Design guide IDP2303(A)
LLC design
Application Note
26
Revision 2.0
2017-05-03
Figure 19
Voltage gain
𝑮
Vs normalized frequency
𝑭
The curve where
𝑄𝑒 = 0.75
can achieve the required peak gain,
𝐺
𝑝𝑘
, which is
𝑀
𝑚𝑎𝑥
,
𝐺
𝑝𝑘
= 𝑀
𝑚𝑎𝑥
∗ 1.05 =
1.14
Having found the proper
𝑄𝑒
, we can calculate
𝐶
𝑟
,
𝐿
𝑟
and
𝐿
𝑝
as follows:
𝐶
𝑟
=
1
2𝜋 ∗ 𝑄𝑒 ∗ 𝑓
𝑟∗
𝑅
𝑒𝑓𝑓
=
1
2𝜋 ∗ 0.75 ∗ 100 ∗ 10
3
∗ 218
= 9.8 𝑛𝐹
If a 10 nF capacitor is selected as
𝐶
𝑟
, then, in the actual design,
𝑄𝑒
must be re-calculated as
𝑄𝑒 =
1
2𝜋 ∗ 𝐶
𝑟
∗ 𝑓
𝑟∗
𝑅
𝑒𝑓𝑓
= 0.74
𝐿
𝑟
=
1
(2𝜋𝑓
𝑟
)
2
𝐶
𝑟
=
1
(2𝜋 ∗ 100 ∗ 10
3
)
2
∗ 10 ∗ 10
−9
= 253 𝑢𝐻
𝐿
𝑝
= 𝑚𝐿
𝑟
= 1.1 𝑚𝐻
The value of
𝐿
𝑝
can be achieved by adjusting the gap length.
From the gain curve with
𝑄𝑒 = 0.74
in Figure 19, the normalized frequency
𝐹
𝑚𝑖𝑛
= 0.87
can be located to
achieve the required maximum gain
𝐺
𝑝𝑘
= 1.08
, for the condition of minimum input voltage at full load.
Accordingly, the actual minimum frequency
𝑓
𝑚𝑖𝑛
is:
𝑓
𝑚𝑖𝑛
= 𝐹 ∗ 𝑓
𝑟
= 0.87 ∗ 100 = 87 𝑘𝐻𝑧
Similarly, the normalized frequency
𝐹
𝑛𝑜𝑚
= 1.1
can be located to achieve gain,
𝐺 = 𝑀
𝑛𝑜𝑚
= 0.94
for
nominal input voltage at full load. Accordingly, the actual nominal frequency
𝑓
𝑛𝑜𝑚
is:
𝑓
𝑛𝑜𝑚
= 𝐹 ∗ 𝑓
𝑟
= 1.1 ∗ 100 = 110 𝑘𝐻𝑧
