Technology of Q 2500 colour TV set
Technology of Q 2500 colour TV set Technology of Q 2500 colour TV set
Technology of Q 2500 colour TV set2.2.2 Start-upThe power supply section is started up by anadditional starting circuit. The required voltageis rectified using a diode path in the bridgerectifier. It is fed via resistor R 621 to pin 6 inTDA4605 where capacitor C 622 is thenslowly charged. During this charging phase,capacitor C 637 is charged to 6.6 V on pin 2via an internal IC path. A reference voltage of1V that is required during the start-up phaseand later during normal operation is also generatedin the IC.If the voltage at pin 6 reaches 12 V the IC operatesand makes the switching transistorconducting via pin 5. A current flows throughboth the transformer working winding and theswitching transformer drain source path. Duringthis period, magnetic energy is beingstored in the transformer. During this conductivephase, a drain current simulator C 637,integrated in the IC, is charged on pin 2. If theinternal reference value of 1 V is reached theIC blocks the switching transistor. The magneticfield in the transformer breaks down andthis induces voltages in the windings.The start-up procedure recommences and thesystem swings/oscillates to normal operation.This is arrived at when a voltage of 400 mVhas built up at pin 1.2.2.3 Normal and control operationA static state is set in normal operation atconstant load. The operating voltage for the ICis drawn from the transformer winding pin 5-6and rectified with D622. The voltage at pin 6 inthe IC is then 11 V. The control input at pin 1is 400 mV and the duty cycle for the zero passagedetector at pin 8 in the IC is set.The switching transistor is controlled with afixed frequency of between 20 and 40 kHz,which corresponds to the instantaneous load.If the load changes, the duty cycle at pin 8 inthe IC also changes. The negative edge indicatesto the IC when the energy stored in thetransformer has been dissipated. If the loadincreases, this occurs more quickly and the ICreduces the control frequency. If the load decreases,the control frequency increases. Thismeans that load variations between approx.40 and 260 W and mains voltage fluctuationsbetween 180 and 270 V can be compensatedfor.In order to achieve a higher UB voltage stability,regulation on the secondary side is nowused which influences the primary circuit’s I669 opto-coupler. The I 669 opto-coupler iscontrolled via I 670. The control mechanism isinfluenced via several paths:- With d.c. voltage via R 663- Alternating, by coupling to the R 680 diode.- When not under load, via D 672The operating voltage is set with the P662potentiometer. A small resistance here indicatesa high value of UB. Correspondingly, ahigh value of R means a low value for UB.If UB increases, e.g. due to a smaller load onthe line output stage, UB exerts a strongerinfluence via R 663 at the input of I 670. Thisresults in the I 670 cathode outputting asmaller voltage. The photodiode in I 669 receivesmore current through R 665. The pathbetween pin 3/4 of I 669 has a lower resistanceallowing for a higher voltage on pin 1 ofI 611. Component I 611 then regulates outputvoltage until a value of 400 mV on pin 1 isreached again. If the value of UB becomeslower, then the regulation process is exactlythe inverse of that described here.Via D 680, R 669 and C 669, ripple voltagecomponents are coupled into the regulatingcircuit. In particular, this branch prevents a low50 hz ripple voltage arising on UB. With 60 hzsignal sources (NTSC, PAL 60 Hz or VGAoperation), this would result in humming interferencein the picture.Document Q 2500 16 © Loewe ProCollege
Technology of Q 2500 colour TV setWithout secondary-side regulation, e.g. inSAT standby operation, UB would increase toapprox. 180 V. In this mode of operation,other voltages (e.g. the bias voltages for the U5, U 12 branch) do not alter significantly. Stabilizationis also guaranteed for these voltagesin this mode of operation.Due to the secondary-side stabilization withUB as stabilization factor, UB without any loadwould naturally remain constant. In this case,the power supply would regulate down fromapprox. 180 V (without secondary-side regulation)to the previously set value of UB, typically146 V. This would also result in all othervoltages being reduced. The bias voltages forthe stabilized voltages would be too low andthey would no longer be stabilized. The digitalelectronics would search for errors.In order to prevent this, the regulation acts viaU 7. This is accomplished via components D672, R 693 and 666.U 7 also drops when the line-output stage isdisconnected. When this has reached a valueof approx. 6V, the D 666 Schottky diode voltagehas also fallen sufficiently for it becomesconducting. A further drop in operating voltageis prevented by the I 670 input. Stabilization ofsecondary side voltages, e.g. for U 5 and U12, still operate properly in this mode ofoperation.The indicator for this is the voltage on pin 6 inTDA4605. During normal operation this voltageis approx. 