Drive circuit for multiple scan rate horizontal deflection circuit

A drive circuit for a horizontal deflection circuit that is operable at a number of horizontal scan rates includes a pair of transistors coupled in a push-pull arrangement. The voltage supplies for the drive circuit transistors, which provide the forward and reverse drive for the horizontal output transistor, produce voltage levels that are dependent upon the horizontal scanning rate that is selected in order to minimize power dissipation during switching of the horizontal output transistor.

This invention relates to horizontal deflection circuits and in particular 
to horizontal deflection circuits having multiple scan rates. 
In recent years, the quality of the picture displayed by commercial 
television receivers has been greatly improved. Computer monitors or word 
processing display terminals, however, may require even greater resolution 
and sharpness than is currently possible with ordinary receivers. Further 
improvements in picture quality may be obtained through improvements in 
color picture tubes and signal processing, and through modification of 
current signal processing standards. Modification of signal processing 
standards should, however, be compatible with the circuitry in receivers 
already in use and should not make them obsolete. 
A technique which improves the observed picture quality provides a change 
in the horizontal scanning system from the current line interlaced to a 
non-interlaced, or progressive scan system. This would involve an increase 
in the horizontal scanning rate from 15.75 KHz to 31.5 KHz, or 63 KHz, but 
would not require any change in the transmitted signal. Therefore, there 
would be no effect on reproduction of pictures by receivers already in 
use. Benefits of the non-interlaced scanning system for monitors and video 
display terminals would be the almost complete elimination of such picture 
artifacts as large area flicker, inter-line flicker, line breakup and line 
crawl. 
At high scanning or deflection rates, such as 31.5 KHz and 63 KHz, power 
dissipation in the horizontal output transistor is considerably higher 
than at a normal receiver scanning rate of 15.75 KHz. Proper adjustment of 
the horizontal drive waveform is necessary in order to minimize this power 
dissipation. The negative portion of the drive waveform is extremely 
critical, as that represents the interval when stored charge in the output 
transistor is removed as the transistor is being turned off. In a 
conventional television receiver, the negative drive that sweeps out the 
stored charge is provided by a secondary winding of a horizontal driver 
transformer. Leakage inductance of the transformer and external components 
are used to shape the drive waveform as desired. 
At high scanning rates, the leakage inductance of the driver transformer 
may not be low enough to provide the required amount of negative drive to 
the horizontal output transistor in order to remove the stored charge. 
Also, at high scanning rates, it is desirable to remove the stored charge 
at a faster rate in order to maintain a desirable trace interval to 
retrace interval ratio. It is also desirable to provide a means for 
switching between scanning rates to provide additional versatility in the 
use of video display equipment. 
In accordance with an aspect of the present invention, a horizontal 
deflection circuit operable at first and second horizontal scan rates 
comprises a circuit that provides a drive signal having first signal 
levels in response to the selection of the first scan rate and second 
signal levels in response to the selection of the second scan rate. 
Circuitry responds to the drive signal and produces horizontal deflection 
signals.

