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Different articles stated that transformers can be used in AC power systems and not DC power systems as described here. but in flyback converters used in LED applications such as this one. Current supplied to LED's must be in DC form, so how does in such a case transformers work?

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    \$\begingroup\$ Where did you learn that "transformers can't be used in DC power systems"? It is cow manure. The correct statement is "Transformers can't convert DC power the way they convert AC power" - there's no reason they can't be used in DC power systems, so long as you don't try to use them to convert DC power the same way as AC. \$\endgroup\$ Commented Jun 20, 2017 at 1:31
  • \$\begingroup\$ I have added the link to Department of Physics-University of Illinois which has stated that Transformers cannot be used in DC power systems. you can open the link and read what they have written. \$\endgroup\$
    – ahm_zahran
    Commented Jun 20, 2017 at 1:54
  • \$\begingroup\$ Yes that's what the question being answered there says. Do you know the difference between "a DC transformer" and "a transformer used in a DC power system"? My computer uses DC power and yet it contains a transformer because the transformer is in a part of the system that is using AC. Did you read the second question on the page you linked to? \$\endgroup\$ Commented Jun 20, 2017 at 2:12
  • \$\begingroup\$ You can post an answer for how transformers are used in DC circuits. \$\endgroup\$
    – ahm_zahran
    Commented Jun 20, 2017 at 2:16
  • \$\begingroup\$ Note that in a flyback the transformer is operated as a set of coupled inductors. The primary and secondary don't conduct at the same time, core is charged on primary, discharged on secondary, and the cycle repeats. \$\endgroup\$
    – sstobbe
    Commented Jun 20, 2017 at 2:18

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A DC-to-DC converter including a transformer will always have a DC-to-AC conversion stage on the transformer input (high-frequency chopping via MOSFETs) and an AC-to-DC conversion stage (rectification and filtering) on the transformer output.

Even complex AC-to-DC power supplies will have multiple levels of conversion:

Mains AC -> DC via rectification -> high frequency AC -> transformer -> DC via rectification / filtering

The key here is that in both cases, the transformer is not exposed to DC - it is exposed to AC. The assertions made in your reference texts continue to hold true.

A flyback transformer, as others have stated, doesn't act like a normal transformer in that the primary stores energy when the switches are conducting and delivers energy out the secondary when the switches are off. Regardless of this fact, the same truism holds - the transformer will be exposed to AC on the input side and on the output side there will be rectification and filtering to create DC from AC.

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  • \$\begingroup\$ When using transformers in AC/DC rectification does it mean that the load has been flyback isolated? \$\endgroup\$
    – ahm_zahran
    Commented Jun 20, 2017 at 14:57
  • \$\begingroup\$ The transformer provides galvanic isolation, yes. \$\endgroup\$ Commented Jun 20, 2017 at 21:13
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A flyback transformer is a storage coil integrated with a transformer. The main property of a storage coil is the air gap. Inside the magnetic field of that air gap, the energy is stored between switch-on and free-wheeling periods of the controlling circuit.

A normal transformer doesn't have an air gap, because an air gap would require it do be much bigger for the same power rating.

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  • \$\begingroup\$ 'The main property of a storage coil is the air gap' - learning something new here. Can you elaborate? \$\endgroup\$ Commented Jun 23, 2017 at 8:40
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    \$\begingroup\$ Please see electronics.stackexchange.com/questions/77797/… \$\endgroup\$
    – Janka
    Commented Jun 23, 2017 at 8:46
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    \$\begingroup\$ They have an iron powder core. In these ones, the air gap is between the individual powder pieces. You can spot powder cores by their thick coating because of their vulnerability for rusting. \$\endgroup\$
    – Janka
    Commented Jun 23, 2017 at 9:06
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When the switch is on, a dc voltage is applied across the primary, the current increases linearly therefore as v_L = LdI/dt (the primary is just an inductor) and energy is stored in the magnetic field. When the switch turns off, the field collapses and the energy is magnetically coupled to the secondary through the transformers core. Transformers operate on the principle of changing current, this is why you hear that they are not used with DC, since DC doesn't have changing current. However, using a mosfet or IGBT with a high switching frequency, we can produce the 'AC' that transformers need to work.

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  • \$\begingroup\$ espionn does it mean that flyback isolation always have a 50% duty cycle because of the MOSFET switching ? \$\endgroup\$
    – ahm_zahran
    Commented Jun 20, 2017 at 14:57
  • \$\begingroup\$ No. not always 50% duty cycle. \$\endgroup\$ Commented Jun 20, 2017 at 21:13
  • \$\begingroup\$ @adam It means that it is a design consideration. taking in consideration the charge-discharge time of the capacitor connected in parallel with the secondary coil. \$\endgroup\$
    – ahm_zahran
    Commented Jun 21, 2017 at 1:49
  • \$\begingroup\$ "Design consideration" does not always equate to "always has". It depends on the situation. \$\endgroup\$ Commented Jun 21, 2017 at 14:08

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