Philips L01.1L AC, Service Manual

Circuit Description

EN 78

L01.1L AC

9.

Figure 9-12 Burst mode waveforms

9.6.3

Protection Events

The SMPS IC 7520 has the following protection features:

Demagnetization 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 demagnetization 
of transformer 5520 is complete. 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(netization) sense is realized by an internal circuit 
that guards the voltage (V_demag) at pin 4 that is connected to 
V_cc winding by resistor R1 (R3522). 
The figure below shows the circuit and the idealized waveforms 
across this winding.

Figure 9-13 Demagnetization protection

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_cc). This is 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 
demagnetization resistor R3522. 

Over Current Protection
The internal OCP protection circuit limits the 'sense' voltage on 
pin 5 to an internal level.

Over Power Protection
During the primary stroke, the rectified AC input voltage is 
measured by sensing the current drawn from pin 4 (DEM). This 
current is dependent on the voltage on pin 9 of transformer 
5520 and the value of R3522. The current information is used 
to adjust the peak drain current, which is measured via pin 
I_sense. 

Short Winding Protection
If the 'sense' voltage on pin 5 exceeds the short winding 
protection voltage (0.75 V), the converter will stop switching. 
Once V_cc drops below the UVLO level, capacitor C2521 will 
be recharged and the supply will start again. This cycle will be 
repeated until the short circuit is removed (safe restart mode).
The short winding protection will also protect in case of a 
secondary diode short circuit.
This protection circuit is activated after the leading edge 
blanking time (LEB).

LEB  time
The LEB (Leading Edge Blanking) time is an internally fixed 
delay, preventing false triggering of the comparator due to 
current spikes. This delay determines the minimum 'on' time of 
the controller. 

Over Temperature protection
When the junction temperature exceeds the thermal shutdown 
temperature (typ. 140 deg. C), the IC will disable the driver. 
When the V_cc voltage drops to UVLO, the V_cc capacitor will 
be recharged to the V(start) level. If the temperature is still too 
high, the V_cc voltage will drop again to the UVLO level (Safe-
Restart mode). This mode will persist until the junction 
temperature drops 8 degrees typically below the shutdown 
temperature.

Mains dependent operation enabling level
To prevent the supply from starting at a low input voltage, which 
could cause audible noise, a mains detection is implemented 
(Mlevel). This detection is provided via pin 8, which detects the 
minimum start-up voltage between 60 and 100 V. As previous 
mentioned, the controller is enabled between 60 and 100 V.
An additional advantage of this function is the protection 
against a disconnected buffer capacitor (C_in). In this case, the 
supply will not be able to start-up because the V_cc capacitor 
will not be charged to the start-up voltage.

V

(start)

V

(UVLO)

V

CC

V

STAB

I

L

Active/
inactive

t1

t2

t3

V

µ

C

Soft start

 Burst mode waveforms

CL 16532020_082.eps

100401

R

1

R

2

D

V

CC

winding

I

(ovp)(demag)

I

(opp)(demag)

Demag

4

Ctrl

Gnd

Vcc

Drain

HVS

Driver

Sense

3

2

1

5

6

7

8

configuration

A

B

0V

OUT

S

Vcc

V

N

N

⋅

IN

P

Vcc

V

N

N

⋅

Demagnetization

V

GATE

V

WINDING

Magnetization

V

demag

Comparator
threshold
V

demag

0.7V

0V

-0.25V