Current regulating circuit for magnetic deflection systems

The current in the deflection coil of a video display system is converted into a DC voltage and applied to a peak detector. Peak values of the DC voltages as stored by the peak detector are amplified and then compared with a reference voltage at the input of a comparator amplifier. The comparator amplifier provides an output as a base drive to a regulating transistor that has an emitter-collector junction in series with the deflection coil and an unregulated source of DC power.

BACKGROUND OF THE INVENTION 
This invention relates to a circuit for regulating the current through an 
inductor, and more particularly to a circuit for regulating the current in 
the deflection coil of an image display tube to maintain a fixed image 
width. 
In an image reproduction system having a cathode ray tube the image width 
is controlled by a horizontal deflection coil functioning as an inductor 
in a circuit that produces, through the coil, a sawtooth waveform of 
current. This sawtooth waveform is centered plus and minus of zero to 
maintain the image centered on the cathode ray tube. 
The classic circuit for generating a current having a sawtooth waveform 
includes, along with the inductor coil, an unregulated voltage source and 
a charging capacitor in series with the inductor. A damping diode is 
connected in parallel with capacitor and, in addition, a switching 
transistor is connected in series with the inductor and responsive to a 
periodically applied signal that establishes the frequency of the 
waveform. 
For this classic circuit, the mathematical relationship for the current, 
i.sub.p, in the inductor coil is as follows: 
EQU i.sub.p = V.sub.c t.sub.trace /2L, (1) 
where 
V.sub.c is the voltage of the direct current source, 
T.sub.trace is the trace time, and 
L is the inductance of the deflection coil. 
From this mathematical expression, it will be clear that any variation due 
to changes in the t.sub.trace, V.sub.c or L will effect the deflection 
coil currents, and this in turn varies the size of the image display on 
the cathode ray tube. Specifically, the amplitude of the sawtooth 
waveform, and thus the deflection coil current, varies with each of the 
parameters of equation (1). Since the inductance of the deflection coil, 
L, can be maintained fairly constant, the waveform primarily varies with 
the level of the applied voltage, and the frequency of the sawtooth 
waveform, that is, t.sub.trace. 
Heretofore, whenever a change occurred in the voltage of the direct current 
source as applied to the deflection coil or the frequency of the waveform 
changed, a manual adjustment was required to return the image display to 
its original width. Considerable inconvenience results if everytime a 
change occurs someone must make a manual adjustment to correct the image 
width. 
Various attempts have been made to eliminate the requirement for a manual 
adjustment when a change in image width occurred due to a change in 
current through the deflection coil. In one such prior art system, the 
high voltage generating circuit of the cathode ray tube is connected with 
a shunt regulator circuit in such a manner that the sum of the current 
flowing to the high voltage load circuit of the cathode ray tube and the 
current flowing through the shunt regulator circuit is maintained at a 
constant value. As a result, the horizontal output stage is operated 
continuously under a maximum load condition thereby adversely affecting 
the reliability of the display. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a current regulating circuit is 
provided that stabilizes the current through an inductor by regulating the 
voltage applied thereto. Further in accordance with the present invention, 
changes in the sweep current waveform at the deflection coil of a video 
display are minimized by regulating the applied voltage level. The present 
invention provides a circuit for regulating inductor current against 
changes in frequency of a switching signal and changes in voltage level of 
a power source. 
In accordance with the present invention, a circuit for regulating the 
current through an inductor includes a sensor responsive to the current in 
the inductor and generating a DC voltage varying therewith. This DC 
voltage is applied to a peak detector that provides peak values of the DC 
voltage to a voltage comparator that also is connected to a reference 
voltage source. The voltage comparator generates a control voltage varying 
with the difference between the peak values of the DC voltage and the 
reference voltage. A control regulator connected between a power source 
and the inductor responds to the control voltage and regulates the current 
in the inductor in accordance with variations in the control voltage. 
A more complete understanding of the invention and its advantages will be 
apparent from the specification and claims and from the accompanying 
drawings illustrative of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, there is shown a circuit for magnetically deflecting 
the scanning beam of a cathode ray tube wherein an inductor 10 functions 
as the deflection coil and is in series with a charging capacitor 12 
forming a tank circuit connected between a voltage supply terminal 14 and 
ground. In parallel with the charging capacitor 12 is a damping diode 16 
with both the charging capacitor and the damping diode in parallel with a 
scanning switch 18. Typically, the scanning switch 18 is a transistor 
controlled by scanning pulses applied to a control terminal 20, and it is 
the repetition rate of these pulses that establishes the trace time, 
t.sub.trace, of equation (1). 
