X-RAY PROTECTION CIRCUIT

Abstract: An x-ray protection circuit for a video processor, to disable a horizontal output pulse when a voltage level on the x-ray input exceeds a reference voltage. The x-ray protection circuit comprises a comparator and a bistable device, and incorporates a reset circuit. The reset circuit enables the initialisation of the protection circuit whilst the power is switched on.

Description: X-RAY PROTECTION CIRCUIT FIELD OF THE INVENTION The present invention relates to an x-ray protection circuit for a video processor to prevent excessive voltages on the EHT (extra high tension) of the tube of a television set. In particular, the present invention relates to an improved protection circuit which utilises a "power-on-reset" circuit, to ensure that the x-ray output would not be held in the wrong state, unable to automatically correct itself. Such a circuit enables automatic correction without the requirement of having to switch the power off and then on again, on the television set. DESCRIPTION OF THE PRIOR ART X-ray protection circuits are used in video processors to prevent excessive voltage on the EHT (extra high tension) of the tube of a television set. They detect the line flyback amplitude, and disable the horizontal output pulse when the amplitude of the flyback exceeds a certain limit. A prior art protection circuit is shown in Fig. 1. The circuit of Fig. 1 comprises a band gap comparator 1, which has a reference voltage input, typically 1.3V, and, an x-ray input. When the x-ray input exceeds the (typically 1.3V) threshold, this triggers an analog d-latch (thyristor structure) 2 such that its output (the x-ray output) will be high. The analog d-latch will memorise the x-ray detection, and will do so until the supply voltage V cc is reduced to below a predetermined value, typically 3V. A problem with such prior art circuits is that the analog d-latch (thyristor structure) is easily triggered by parasitic spikes generated by the ESD (electrostatic discharge). This results in such protection circuit being held in the wrong state, and it is not able to automatically correct itself. To correct this occurrence, the television is required to be switched off and then on again, such that the analog d-latch is reset. SUMMARY OF THE INVENTION The present invention seeks to provide an x-ray protection circuit for a video processor, which overcome the disadvantages of the prior art. The present invention also seeks to provide an x-ray protection circuit for a video processor which ensures that the x-ray output is provided in a safe-state. In one broad form, the present invention provides an x-ray protection circuit for a video processor to disable a horizontal output pulse when a voltage level on the x-ray input exceeds a reference voltage characterised in that said protection circuit comprises a reset circuit to initialise said protection circuit whilst power is switched on. Preferably, said reset circuit detects the occurrence of a parasitic spike and generates a reset signal whilst power is switched on. Also preferably, the x-ray protection circuit further comprises: a comparator, to provide a comparator output signal when an x-ray input voltage exceeds a reference voltage; and, a bistable device, to receive said reset signal and said comparator output signal, and provide an x-ray output signal to disable the horizontal output pulse when a voltage level on the x-ray input exceeds a reference voltage. Preferably, said comparator is a bandgap comparator. Also preferably, said reference voltage is about 1.3V. In preferred forms, said bistable device is an analog or digital memory circuit with two stable states. Preferably, said bistable circuit is a flip-flop. Perhaps more preferably, said bistable circuit is an RS flip-flop. In a most preferred form, said reset circuit is connected to the reset (R) input, said comparator is connected to the set (S) input, and said x-ray output is connected to the output Q of said RS flip-flop. Also preferably, output QB of said RS flip-flop is connected to the BUS, the BUS being connected to the reset (R) input to additionally recover the x-ray if it is wrongly triggered by the parasitic spike. In another preferred form, the present invention provides an x-ray protection circuit for a video processor, comprising: a reset circuit for correct initialisation at power on and to detect the occurrence of a parasitic spike in said video processor and provide a reset signal in response thereto; a comparator, to provide a comparator output signal when an x-ray input voltage exceeds a reference voltage; and, a bistable circuit, to receive said reset signal and said comparator output signal and provide a memorised x-ray output signal to disable the horizontal output pulse when a voltage level on the x-ray input has exceeded a reference voltage. