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Flyback transformer confusion

  1. Jul 16, 2013 #1
    Hey everyone,

    I'm a bit confused about how exactly the transformer works. What I have gathered from wikipedia is that basically what is happening is a low voltage DC is pulsed through the primary winding via a transistor, causing current build-up. When the pulse is through, the current in the primary ceases, and the stored energy in the iron loop induces a current in the feedback coil. The resultant current is at a very high voltage and it decreases until the current no longer is running, at which point the process is repeated. Is this correct? Why does the transformer induce such a large voltage in the feedback coil? Also, what does it have to do with the turn ratios in the coils? How can the proper ratio be figured out? And finally, why are the number of turns for the feedback coil supposed to be less than the primary coil?

    Thank you for any and all explanations.
     
  2. jcsd
  3. Jul 16, 2013 #2
    That wiki article is one not that explicit in useful information, so you might want to expand our search onto other flyback transformer sites.

    The flyback transformer does not function in the same manner as a typical 2 winding transformer where current flows simultanously in both the primary and secondary circuits. Instead one can think of it as an inductor with the identical magnetic field of the inductor linkng the two coils. The primary builds up the stored energy in the magnetic field and the secondary utilizes the stored energy for its particular circuit. The turns ratio will determine the voltage-current scaling between the primary and secondary.

    The current in the primary is ramp fed - ie the current increase from zero to a value building up the magnetic field in the inductor. A diode in the secondary circuit does not allow any current during this phase of operation.

    When the primary current ends, the magnetic field releases its energy through the secondary circuit with a decreasing current also in a ramp like manner.

    Why a ramp? Well that follows from the formula for an inductor v = L di/dt.

    For a television or monitor CRT, the secondary is of more turns than the primary and there will be a voltage increase in the secondary of around 20kV. This voltage is applied between the electron gun and the surface of the screen and is what accelerates the electrons to the screen to give them enough energy to energize the phospherous layer of the screen, this voltage is good for around a 14 inch CRT. Larger monitors or television use even a higher voltage achieved through a capacitive voltage multiplier circuit.

    The flyback operates around 15khz on a typical television. The primary circuit is fed from or feeds, depending upon the circuitry, the movement of the electron stream from left to right on the CRT, so that is why a ramp circuit is important. The televsion has 525 lines per screen and it updates each screen 30 times per second.
    Multiplying 30 refreshes x 525 lines gives about 15kHz for an older style television.

    The secondary operates when the primary circuit has moved the electron stream to the far right and has to move back to the left again. You can well see that the secondary doesn't have much time to perform its function, so the magnetic field has to release its energy quickly. doing so induces the high voltage and subsequently the lower overall current in the higher turns ratio secondary.
     
    Last edited: Jul 16, 2013
  4. Jul 17, 2013 #3

    Averagesupernova

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    Voltage typically isn't that high. Most 27 and larger CRT TVs will have around 30 KV at the second anode button on the tube. This voltage comes from a voltage tripler which is fed by the flyback. So naturally the voltage out of the flyback probably around 10,000 volts peak.
     
  5. Jul 18, 2013 #4
    Good edit. Where in my memory banks I got 20kv from I don't know.
     
  6. Jul 18, 2013 #5

    Averagesupernova

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    Oh there are smaller TVs that have 20kv on the second anode and black and white TVs were less.
     
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