Philips cdr 775 service manual
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Page 1
Faultfinding Guide GB 60 CDR779 8. capacitor at the Vcc pin will discharge because the primairy auxiliary voltage, coming from winding7-9 is below the Vcc voltage. At some moment during t2, the primary auxiliary voltages reaches the same level as Vcc. This primary auxiliary voltage now determines the Vcc voltage t3: regulation The output voltage of the power supply is in regulation t4: overload When the output is shortened, the supply voltage of the circuit will decrease and after some time drop below the lower threshold voltage. At that moment, the output will be disabled and the process of charging the Vcc capacitor starts again. If the output is still shorted at the next t2 phase, the complete start-and stop sequence will repeat. The power supply comes in a hiccup mode. Figure 8-12 8.2.5 Regulation Figure 4 shows the most relevant signals during the regulation phase of the power supply. The oscillator voltage ramps up and down between V1 and V2. The voltage at the current sense terminal is compared every cycle with the output of the error amplifier Vcomp. The output is switched off when the current sense level exceeds the level at the output of the error amplifier. TimeON phase : A drain current will flow from the positive supply at pin 1 through the transformers primary winding, the MOSFET and Rsense to ground. As the positive voltage at pin 1 of the transformer is constant, the current will increase linearly and create a ramp dependent on the mains voltage and the inductance of the primary winding. A certain amount of energy is stored in the transformer in the form of a magnetic field. The polarity of the voltages at the secundary windings is such that the diodes are non-conducting. TimeDIODE phase : When the MOSFET is switched off, energy is no longer supplied to the tranformer. The inductance of the tranformer now tries to maintain the current which has been flowing through it at a constant level. The polarity of the voltage from the transformer therefore becomes reversed. This results in a current flow through the tranformers secondary winding via the diodes, electrolytic capacitors and the load. This current is also ramp shaped but decreasing. TimeDEAD phase : when the stored energy has been supplied to the load, the voltage from the secondary windings falls below the output voltage(held constant by the electrolytic capacitors) plus the threshold voltage of the diodes. The current in the secondary winding stops flowing. At this point, the drain voltage of the MOSFET is not yet zero because C2609 between drain and source contains a certain charge. This charge will start a sine-shaped ringing together with the transformers self-induction. The oscillator will start a next cyclus which consists of the described three phases. The time of the different phases depends on the mains voltage and the load. TimeDEAD is maximum at an input of 400VDC and minimum load, it will be zero at an input of 100VDC and overload. Figure 8-13 Figure C : Start-up sequence 1mA t1 Vo 0 OUTPUT t2 t3 t4 short Vcc Vc2134 Icc20mA 0V 10V 16V 12V CL 06532151_020.eps 271100 Vosc Idiodes Vgate Vdrain Idrain Vsense Vcomp0 V2 V1 Ton Tdiode Tdead CL 06532151_021.eps 271100
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Faultfinding Guide GB 61 CDR779 8. 8.2.6 Oscillograms Figure 8-14 8.2.7 Circuit description Input circuit The input circuit consists of a lightning protection circuit and an EMI filter. The lightning protection comprises R3120, gasarrestor 1125 and R3124. The EMI filter is formed by C2120, L5120, C2125 and R3124. It prevents inflow of noise into/from the mains. Primary rectifier/smoothing circuitThe AC input is rectified by rectifier bridge 6102 and smoothed into C2121. The voltage over C2121 is approximately 300V. It can vary from 100V to 390V. Start circuit and Vcc supplyThis circuit is formed by R3123, R3134, C2129, D6129, R3129, R3111, C2133 and C2111. When the power plug is connected to the mains voltage, the stabilised voltage over D6129(24V) will charge C2133 via R3129. When the voltage reaches 14,5V across C2111, the control circuit of IC7110 is turned on and the regulation starts. During regulation, Vcc of IC7110 will be supplied by the rectified voltage from winding 7-9 via L5132, D6132 and C2133. Control circuit The control circuit exists of IC7110, C2102, C2104, C2107, C2109, C2110, R3102, R3103, R3104, R3107, R3108, R3109 and R3110. C2102 and R3110 define the frequency of the oscillator. Power switch circuitThis circuit comprises MOSFET 7125, Rsense 3126, 3127 and 3128, R3125, C2127, L5125, R3112 and R3113. R3125 is a pull-down resistor to remove static charges from the gate of the MOSFET. Regulation circuitThe regulation circuit comprises opto-coupler 7200 which isolates the error signal from the control IC on the primary side and a reference component 7201. The TL431(7201) can be represented by two components: a very stable and accurate reference diode a high gain amplifier Figure 8-15 TL431 will conduct from cathode to anode when the reference is higher than the internal reference voltage of about 2.5V. If the reference voltage is lower, the cathode current is almost zero. The cathode current flows through the LED of the opto- coupler. The collector current of the opto-coupler flows through R3106, producing an error voltage, connected to voltage feedback pin 14 of IC7110. Overvoltage protection circuit This circuit consist of D6114, C2114, R3115and R3116. Oscillograms PM3394B ch2 ch3 ch1 CH1 2 CH2 CH3 2 V~ ALT MTB5.00us- 0.90dv ch1- 1 2 3 T ch1 : Drain voltage ch2 : Drain current ch3 : Gate voltage PM3394B ch3 ch1 CH1 1 CH3 50mV~ ALT MTB5.00us- 0.90dv ch1- 1 3 T ch1 : Drain voltage ch2 : Oscillator voltage PM3394B ch3 ch1 CH1 1 CH3 20mV~ ALT MTB5.00us- 0.90dv ch1- 1 3 T ch1 : Drain voltage ch3 : Sense voltage CL 06532151_022.eps 301100 A 2.5VR K CL 06532151_023.eps 271100