AN11524
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© NXP Semiconductors N.V. 2015. All rights reserved.
Application note
Rev. 1 — 17 February 2015
28 of 54
NXP Semiconductors
AN11524
NXQ1TXA6 Evaluation Board
7. Critical
components
As mentioned in
, certain components are critical in the design. In this section,
we discuss these components, associated design considerations and potential pitfalls.
7.1 Power stage
7.1.1 Capacitor in LC tank circuit
The capacitor value in the tank circuit must be correct or the system does not function
properly. The Wireless Power Consortium (WPC) specify the capacitor values. To verify
that the correct value is used in the base station design, refer to WPC specifications.
For a 3-coil A6 type base station design, WPC specifies a value of 147 nF to be used with
charging coil closer to the charging pad. It specifies using 136 nF for a charging coil
further away from the charging pad.
As an example,
depicts the LC tank circuit of coil 1, which is the furthest from
the interface surface. The capacitance of C210 and C211 is 68 nF each. Total capacitance
combined is 136 nF. Beside capacitor value, the dielectric must be C0G/NP0 type, else
efficiency is lower and Qi compliance can be problematic.
The voltages in the tank circuits can swing as much as 100 V
p-p
in the NXQ1TXA6
Evaluation Board. It is therefore important to choose the correct voltage rated capacitors.
The capacitors C210 and C211 in
, are 100 V rated.
7.1.2 Half-bridge driver and MOSFETs
For +12 V system such as the A6 type base station, the maximum base station operating
conditions lead to MOSFET requirements of a maximum 30 V, minimum 4 A and a
maximum Rds
on
40 m
. In NXQ1TXA6 Evaluation Board, an N-Channel Trench MOSFET
NX2020N2 (T203 and T204) is used in combination with NWP2081 (U201) half-bridge
controller IC.
Fig 13. LC tank circuit of coil 1
aaa-015893
C211
GND1
C210
L1-1
[5]
UC1_2
COIL1C
COIL1T
CL1C
CL1T