Philips L9.2A, Service Manual

Circuit description new circuits

GB 57

L9.2A

9.

of this paragraph. This cycle will repeat itself and can be 
noticed by an audible hick-up sounding noise.

•

Take for IC7520: During start-up a voltage across winding 
8 - 9 is gradually built up. At the moment the voltage across 
winding 8 - 9 reaches approx. (14.5 Volts, D6540 start 
conducting and takes over the supply voltage Vpin 1 of 
IC7520 (take over current is approx. 17mA). 

Note: This power supply is a SMPS (= Switched Mode Power 
Supply) and not a SOPS (= Self Oscillating Power Supply).

9.3

Control circuitry

9.3.1

IC7520 control mechanisms

IC7520 controls the T-on time of FET 7518 in four different 
ways:
•

 "Secondary-output-sensing" controls the secondary 
output voltages via the feedback voltage pin 14

•

"Primary current sensing" control due to the mains voltage 
via the current sense voltage pin 7

•

"Demagnetization control" prevents the transformer T5545 
from going into saturation via the so-called "DEMAG" 
function at pin 8

•

Mains voltage control via R3514 and R3516

9.3.2

Secondary voltage sensing (pin 14 of IC7520)

When the output vVBATT increases (due to a reduction 
in the load ) the current through the led in the opto-coupler 7581 
will increase due to the fact that the series-resistor in regulator 
7570 decreases. An increase in opto-coupler led-current 
(7581) results in a decrease in the Vce of transistor 7581, 
therefore the voltage across capacitor 2576 increases. This will 
reduce the on-time of  FET 7518 due to an increase of the 
voltage present on pin 14.
In the event of an increase of the load (decrease of output 
vVBATT ), the control circuit will work in the opposite 
way to the explanation above.

9.3.3

Primary sensing (pin 7 of IC7520)

The current sense voltage at pin 7 is used to measure the 
primary current through FET7518. The primary current is 
converted into a voltage by R3518. R3514. 3516. couples a 
part of the main voltage to the same pin 7 of IC 7520 by dividing 
this sample of the voltage. 
Hence the higher the input voltage the more the primary current 
is limited. In this way the maximum output power of the power-
supply is limited.

9.3.4

Demagnetization control (pin 8 of IC7520) 

Winding 8 - 9 has the same polarity as the secondary winding 
that supplies the load. When FET 7518 is turned off the voltage 
at winding 9 becomes positive. The power supply transfers the 
stored energy at the secondary side. Until the transformer is 
demagnetized the voltage on the winding remains positive. At 
the moment that the energy is fully transferred to the load, the 
voltage at pin 9 of the transformer becomes negative. 
Additionally with a certain dead time the voltage at control pin 
8 of IC 7520 also drops below zero which releases the output 
buffer (pin 3) and a new cycle starts.

9.3.5

Peak current limiting

An internal clamp at pin 7 allows peak current limiting to be 
achieved . This pin can never exceed 1V DC and so the 
maximum primary current through FET 7518, and also the 
maximum output power is determined. In case of an output 
being short-circuited or loaded excessively, the I-prim becomes 

too high which is detected by pin 7. As a result the primary 
current is limited to its maximum value and the secondary 
voltages will drop. The voltage at pin 1, which is coupled with 
the output voltage, will also drop. When the voltage at pin 1 
drops below the 9V, IC7520 will stop functioning and the output 
voltage will rapidly drop to zero.
Via start-up circuitry 3510, 3530 and 3529 the voltage 
originating from the 230V AC mains is used to start-up IC7520 
via the supply pin 1. As soon as this voltage reaches the 14.5V, 
IC7520 starts functioning. If the load is still too much or the 
output is short-circuited the same cycle will happen again. This 
fault condition can be clearly identified as the power supply will 
be loudly tripping. 

9.3.6

Slow-start

As soon as Vpin 1 > 14.5V the SMPS will start-up. During the 
slow-start procedure both the frequency and the duty cycle will 
be built up slowly. The duty cycle will initially slowly increase 
commencing with the absolute lowest possible duty cycle. The 
maximum duty cycle is determined by C2530 at pin 11 of 
IC7520, as C2530 is uncharged at start-up. 

9.3.7

Standby mode

In standby mode the SMPS switches to the so-called "reduced 
frequency mode" and runs at about 20 kHz. During standby the 
SMPS only has to deliver a minimal level of output power. The 
minimal load threshold level is determined by R3532 at pin 12. 
In the L9 chassis the SMPS does not have a burst mode in 
standby but only a reduced frequency mode of about 20 kHz as 
stated above. In normal operation mode the internal oscillator 
is around 40 kHz. This frequency is controlled by C2531 at pin 
10 of IC7520 and by R3537 at pin 16 of IC7520. In standby 
mode the frequency of operation is determined by R3536 at pin 
15 of IC7520.

9.3.8

Protections

Over voltage protection of the secondary voltages.
After start-up the supply voltage pin 1 will be "taken over" by  
winding 8 - 9.  Pin 1 of IC 7520 is used to detect an over voltage 
situation on the secondary side of the transformer. If this 
voltage exceeds 17V (typically the output buffer is disabled, 
and IC 7520 goes into over voltage protection and a complete 
restart sequence is required. Check in this case IC7520, 
IC7581 and the secondary vVBATT ( +140V ).
REMARK: In the event of the over voltage situation remaining 
present, the SMPS will go in protection, start up cycle, 
protection, etc. The standby led on the front of the set starts 
flashing.

Under voltage protection of the secondary voltages
If the supply voltage at pin 1 of IC 7520 drops below 9V 
because of a short-circuit or excessive load, the  drive pulse 
present at pin 3 will be disabled and IC7520 will switch off the 
complete SMPS. Capacitor C2450 is charged up via start-up 
resistors 3510, 3530 and 3529, however once the voltage 
exceeds 14.5V start up threshold, the SMPS will once again 
commence a re start cycle. 
In the event of the under voltage situation remaining, the SMPS 
will again go in protection mode, start up cycle, protection,  etc. 
and so the cycle repeats. This effect is highly audible.

9.4

Audio processing

The following systems are available:
•

BASIC : MONO/AV STEREO ( M,BG, I and DK : single or 
dual system )