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Active Frame Flyback Circuit for CRT

IP.com Disclosure Number: IPCOM000043422D
Original Publication Date: 1984-Aug-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 3 page(s) / 56K

Publishing Venue

IBM

Related People

Morrish, AJ: AUTHOR

Abstract

To maximize the duty cycle and brightness of the image displayed by a raster-scanned CRT the vertical flyback time of the current in the deflection yoke is required to be decreased. A frame flyback circuit is described which switches in a higher voltage to the amplifier output stage during the flyback period to allow a faster rate of change during the flyback period. This switching is controlled by a pulse-shaping network which limits the instantaneous power dissipated in the amplifier and prevents fast switching transients which could disturb normal operation. The basic circuit with typical waveforms shown in Fig. 1 achieves the function of fast flyback. When the voltage at the output reaches a predetermined level, set by the Zener diode Z voltage, the transistor Q1 turns on, thus turning on the 'pull-up' transistor Q2.

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Active Frame Flyback Circuit for CRT

To maximize the duty cycle and brightness of the image displayed by a raster-scanned CRT the vertical flyback time of the current in the deflection yoke is required to be decreased. A frame flyback circuit is described which switches in a higher voltage to the amplifier output stage during the flyback period to allow a faster rate of change during the flyback period. This switching is controlled by a pulse-shaping network which limits the instantaneous power dissipated in the amplifier and prevents fast switching transients which could disturb normal operation. The basic circuit with typical waveforms shown in Fig. 1 achieves the function of fast flyback. When the voltage at the output reaches a predetermined level, set by the Zener diode Z voltage, the transistor Q1 turns on, thus turning on the 'pull-up' transistor Q2. This reverse biases the supply rail Vs1 diode D2 and switches in the higher voltage Vs2 supply, in this case the line output stage supply rail. Fast fly back times of typically 250 ms can be achieved in a yoke of 8 mH inductance using an 85 V supply rail and a peak to peak current of
1.5 amps. A disadvantage of this basic circuit is that the three output transistors Q2, Q3, Q4 have undesirably high instantaneous power dissipations at the start and end of flyback. It was found that the maximum instantaneous power for each transistor exceeded 40 W, the absolute maximum allowed. It was also found that this basic circuit did not perform satisfactorily as the noise transients caused by the fast switching caused line pairing errors and also character distortion at the top of the screen due to a long and variable settling time, despite excessive decoupling. Another problem experienced was the variability of switching threshold caused by the tolerance of the supply rail and Zener diode; this required a reduction in the Zener voltage to ensure the flyback stage always worked, but in turn this reduced the effective range of amplifier output. The improved circuit shown in Fig. 2 overcomes these problems. The thresh...