Sony STR-DE845 Training Manual page 32

This switching power supply actually has four modes of operation: OFF, Standby, Power ON and power ON in
DVD. In the OFF mode, S901 is open. In the standby mode, S901 closes, the oscillator (IC901) outputs in bursts
and the error regulator (IC902) is disabled (by D912) to prevent the oscillator from shutting down. In standby, the
power supply only outputs 5.6 volts (and 19Vdc from the 27V line). In the power ON mode when P Cont 1 goes
HIGH, the oscillator switches to a continuous sine wave output and the error regulator is enabled to maintain a
stable 27Vdc output. In the power ON in DVD mode, P Cont 2 also goes HIGH, turning ON regulator IC906.
IC906 outputs 3.3 Vdc to the DVD board.
Power Supply Operation – Standby Mode
When front panel S901 is closed, AC is input to the switching power supply and the supply enters the Standby
Mode to output 5.6Vdc. At one of the AC inputs of bridge rectifier D902, 62Vdc is available with respect to hot
ground. This voltage is regulated by Q903 down to 20Vdc and applied to switches Q901-Q902. In the standby
mode, only 11Vdc is passed by the switches into Switching Regulator IC901 at Vcc. With 11Vdc input IC901, the
oscillator outputs bursts of 200kHz. See Waveform 9-1.
Channel 1
Time base
The burst output of IC901 is applied to power transformer T901. T901 outputs two (standby) voltages, listed
in Chart 9-2:
9Vdc
19Vdc
To keep T901's output voltages present but reduced in the standby mode, the error regulator (IC902) must be
disabled. That is the function of D912. To prevent IC901's oscillator from shutting off, D912 is enabled to bring
up the error input voltage at IC901/pin 1 to within the operating range of 0.8V – 4Vdc. See Figure 9-3. D912 is
enabled using inverters Q905 and Q908. In the standby mode, P Cont 1 = 0Vdc. This voltage is inverted by
Q905 and Q908, placing 0Vdc (gnd) at D912's Anode, enabling it. D912 clamps the voltage into PC901 (disabling
IC902) and PC902 outputs 9.5V at pin 3. This 9.5V output voltage is reduced by R912 to produce 1.1Vdc at
IC901/pin 1 to keep the oscillator alive.
ch1: pkpk= 246 V
ch1: freq= 11.6kHz
T
1
CH1!50.0 V= MTB1.00ms- 0.78dv ch1+
Waveform 9-1 - Standby Mode IC901 Output
Name
Drain Output
Chart 9-2 - T901 Output Voltages (after rectification)
Standby Mode
9. DAV-C700 Switching Type Power Supply
Location
C916 / IC901/pin 3
1msec/div
Power ON
13Vdc
27Vdc
30
Voltage/div
248Vp-p