Application Note 20 2011-07-06
With
CS
V < 0.4V,
CL
V starts to drop below 2V, extra current is drawn from SRD pin, thereby the actual
SR on time is reduced with the load decreased.
For filter purpose,
CL
C is chosen to be 47nF.
Figure 10 SR on time versus SRD resistance
2.4.2 Turn-on delay
delayon
T
_
- Vres pin
When the input voltage is higher than resonant voltage, the LLC converter secondary switches are
working in CCM condition. Certain recovery time of the SR MOSFET body diode is required depending on
the current to turn-off. For better performance, the other SR MOSFET should be turn on after the
recovery phase. The turn-on delay function is built in ICE2HS01G for such purpose. When the sensed
input voltage at VINS pin is higher than the reference voltage set by Vres pin according to the resonant
voltage, SR turn-on delay is added, i.e, the SR MOSFETs are turn on 250ns after the corresponding
primary MOSFETs are turned on.
The nominal bus voltage at resonant point is:
[28]
The corresponding voltage at VINS pin is 1.59V. To allow the turn-on delay for input voltage above this
resonant point, we can set the voltage divider
1res
R and
2res
R connected at VRES pin accordingly. We
select
Ω= kR
res
12
1
, and
2res
R can be calculated to be 5.2kΩ. To disable the turn-on delay during normal
operation, we can set the voltage at
res
V to be 1.07x1.59=1.7V. Accordingly, Ω= kR
res
12
1
,
and
Ω= kR
res
2.6
2
.
VVVnV
fOres
3.399)1.012(*5.16*2)(*2
+=
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