OVER-CURRENT PROTECTION CIRCUIT IN LINE DEFLECTION CIRCUITS

Abstract: A line deflection circuit includes circuitry for protecting the line output transistor (Q1) from failure due to an over-current condition. The circuitry includes a current detector (R3) for detecting the current in the collector-emitter path of the line output transistor (Q1), and an interrupter (SCR) for interrupting the drive signal (Vd) to the line output transistor (Q1) when the detected current exceeds a predetermined level.

Description: OVER-CURRENT PROTECTION CIRCUIT IN LINE DEFLECTION CIRCUITS The subject invention relates to line deflection circuits in television receivers. In a typical cathode ray tube (CRT) television receiver, a high voltage supply derived from an AC line is used to provide a DC collector supply voltage for a horizontal deflection output transistor The collector supply voltage is coupled through a primary of a flyback transformer to the horizontal output transistor. As is well known, the horizontal output transistor is used to supply an alternating current to the primary of the flyback transformer. This current, in turn, induces a relatively high voltage in a flyback secondary from which is derived the anode voltage for the CRT. Additionally, a horizontal deflection coil in series with a capacitor is connected to the collector of the horizontal output transistor. The base of the horizontal output transistor is coupled through a resistor to the secondary winding of a drive transformer The primary winding of the drive transformer connects the above-noted DC collector supply voltage to the collector of a horizontal drive transistor. The base of the horizontal drive transistor then receives horizontal signals from the horizontal oscillator. Ordinarily, the horizontal output transistor is dimensioned such that it operates in saturation. However, situations arise causing the horizontal output transistor to exceed its safe operating area (SOA). These situations, which may include normal start-up of the television receiver, RF interference, CRT arcing, and lightning transients, cause the normally occurring drive pulses from the horizontal drive transistor to be interrupted. The missing drive pulses cause the horizontal output transistor to turn on longer than normal thus causing a large current to flow through the horizontal output transistor The flyback transformer eventually saturates causing an even greater increase in the transistor current. The horizontal output transistor does not have enough base drive to handle the large collector current so it pulls out of saturation. The collector-emitter voltage developed across the horizontal output transistor at this time may now exceed the SOA of the transistor and as such, failure may result. U.S. Patent 4, 185,234 discloses an isolated DC collector supply voltage sensing and high-voltage shutdown circuit which detects when the DC collector supply voltage exceeds a predetermined level and then stops operation of the horizontal deflection circuit by effectively short circuiting the output from the horizontal oscillator thereby terminating the drive signal for the horizontal drive transistor. While this circuit will stop the operation of the horizontal deflection circuit, it will not detect the over-current conditions on the horizontal output transistor which result from situations other than an over-voltage condition on the high voltage line. It is an object of the present invention to prevent the line output transistor from failure due to over-current. To this end a first aspect of the invention provides a line deflection circuit as defined in claim 1. A second aspect provides a display apparatus as defined in claim 9. Advantageous embodiments are defined in the dependent claims. In a preferred embodiment of the invention, the detecting means includes a resistor arranged between the second main terminal of the line output transistor and the reference terminal, and the interrupting means includes a silicon controlled rectifier (SCR) connecting the first main terminal of the line drive transistor to the reference terminal. The control electrode of the SCR is coupled to the second main terminal of the line output transistor. In the above circuit, when the current sensed through the resistor gets to such a level that the voltage across the resistor exceeds the gate threshold voltage of the SCR, the SCR will turn on thus removing a drive pulse to the line output transistor, thereby turning it off. On the arrival of a next line drive pulse, the line drive transistor is turned on which removes the latch current from the SCR, thereby turning the SCR off and thus resetting the circuit. Applicant has found that the point were the drive pulse is removed can be anywhere in the line drive path, even at the control terminal of the line output transistor. The present location is preferred due to low currents less susceptibility to CRT arc currents that may be present at the control terminal of the line output transistor. With the above and additional objects and advantages in mind as will hereinafter appear, the invention will be described with reference to the accompanying drawings, in which: Fig. 1 shows a schematic diagram of a portion of a known horizontal deflection circuit; and Fig. 2 shows a schematic diagram of