Quote:
Originally Posted by FERNSEH
It has to borne in mind that it is the second cycle of the flyback oscillation that has to be suppressed, damp or suppress the first cycle and the retrace energy will be lost.
DFWB.
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A bit of a simplification, its more about 1/4 and half cycles:
The H output stage epitomizes the two key aspects of everything we love about electronics, electric and magnetic fields and the energy exchange between those in resonant circuits.
At the end of the scan, on the right hand side of the raster, there is maximum magnetic field energy stored in the yoke and line output transformer. The device sustaining that current (the H output valve or the transistor) is driven into abrupt cut-off. The is leaves a resonant circuit of the inductance of the yoke, the H output transformer, their winding self capacitances and any tuning capacitance (often added in transistor circuits) to form a resonant circuit at around 60 to 80 kHz.
As the magnetic field energy collapses it is transferred to the electric field energy of the capacitance as it charges up in a sinusoidal manner. One quarter of a cycle into this oscillation, all of the magnetic field energy of the total inductance is transferred to the electric field energy of the capacitance. At this moment the flyback voltage pulse has peaked, and is rectified to develop the EHT. The voltage on the collector of the output transistor (or the plate of the output valve ) has peaked and there is no magnetic field energy at that moment, because, the yoke current is zero (of course it should be, as its half way into the flyback, the beam is obviously in the screen center line).
Then, on the next quarter cycle the voltage starts to drop, energy is returned from the electric field of the capacitance back to the magnetic field of the transformer and yoke. When the voltage hits zero, the energy now is all transferred back to the magnetic field of the yoke & output transformer. This is the start of a new scanning line, on the left side of the raster.
Of course, these oscillations would continue, and the plate or collector voltage then swing negative (after all the output device is still switched off) However, this is where the damper(efficiency) diode comes into conduction. This immediately damps the oscillations and causes the stored magnetic energy to 1) be returned to the power supply and 2) result in a near linear decay in yoke current to scan the left side of the raster. As the damped current decays away toward the center of the raster, the output device is turned on again.
So the flyback voltage spike you see represents a 1/2 cycle of the oscillation in the range of 60 to 80 kHz.
It was this cycle that inspired Blumlein to realize that H deflection circuits really only needed to be energy control and management circuits. If the bulk of the energy used to scan the beam to the right side of the raster could be recovered to scan the left side, all that would be required would be enough injection of energy to overcome losses. This was quite unlike earlier designs where the oscillations at flyback were merely damped away with resistors and the opportunity to recover a high peak voltage from a high Q resonant circuit, with an overwind on the transformer was lost.