The system thus far described supplies a current having a sawtooth waveform 
22, illustrated in FIG. 2, across the horizontal deflection coil or 
inductor 10 of the yoke assembly (not shown) comprising parts of an image 
reproduction device. The current in the deflection coil is a maximum 
(i.sub.p) on the termination of a line scan and flows, at a rapidly 
decreasing rate, through the charging capacitor 12 at the initiation of a 
line scan return. 
A signal applied to the terminal 20 turns on the scanning switch 18 for the 
duration of a scanning pulse. This closes a circuit connecting the 
deflection coil 10 to a DC voltage source at the terminal 14 and energy is 
transferred therefrom to the deflection coil thereby causing current to 
flow through the coil in the direction and reaching a maximum (ip) 
indicated by the positive going portion of the waveform 22 of FIG. 2. At 
this instant switch 18 is opened. With the opening of the scanning switch 
18, the energy stored in the coil is transferred to the charging capacitor 
12 causing current to again flow through the deflection coil in the 
direction indicated by the negative going portion of the waveform 22. The 
peak, or flyback, voltage produced on terminal 24 is given by: 
EQU V.sub.f = i.sub.p .sqroot.L/C (2) 
in accordance with this invention, the current producing the sawtooth 
waveform 22, which is utilized in the deflection 10 for horizontal 
scanning of an electron beam in a cathode ray tube, displays a constant 
peak amplitude independent of changes in the variables of equation (1). By 
maintaining a constant peak amplitude or magnitude for the deflection coil 
current the horizontal dimension of the image in a cathode ray tube is 
held fixed. 
To maintain a constant peak-to-peak amplitude for the current of waveform 
22, the voltage level applied to the terminal 14 is varied in accordance 
with the retrace voltage sensed at a terminal 24 connected to the 
deflection coil 10. An RC network having a small time constant and 
including a coupling capacitor 26 and a resistor 28 converts the current 
of the waveform 22 into a retrace voltage across the resistor 28. This 
retrace voltage is applied to the input of a buffer amplifier 30 that 
provides isolation of the regulating circuit of the present invention from 
the image scan circuit. 
This output voltage from the amplifier 30 is applied to a peak detector 
consisting of a diode 32 and a capacitor 34 connected to ground. 
The peak detector receives the output of the buffer amplifier 30, which is 
a voltage varying plus or minus of zero and related to the current through 
the deflection coil 10. As the name implies, the peak detector rectifies 
the output of the buffer amplifier 30 by means of the diode 32 and 
converts it into essentially a DC voltage for charging the capacitor 34. 
The charge on the capacitor 34 is thus a stored voltage representing the 
peak values of the flyback voltage and therefore proportional to the peak 
current in accordance with equation (2). These peak voltage values are 
applied to an input of a buffer amplifier 36 that produces an output 
through a resistor 38 for comparison with a reference voltage. 
To generate the reference voltage, a resistor 40 is connected in series 
with a resistor 42 between the terminal 14 and a reference supply 
connected to a terminal 44. The reference supply generates an output 
voltage in any conventional manner, such as by a Zener diode network 
connected to an unregulated supply. 
With the interconnection of the resistors 40 and 42 as shown, the voltage 
at a summing junction 46 varies directly with the difference between the 
voltages at the terminals 14 and 44. This voltage is compared with the 
output of the amplifier 36 through the resistor 38. The resulting voltage 
at the summing junction 46 is then applied to the noninverting input 
terminal of a control amplifier 48. The amplifier 48 generates a control 
voltage to a regulating transistor 50 having an emitter-collector junction 
connected between the terminal 14 and an unregulated DC voltage supply at 
a terminal 52. 
Considering equation (1) above, the deflection coil current, i.sub.p, 
varies directly with the voltage V.sub.c applied to the terminal 14. 
Assuming that the inductance L of the deflection coil 10 remains constant, 
then from equation (1) any variation in the frequency, that is, the trace 
time t.sub.trace, will change the deflection coil current in a direct 
relationship. To maintain a fixed amplitude for the deflection coil 
current, and thus maintain the constant horizontal deflection width for a 
cathode ray tube display, the voltage, V.sub.c, applied to the terminal 14 
must vary inversely with the trace time. 
