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f
f
f
f
Figure 7
Waveforms for LLC converter with
>
(left) and
<
(right)
sw
r
sw
r
It can be seen from the waveforms in Figure 7 (left) that to ensure safe switching, the switch-on of the SR
MOSFET (see
) need to be a certain time AFTER the switch-on of the primary side switch(see
);
V
V
SLG
LG
while switch-off of the SR MOSFET(see
) needs to be certain time BEFORE the switch-off of primary
V
SLG
side switch(see
), in order to compensate the propagation delay of the gate signals from IC to the
V
LG
actual MOSFET. In this operation condition (
>
), the SR MOSFET conduction period (on-time)
f
f
sw
r
depends on the primary gate switching frequency.
From Figure 7 (right), the current flowing through the SR MOSFET (see
) goes to zero before the
I
SL
switch-off of the primary switch. To avoid the current going into negative, the SR MOSFET need to be
turn off just before the current goes to zero. In this condition, the SR MOSFET on-time is almost constant
and nearly half of the resonant period.
The control of SR in ICE2HS01G consists of four main parts: on-time control, turn-on delay, advanced
turn-off delay and protections, with the block diagram shown in Figure 8.
Application Note
17
2011-07-06
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