Demagnetization; Minimum And Maximum ‘On-Time’; Overvoltage Protection (Ovp) - TCL SY33 Service Manual

Philips Semiconductors
GreenChip™II SMPS control IC

Demagnetization

The system will be in discontinuous conduction mode all
the time. The oscillator will not start a new primary stroke
until the secondary stroke has ended.
Demagnetization features a cycle-by-cycle output
short-circuit protection by immediately lowering the
frequency (longer off-time), thereby reducing the power
level.
Demagnetization recognition is suppressed during the first
t time. This suppression may be necessary in
suppr
applications where the transformer has a large leakage
inductance, at low output voltages and at start-up.
If pin DEM is open-circuit or not connected, a fault
condition is assumed and the converter will stop operating
immediately. Operation will recommence as soon as the
fault condition is removed.
Minimum and maximum 'on-time'
The minimum 'on-time' of the SMPS is determined by the
Leading Edge Blanking (LEB) time. The IC limits the
'on-time' to 50 µs. When the system desires an 'on-time'
longer than 50 µs, a fault condition is assumed (e.g.
removed C in Fig.12), the IC will stop switching and enter
i
the safe restart mode.

OverVoltage Protection (OVP)

An OVP mode is implemented in the GreenChip series.
This works for the TEA1506 by sensing the auxiliary
voltage via the current flowing into pin DEM during the
secondary stroke. The auxiliary winding 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, an internal
counter starts counting subsequent OVP events. The
counter has been added to prevent incorrect OVP
detections which might occur during ESD or lightning
events. If the output voltage exceeds the OVP trip level a
few times and not again in a subsequent cycle, the internal
counter will count down with twice the speed compared
with counting-up. However, when typical 10 cycles of
subsequent OVP events are detected, the IC assumes a
true OVP and the OVP circuit switches the power
MOSFET off. Next, the controller waits until the UVLO
level is reached on pin V
capacitor C will be recharged to the V
VCC
2003 Sep 09
. When V drops to UVLO,
CC CC
level.
start
TEA1506P; TEA1506AP;
TEA1506T; TEA1506AT
Regarding the TEA1506, the IC will not start switching
again. Subsequently, V
level, etc.
Operation only recommences when the V
below a level of about 4.5 V.
Regarding the TEA1506A, when the V
switching starts again (safe restart mode) when the V
level is reached. This process is repeated as long as the
OVP condition exists.
The output voltage V
can be set by the demagnetization resistor, R
V
=
( )
o OVP
N
s
{ ×
----------- I R
(OVP)(DEM)
N
aux
where N is the number of secondary turns and N
s
number of auxiliary turns of the transformer.
Current I is internally trimmed.
(OVP)(DEM)
The value of R can be adjusted to the turns ratio of the
DEM
transformer, thus making an accurate OVP possible.
8
Product specification
will drop again to the UVLO
CC
CC
level is reached,
start
at which the OVP function trips,
o(OVP)
}
+ V
DEM clamp(DEM)(pos)
voltage drops
start
:
DEM
is the
aux