WO1998028910A1

WO1998028910A1 - Constant current horizontal scan generator

Info

Publication number: WO1998028910A1
Application number: PCT/US1997/022781
Authority: WO
Inventors: William Vincent Fitzgerald
Original assignee: Thomson Consumer Electronics, Inc.
Priority date: 1996-12-20
Filing date: 1997-12-11
Publication date: 1998-07-02
Also published as: AU5697698A

Abstract

A power supply circuit (101) provides a variable voltage (VS) to a scan generator circuit (102) for generating a horizontal deflection current (IH) for a horizontal deflection coil (103) of a cathode-ray tube. The circuit (101) senses the peak-to-peak amplitude of the horizontal deflection current (IH) and adjusts the magnitude of the voltage (VS) applied to the scan generator circuit (102) to maintain the horizontal deflection current (IH) at a substantially constant peak-to-peak amplitude.

Description

CONSTANTCURRENTHORIZONTALSCANGENERATOR

BACKGROUND

The present invention relates generally to video display apparatus and, in particular, to power supply circuits for scan circuits that provide horizontal deflection currents for driving th e horizontal deflection yoke coil of a cathode ray tube of the video display apparatus.

Video display apparatus, for example television receivers, computer monitors, video display terminals, and the like, typically include a cathode ray tube (CRT) which displays video images b y deflecting an electron beam with a yoke having horizontal an d vertical deflection coils. The electron beam is swept across th e CRT face or screen to display horizontal scan lines on the face of the CRT using a horizontal deflection current, typically having a sawtooth waveform, applied to the horizontal deflection yoke coil of the CRT.

Video display apparatus typically include the CRT itself an d its deflection yoke as well as additional elements such as a scan generator circuit for generating the horizontal deflection current, and a scan power supply circuit that provides a scan power supply voltage to the scan generator circuit. Such scan generator circuits typically generate the sawtooth-shaped horizontal deflection current, where the sawtooth waveforms have a peak-to-peak current amplitude that is directly proportional to the magnitude of the scan power supply voltage. The peak-to-peak current amplitude of the horizontal deflection current is maintained at a substantially constant, predetermined amount for a given CRT to obtain the proper length of the displayed scan lines. The scan lines are displayed at a given scan frequency. The scan frequency utilized depends on a variety of factors, such a s the vertical rate of displaying a new field of scan lines and the number of scan lines per field. For example, in the NTSC system, the " IH" scan frequency of approximately 15,734 Hz may b e utilized in a system having 262.5 scan lines per field, with tw o fields per frame and approximately 30 frames per second. The "2H" scan frequency of approximately 31 ,468 Hz may also b e utilized, for 525 scan lines per field, with one field per frame, a t approximately 59.94 frames per second.

One problem of such video display apparatus that can utilize several horizontal scanning frequencies is that changes in the scan frequency cause the peak-to-peak amplitude of the horizontal deflection current to change. For example, if the scan frequency increases, the peak-to-peak magnitude of the deflection c urrent decreases, causing scan lines that are too short to be displayed on the CRT face.

In one approach designed to address this problem, a separate, fixed scan power supply voltage is used for each scan frequency. Thus, for a given CRT and scan frequency, a fixed scan power supply is utilized that will cause the scan generator circuit to generate a horizontal deflection current having the proper peak-to-peak amplitude. However, providing such multiple scan power supplies entails additional cost and complexity. Additionally, in this approach, the video display apparatus is limited to a finite number of discrete scan frequencies. In another approach, a circuit is provided that monitors the retrace voltage pulse that appears across the inductance of th e horizontal deflection coil of the CRT yoke, and is regulated to keep the retrace voltage pulse substantially constant. In both thi s approach and the fixed scan power supply approach discussed above, the peak-to-peak deflection current magnitude m a y fluctuate due to changes in circuit parameters that may occur o ver time, such as changes in the inductance of the horizontal deflection coil of the CRT yoke.

SUMMARY A power supply circuit provides a variable voltage to a scan generator circuit for generating a horizontal deflection current for a horizontal deflection coil of a cathode-ray tube. The circuit senses the peak-to-peak amplitude of the horizontal deflection current and adjusts the magnitude of the voltage applied to the scan generator circuit to maintain the horizontal deflection current at a substantially constant peak-to-peak amplitude. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of inventive arrangements described herein.

