Thomson Consumer Electronics S.a.

Thomson Multimedia S.a.

Koninklijke Philips Corporation N.v.

Chun Hsing Wu

Wolfgang Heidebroek

Kian P Hoh

The processing unit () for generating a scan velocity modulation signal (DS) has an input for receiving a video signal (VS) and an output for supplying a modulation signal (DS). The unit () comprises a series connection of, successively, a coring circuit (C), a differentiator (D), and a limiter (L). The display device () comprises a cathode ray tube () and the processing unit ().

Generating a scan velocity modulation signal

A controller, which is normally used for controlling different modes is used for circuit protection. The suipply voltage and control signals from the switched mode power supply to the controller are modified when a short circuit occurs. In response thereto, the controller switches 'off' a mode-control signal and the switched mode power supply operates in standby mode.

Protection circuit for a switch mode power supply

Due to low output voltage during flyback mode of start phase the secondary current cannot be fully discharged in time. The current buildup will eventually damage the output transistor if the current is not limited. In order to prevent the current build up during start phase, the storage time of the output transistor of the SMPS has to be reduced. During the startup phase a negative bias capacitor is not fully charged. Thereby the base of an output transistor is not biased sufficiently when the transistor is switched off. So the transistor could not operate safely if the collector voltage is higher than Vceo. It is the object to keep charges, which are stored on the capacitor during startup phase and to reduce discharging of the capacitor. According to this invention means are provided between the base and the emitter of the output transistor. It is preferred that said means are realised by a resistor and a diode, connected in series.

Startup protecting for a switch mode power supply

A television receiver has a switched mode power supply controller, which may be deflection synchronized, which produces pulses in a power transformer. To power the controller during startup, before the internal power supply has started, a storage capacitor on the power supply input of the controller is coupled through a current limiting resistance to a rectified voltage from the AC mains so as to charge the storage capacitor during first polarity phases of the AC mains. The current limiting resistance includes a positive temperature coefficient (PTC) element which increases its impedance as it heats, to reduce power dissipation after initial connection of the television receiver to AC mains. In addition, a diode is coupled in a current path to the storage capacitor to charge the capacitor to the same polarity during opposite polarity phases of the AC mains. The two paths for charging the capacitor allow a relatively smaller value for the current limiting resistance, which remains coupled to the storage capacitor, and provides sufficient power to the controller without undue power dissipation over a wide range of power mains voltages.

Television power supply with polarity-independent charging paths

In a switch mode power supply of a video apparatus, a capacitor develops a supply voltage that is coupled to a main current conducting electrode of a driver transistor. The driver transistor drives a switching power transistor. An arrangement charges the charging capacitor when the video apparatus operates in standby mode. A delay circuit introduces a delay time between a time when the capacitor begins to be charged and the time when a switching control signal is applied to the driver transistor for operation in a run mode. Consequently, the capacitor develops sufficient voltage that insures sufficient base current to turn on the power transistor in to saturation prior to the time the switching control signal is applied to the driver transistor. Thus, a possible damage to the power transistor is prevented.

Switch mode power supply circuit

A television receiver has a switched mode power supply controller, which may be deflection synchronized, which produces pulses in a power transformer. To power the controller during startup, before the internal power supply has started, a storage capacitor on the power supply input of the controller is coupled through a current limiting resistance to a rectified voltage from the AC mains so as to charge the storage capacitor during first polarity phases of the AC mains. The current limiting resistance includes a positive temperature coefficient (PTC) element which increases its impedance as it heats, to reduce power dissipation after initial connection of the television receiver to AC mains. In addition, a diode is coupled in a current path to the storage capacitor to charge the capacitor to the same polarity as during opposite polarity phases of the AC mains. The two paths for charging the capacitor allow a relatively smaller value for the current limiting resistance, which remains coupled to the storage capacitor, and provides sufficient power to the controller without undue power dissipation over a wide range of power mains voltages.

Television startup current regulation

In a television receiver, a microprocessor controls a horizontal deflection circuit which is coupled to run mode power supply to define a standby operating mode and a run operating mode. The microprocessor has a power-up reset input terminal which initializes operation upon application of AC mains power. The run mode power supply is coupled to at least one load which is subject to overcurrent or overvoltage faults. A fault detection circuit detects fault conditions in the run mode power supply loads such as overvoltage or overcurrent and anomalous vertical deflection current. Upon fault detection the fault detection circuit triggers a power up reset circuit which applies a momentary reset pulse to the power-up reset input of the microprocessor. Upon receipt of the reset pulse, the microprocessor controls the horizontal deflection circuit to place the run mode power supply in the standby operating mode. Thus power is removed from the run mode load subject to the overvoltage or overcurrent or anomalous condition, and the television receiver assumes a standby condition.