a line deflection circuit incorporating the protection circuit of the subject invention. Fig. 1 shows a schematic diagram of a portion of a known line deflection circuit. In a typical cathode ray tube (CRT) television receiver, a high voltage supply derived from an AC line is used to provide a DC collector supply voltage B+ for a horizontal deflection output transistor Ql . The collector supply voltage B+ is coupled through a primary of a flyback transformer Tl to the horizontal output transistor Ql . As is well known, the horizontal output transistor is used to supply an alternating current to the primary of the flyback transformer Tl . This current, in turn, induces a relatively high voltage in a flyback secondary from which is derived the anode voltage Va for the CRT. Additionally, a horizontal deflection coil Ly in series with a capacitor Cs is connected to the collector of the horizontal output transistor Ql . The base of the horizontal output transistor Ql is coupled through a resistor Rl to the secondary winding of a drive transformer T2. The primary winding of the drive transformer T2 connects the above-noted DC collector supply voltage B+ to the collector of a horizontal drive transistor Q2. The base of the horizontal drive transistor Q2 then receives horizontal signals from the horizontal oscillator. Fig. 2 shows a portion of line deflection circuit substantially similar to that shown in Fig. 1. In addition, a resistor R3 is inserted between the junction point of the secondary of the line drive transformer T2 and the emitter of the line output transistor Ql and ground. This resistor R3 then forms detecting means for detecting the current through the collector-emitter path of the line output transistor Ql . A silicon controlled rectifier SCR is connected between the collector of the line drive transistor Q2 and ground. The control electrode of the SCR is connected, through resistor R4, to the above-noted junction point. In operation, the current through the resistor R3 causes a voltage to appear across the resistor R3. When this voltage exceeds the gate threshold of the SCR (approx. 0.6 volts), the SCR is turned on thereby short circuiting the drive signal from the line drive transistor Q2 to primary winding of the line drive transformer T2. This removes the drive signal Vd to the base of the line output transistor Ql thereby shutting it off. While the turning off of the line output transistor Q l interrupts the current and eliminates the voltage drop across resistor R3, the SCR remains latched due to the current passing through it from the primary winding of the line drive transformer T2. Upon the occurrence of a next line drive pulse on the base of the line drive transistor Q2, this current is drained allowing the SCR to turn off thereby resetting the protection circuit. Applicant has found that the value of resistor R3 should be approximately 0.075 ohms, with this value being adjustable to determine the "trip point" of the protection circuit. The resistor R4 (approx. 47 ohms) is used to control the sensitivity of the SCR. The low resistance from the gate to cathode results in dv/dt desensitization, anode to gate leakage current desensitization, and greater noise immunity. With regard to capacitor C l , while in prior art line deflection circuits the value of Cl is usually 1 μF, Applicant has found it necessary to reduce this value to 0.22 μF to allow for a quicker start-up after the SCR unlatches which results in good line drive during restart. Numerous alterations and modifications of the structure herein disclosed will present themselves to those skilled in the art. However, it is to be understood that the above described embodiment is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims. The line deflection circuit can be used to deflect the lines in the horizontal direction, whereby the lines succeed each other in the vertical direction, as is common practice in display apparatus for display of text or pictures. The line deflection circuit can also be used to deflect the lines in vertical direction, whereby the lines succeed each other in the horizontal direction as is the case in the so called transposed scan systems. Instead of the bipolar horizontal output transistor Ql also a field effect transistor or any other suitable controllable switching device may be used. The controllable switching device Ql comprises a first main terminal and a second main terminal for forming a main current path and a control terminal to influence the main current path. If the controllable switching device is a NPN bipolar transistor the first main terminal is the collector, the second main terminal in the emitter and the bipolar transistor instead of the bipolar horizontal drive transistor Q2 also a field effect transistor may be used. The horizontal drive transistor comprises a first main terminal , a second main terminal and a control terminal for controlling a current which flows in a main current path which is arranged between the first and the second main terminal. If the horizontal drive transistor Ql is a bipolar transistor, the first main terminal is the collector, the second main terminal is the emitter and the control terminal is the base of the bipolar drive transistor Ql . The reference terminal is preferably connected to ground.

