thedetected zero-crossing to the switch-on of the main
switch
t
delay
, theoretically:
T osc
?t
= ------------ – t
delay
4
[2]
To avoid mistriggering caused by the voltage spike
across the shunt resistor at the turn on of the main
power switch, a leading edge blanking time, t
LEB
, is
applied to the output of the comparator. In other words,
once the gate drive is turned on, the minimum on time
of the gate drive is the leading edge blanking time.
In addition, there is a maximum on time, t
OnMax
,
limitation implemented in the IC. Once the gate drive
has been in high state longer than the maximum on
time, it will be turned off to prevent the switching
frequency from going too low because of long on time.
This time delay should be matched by adjusting the
time constant of the RC network which is calculated as:
τ
td
= C
R zc1
?
R zc2
-
?
--------------------------------
zc R
+R
zc1
zc2
[3]
3.4
3.3.1.3
Ringing suppression time
After MOSFET is turned off, there will be some
oscillation on V
DS
, which will also appear on the voltage
on ZC pin. To avoid that the MOSFET is turned on
mistriggerred by such oscillations, a ringing
suppression timer is implemented. The time is
dependent on the voltage
v
ZC
. When the voltage
v
ZC
is
lower than the threshold
V
ZCRS
, a longer preset time
applies, while a shorter time is set when the voltage
v
ZC
is higher than the threshold.
3.3.1.4
Switch on determination
After the gate drive goes to low, it can not be changed
to high during ring suppression time.
After ring suppression time, the gate drive can be
turned on when the ZC counter value is higher or equal
to up/down counter value.
However, it is also possible that the oscillation between
primary inductor and drain-source capacitor damps
very fast and IC can not detect enough zero crossings
and ZC counter value will not be high enough to turn on
the gate drive. In this case, a maximum off time is
implemented. After gate drive has been remained off
for the period of T
OffMax
, the gate drive will be turned on
again regardless of the counter values and V
ZC
. This
function can effectively prevent the switching
frequency from going lower than 20kHz, otherwise
which will cause audible noise, during start up.
3.3.2
Switch Off Determination
In the converter system, the primary current is sensed
by an external shunt resistor, which is connected
between low-side terminal of the main power switch
and the common ground. The sensed voltage across
the shunt resistor v
CS
is applied to an internal current
measurement unit, and its output voltage V
1
is
compared with the regulation voltage V
FB
. Once the
voltage V
1
exceeds the voltage V
FB
, the output flip-flop
is reset. As a result, the main power switch is switched
off. The relationship between the V
1
and the v
CS
is
described by:
V 1 = 3.3
?
V cs + 0.7
[4]
Current Limitation
There is a cycle by cycle current limitation realized by
the current limit comparator to provide an overcurrent
detection. The source current of the MOSFET is
sensed via a sense resistor R
CS
. By means of R
CS
the
source current is transformed to a sense voltage V
CS
which is fed into the pin CS. If the voltage V
CS
exceeds
an internal voltage limit, adjusted according to the
Mains voltage, the comparator immediately turns off
the gate drive.
To prevent the Current Limitation process from
distortions caused by leading edge spikes, a Leading
Edge Blanking time (t
LEB
) is integrated in the current
sensing path.
A further comparator is implemented to detect
dangerous current levels (V
CSSW
) which could occur if
one or more transformer windings are shorted or if the
secondary diode is shorted. To avoid an accidental
latch off, a spike blanking time of t
CSSW
is integrated in
the output path of the comparator .
3.4.1
Foldback Point Correction
When the main bus voltage increases, the switch on
time becomes shorter and therefore the operating
frequency is also increased. As a result, for a constant
primary current limit, the maximum possible output
power is increased, which the converter may have not
been designed to support.
To avoid such a situation, the internal foldback point
correction circuit varies the V
CS
voltage limit according
to the bus voltage. This means the V
CS
will be
decreased when the bus voltage increases. To keep a
constant maximum input power of the converter, the