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the following detailed description of a preferred but non-limiting embodiment thereof, described in connection with the accompanying drawings, wherein: Fig. 1 illustrates a schematic diagram of a prior art x-ray protection circuit; Fig. 2 illustrates a schematic diagram of a preferred embodiment of the present invention; Fig. 3 illustrates a schematic diagram of a further preferred embodiment of the present invention; Fig. 4 details a preferred implementation of the reset circuit; and, Fig. 5 illustrates a preferred implementation of the bistable circuit of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Throughout the drawings, like numerals will be used to identify similar features, except where expressly otherwise indicated. As hereinbefore described, a prior art protection circuit is illustrated in Fig. 1. A main disadvantage of such a prior art circuit is that the circuit can be held in the wrong state, unable to automatically correct itself. To correct the occurrence of a wrong state for such a prior art circuit, the television is required to be switched off and then on again, such that the analog d-latch is reset. A preferred embodiment of the x-ray protection circuit in accordance with a preferred embodiment of the present invention is shown in Fig. 2. Fig. 2 illustrates that the x-ray protection circuit primarily comprises a bistable circuit 4, a reset circuit 5, and a comparator 6. The bistable circuit 4 may be embodied as an RS flip-flop which has two stable output states. It should however be appreciated that other forms of bistable circuitry might be provided, and such circuit could be digital or analog. The x-ray output provides the signal to disable the horizontal output pulse of the video processor when a parasitic spike is detected. The inputs of the bistable circuit 4 are shown in Fig. 2 as the Reset (R) and Set (S) inputs. The Reset (R) input is provided from the power-on-reset circuit 5 and enables the correct initialisation of the bistable circuit 4. This power-on-reset circuit 5 is therefore used to initialise the flip-flop or bistable circuit 4 when the power is switched on. This was the primary disadvantage of the prior art - the power had to previously be switched off and then on again. Since the power-on-reset circuit 5 is the weakest component to parasitic spikes, it may be used to reset the flip-flop 4 when a spike occurs. However, if the x-ray input of comparator 6 is higher than prescribed reference voltage, which is typically 1.3V, the flip-flop 4 will be automatically set (and horizontal output pulse is disabled). It should therefore be appreciated that the circuit shown in Fig. 2 is a protection circuit which is able to "correct" itself. A further embodiment of the invention is shown in Fig. 3. Fig. 3 shows a similar circuit to that of Fig. 2 but further includes bus connections. These two additional signal lines connected to bus, provide additional protection to recover the x-ray signal if it is wrongly triggered by the spike generated by the ESD (electrostatic discharge). In operation, the circuitry works as follows. During the power ramping up, the power on reset circuit will reset the RS flip-flop, so that x-ray is in the untriggered state. When the voltage at the x-ray input exceeds the 1.3 V threshold, the event will trigger the RS flip-flop and the x-ray will change into the triggered state. The RS flip-flop will memorise the x-ray detection. It can hold on until the supply voltage V cc is reduced to a certain voltage so that the power-on-reset circuit will generate a reset signal. Referring to the power-on-reset circuit of Fig. 4 I^CV^VbeVR I 2 =(V„- Vbe)/2R Therefore, 1) when V cc < = Vbe, 1=0 2) when Vbe <V cc <3Vbe, I >0 3) when V cc > =3Vbe, 1=0 This power-on-reset circuit is different from the general power-on-reset circuit. After the power ramping up, the general power-on-reset circuit has some output current. In this circuit, since V cc > 3Vbe, so there is not output current. The transistor Q6 is blocked (collector is high impedance). When a parasitic spike generated by the ESD occurs, Q6 is quite easy to trigger. This is proven by doing the simulation that during the spike of the supply voltage V cc , ground and substrate, the circuit is easy to give a current spike at I output. The RS flip-flop circuit is as shown in Fig. 4. The RS flip-flop is different from ordinary RS flip-flop. The inverter invl, inv2, and inv5 are the additional components compared to the ordinary RS flip-flop. If the circuit is triggered at the same time by both the R and S inputs, due to the inverter inv5, the result is Q=0 