Protection Events; Demagnetisation Sense; Over Voltage Protection; Over Current Protection - Philips L01.1E Service Manual

• 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
stabiliser (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.
V
IN
V
CC
1 Vcc Drain 8
2 Gnd HVS 7
C
Vcc
3 Ctrl Driver 6
4 Demag Sense 5
R
1
Basic Burst mode configuration
Figure 9-14
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 ms): 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 behaviour.
For a more detailed description of one burst cycle, three time
intervals are defined:
• t1: Discharge of V
CC
first interval, energy is transferred, which result in a
ramp-up of the output voltage (V
stabiliser. 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 R
in series with the opto coupler, to limit the current going
into the Ctrl pin. Meanwhile the V
discharged but has to stay above V
• t2: Discharge of V
CC
the second interval, the V
output voltage will decrease depending on the load.
• t3: Charge of V when gate drive is inactive The third
CC
interval starts when the UVLO is reached. The internal
current source charges the V
start capacitor is recharged). Once the V
charged to the start-up voltage, the driver is activated
and a new burst cycle is started.
in front of the linear
STAB
V
STAB
Linear
stabilizer
C
STAB
Burst-Mode stand-by on/off
from microprocessor
Current pulse
generator
CL 16532020_081.eps
when gate drive is active During the
) in front of the
STAB
(R3519) is placed
1
capacitor is
CC
.
UVLO
when gate drive is inactive During
is discharged to V . The
CC UVLO
capacitor (also the soft
CC
capacitor is
CC
Circuit Description
Active/
inactive
I
L
V
STAB
V
CC
V
µC
9.6.3

Protection Events

The SMPS IC7520 has the following protection features:

Demagnetisation sense

This feature guarantees discontinuous conduction mode
operation in every situation. The oscillator will not start a new
primary stroke until the secondary stroke has ended. This is
to ensure that FET 7521 will not turn on until the
100401 demagnetisation of transformer 5520 is completed. The
function is an additional protection feature against:
• saturation of the transformer,
• damage of the components during initial start-up,
• an overload of the output.
The demag(netisation) sense is realised by an internal circuit
that guards the voltage (Vdemag) at pin 4 that is connected
to V winding by resistor R
CC
shows the circuit and the idealised waveforms across this
winding.
1
2
3
4
I I
(opp)(demag)
(ovp)(demag)
configuration
A
B

Over Voltage Protection

The Over Voltage Protection ensures that the output voltage
will remain below an adjustable level. This works by sensing
the auxiliary voltage via the current flowing into pin 4 (DEM)
during the secondary stroke. This voltage is a well-defined
replica of the output voltage. Any voltage spikes are
averaged by an internal filter.
If the output voltage exceeds the OVP trip level, the OVP
circuit switches the power MOSFET 'off'.
Next, the controller waits until the 'under voltage lock out'
level (UVLO = ± 9 V) is reached on pin 1 (V
followed by a safe restart cycle, after which switching starts
again. This process is repeated as long as the OVP condition
exists. The output voltage, at which the OVP function trips, is
set by the demagnetisation resistor R3522.

Over Current Protection

The internal OCP protection circuit limits the 'sense' voltage
on pin 5 to an internal level.
L01.1E 9.
t2
t1
t3
Soft start
Burst mode waveforms
CL 16532020_082.eps
Figure 9-15
(R3522). The Figure below
1
V
Vcc Drain 8 GATE
Gnd HVS 7
Demagnetization
Ctrl Driver 6
Demag Sense 5
V
WINDING
R
1
0V
D
R
2
V
CC
winding
Magnetization
V
demag
0V
Figure 9-16
GB 63
V µC
V
(start)
V
(UVLO)
100401
N
⋅
Vcc
V
OUT
N
S
N
⋅
Vcc V
IN
N
P
Comparator
0.7V
threshold
V
demag
-0.25V
). This is
CC