Fig. 2 is a schematic diagram of the inventive arrangements shown in Fig. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to Fig. 1, there is shown a block diagram of a video display apparatus 100 in accordance with the pres ent invention. Video display apparatus 100 includes scan power supply circuit 101 , scan generator circuit 102, and horizontal deflection coil 103 of the CRT yoke (not shown). The scan power supply circuit 101 monitors the horizontal deflection current IH applied to the horizontal deflection coil 103 of the CRT yoke b y scan generator circuit 102, and varies the magnitude of the scan power supply voltage Vs delivered to scan generator circuit 1 02 so that the horizontal deflection current IH maintains a constant peak-to-peak magnitude, within a given range of scan frequencies.

The peak-to-peak current magnitude of current IH generated b y scan generator circuit 102 is proportional to the magnitude of th e scan power supply voltage Vs applied by scan power supply circuit 101.

Referring now to Fig. 2, there is shown a schematic diagram of the scan power supply circuit 101 of video display apparatus

100 of Fig. 1. Terminals 221 of scan power supply circuit 101 are coupled to horizontal deflection coil 103 so that horizontal deflection current IH is passed through the secondary winding of current transformer 201. Terminal 222 is coupled to scan generator circuit 102 to supply scan generator circuit 102 with scan power supply voltage Vs. Current transformer 201 is coupled at its secondary winding 202 to terminals 221 and at i ts primary winding 203 to ground and to resistor Rl and capacitor

Cl . The other terminal of resistor Rl is coupled to ground, and th e other terminal of capacitor Cl is coupled to the cathode of diode

D6 and anode of diode D5. The anode of diode D6 is coupled to ground, and the cathode of diode D5 is coupled through resistor R2 to potentiometer PI and to ground through capacitor C2. The other terminal of potentiometer PI is coupled to ground through resistor R3. The wiper terminal of potentiometer PI is coupled to one terminal of resistor R5 and to the other terminal of resistor R5 through series-connected resistor R4 and capacitor C3. The o ther terminal of resistor R5 is coupled to. ground through resistor R6 and to the anode of diode Dl. The cathode of diode Dl is coupled to ground through resistor R7, to a control input terminal 232 of a variable voltage power supply portion 230, and to the junction of series-connected resistors R33 and R32 through Zener diode Z2. Power supply voltage V4 is coupled to input terminal 231 of variable voltage power supply 230. Output terminal 222, which provides scan supply voltage Vs, is coupled to ground through th e series-coupled resistors R32 and R33.

Within variable power supply 230, control input terminal 232 is coupled to the base terminal of transistor Ql . The emi tter terminal of transistor Ql is coupled through resistor R8 to th e cathode of Zener diode Zl and to a terminal of resistor R9, th e other terminal of which is coupled to power supply voltage V2. The anode of Zener diode Zl is coupled to ground. The collector terminal of transistor Ql is coupled through resistor R10 to th e base terminal of transistor Q4 and, through resistor R19, to a terminal of resistor Rl l and to power supply voltage V3. The emitter terminal of transistor Q4 is coupled to the other terminal of resistor Rl l, and the collector terminal of transistor Q4 is coupled to ground through capacitor C4 and, through resistor R20, to the base terminal of transistor Q2, to ground through capacitor C5, to the collector terminal of transistor Q5, and to a terminal of resistor R30. The other terminal of resistor R30 is coupled to ground through resistor R31 and to the source terminal S of transistor Q3. The emitter terminal of transistor Q2 is coupled to ground and the collector terminal of transistor Q2 is coupled to th e base terminal of transistor Q5 and to a terminal of resistor R22. The other terminal of resistor R22 is coupled through resistor R21 to the emitter terminal of transistor Q5, to capacitor C6, to a terminal of resistor R23, and to the gate terminal G of transistor Q3 through resistor R25. The other terminal of resistor R23 is coupled at power input terminal 231 to power supply voltage V₄ and to a terminal of winding 212 of transformer 210. The o ther terminal of winding 212 is coupled to the drain terminal D of transistor Q3 and, through capacitor C7, to ground. The o ther terminal of capacitor C6 is coupled through resistor R24 to a terminal of winding 21 1 of transformer 210, the other terminal of which is coupled to ground. A terminal of winding 213 of transformer 210 is coupled through a parallel-connection of diode D2 and capacitor C9 to ground, and the other terminal of winding 213 is coupled to terminal 222, which provides scan supply voltage Vs. Terminal 222 is coupled to ground through resistor R34 and capacitor C8.

