Circuit Descriptions, Abbreviation List, and IC Data Sheets
9.
Figure 9-11
Point P1 is the minimum frequency f
MIN
that occurs at the
specified minimum input voltage and maximum output power
required by the application. Of course the minimum frequency
has to be chosen above the audible limit (>20 kHz).
Start-Up Sequence
When the rectified AC voltage V
IN
(via the centre tap connected
to pin 8) reaches the Mains dependent operation level (Mlevel:
between 60 and 100 V), the internal ‘Mlevel switch’ will be
opened and the start-up current source is enabled to charge
capacitor C2521 at the V
CC
pin as shown below.
The ‘soft start’ switch is closed when the V
CC
reaches a level of
7 V and the ‘soft start’ capacitor C
SS
(C2522, between pin 5
and the sense resistor R3526), is charged to 0.5 V.
Once the V
CC
capacitor is charged to the start-up voltage V
CC-
start
(11 V), the IC starts driving the MOSFET. Both internal
current sources are switched ‘off’ after reaching this start-up
voltage. Resistor R
SS
(3524) will discharge the ‘soft start’
capacitor, such that the peak current will slowly increase. This
to prevent ‘transformer rattle’.
During start-up, the V
CC
capacitor will be discharged until the
moment that the primary auxiliary winding takes over this
voltage.
Figure 9-12
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 9-13
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
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.
V
IN_MAX
P
OUT_MIN
P
OUT_MAX
power
switching
frequency
f
MIN
f
MAX
V
IN_MIN
P1
P2
QR frequency characteristics at different input voltages
CL 16532020_077.eps
100401
0.5V
+
-
I
L
R
SENSE
I
SS
V
OCP
R
SS
C
SS
soft start
V
IN
8
5
1
Mlevel
I
in(Vcc)
V
SENSE
2
C
Vcc
V
CC
Charging of V
CC
capacitor
taken over by the winding
V
(start)
=11V
C
Vcc
charged
by current
V
OUT
V
GATE
V
CC
V
SENSE
I
L
τ
= R
SS
⋅
C
SS
≈
7V
Start-up sequence
CL 16532020_078.eps
110401
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
CL 16532020_080.eps
100401
