good luck
Goldsmith

So you basically use the property of the inductor to couple the energy from one circuit to another through magnetic flux but it doesn't couple electrically, right?
 

Yes , it is the definition of coupling . the circuit is electromagnetically !
 

Thank you so much goldsmith. You were very helpful with that explanation.

So that means if we touch in the secondary of a transformer, that is not connected to ground we'll not be in danger?
 

Hi again
Of course , there won't be any problem if you touch one of the wires of transformer ( one of the out put wires ) because there isn't any connection to the ground .
Best Wishes
Goldsmith
 

Even for a significante value of voltage at the terminals? Imagine 500V?
 

Yes ! suppose that you have the voltages nearby , 30KV . if you touch one of the heads of secondary , there won't be any problem . but if you touch both of them the voltage across your hands will be 30KV ! then you'll change to the powder !
 

Well, if you grab the secondary, with those 30kV, the path will close through your body because you have your feets on the ground/street/floor, right? That is not a problem?
 

No no no ! it is not because your legs are on ground ! recal , if you touch just one of the wires of secondary there won't be any problem . but both of them is pretty dangerous for 30 kv ! . consider the the voltage across the secondary is 50 volts , it isn't dangerous for your body if you touch both of the wires . because it's voltage is low . but if the voltage is 30 kv , if you touch both of the out put wires , it will be dangerous , because 30 kv is across your hands . thus current can follow through your body . from top head of out put to the other head of it . if you had not this transformer , it was enough that you touch , phase wire , just , then you was not alive ! but the advantage of transformer is isolation between ground and his imaginary ground .
 

Okay clear me this doubt. We don't have any trasformer. On one side we have a supply of about 30 kV whose one end is connected to earth and whose other end is extended to you and when you touch this end will you get shock? Do you mean the circuit gets closed through the earth no matter wherever you are?
 

Hi charles
touch this end will you get shock
of course , but i think you'll become die if you touch this voltage !
Do you mean the circuit gets closed through the earth no matter wherever you are?
Yes , quite right .
But transformer is isolated from ground hence if you touch one wire , there won't be any problem . but both of them is dangerous . You understand ?
Best Wishes
Goldsmith
 

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