and QB = 1 , and since the R signal has two additional gates delay, so after triggering, the circuit will keep in the untriggered state. So that means the S input will be inhibited if the R input and S input are triggered at the same time. The x-ray has two states: the triggered state and the untriggered state. The triggered state is not a safe state for the x-ray since it can not correct itself if it is the wrong state. The untriggered state is a safe state for the x-ray since it will immediately change into the triggered state if it is the wrong state. So if the x-ray is in the untriggered state before the parasitic spike generated by the ESD, it also should keep in the untriggered state after the parasitic spike generated by the ESD. If the x-ray is in the triggered state before the parasitic spike generated by the ESD, it can be either in the triggered or untriggered states since the untriggered state will be triggered immediately if the input voltage is still higher than 1.3V. When the parasitic spike generated by the ESD comes, since the power-on-reset circuit is triggered, the RS flip-flop will be reset and this will make the x-ray into the untriggered state whatever it is in the triggered or untriggered state before the parasitic spike generated by the ESD. Thus the x-ray circuit will be in the safe state. So this circuit can work with the parasitic spike generated by the ESD. The circuit also has two additional signal lines connected with the bus. Once is used to send the state of the RS flip-flop to the bus, another is used to reset the RS flip-flop with the reset signal from the bus. So the microcontroller can control the x-ray through the bus. If the microcontroller detects the x-ray is in the triggered state, it can send the reset signal to the x-ray and detect the new state of the x-ray. If the x-ray is still in the triggered state, this means that the last triggering is the correct triggering; otherwise the last triggering is the wrong triggering. In this way, even the x-ray is wrongly triggered by the parasitic spike generated by the ESD, the microcontroller still can recover it. So this circuit can work with the parasitic spike generated by the ESD. It will therefore be appreciated that the present invention provides a circuit which has one safe state, by using a power-on-reset circuit which is sensitive to the parasitic spike generated by the ESD (electrostatic discharge), and an RS flip-flop to ensure that the bi-state circuit is in the safe state after the parasitic spike is generated by the ESD (electrostatic discharge). The circuit of the present invention has the advantage that it can operate with the parasitic spike generated by the ESD with minimal additional cost. Furthermore, it can be integrated in the STV 2128 (video processor). Tests performed by the Applicant, indicate that this circuit provides far more accurate protection that conventional prior art protection circuits, such as illustrated in Fig. 1. It will be appreciated that variations and modifications to the present invention will become obvious to persons skilled in the art. All such variations and modifications should be considered to fall within the spirit and scope of the invention. It will be also appreciated that other applications of the invention will become apparent to persons skilled in the art, whereby a bi-state circuit with one safe state is required. All such variations and modifications should be considered to be within the spirit and scope of the present invention as hereinbefore described and as hereinafter claimed.

Claims: The claims defining the present invention are as follows: 1. An x-ray protection circuit for a video processor to disable a horizontal output pulse when a voltage level on the x-ray input exceeds a reference voltage, characterised in that said protection circuit comprises a reset circuit to initialise said protection circuit whilst power is switched on. 2. An x-ray protection circuit as claimed in claim 1, wherein said reset circuit detects the occurrence of a parasitic spike and generates a reset signal whilst power is switched on. 3. An x-ray protection circuit as claimed in claim 2, further comprising: a comparator, to provide a comparator output signal when an x-ray input voltage exceeds a reference voltage; and, a bistable device, to receive said reset signal and said comparator output signal, and provide a memorised x-ray output signal to disable the horizontal output pulse when a voltage level on the x-ray input exceeds a reference voltage. 4. An x-ray protection circuit as claimed in claim 3, wherein said comparator is a bandgap comparator. 