Infineon ICE2HS01G - Page 17

Application Note 17 2011-07-06
Figure 7 Waveforms for LLC converter with
sw
f >
r
f (left) and
sw
f <
r
f (right)
It can be seen from the waveforms in Figure 7 (left) that to ensure safe switching, the switch-on of the SR
MOSFET (see
SLG
V ) need to be a certain time AFTER the switch-on of the primary side switch(see
LG
V );
while switch-off of the SR MOSFET(see
SLG
V ) needs to be certain time BEFORE the switch-off of primary
side switch(see
LG
V ), in order to compensate the propagation delay of the gate signals from IC to the
actual MOSFET. In this operation condition (
sw
f >
r
f ), the SR MOSFET conduction period (on-time)
depends on the primary gate switching frequency.
From Figure 7 (right), the current flowing through the SR MOSFET (see
SL
I ) goes to zero before the
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.