Design guide IDP2303(A)
LLC design
Application Note
30
Revision 2.0
2017-05-03
𝐼
𝐷_𝑟𝑚𝑠_12
=
𝜋
4
𝐼
𝑜
=
𝜋
4
∗ 3 = 2.36 𝐴
𝐼
𝐷_𝑟𝑚𝑠_24
=
𝜋
4
𝐼
𝑜
=
𝜋
4
∗ 3.5 = 2.75 𝐴
Considering the voltage overshoot due to the stray inductance, 60 V/20 A and 100 V/30 A Schottky diodes are
selected as the rectifier diodes for the 12 V and 24 V channels, repectively.
4.3
Control parameters and protections
4.3.1
VCO frequency curve
During normal LLC operation, a voltage-controlled oscillator (VCO) generates the LLC converter switching
frequency
f
HB
based on the average LLC feedback voltage
V
HBFB.
With a DPdigital controller, a VCO curve based on switching period can be easily implemented digitally. The
curve of the LLC switching period
T
HB
with respect to the feedback voltage
V
HBFB
is shown in the figure below.
Figure 21
VCO curve
The VCO curve consists of three linear sections with different slew rates. As shown in the lower half of Figure
21, the line in area II (normal operation) has much lower slew rate than the line in areas I (light load) and III
T
HB
T
MaxVCO
0
V
HBFB
f
_MaxVCO
f
HB
f
_MinVCO
f
_NomVCO
0
f
_LLVCO
T
LLVCO
T
MinVCO
T
NomVCO
f
_HLVCO
T
HLVOC
I
II
III
V
HBFB
V
_NomVCO
V
_LLVCO
V
_HLVCO
V
_MaxVCO