In the accompanying drawing, 
FIG. 1 is a schematic and block diagram of a horizontal deflection circuit 
incorporating a drive circuit in accordance with an aspect of the present 
invention; 
FIG. 2 illustrates waveforms associated with the drive circuit sown in FIG. 
1; and 
FIG. 3 is a schematic and block diagram of a portion of the circuit sown in 
FIG. 1, illustrating additional features. 
Referring to FIG. 1, there is shown a horizontal drive circuit 10 and a 
horizontal output circuit 11. 
A horizontal rate signal having a selected frequency (based on the selected 
horizontal scan or deflection rate) is applied to drive circuit 10 from a 
horizontal oscillator (not shown). The signal from the horizontal 
oscillator is applied through a coupling capacitor 12 to the bases of 
transistors 13 and 14, connected in a push-pull arrangement. The emitters 
of transistors 13 and 14 are connected together and provide a drive signal 
to the base of the horizontal output transistor 15 of horizontal output 
circuit 11 without the need for a driver transformer. Horizontal output 
circuit 11 also includes a damper diode 16, retrace capacitor 17, 
horizontal deflection coils 20 and S-shaping capacitor 21. Power is 
provided to output circuit 11 from a source of voltage B+ via a winding 22 
of a high voltage transformer (not shown). The drive signals from drive 
circuit 10 switch transitor 15 between conductive and nonconductive states 
at the desired horizontal scan or deflection frequency. Retrace pulses 
appearing across retrace capacitor 17 and winding 22 when transistor 15 is 
switched off are commonly used via the high voltage transformer to 
generate the high voltage that is applied to the anode of a picture tube 
(not shown). 
As previously described, it is important to remove the charge stored in the 
base region of transistor 15 as rapidly as possible when transistor 15 is 
switched off from its saturated conduction state in order to reduce the 
power dissipation of transistor 15 as much as possible. In accordance with 
an aspect of the present invention, drive circuit 10 incorporates multiple 
level voltage sources 23 and 24 which are coupled to the collectors of 
transistors 13 and 14, respectively. Multiple level voltage source 23 
provides a positive voltage which determines the forward drive level for 
transistor 15. The magnitude or level of the voltage provided by voltage 
source 23 is dependent on the horizontal scanning or deflection rate that 
is selected. For example, to maintain a constant trace to retrace interval 
ratio, the inductance of the horizontal deflection coils 20 is decreased 
as the horizontal scanning frequency increases. This causes the forward 
drive current requirement of transistor 15 to increase in order to 
maintain a given deflection current, which in turn requires a higher 
voltage requirement from voltage source 23. This is illustratively shown 
in FIGS. 2A, 2B and 2C, which shows the base or drive current waveforms of 
horizontal output transistor 15 at horizontal scanning rates of 15.75 KHz, 
31.5 KHz and 63 KHz, respectively. The output of voltage source 23, as 
shown in FIG. 1, is controlled by a signal at a terminal 25. This signal 
is representative of the scanning rate selected. 
Multiple level voltage source 24 provides a negative voltage level to the 
collector of transistor 14 which determines the negative drive level for 
transistor 15. As the horizontal scanning frequency is increased, the 
negative drive current requirement, and hence the magnitude of the voltage 
level of the voltage source 24 is increased, as shown illustratively by 
the base current waveforms of FIGS. 2A, 2B and 2C. The voltage level 
provided by voltage source 24 as shown, is controlled by a signal at 
terminal 26. This signal is also representative of the selected horizontal 
scan rate. 
It is possible to maintain a constant retrace interval by maintaining 
constant inductance deflection coils at different frequencies. Although 
the horizontal output circuit voltage supply requirements may change, the 
forward drive level for the horizontal output transistor would remain 
constant. A multiple level supply, such as voltage source 23, would not be 
necessary in this case. However, the negative drive requirement would 
change as the scanning frequency changed. Therefore, a multiple level 
supply, such as voltage source 24, would be needed. 
The operation of a multiple scan rate deflection circuit is described in 
detail in U.S. patent application Ser. No. 497,950, filed May 25, 1983, in 
the names of W. F. Wedam, et al., entitled "POWER SUPPLY AND DEFLECTION 
CIRCUIT PROVIDING MULTIPLE SCAN RATES", and is hereby incorporated by 
reference. Patent application Ser. No. 497,950 describes an illustrative 
arrangement for producing a signal representative of the horizontal 
scanning rate selected, which could be applied, for example, to terminals 
25 and 26 of FIG. 1. Patent application Ser. No. 497,950 also describes 
arrangements for changing the S-shaping capacitance and the horizontal 
output circuit supply voltage when the horizontal scanning frequency is 
changed. Multiple level voltage sources 23 and 24 could include, for 
example, some form of switching circuitry which would control the 
magnitude of the voltage levels produced by voltage sources 23 and 24. 
FIG. 3 shows a drive circuit similar to that shown in FIG. 1 which includes 
components that further aid in shaping of the drive waveform for the 
horizontal output transistor. 
Inductor 27 provides shaping of the collector current for transistor 13. 
Diode 30 returns some power to the voltage source during the negative 
drive interval. Shaping of the base current waveform for the horizontal 
output transistor is also provided by inductors 31 and 32. 
The source of voltage for transistor 13, which provides the forward drive 
to horizontal output transistor 15 is shown as separate voltage sources 
33, 34 and 35, which illustratively provide voltage levels for operation 
at horizontal scanning rates of 15.75 KHz, 31.5 KHz and 63 KHz, 
respectively. Additional voltage sources for additional scanning 
frequencies are of course possible. A switching circuit 36, schematically 
shown, selects the appropriate voltage source based on the selected 
scanning rate representative signal at terminal 25. 
The source of voltage for transistor 14, which provides negative or reverse 
drive for transistor 15, is shown as individual voltage sources 37, 40 and 
41 which provide voltage levels for operation at horizontal scanning rates 
of 15.75 KHz, 31.5 KHz and 63 KHz, respectively. A switching circuit 42, 
similar to switching circuit 36, selects the proper voltage sources in 
response to the signal at terminal 26.