GB2280071A - Protection circuit for a power supply within a TV receiver - Google Patents

Abstract

A switch mode power supply or line output stage in a TV receiver has a power switching transistor TP10 connected in series with the primary winding of a transformer Tr and a current sensing resistor RP30. A line synchronized sawtooth generating circuit 3 provides a switching voltage for the transistor. Circuit means are provided between the sensing resistor RP30 and sawtooth generating circuit 3 such that if the voltage at the sensing resistor RP30 exceeds a threshold value, the circuit 3 is inhibited from generating the switching voltage Us. Additionally, a second threshold circuit may be connected to the sensing resistorwhich, when activated, shuts down the power supply unit of the TV receiver. The second threshold value lies beyond the first threshold value. Switching transistor TP10 is turned off without any delay when current through it exceeds a predetermined value. If current through the transistor is further increased, the power supply unit of the TV receiver is switched off by the second threshold circuit. <IMAGE>

Description

Protection circuit for a power supply within a TV receiver The invention relates to a protection circuit according to the introductory part of claim 1. Said power supply might be a switch mode power supply, called SMPS, or the line output stage of television receiver. In both cases a power switching transistor is provided, connected in series with the primary winding of the transformer and having a current sensing emitter resistor and further including a synchronized sawtooth generating circuit for generating a switching voltage for said switching transistor.

Such circuits need additional protection circuit means for limiting excessive current through the switching transistor.

Circuits according to prior art generally use an averaging process to detect overload. Due to averaging said circuits are unable to react quickly to the fault causing too high current through said switching transistor.

It is an object of the invention to provide a protection circuit for a power supply of the kind described providing reliable and quick protection in case of fault or overload.

The invention relates to a protection circuit for power supply within a TV receiver including a power switching transistor connected in series with the primary winding of the transformer and having a current sensing emitter resistor and further including a synchronized sawtooth generating circuit for generating a switching voltage for said switching transistor.

According to the invention circuit means are provided between said sensing resistor and said sawtooth generating circuit effecting that voltage at said sensing resistor exceeding a threshold value inhibits said generating circuit from generating said switching voltage.

The circuit according to the invention provides a line-by-line current limiter which is able to limit the excess current immediately without employing an averaging process. If once only for a very short time, the current through the switching transistor exceeds a predetermined value immediately without any delay current through said transistor is switched off via said sawtooth generating circuit normally generating switching voltage for said transistor. That means that said sawtooth generating circuit has a double function. On the one hand it provides switching voltage for said switching transistor during normal operation. On the other hand it has a protection function switching off said transistor if its current becomes too high.

Using the invention it is not necessary to shut off the power supply itself. In limiting the excessive current the invention makes use of the oscillator circuit which is connected to the switch mode power supply or the horizontal synchronization as explained below. By making use of the oscillator circuit a reliable and economical solution is available to limit excess current without shutting the power supply itself off.

According to a preferred embodiment of the invention a terminal with said synchronizing pulse is connected via a differentiation network to the base of the transistor being connected in parallel to a capacitor which is charged via a resistor and discharged by said transistor for generating said sawtooth voltage. A threshold circuit may be provided between said sensing resistor and the output of said differentiating circuit effecting that in case voltage at said sensing resistor exceeds a threshold, said second transistor is turned on or off for turning off said switching transistor.

According to another preferred embodiment a second threshold circuit is connected to said sensing resistor which, when activated, shuts down the power supply unit of the receiver. The threshold value of said second threshold circuit lies beyond the threshold value of said first threshold circuit. Within said solution two protection circuits are connected to the same point, i.e. said sensing resistor in the emitter path of said switching resistor. In this way an accurate and reliable second level protection is provided to shut down the power supply when current exceeds the second threshold level.

In order that the invention may more readily be understood, a description is now given by means of examples, reference being made to the drawing showing two circuits according to the present invention.

In the drawing Fig. 1 shows a power supply circuit including a protection circuit according to the invention and Fig. 2 a circuit according to the embodiment of the invention including a second protection circuit.

Fig. 1 shows a power supply circuit for a TV receiver which might be a switch mode power supply SMPS or a line output stage.

The power switching transistor TPl0 is connected to the primary winding 1 of transformer Tr the secondary winding 2 of which may provide operating voltages or feeding a deflection coil. The emitter of said switching transistor TPl0 is connected to ground via a current sensing resistor RP30. A sawtooth generating circuit 3 is provided being synchronized from terminal 4 by a line impulse A and generating at emitter of transistor TP04 a sawtooth voltage 5 controlling impulse generator 8 generating switching voltage Us which is fed to base of transistor RP30 for periodically switching on and off said transistor.

In the following said circuit will be described more in detail.

During normal operation of the power supply, a current flows through the resistor RPl2 to charge up the capacitor CP03. The capacitor CP03 will charge until a stage whereby a pulse current appears at the resistor RP67. This pulse is from the integrated chip ILOl. This pulse has a frequency equal to the horizontal scanning frequency of the raster. The positive edge of the pulse current flowing through the resistor RP67 is differentiated by the capacitor CP04 and resistor RPl0 into a very short triggering pulse (an impulse) of positive magnitude. This impulse will cause a current to flow through the diode DP05, the resistor RP09 and diode DP04 to ground.When the voltage dropped across the resistor RP09 and diode DP04 is about 0,6V, the transistor TP0l will be turned on and the base of the transistor TP02 which is tied to the collector of the transistor TP0l will be pulled low. In doing so, the transistor TP02 will turn "ON".

