Iii - Regulation - Sony STR-DE845 Training Manual

To do this, P.Cont 1 turns ON Q915, which forces Inverter IC907 to output a HIGH at pin 4 for PC902. The
transistor in isolator PC902 turns OFF, allowing switches Q901 and Q902 to fully conduct (via R905). As a result,
the entire regulated 20Vdc from regulator Q903 (emitter) is applied through Q902 (e-c) into switching regulator
IC901/pin 4. The higher Vcc voltage causes IC901 to change from a burst output to a continuous sine wave
output in the Power ON Mode. The continuous sine wave output is shown in Waveform 9-2.
Waveform 9-2 - Normal IC901 Output at Power ON – 213kHz (205kHz in DVD)
Channel 1
Time base
The HIGH at P.Cont 1 has a second purpose. It is also used to enable error regulator IC902. In the Power ON
mode when P.Cont 1 is 5Vdc, D912 is switched out of the circuit. Q908 turns ON and Q905 turns OFF so D912's
anode is floating and effectively "out of the circuit". When D912 is no longer clamping IC902/pin 2's output
voltage, IC902 can regulate the 27Vdc output from T901.
Figure 9-3 shows the regulating circuit. The regulation components consist of IC902 and PC901. Both act as
inverters. IC902 is the error regulator, monitoring the 27 volt line at pin 1 (Pwr On = 6.3Vdc). PC901 is an
optical-isolator, isolating the cold ground from the hot ground signal. PC901 also acts to invert the signal from
IC902. If the 27V-output line increases, the resultant correction voltage input the switching regulator IC901/pin
1 also goes high to compensate.

III - Regulation

The power supply output regulation was not fully explained in the switching power supply operation. The mystery
of how the DC error correction voltage input IC901/pin 1 controls the power supply output voltage is not necessary
for repairs, but is important to completely understand the switching power supply regulation concept.
Concept
In this switching power supply there is a frequency controlled oscillator (IC901) and a resonate circuit load. The
resonate circuit is formed by power transformer T901 (the inductor = L), and C916 (the capacitor = C). These
parts are located at the top of Figure 9-3.
PM3394, FLUKE & PHILIPS
ch1: pkpk= 301 V
ch1: freq= 213kHz
T
1
CH1!50.0 V= MTB2.00us- 0.54dv ch1+
Name Location
Drain Output C916 / IC901/pin 3
9. DAV-C700 Switching Type Power Supply
Voltage/div
300Vp-p
2usec/div
32