From an analysis of the regulating circuit of FIG. 1 connected to terminals 
14 and 24, a voltage developed across the charging capacitor 12 is coupled 
through the capacitor 26 to produce a voltage across the resistor 28 which 
is applied to the input of the buffer amplifier 30. At the output of the 
buffer amplifier 30 the peak detector, comprising the diode 32 and the 
capacitor 34, rectifies the waveform of FIG. 2 into essentially a DC 
voltage that is stored on the capacitor 34. This voltage as stored in the 
capacitor 34 is the peak value of the essentially DC voltage and is input 
to a buffer amplifier 36 that generates a voltage at the summing junction 
46 for comparison with a reference voltage as established by resistors 40 
and 42. This reference voltage is set at a value to produce a desired 
deflection coil current, and any variation thereof at the output of the 
amplifier 36 appears as an error signal to the input of the control 
amplifier 48. A control voltage now generated at the ouput of the 
amplifier 48 controls the conduction level of the regulating transistor 50 
to adjust the voltage, V.sub.c, see equation (1), applied to the terminal 
14. Thus, any change in the peak current through the deflection coil 10 
will appear as a change in the voltage stored in the capacitor 34 which 
produces an error signal at the summing junction 46 to control conduction 
of the regulating transistor 50. The feedback circuit of FIG. 1 thus 
regulates the deflection coil current by varying the applied voltage at 
the terminal 14. 
Referring to FIG. 3, there is shown an alternate embodiment of the 
invention wherein the deflection coil 10 is in series with the charging 
capacitor 12 and also in series with a sensing resistor 54. As in the 
embodiment of the invention shown in FIG. 1, the switch 18 responds to a 
scanning pulse on a terminal 20 to control the current build up in the 
coil 10. The damping diode 16 is in parallel with the charging capacitor 
12 to control the negative excursion of the deflection current below the 
zero axis line, as explained previously. 
While the circuit of FIG. 1 responds to the retrace voltage to stabilize 
the deflection coil current against changes in input frequency, the 
embodiment of FIG. 3 stabilizes the actual deflection coil current by 
sensing this current through the resistor 54. 
At the interconnection of the resistor 54 and the deflection coil 10, an RC 
network, comprising a capacitor 56 and a resistor 58, responds to the 
voltage developed across the resistor 54 to provide a voltage across the 
resistor 58 as an input to a buffer amplifier 60, again functioning as an 
isolation amplifier. Because the circuit of FIG. 3 responds continuously 
to the voltage developed across the resistor 54, the time constant of the 
RC network of capacitor 56 and resistor 58 is relatively large. This 
measurement of deflection coil current minimizes any change in the peak 
deflection coil current as given in equation (1). 
An output from the buffer amplifier 60 is applied to a peak detector 
comprising a diode 62 and a storage capacitor 64. Again the voltage stored 
across the capacitor 64 is essentially a DC voltage having a value 
directly related to the peak current in the deflection coil 10. This 
voltage is applied through a buffer amplifier 66 to a summing junction 68 
through a resistor 70. 
At the summing junction 68 there is developed a reference voltage related 
to the desired peak value of the deflection coil current. This reference 
voltage is established by a resistor 72 in series with a resistor 74, the 
latter connected to a conventional reference voltage supply. Any error 
signal generated at the summing junction 68 is applied to the input of a 
control amplifier 76 that produces a control voltage to the base electrode 
of a regulating transistor 78 having an emitter-collector junction in 
series with the resistor 54. Again, the collector junction of the 
transistor 78 is connected to an unregulated voltage supply. 
In operation, the circuit of FIG. 3 operates essentially as that of FIG. 1. 
Actual current in the deflecton coil 10 is sensed and any variation 
thereof from a desired peak value changes the control voltage to the 
regulating transistor 78 to return the deflection coil current peak value 
to the desired level. Thus, the horizontal width of an image on a cathode 
ray tube is maintained constant by regulating the deflection coil current. 
While several embodiments of the invention, together with modifications 
thereof, have been described in detail herein and shown in the 
accompanying drawings, it will be evident that various further 
modifications are possible without departing from the scope of the 
invention.