Claims: Claims: 1. A line deflection circuit comprising: a flyback transformer having a primary winding and a secondary winding, said secondary winding providing an anode voltage for a cathode ray tube; means for supplying a D.C. voltage to a first end of the primary winding of said flyback transformer; a line output transistor having a first main terminal coupled to a second end of the primary winding of the flyback transformer, a second main terminal cou/pled to a reference terminal, and a control terminal tor controlling a current flowing in a main current path being arranged between the first and the second main terminal, a drive transformer having a primary winding and a secondary winding, said secondary winding having a first end coupled to the control terminal of the line output transistor and a second end coupled to the second main terminal of said line output transistor, and said primary winding having a first end also coupled to said supplying means; a line drive transistor having a first main terminal coupled to a second end of the primary winding of said drive transformer, a second main terminal coupled to the reference voltage, and a control terminal; means for providing a line drive signal coupled to the control terminal of said line drive transistor; detecting means tor detecting the current through the ma current path of said line output transistor; and interrupting means, coupled to said detecting means, for interrupting a drive current to the control terminal ot said line output transistor when the current in the main current path exceeds a predetermined level. 2. A line deflection circuit as claimed in claim 1 , wherein said detecting means comprises a resistor coupled between the second main terminal of said line output transistor and the reference terminal 3. A line deflection circuit as claimed in claim 1 , wherein said interrupting means comprises a controllable switch coupling a path between said line drive transistor and said line output transistor to the reference terminal, a control input of said controllable switch being connected to said detecting means. 4. A line deflection circuit as claimed in claim 3, wherein said controllable switch couples the first main terminal of said line drive transistor to the reference terminal. 5. A line deflection circuit as claimed in claim 3, wherein said controllable switch couples the secondary winding of said line drive transformer to the reference terminal. 6. A line deflection circuit as claimed in claim 3, wherein said controllable switch comprises a silicon controlled rectifier (SCR). 7. A line deflection circuit as claimed in claim 6, wherein said detecting means comprises a resistor coupled between the second main terminal of said line output transistor and the reference terminal , and a control input of said SCR is coupled to the second main terminal of said line output transistor. 8. A line deflection circuit as claimed in claim 7, wherein said interrupting means further comprises a resistor connecting the control input of said SCR to the second main terminal of the line output transistor. 9. A display apparatus comprising: a cathode ray tube with a line deflection coil , and a line deflection circuit comprising: a flyback transformer having a primary winding and a secondary winding, said secondary winding providing an anode voltage for a cathode ray tube; means for supplying a D.C. voltage to a first end of the primary winding of said flyback transformer; a line output transistor having a first main terminal coupled to a second end of the primary winding of the flyback transformer, a second main terminal coupled to a reference terminal, and a control terminal for controlling a current flowing in a main current path being arranged between the first and the second main terminal, a drive transformer having a primary winding and a secondary winding, said secondary winding having a first end coupled to the control terminal of the line output transistor and a second end coupled to the second main terminal of said line output transistor, and said primary winding having a first end also coupled to said supplying means; a line drive transistor having a first main terminal coupled to a second end of the primary winding of said drive transformer, a second main terminal coupled to the reference voltage, and a control terminal; means for providing a line drive signal coupled to the control terminal of said line drive transistor; detecting means for detecting the current through the main current path of said line output transistor; and interrupting means, coupled to said detecting means, for interrupting a drive current to the control terminal of said line output transistor when the current in the main current path exceeds a predetermined level.

Inventors: JENNINGS JAMES R, JENNINGS, JAMES, R.
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H04N3
IPC codes: G09G1, H03K4, H03K17, H04N3