FAN6248HC/HD/LC/LD
www.onsemi.com
11
APPLICATION INFORMATION
Basic Operation Principle
FAN6248 controls the SR MOSFET based on the
instantaneous drain-to-source voltage sensed across DRAIN
and SOURCE pins. Before SR gate is turned on, SR body
diode operates as the conventional diode rectifier. Once the
body diode starts conducting, the drain-to-source voltage
drops below the turn-on threshold voltage V
TH_ON
which
triggers the turn-on of the SR gate. Then the drain-to-source
voltage is determined by the product of turn-on resistance
R
ds_on
of SR MOSFET and instantaneous SR current. When
the drain-to-source voltage reaches the turn-off threshold
voltage V
TH_OFF
as SR MOSFET current decreases to near
zero, FAN6248 turns off the gate. If a SR dead time is larger
or smaller than the dead time regulation target t
DEAD
,
FAN6248 adaptively changes internal offset voltage to
compensate the dead time. In addition, to prevent cross
conduction SR operation, FAN6248 has 200 ns of turn-on
blocking time just after alternating SR gate is turned off.
SR Turn-off Algorithm
Since a SR turn-off method determines SR conduction
time and stable SR operation, the SR turn-off method is one
of important feature of SR controllers. The SR turn-off
method can be classified into two methods. The first method
uses present information by an instantaneous drain voltage.
This method is widely used and easy to realize, and can
prevent late turn-off. However, it may show premature
turn-off by parasitic stray inductances caused by PCB
pattern and lead frame of SR MOSFET. The second method
predicts SR conduction time by using previous cycle drain
voltage information. Since it can prevent the premature
turn-off, it is good for the system with constant operating
frequency and turn-on time. However, in case of the
frequency varying system, it may lead late turn-off so that
negative current can flow in the secondary side.
To achieve both advantages, FAN6248 adopts mixed type
control method as shown in Figure 21. Basically the
instantaneous drain voltage V
Drain
is compared with
V
TH_OFF
to turn off SR gate. Then, the offset voltage V
offset
,
which is determined by the product R
offset
and I
offset
, is added
to V
Drain
in order to compensate the stray inductance effect
and maintain 280 ns of t
DEAD
regardless of parasitic
inductances. R
offset
is an external resistor in Figure 1 and
I
offset
is an internal modulation current in Figure 2.
Therefore, FAN6248 can show robust operation with
minimum dead time.
Figure 21. SR Turn-off Algorithm
V
Drain
Gate
V
SAW
V
TH_off
SR off
SQ
R Q
SR on
Voffset
control
V
offset
=R
offset
x I
offset
=instantaneous Vdrain type
Previous cycle information
=Prediction type
Present information+Previous cycle information
S
= Mixed type control
Adaptive Dead Time Control
The stray inductances of the lead frame of SR MOSFET
and PCB pattern induce positive voltage offset across
drain-to-source voltage when SR current decreases. This
makes drain-to-source voltage of SR MOSFET larger than
the product of R
ds_on
and instantaneous SR current, which
results in premature turn-off of SR gate. Since the induced
offset voltage changes as load condition changes, the dead
time also changes with load variation. To compensate the
induced offset voltage, FAN6248 has a adaptive virtual
turn-off threshold voltage as shown in Figure 22 with
a combination of variable internal turn-off threshold
voltages V
TH_OFF1
and V
TH_OFF2
(2 steps) and modulated
offset voltage V
offset
(16 steps). The virtual turn-off
threshold voltage can be expressed as:
Virtual V
TH_OFF
+ V
TH_OFF
* V
offset
(eq. 1)
In FAN6248HC(D) version, if a dead time T
DEAD
is larger
than 280 ns of t
DEAD_H
, as shown in Figure 23, V
offset
is
decreased by one step in next switching cycle. As a result,
the dead time is decreased by increase of virtual V
TH_OFF
,
and becomes close to t
DEAD_H
, as shown in Figure 24. If the
dead time is smaller than t
DEAD_H
, the dead time is increased
by the virtual V
TH_OFF
decrease. Thus, the dead time is
maintained at around t
DEAD_H
regardless of parasitic
inductances.
Figure 22. Virtual V
TH_OFF
V
Drain
SR gate
Virtual V
TH_OFF
SR off
SQ
R Q
SR on
=V
TH_OFF
−V
offset
V
TH_ON
