Obsolete Product(s) - Obsolete Product(s)
VIPer12ADIP / VIPer12AS
9/15
FEEDBACK PIN PRINCIPLE OF OPERATION
A feedback pin controls the operation of the
device. Unlike conventional PWM control circuits
which use a voltage input (the inverted input of an
operational amplifier), the FB pin is sensitive to
current. Figure 9 presents the internal current
mode structure.
The Power MOSFET delivers a sense current I
s
which is proportional to the main current Id. R2
receives this current and the current coming from
the FB pin. The voltage across R2 is then
compared to a fixed reference voltage of about
0.23 V. The MOSFET is switched off when the
following equation is reached:
By extracting I
S
:
Using the current sense ratio of the MOSFET G
ID
:
The current limitation is obtained with the FB pin
shorted to ground (V
FB
= 0 V). This leads to a
negative current sourced by this pin, and
expressed by:
By reporting this expression in the previous one, it
is possible to obtain the drain current limitation
I
Dlim
:
In a real application, the FB pin is driven with an
optocoupler as shown on figure 9 which acts as a
pull up. So, it is not possible to really short this pin
to ground and the above drain current value is not
achievable. Nevertheless, the capacitor C is
averaging the voltage on the FB pin, and when the
optocoupler is off (start up or short circuit), it can be
assumed that the corresponding voltage is very
close to 0 V.
For low drain currents, the formula (1) is valid as
long as IFB satisfies I
FB
< I
FBsd
, where I
FBsd
is an
internal threshold of the VIPer12A. If I
FB
exceeds
this threshold the device will stop switching. This is
represented on figure 4, and I
FBsd
value is
specified in the PWM COMPARATOR SECTION.
Actually, as soon as the drain current is about 12%
of Idlim, that is to say 50 mA, the device will enter
a burst mode operation by missing switching
cycles. This is especially important when the
converter is lightly loaded.
It is then possible to build the total DC transfer
function between I
D
and I
FB
as shown on figure 10.
This figure also takes into account the internal
blanking time and its associated minimum turn on
time. This imposes a minimum drain current under
which the device is no more able to control it in a
linear way. This drain current depends on the
primary inductance value of the transformer and
the input voltage. Two cases may occur,
depending on the value of this current versus the
fixed 50 mA value, as described above.
START UP SEQUENCE
This device includes a high voltage start up current
source connected on the drain of the device. As
soon as a voltage is applied on the input of the
converter, this start up current source is activated
as long as V
DD
is lower than V
DDon
. When
reaching V
DDon
, the start up current source is
switched off and the device begins to operate by
turning on and off its main power MOSFET. As the
FB pin does not receive any current from the
optocoupler, the device operates at full current
capacity and the output voltage rises until reaching
gure
:
nterna
urrent
ontro
tructure
60kHz
OSCILLATOR
PWM
LATCH
S
Q
R
0.23V
Id
DRAIN
SOURCE
FB
R1
R2
C
+Vdd
Secondary
feedback
I
FB
Is
1 kΩ
230 Ω
I
S
0.23V
R
2
---- ---- ---- -- I
FB
–=
I
D
G
ID
I
S
⋅ G
ID
0.23V
R
2
---- ---- ---- -- I
FB
–
⋅==
I
FB
0.23V
R
1
--------------–=
I
Dlim
G
ID
0.23V
1
R
2
---- --
1
R
1
------+
⋅⋅=
gure
:
FB
rans
er
unct
on
I
FBsd
I
Dlim
I
FB
t
ONmin
V
2
⋅
IN
L
----- --- --- ------ --- ------ --- --- -------
t
ONmin
V
1
⋅
IN
L
----- --- --- ------ --- ------ --- --- -------
50mA
I
Dpeak
0
Part masked by the
I
FBsd
threshold
