Main Oscillator, Enable, Regulation

Main Oscillator

The main oscillator stage operates like the standby oscillator stage ex- cept that the main oscillator is switched ON/OFF and there are multiple secondaries to supply the needs of the DVD player.

The main oscillator has four parts:

·Enable

·Start

·Run

·Regulation

Enable

Oscillator OFF

The operation of the main oscillator is inhibited by PC102. When the unit is plugged into AC, Ever 5V is made by the standby oscillator stage and is used to turn on the opto-isolator LED in PC102. The LED’s infrared light turns on the phototransistor and it conducts.

PC102’s phototransistor is connected to oscillator transistor Q101’s start voltage applied to the base. When PC201 conducts, the start voltage from R102/R103 is grounded, stopping the oscillator.

Oscillator ON

Q201 removes the voltage to the opto-isolator diode in PC102, permitting the main oscillator stage to run. When PCONT from Interface IC201 goes high at CN203/pin 1, Q201 turns ON. Its conduction grounds the voltage from R211, removing voltage from the LED. PC102 transistor turns off, removing the ground from R103 so the main oscillator transistor Q101 can start.

Start

Once the PCONT control line at CN203/pin 1 goes high, PC102’s transis- tor no longer conducts. The ground is removed from the start voltage at the junction of C111 and R104 so it rises. It will reach 6.6V, limited by the voltage divider formed by R102, R103 and R104.

Sufficient current passes through C111 during this voltage increase to turn ON main oscillator Q101. This is how oscillator transistor Q101 starts conducting (turns ON) and the main oscillator starts.

Run

Q101 Turns ON

When Q101 turns ON, current flows through the main transformer’s pri- mary winding at the upper left of T101. This induces a positive voltage to the lower T101 winding that follows two paths. The first path is through R106, R107, D109 and C113 to Q101’s base to keep it conducting. The second path is through D108 and R109 to bias Q102. Q102 acts like an active resistor at Q101’s base to keep the voltage from becoming exces- sive (protection) and is used later for regulation. Consequently the cor- rect value components and transistors are critical.

When Q101 reaches saturation, there is no longer a change in T101’s primary current. The steady current flow no longer induces a voltage into the lower secondary winding and the secondary voltage decays.

Q101 Turns OFF

The reduction in secondary voltage turns Q101’s OFF. The magnetic field in T101’s primary collapses. D105, limiter, L102 and C110 short its energy. D106 is used to discharge C110 (when Q101 is ON).

The collapsing (changing) magnetic field induces a negative voltage at the lower secondary winding. This negative voltage from T101 passes through R106, R107 and C113 to the base of oscillator transistor Q101. It is used to cut off Q101 to conclude this oscillator cycle.

Regulation

The 3.3-volt output is monitored by error regulator IC201 and used to control the conduction of phototransistor PC101. If the 3.3-volt line rises, the phototransistor conducts harder. This reduces the resistance between the lower secondary winding and Q102’s base, increasing Q102 base current.

The increased base current drives Q102 harder, lowering the oscillator’s base bias voltage and causing the oscillator to be turned OFF sooner. When part of the oscillator waveform is shortened, its frequency increases. This reduces T101 efficiency and the 3.3V output voltage is returned to normal.

IMPORTANT VOLTAGES:

Q101/Collector = 336Vp-p; 150Vdc.

PC101/collector = 2.6Vdc. PC101/emitter = 0.21Vdc

Feedback secondary voltage at D108/cathode = 20Vp-p; 0.21Vdc.