Fault protection using microprocessor power up reset

A television receiver is subject to certain operational conditions which result in poor, unreliable or unusable separated sync pulse signals. During such conditions the use of unsuitable sync signals for synchronization and the like is inhibited to prevent mis-triggering or spurious synchronization. A television receiver contains circuitry for extracting a sync signal, a voltage controlled oscillator (VCO) for generating a scanning signal, and a comparator comparing the scanning signal to the separated sync signal. A microprocessor is used to verify the separate sync signal for invalid or unusable signals and has an output activated during such conditions. The phase comparator has a current output coupled to a integrating capacitor or LPF which develops a varying positive or negative voltage to raise or lower the frequency of the VCO for scanning in phase with the separated sync. During conditions of unusable separated sync pulse signals, the microprocessor output causes the integrating capacitor or LPF to be coupled to a voltage source for forcing the VCO to a maximum frequency. Also, during periods of unusable sync the microprocessor decouples the separated sync signal from the vertical deflection generating circuit, allowing the vertical deflection generating circuits to free run. Setting the horizontal frequency to a maximum value and inhibiting vertical deflection synchronization eliminates instability of on-screen displays during periods of unusable sync.

Television sync and phase detector disable circuit

A television receiver has a switched mode power supply for regulating an output voltage, including a power transformer having a primary winding coupled to an unregulated voltage via a power transistor. The power transistor is controlled by pulses from the output of a controller. In order to sweep the base-emitter charge from the power transistor in order to end collector-emitter conduction, the base of the power transistor is coupled to the output of the pulse generating controller via a capacitor in parallel with at least one diode. During a pulse from the controller, the capacitor is charged to a voltage equal to the forward biased drop across the diode(s). At the end of a pulse at the output of the controller, the voltage across the capacitor provides a negative voltage at the base of the transistor, thus sweeping out the base-emitter charge and sharply turning the transistor off. In order to assure this function of the capacitor even upon plug-in startup, the capacitor is precharged via a resistor coupled between the output of the controller and the unregulated voltage input.

Switched mode power supply with precharge of base-emitter sweep-out

A television receiver has a switched mode power supply for regulating output voltages including the B+ output to a flyback transformer, by generation of output pulses to a power transformer. A controller for the power supply is coupled in two feedback loops, one responsive to a pulse width modulator coupled to the flyback transformer and the other comparing the output voltage with an internal reference for free running operation. The first feedback loop takes precedence and is active in the run mode of the television receiver. In the standby mode when horizontal rate pulses are absent, the second feedback loop takes over. The second feedback loop has a different reference level than the first, such that when switching from the run mode to the standby mode a transition interval occurs in which no pulses are output by the controller, thereby causing the B+ voltage to the flyback transformer to fall. During this transition interval, horizontal scanning signals continue at falling amplitude, and a signal is input to the kinescope drivers, thereby collapsing the picture and draining the ultor voltage on the screen anode. The reoccurrence of controller pulses when the output voltage of the power transformer has fallen to the new lower reference level is used to positively switch operation of the receiver at the conclusion of the transition interval.

Run/standby control with switched mode power supply

A vertical deflection ramp voltage is generated by the charge and discharge of a resistor capacitor combination. The voltage ramp is coupled to an amplifier which drives a current through a deflection coil producing a magnetic deflection field. The ramp voltage produced by the resistor capacitor combination has an inherent exponential shape which is linearized to prevent vertical linearity errors in the deflected electron beam. Linearization is achieved by multiple feedback paths which couple the amplifier output voltage and a voltage sample of the deflection coil current to the ramp forming resistor. These feedback signals are of a polarity such to establish a constant voltage across the resistor which consequently produces a constant current flow through the resistor. Hence with constant current flow in the resistor, a linear voltage ramp is produced across the capacitor and exponential deflection cramping is avoided. The voltage sample of the deflection coil current is coupled from the wiper of a potentiometer, which by other coupling produces degenerative feedback and provides control of deflection amplitude or height.

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Singapore, Singapore

Chun Hsing Wu