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
. The frequency will be
VCO,start
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
= 175
oscH
kHz typically). At 50 mV (V
) the frequency is
VCO,max
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.
frequency limit
f
= 175 kHz
oscH
kH
MinF
FR
QR
V
VCO,start
switching
frequency
V
VCO ,max
f
= 6 kHz
oscL
P
P
OUT_MIN
OUT_MAX
power
Multi mode operation
CL 16532020_080.eps
100401
Figure 9-13
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
reaching UVLO level (fold back during overload),
CC
•
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
winding will not
CC
charge the V
capacitor anymore and the V
voltage will
CC
CC
drop until UVLO is reached. To recharge the V
capacitor, the
CC
internal current source (I
) will be switched ‘on’ to
(restart)(VCC)
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.