In a presently preferred embodiment, the components and parameters of scan power supply circuit 101 have the following values: Rl = 47 Ω (2W); R2 = 100 Ω; R3 = 6.8 KΩ; R4 = 1 KΩ; R5 = 4.7 KΩ; R6 = 10 KΩ; R7 = 100 KΩ; R8 = 680 Ω; R9 = 1 KΩ; R10 = 2 KΩ; Rl l = 680 Ω; R19 = 4.7 KΩ (3W); R20 = 680 Ω; R21 = 470 Ω; R22 = 100 Ω; R23 = 680 KΩ; R24 = 68 Ω (3W); R25 = 15 Ω (3W); R30 = 68 Ω; R31 = 0.18 Ω (3W); R32 = 47 KΩ (3W); R33 = 12 KΩ (1W); R34 = 10 KΩ (3W). Cl = 0.1 μF; C2 = 0.1 μF; C3 = 1 μF; C4 = 0.15 μF; (100 V); C5 = 0.022 μF (100 N); C6 = 1 μF ( 100 V); C7 = 3300 pF (600 V); C8 = 2.06 μF (250 V); C9 = 470 pF (500 V). The secondary winding 202 of transformer 201 has 1 turn and th e primary winding 203 has 40 turns. Potentiometer PI has a resistance of 3 KΩ. Zener diode Zl has a reverse breakdown voltage of 4.7 V; Zener diode Z2 has a reverse breakdown voltage of 47 V. Voltage V₂ = 16 V; V₃ = 16 V; and V₄ = 140 V. Transistor Q3 is a power MOSFET, preferably of type IRF830.

Scan power supply circuit 101 operates as follows. The horizontal deflection current IH flowing through the secondary winding 202 of transformer 201 causes current I2 to flow through the primary winding 203 of transformer 201. Since the primary to secondary turns ratio of transformer 201 is 40: 1 , the magnitude of current I2 is approximately 1/40 of the magnitude of current IH - Current I2 flowing through resistor Rl causes a voltage VR I that is proportional to current IH to be developed across resistor

Rl . The network of capacitors Cl and C2, diodes D5 and D6, a n d resistor R2 provides a means to convert the peak-to-peak voltage VR I into a rectified, substantially-DC voltage V 2 across capacitor C2. This causes a substantially-DC voltage V j to be generated a t the cathode of diode Dl , which is proportional in magnitude to the magnitude of horizontal deflection current IH- Circuit 101 thu s comprises a means for sensing the peak-to-peak amplitude of horizontal deflection current IH, and also for rectifying the peak- to-peak current and for providing a substantially-DC voltage Vj, the magnitude of which represents the peak-to-peak amplitude of horizontal deflection current IH .

Voltage V i is applied as a control input to the control input terminal 232 of variable voltage power supply 230. In th e presently preferred embodiment, variable voltage power supply 230 is a self-oscillating zero-voltage switching power supply, which is configured to provide an output voltage Vs at terminal 222 having a magnitude that varies inversely with the magnitude of the input voltage Vj . The use of a self-oscillating zero- voltage switching topology for variable voltage power supply 230 is merely exemplary. It should be apparent to those skilled in th e art that other switching power supply topologies may be used for the variable voltage power supply 230 in the context of th e inventive arrangements described herein. The network formed by Zener diode Z2 and resistors R32 and R33 serves to limit the maximum magnitude of the voltage Vs, for example in the case where there is no horizontal deflection current IH applied to terminal 221 and the magnitude of voltage V i is consequently zero volts. Transistor Q3 of power supply 230 is repeatedly switched o n and off to cycle current through coil 212. At the beginning of each switching cycle, this current ramps up and flows through resistor R31. When the current reaches a high enough magnitude, th e base voltage of transistor Q2 becomes high enough to turn o n transistor Q2, which causes transistor Q5 to turn on, thereby removing the drive from the gate terminal G of transistor Q3 an d causing the switching cycle of power supply 230 to start o ver again. To control the magnitude of output voltage Vs, the magnitude of voltage V i is compared with reference voltage VREF appearing at the cathode of Zener diode Zl. If the magnitude of voltage Vi becomes large enough, transistor Q4 begins to conduct current, thereby providing a voltage Vc4 across capacitor C4. The voltage Vc4 s divided through resistors R20 and R30. For a higher magnitude of voltage Vc4, which corresponds to a higher magnitude of horizontal deflection current IH, a relatively smaller amount of voltage across resistor R31 is required to cause Q2 to turn on. This causes transistor Q3 to turn off earlier than it would have in the switching cycle, thereby lowering the average current through winding 212 and consequently lowering the magnitude of the output scan voltage Vs. This, in turn, tends to reduce the peak-to-peak amplitude of horizontal deflection current IH generated by scan generator circuit 102, since the peak-to-peak current amplitude of current IH generated by scan generator circuit 102 is proportional to the magnitude of the scan power supply voltage Vs .