The capacitor CP03 will discharge quickly via the resistor RP65, transistor TP02, resistor RP09 and diode DP04 to ground. While discharging, the voltage across the resistor RP09 and diode DP04 will maintain the transistor TP0l "ON" as long as this voltage exceeds the base-emitter breakdown voltage of TP0l. When the capacitor CP03 is discharged to a stage whereby the voltage across the resistor RP09 and diode DP04 is insufficient to turn the transistor TP0l "ON", the transistor TP0l will turn "OFF".

When this happens, the capacitor CP03 will be charged via the resistor RPl2 until the appearance of the next impulse to trigger the transistor TP0l back "ON" again. This results in a sawtooth waveform 5 being generated at the emitter of the transistor TP04 as the charging and discharging of the capacitor CP03 will toggle the transistor TP04 "OFF" and "ON". This sawtooth waveform is used to turn the transistor TPl0 "ON" and "OFF". When the transistor TP0l turns on, the transistor TPl0 will be turned off and vice versa.

The invention makes use of the sawtooth oscillator to limit the excess current flowing through the transistor TPl0. When a large current flows through the transistor TPl0, a large voltage drop across the resistor RP30 will occur. The larger voltage drop, the voltage across the diode DP69, resistor RP89 and RP30 will also increase. If this voltage exceeds the threshold level of the diode DP05 and the base to emitter voltage of the transistor TP0l, current will flow via the resistor RP68, diode DP05 to turn the transistor TP0l " ON11. As mentioned above, when the transistor TP0l is turned on, the transistor TPl0 will turn off.

This limits further current from flowing through the transistor TPl0 and prevents damages to TPl0. By doing so, this invention is able to combine the horizontal synchronization, the oscillator and the protection within the same circuit. By tying the overcurrent sensing from the resistor RP30 to the oscillator circuit via the resistor RP89, diode DP69 and the resistor Rip88 it is able to respond immediately to limit further current excursion through the transistor TPl0 by changing the oscillator's sawtooth waveform. Since the switching frequency of the transistor TPl0 is equal to the horizontal scanning frequency of the television set, every pulse of current passes through the transistor TPl0 and exceeds the threshold set will be limited via the oscillator circuit.By making use of the oscillator circuit 3 a reliable and economical solution is available to limit excess current through TPl0 without shutting the power supply "OFF".

Fig. 2 shows an embodiment wherein a second protection circuit 7 is connected to the current sensing resistor RP30. By the combination of the circuit according to fig. 2, with the protection circuit according to fig. 1 an accurate and sensitive protection circuit can be employed to shut down the power supply if current passing through the transistor TPl0 exceeds a second threshold level.

When a large current flows through transistor TPl0, the voltage dropped across the resistor RP30 will be large enough to cause a current to flow via the resistor RP78, diode DP68 and the resistor RP43 to turn the transistor TR04 on. When the transistor TR04 is turned on, the capacitor CP63 will discharge via the diode DR06 and transistor TR04 to ground. This shuts off the power supply immediately.

By tying the second level protection at the same point as the line-by-line i.e. at the emitter of the transistor TPl0 the second level protection can be made accurately to detect a fault. During the power supply operation, any excessive current through TPl0 will be limited by the line-by-line current limiter to the first threshold level. However, if a large current beyond the second threshold level is detected, the second level protection is turned on. By having the sensing point of the line-by-line and the second level protection at the same point, the second level protection can be made accurately to trigger at a threshold that is higher than the first threshold. This also ensures that the first threshold protection (the line-by-line current limiter) is always activated before the second level protection.

In this invention the following components were used: Resistor RP30 0,15 Ohm Resistor RP77 68 kOhm Resistor RP79 68 kOhm Resistor RP89 68 kOhm Resistor RPl0 3.3 kOhm Resistor PR88 1.8 kOhm Diode DP69 in4148

Claims (6)

1. Claims 1. Protection circuit for a power supply within a television receiver including a power switching transistor (TPl0) connected in series with the primary winding (1) of a transformer (Tr) and having a current sensing emitter resistor (RP30) and further including a synchronized sawtooth generating circuit (3) for generating a switching voltage (Us) for said switching transistor (TPl0),characterised in that circuit means are provided between said sensing resistor (RP30) and said sawtooth generating circuit (3) effecting that voltage at said sensing resistor exceeding a threshold value inhibits said generating circuit from generating said switching voltage (Us).
2. 2. Circuit according to claim 1, characterised in that a terminal (4) with said synchronizing pulse (H) is connected via a differentiation network (6) to the base of a second transistor (TP0l, TP02) being connected in parallel to a capacitor (CP03) which is charged via a resistor (RPl2) and discharged via said transistor for generating said sawtooth voltage (5).
3. 3. Circuit according to claim 2, characterised in that a threshold circuit is provided between said sensing resistor (RP30)and the output of said differentiating network (6) effecting that, in case voltage at said sensing resistor (RP30) exceeds a threshold, said second transistor (TP0l, TP02) is turned on or off for turning off said switching transistor (TPl0).
4. 4. Circuit according to claim 1, characterised in that a second threshold circuit (7) is connected to said sensing resistor (RP30) which, when activated, shuts down the power supply unit of the receiver.
5. 5. Circuit according to claim 4, characterised in that the threshold value of said second threshold circuit lies beyond the threshold value of said first threshold circuit.
6. 6. Protection circuit substantially as herein described with reference to Fig. 1 or to Figs. 1 and 2 of the accompanying drawings.