Application Note
18
2011-07-06
Figure 8
Synchronous rectification control block diagram
2.4.1
On-time control - SRD pin and CL pin
With ICE2HS01’s control scheme, SR MOSFET ‘s turning-off depends on two conditions - turning-off of
the primary gate and the “off” instruction from SR on-time block, where the maximum on-time
max
_
on
T
is
preset. Whichever “off” instruction comes first will trigger the turn-off of the SR MOSFET.
As illustrated in the previous chapter, the
max
_
on
T
depends on the resonant frequency when LLC
converter operates below resonant frequency (
sw
f
<
r
f
)
. Considering the primary side dead time
DEAD
T
and the SR gate turn-on delay
delay
on
T
_
( will be discussed later section 2.4.2), we can preset
max
_
on
T
with
a safe value as below:
us
T
T
f
T
delay
on
DEAD
res
on
31
.
5
25
.
0
32
.
0
88
.
5
2
1
_
max
_
=
−
−
=
−
−
<
[27]
To achieve higher efficiency, a bigger
max
_
on
T
is an advantage, because bigger on-time
means longer SR
MOSFET conduction time and less body diode conduction time, which reduces conduction loss. In actual
design,
max
_
on
T
can be fine-tuned by looking at the similar waveforms in Figure 7, as long as safe
switching is guaranteed.