Thus it can be seen that, as the scan power supply circuit 101 senses a decrease in the peak-to-peak amplitude of the horizontal deflection current IH, the output voltage Vs applied to the scan generator circuit 102 increases, thereby restoring th e peak-to-peak amplitude of the horizontal deflection current IH. Conversely, as the scan power supply circuit 101 senses a n increase in the peak-to-peak amplitude of the horizontal deflection current IH, the output voltage Vs applied to the scan generator circuit 102 decreases, again in such a way to restore th e peak-to-peak amplitude of the horizontal deflection current IH - In this manner, scan power supply circuit 101 provides a variable scan power supply voltage Vs to scan generator circuit 102 so th at a substantially constant peak-to-peak amplitude for the horizontal deflection current IH is maintained over a range of horizontal scanning frequencies.

Claims

CLAIMS:

1. A circuit for generating a voltage (V_s) for a scan generator circuit in a video display apparatus, the circuit comprising: means (201 , Rl) for sensing the peak-to-peak amplitude of a horizontal deflection current (I_H); and means (230) for adjusting the magnitude of the voltage i n accordance with the amplitude of the horizontal deflection current (I_H) such that the amplitude of the horizontal deflection current (I_H) is maintained substantially constant.

2. The circuit of claim 1, wherein the magnitude of th e voltage (V_s) varies inversely with the amplitude of the horizontal deflection current (I_H) .

3. The circuit of claim 1, wherein the sensing means comprises: means (201 , Rl) for providing a first voltage proportional to the horizontal deflection current (I_H); and means (D5, D6) for rectifying the first voltage to provide a substantially-DC second voltage (V_c2) having a magnitude proportional to the magnitude of the horizontal deflection current.

4. The circuit of claim 3, wherein the providing means comprises a transformer (201) having a first winding (202) coupled in series with a horizontal deflection coil (103) of the scan generator circuit and a second winding (203).

5. The circuit of claim 4, wherein the providing me ans further comprises a resistor (Rl) coupled to the second winding (203) for providing the first voltage across the resistor.

6. The circuit of claim 1, wherein the adjusting means comprises a switching power supply circuit (230).

7. The circuit of claim 6, wherein the power supply circuit (230) comprises a self-oscillating, zero-voltage switching topology.

8. An arrangement for generating a horizontal deflection current having a substantially constant peak-to-peak amplitude for a horizontal deflection coil of a CRT, comprising: a scan generator circuit ( 102) having an input terminal for receiving a voltage (V_s) and an output terminal coupled to th e horizontal deflection coil ( 103) for applying the horizontal deflection current (I_H) to the horizontal deflection coil ( 103), wherein the amplitude of the horizontal deflection current is proportional to the magnitude of the voltage; and a power supply circuit ( 101 ) having an input terminal (22 1 ) coupled to the horizontal deflection coil for receiving th e horizontal deflection current and an output terminal (222) coupled to the input terminal of the scan generator circuit for providing the voltage to the scan generator circuit, wherein the power supply circuit senses the amplitude of the horizontal deflection current and adjusts the magnitude of the voltage provided to the scan generator circuit in accordance with the amplitude of the horizontal deflection current.

9. The arrangement of claim 8, wherein the power supply circuit (101) adjusts the magnitude of the voltage (V_s) inversely with the amplitude of the horizontal deflection current (I_H) .

10. The arrangement of claim 8, wherein the power s upply circuit (101) comprises: a transformer (201) having a first winding (202) coupled i n series with the horizontal deflection coil ( 103) and a second winding (203) coupled to a resistor (Rl) for providing across th e resistor a first voltage proportional to the horizontal deflection current (I_H); and a rectifier (D5, D6) for rectifying the first voltage to provide a substantially-DC second voltage (N_c2) having a magnitude proportional to the amplitude of the horizontal deflection current.

11. The arrangement of claim 10, wherein the power supply circuit (101 ) further comprises a switched-mode power s upply circuit (230) for adjusting the magnitude of the voltage (V_s) provided to the scan generator circuit ( 102) responsive to said second voltage (N_c2) .

12. The arrangement of claim 1 1 , wherein the switched- mode power supply circuit (230) comprises a self-oscillating, zero- voltage switching topology.