9 Designing a 5 W Qi BPP or 15 W Qi EPP wireless power receiver
based on STEVAL-WLC38RX evaluation board
The design should begin with external component selection, as those components have a significant impact on
the board performance. Modifications of the external components are also harder to implement after the board is
manufactured and assembled.
9.1 External component design
STWLC38 requires only a few external components to work properly. The design should begin with coil and series
resonant capacitor selection, as those are the crucial part of the design and affect most of the device’s functions.
9.1.1 Coil selection
Coil selection is a complex problem, as there are several factors which influence the resulting performance. When
selecting the coil, the designer should consider the coil’s inductance, DCR, ACR, current rating, dimensions,
maximum z-distance and layout.
DCR, ACR (DC and AC resistance) should be kept as low as possible to minimize power loss caused by the
current flowing through the coil.
Dimensions, maximum z-distance and coil layout must be considered to achieve the best coupling possible, as
better coupling results in higher efficiency. Similar sized coils might offer a higher coupling factor, on the other
hand a different sized RX/TX coil might offer greater freedom of positioning.
Using the default coil combination found in ST evaluation kits should be a good starting point. For more
information regarding coil selection, please refer to AN5961.
9.1.2 Series and parallel resonant capacitor selection
Selection of the series and parallel capacitors is greatly influenced by the transmitting coil design. The value
should be selected so that the resulting circuit has its resonant peak located at, or very close to, 100 kHz. The
following formula can be used to calculate the series capacitance value:
f =
1
2 ⋅ π ⋅ L ⋅ C
C =
1
L ⋅ 2 ∙ π ∙ f
2
The parallel capacitance can be omitted, as it is only used for backwards compatibility with certain power
transmitters (required during Qi certification).
Voltage rating of the capacitors should consider that during operation, voltage generated across the capacitors is
usually much higher than the input/output voltage of the system. Also, a quick movement of the receiving coil from
a misaligned position to the transmitting coil center may generate a voltage peak, as the regulation loop is quite
slow. Therefore, the minimum recommended voltage rating of the series resonant capacitors is 50 V. X5R/X7R
dielectric capacitors are recommended if price is the deciding factor, C0G dielectric capacitors are recommended
for better performance (lower derating caused by voltage biasing and temperature shift).
For more information regarding capacitor value selection, please refer to Wireless Power Consortium
documentation, which describes capacitors used in the Qi certified topologies.
9.1.3 VRECT and VOUT capacitor selection
VRECT and VOUT capacitor value selection is mostly a tradeoff between load transient response time and
capacitor cost and/or size. A capacitance of 30 uF is recommended for a 15 W application. For lower power
applications, the capacitance on VOUT and VRECT can be reduced depending on power scaling.
The VRECT capacitor voltage rating should respect the maximum input voltage. Voltage peaks generated by
quick coil movements should also be a concern when determining the voltage rating. Leaving a greater margin is
advisable. The VOUT capacitor voltage rating is determined by the selected output voltage. Ceramic X5R or X7R
(preferred) dielectric capacitors are recommended for both VRECT and VOUT.
Capacitor derating caused by temperature shift and DC bias should also be considered, as the capacitance
decrease caused by those effects may be quite significant. Please refer to the capacitor datasheet for more
information.
9.1.4 ASK modulation capacitor selection and configuration
STEVAL-WLC38RX features two pairs of modulation capacitors, with each pair having a different capacitance
value – 10 nF for COMA, 22nF for COMB.
UM3154
Designing a 5 W Qi BPP or 15 W Qi EPP wireless power receiver based on STEVAL-WLC38RX evaluation board
UM3154 - Rev 2
page 55/81
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