11 V. If the load increases, dueto a short circuit on the secondary side, forexample, the voltage on pin 6 drops. If it dropsbelow 7V, the logic circuit switches off. Thesame occurs if the voltage on pin 6 exceeds15 V due to complete discharge or a fault inthe control circuit. There is also a second protectivecircuit to safeguard against overloading.If the current flowing through the workingwinding in the transformer and therefore alsothrough the switching transistor is so greatthat the voltage on pin 3 drops below 1 V, thepower supply also cuts out.In addition, integrated thermal protectionswitches the system off at chip temperaturesof over 150°.2.2.5 Power Factor ControlAll TV sets with power consumption of morethan 75 W, and delivered after 01.01.2001,must be fitted with a Power Factor Control(PFC) circuit.The circuit power supplies used up to nowoverlay a pulsed current on the sinusoidalmains alternating voltage. The Power FactorControl circuit ensures current extraction isalmost completely sinusoidal. This function isimplemented by changing the power supply’sinput rangePower Factor Control2.2.4 Protective operationT639The IC contains a protective circuit to preventthe control frequency from entering rangeswhere it would otherwise damage the switchingtransistor during over- and underload. Insuch cases the system is shut down.230V~R613D613D617C611L618C628D623C620Document Q 2500 17 © Loewe ProCollege
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<strong>Technology</strong> <strong>of</strong> Q <strong>2500</strong> <strong>colour</strong> <strong>TV</strong> <strong>set</strong>Without secondary-side regulation, e.g. inSAT standby operation, UB would increase toapprox. 180 V. In this mode <strong>of</strong> operation,other voltages (e.g. the bias voltages for the U5, U 12 branch) do not alter significantly. Stabilizationis also guaranteed for these voltagesin this mode <strong>of</strong> operation.Due to the secondary-side stabilization withUB as stabilization factor, UB without any loadwould naturally remain constant. In this case,the power supply would regulate down fromapprox. 180 V (without secondary-side regulation)to the previously <strong>set</strong> value <strong>of</strong> UB, typically146 V. This would also result in all othervoltages being reduced. The bias voltages forthe stabilized voltages would be too low andthey would no longer be stabilized. The digitalelectronics would search for errors.In order to prevent this, the regulation acts viaU 7. This is accomplished via components D672, R 693 and 666.U 7 also drops when the line-output stage isdisconnected. When this has reached a value<strong>of</strong> approx. 6V, the D 666 Schottky diode voltagehas also fallen sufficiently for it becomesconducting. A further drop in operating voltageis prevented by the I 670 input. Stabilization <strong>of</strong>secondary side voltages, e.g. for U 5 and U12, still operate properly in this mode <strong>of</strong>operation.The indicator for this is the voltage on pin 6 inTDA4605. During normal operation this voltageis approx. 11 V. If the load increases, dueto a short circuit on the secondary side, forexample, the voltage on pin 6 drops. If it dropsbelow 7V, the logic circuit switches <strong>of</strong>f. Thesame occurs if the voltage on pin 6 exceeds15 V due to complete discharge or a fault inthe control circuit. There is also a second protectivecircuit to safeguard against overloading.If the current flowing through the workingwinding in the transformer and therefore alsothrough the switching transistor is so greatthat the voltage on pin 3 drops below 1 V, thepower supply also cuts out.In addition, integrated thermal protectionswitches the system <strong>of</strong>f at chip temperatures<strong>of</strong> over 150°.2.2.5 Power Factor ControlAll <strong>TV</strong> <strong>set</strong>s with power consumption <strong>of</strong> morethan 75 W, and delivered after 01.01.2001,must be fitted with a Power Factor Control(PFC) circuit.The circuit power supplies used up to nowoverlay a pulsed current on the sinusoidalmains alternating voltage. The Power FactorControl circuit ensures current extraction isalmost completely sinusoidal. This function isimplemented by changing the power supply’sinput rangePower Factor Control2.2.4 Protective operationT639The IC contains a protective circuit to preventthe control frequency from entering rangeswhere it would otherwise damage the switchingtransistor during over- and underload. Insuch cases the system is shut down.230V~R613D613D617C611L618C628D623C620Document Q <strong>2500</strong> 17 © Loewe ProCollege
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