5. An x-ray protection circuit as claimed in claims 3 or 4, wherein said reference voltage is about 1.3V. 6. An x-ray protection circuit as claimed in any one of claims 3 to 5, wherein said bistable device is an analog or digital memory circuit with two stable states. 7. An x-ray protection circuit as claimed in any one of claims 3 to 6, wherein said bistable circuit is a flip-flop. 8. An x-ray protection circuit as claimed in any one of claims 3 to 7, wherein said bistable circuit is an RS flip-flop. 9. An x-ray protection circuit as claimed in claim 8, wherein said reset circuit is connected to the reset (R) input, said comparator is connected to the set (S) input, and said x-ray output is connected to the output Q of said RS flip-flop. 10. An x-ray protection circuit as claimed in claim 9, wherein output QB of said RS flip- flop is connected to the BUS, the BUS being connected to the reset (R) input to additionally recover the x-ray if it is wrongly triggered by the parasitic spike. 11. An x-ray protection circuit for a video processor, comprising: a reset circuit, to detect the occurrence of a parasitic spike in said video processor and provide a reset signal in response thereto; a comparator, to provide a comparator output signal when an x-ray input voltage exceeds a reference voltage; and, a bistable circuit, to receive said reset signal and said comparator output signal and provide a memorised x-ray output signal to disable the horizontal output pulse when a voltage level on the x-ray input has exceeded a reference voltage. 12. An x-ray protection circuit substantially as herein described with reference to the accompanying drawings. AMENDED CLAIMS [received by the International Bureau on 23 March 1998 (23.03.98); original claims 1-12 replaced by amended claims 1-11 (2 pages)] The claims defining the invention are as follows: 1. An x-ray protection circuit for a video processor to disable a horizontal output pulse when a voltage level on the x-ray input exceeds a reference voltage or when a parasitic spike is detected, characterised in that said protection circuit includes a reset circuit to initialise said protection circuit whilst power of said video processor is switched on. 2. An x-ray protection circuit in claim 1, further including: a comparator, to provide a comparator output signal when an x-ray input voltage exceeds a reference voltage; and a bistable device, to receive said reset signal and said comparator output signal, and provide a memorised x-ray output signal to disable the horizontal ouφut pulse when a voltage level on the x-ray input exceeds a reference voltage or a parasitic spike is detected. 3. An x-ray protection circuit as claimed in claim 2, wherein said comparator is a bandgap comparator. 4. An x-ray protection circuit as claimed in claims 2 or 3, wherein said reference voltage is about 1.3V. 5. An x-ray protection circuit as claimed in any one of claims 2 to 4, wherein said bistable device is an analog or digital memory circuit with two stable states. 6. An x-ray protection circuit as claimed in any one of claims 2 to 5, wherein said bistable circuit is a flip-flop. 7. An x-ray protection circuit as claimed in any one of claims 2 to 6, wherein said bistable circuit is an RS flip-flop. 8. An x-ray protection circuit as claimed in claim 7, wherein said reset circuit is connected to the reset (R) input, said comparator is connected to the set (S) input, and said x-ray output is connected to the output Q of said RS flip-flop. 9. An x-ray protection circuit as claimed in claim 8, wherein output QB of said RS flip- flop is connected to the BUS, being connected to the reset (R) input to additionally recover the x-ray if it is wrongly triggered by the parasitic spike. 10. An x-ray protection circuit for a video processor including: a reset circuit, to detect the occurrence of a parasitic spike or a voltage level on the x-ray input of said video processor exceeding a reference voltage and provide a reset signal in response thereto; a comparator, to provide a comparator output signal when a parasitic spike is detected or when an x-ray input voltage exceeds a reference voltage; and, a bistable circuit, to receive said reset signal and said comparator output signal and provide a memorised x-ray output signal to disable the horizontal output pulse when a parasitic spike has occurred or when a voltage level on the x-ray input has exceeded a reference voltage. 11. An x-ray protection circuit substantially as herein described with reference to the accompanying drawings.

Inventors: DESPREZ-LE GOARANT YANN, ZHOU DONGLIN, DESPREZ-LE GOARANT, YANN, ZHOU, DONGLIN
Applicants: DESPREZ LE GOARANT YANN, SGS THOMSON MICROELECTRONICS A, ZHOU DONGLIN, DESPREZ-LE GOARANT, YANN, SGS-THOMSON MICROELECTRONICS ASIA PACIFIC (PTE) LTD., ZHOU, DONGLIN
Application year: 1997
Main classification: H04N3
IPC codes: H04N3