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File name Tatung_TM01_Monitor.pdf

SECTION 3 ISOLATED SWITCHED-MODE P O W E R SUPPLY 3.1. CIRCUIT DESCRIPTION _I . The power supply is a self-oscillating switching type of converter which, under normal conditions, operates at a free-running frequency l of between 20 and 3OkHz. A l control functions are performed within 1801 (TDA4600). The output transistor, Q801, switches the primary winding of the transformer, T801, across the rectified mains voltage developed on the reservoir capacitor, C804. The output voltages, (121V and 18V, are generated at taps on the secondary winding and rectified and smoothed by D812, D811, C822, C826 and C821. The base drive for Q801 is generated in I801, the current at pin 8 controlling the switch-on period and that at pin 7 controlling the turn-off conditions of Q801. R808, R810 and C813 develop a sawtooth voltage at pin 4 which simulates the collector current of Q801. This is used to generate a sawtooth base drive at pin 8 to avoid over-saturation of Q801. 0803 and C314 rectify and smooth a voltage from the feedback winding on T801 which is proportional to the output voltages. This voltage is attenuated by R313, R312, R807 and R806 with respect to a reference voltage at pin 1 and applied to pin 3. Any changes in the output are thus transmitted via pin 3 to the control logic and the base current amplifier e within the I.C. In this way the frequency and duty cycle of the output pulses are adjusted to correct for the changes. R813 (set H.T.) adjusts the proportion of voltage fed back and hence adjusts the output vol+age. In order to complete the oscillator feedback loop R8i4 feeds an attenuated voltage from the feedback winding on T801 and pin 2. This enables the I.C. to identify the points at which the output pulse crosses the zero voltage level and so provide correctly timed base drive pulses to Q801. The supply for the I.C. is developed by D806 and C808 from a third winding on T801. D805 and R802 provide a start-up supply for the I.C. If an overload occurs on the 121V output voltage, the frequency and the duty cycle are reduced, thus limiting the power supplied to the load, If the overload becomes a short circuit, the frequency reduces further to 1.4kHz and the power output is limited to a low level. 3.2. VOLTAGE ADJUSTMENT with the receiver operating with a normal picture signal, reduce the brightness control for zero beam current. Adjust the 'Set H.T.' preset potentiometer R813 to give a meter reading of 117V at TP45, the supply voltage- to the line scan output transistor. 3-l 3.3. VOLTAGE MEASUREMENTS low Measured with a multimeter (2OkOhm/V) cn a normal picture displayed unless otherwise state? d.c. range With Integrated Circuit I801 (TDA4600) Rectified mains across C 8 0 4 330V d.c. (iOOOV range) IMPORTANT: All other voltages on I801 and Q801 are measured with respect to the heat sink tab of I801 as a reference level o for the power supply. Integrated Circuit 1801 Pin 1 2 3 4 5 V 4.4 0.1 2.2 2.2 6.3 Pin 6 7 8 9 V 0 1.8 1.8 16.5 (3