Circuit Description
GB 62
L01.1E
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.
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
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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
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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
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