Philips L01.1L AC, Service Manual

Circuit Description

EN 77

L01.1L AC

9.

The moment that the voltage on pin 1 drops below the 'under 
voltage lock out' level (UVLO = 

±

 9 V), the IC will stop switching 

and will enter a safe restart from the rectified mains voltage.

Operation
The supply can run in three different modes depending on the 
output power:
•

Quasi-Resonant mode (QR). The 'QR' mode, described 
above, is used during normal operation. This will give a 
high efficiency.

•

Frequency Reduction mode (FR). The 'FR' mode (also 
called 'VCO' mode) is implemented to decrease the 
switching losses at low output loads. In this way, the 
efficiency at low output powers is increased, which enables 
power consumption smaller than 3 W during stand-by. The 
voltage at the pin 3 (Ctrl) determines where the frequency 
reduction starts. An external Ctrl voltage of 1.425 V 
corresponds with an internal VCO level of 75 mV. This 
fixed VCO level is called V_vco,start . The frequency will be 
reduced in relation to the VCO voltage between 75 mV and 
50 mV (at levels larger than 75 mV, Ctrl voltage < 1.425V, 
the oscillator will run on maximum frequency f_oscH = 175 
kHz typically). At 50 mV (V_vco,max), the frequency is 
reduced to the minimum level of 6 kHz. Valley switching is 
still active in this mode.

•

Minimum Frequency mode (MinF). At VCO levels below 
50 mV, the minimum frequency will remain on 6 kHz, which 
is called the 'MinF' mode. Because of this low frequency, it 
is possible to run at very low loads without having any 
output regulation problems.

Figure 9-10 Different supply modes

Safe-Restart Mode
This mode is introduced to prevent the components from being 
destroyed during eventual system fault conditions. It is also 
used for the Burst mode. The Safe-Restart mode will be 
entered if it is triggered by one of the following functions:
•

Over voltage protection,

•

Short winding protection,

•

Maximum 'on time' protection,

•

V_cc reaching UVLO level (fold back during overload),

•

Detecting a pulse for Burst mode,

•

Over temperature protection.

 
When entering the Safe-Restart mode, the output driver is 
immediately disabled and latched. The V_cc winding will not 
charge the V_cc capacitor anymore and the V_cc voltage will 
drop until UVLO is reached. To recharge the V_cc capacitor, 
the internal current source (I_(restart)(vcc) ) will be switched 
'on' to initiate a new start-up sequence as described before. 
This Safe-Restart mode will persist until the controller detects 
no faults or burst triggers.

Standby 
The set goes to Standby in the following cases:
•

After pressing the 'standby' key on the remote control.

•

When the set is in protection mode.

In Standby, the power supply works in 'burst mode'. Burst 
mode can be used to reduce the power consumption below 1 
W at stand-by. During this mode, the controller is active 
(generating gate pulses) for only a short time and for a longer 
time inactive waiting for the next burst cycle.
In the active period, the energy is transferred to the secondary 
and stored in the buffer capacitor C_stab in front of the linear 
stabilizer (see figure below). During the inactive period, the 
load (e.g. microprocessor) discharges this capacitor. In this 
mode, the controller makes use of the Safe-Restart mode.

Figure 9-11 Supply standby mode (burst mode)

The system enters burst mode standby when the 
microprocessor activates the 'Stdby_con' line. When this line is 
pulled high, the base of TS7541 is allowed to go high. This is 
triggered by the current from collector TS7542. When TS7541 
turns 'on', the opto-coupler (7515) is activated, sending a large 
current signal to pin 3 (Ctrl). In response to this signal, the IC 
stops switching and enters a 'hiccup' mode. This burst 
activation signal should be present for longer than the 'burst 
blank' period (typically 30 s): the blanking time prevents false 
burst triggering due to spikes.
Burst mode standby operation continues until the 
microcontroller pulls the 'Stdby_con' signal low again. The 
base of TS7541 is unable to go high, thus cannot turn 'on'. This 
will disable the burst mode. The system then enters the start-
up sequence and begins normal switching behavior.
 
For a more detailed description of one burst cycle, three time 
intervals are defined:
•

t1: Discharge of V_cc when gate drive is active. During 
the first interval, energy is transferred, which result in a 
ramp-up of the output voltage (V_stab) in front of the 
stabilizer. When enough energy is stored in the capacitor, 
the IC will be switched 'off' by a current pulse generated at 
the secondary side. This pulse is transferred to the primary 
side via the opto coupler. The controller will disable the 
output driver (safe restart mode) when the current pulse 
reaches a threshold level of 16 mA into the 'Ctrl' pin. A 
resistor R1 (R3519) is placed in series with the opto 
coupler, to limit the current going into the 'Ctrl' pin. 
Meanwhile the V_cc capacitor is discharged but has to stay 
above V_uvlo .

•

t2: Discharge of V_cc when gate drive is inactive. 
During the second interval, the V_cc is discharged to 
V_uvlo. The output voltage will decrease depending on the 
load.

•

t3: Charge of V_cc when gate drive is inactive. The third 
interval starts when the UVLO is reached. The internal 
current source charges the V_cc capacitor (also the soft 
start capacitor is recharged). Once the V_cc capacitor is 
charged to the start-up voltage, the driver is activated and 
a new burst cycle is started.

frequency limit

P

OUT_MIN

                          P

OUT_MAX

f

oscL

 = 6 kHz

f

oscH

 = 175 kHz

kH

MinF       FR                                     QR

power

switching
frequency

V

VCO,start

V

VCO,max

            Multi mode operation

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Demag

4

Ctrl

Gnd

Vcc  Drain

HVS

Driver

Sense

3

2

1

5

6

7

8

Burst-Mode stand-by on/off
from microprocessor

Linear
stabilizer

V

STAB 

V

µ

C

C

STAB

C

Vcc

V

CC

R

1

Current pulse
generator

V

IN

Basic Burst mode configuration

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