power measurements in (wpt) systems
TRANSCRIPT
Power measurements in (WPT) systems
Laurens Swaans &James Lo March 2017
CONFIDENTIAL
2 CONFIDENTIAL
Questions to answer
o What is a power measurement?
o Why is power important?
o Where do we measure power?
o How to measure AC power?
o What model to use?
o What do the signals look like?
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What is a power
measurement?
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What is power measurement?
o Power: β Indication of the energy usage per time unit (rate of work)
β Expressed in Joule per second (SI), or Watt (more common)
β Electrical power: π π‘ = π£ π‘ β π π‘
o DC power β ππ·πΆ = ππ·πΆ β πΌπ·πΆ
o AC power β Instantaneous power: π π‘ = π£ π‘ β π π‘
β RMS values power: π π‘ = ππ ππ β πΌπ ππ β cosπ β’ RMS= Root Mean Square π = the phase shift between voltage and current
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Why is power important?
VS
o Burning 1kg of coal vs 1kg of TNT
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Why is power important?
o Burning 1kg of coal vs 1kg of TNT β the coal has a lot more energy than the TNT, but
β the TNT has a lot more power than the coal β’ It takes much longer to burn all the coal than all the TNT
o Energy is accumulated power over time β The heat generated until all coal is burned up, or
β All power into the battery of a phone until it is fully charged
o Power is the amount of energy per time unit β The pressure waves generated by the TNT explosion, or
β The charge-speed of your phone while charging (% per minute)
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Why is power important?
o Foreign Object Detection (FOD)
Receiver coil
Transmitter coil
Magnetic flux
Received Power ππ π
Power loss < 250mW ππΏ = πππ β ππ π
Power loss > 250mW ππΏ = πππ β ππ π
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Where do we measure power?
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A simple model
o In Wireless Power Transfer (WPT) systems: β Transmitter input: ππ₯πΌπ
β Transmitter output: ππ₯πππ
β Receiver input: π π₯πΌπ
β Receiver output: π π₯πππ
V Power
Transmitter Power
Receiver
Load
ππ₯πΌπ ππ₯πππ π π₯πΌπ π π₯πππ
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Conventional powers
o TxIN: β External DC power supply: π = ππ·πΆ β πΌπ·πΆ
β Mains connected transmitter: π = π΄ππΊ π π‘ β πΌ π‘
o RxOUT: β Battery or other DC load: π = ππ·πΆ β πΌπ·πΆ
o System efficiency: πΌπΊππΊ =πΉππΆπΌπ»
π»ππ°π΅
β Note the possibility to distinguish between energy efficiency and power efficiency V
Power Transmitter
Power Receiver
Load
ππ₯πΌπ ππ₯πππ π π₯πΌπ π π₯πππ
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WPT characteristic powers
o TxOUT: β AC power generated by the coil: π = π΄ππΊ π π‘ β πΌ π‘
o RxIN: β AC power generated by the coil: π = π΄ππΊ π π‘ β πΌ π‘
o Efficiencies: β Antenna: πΌπ¨π΅π» =
πΉππ°π΅
π»ππΆπΌπ»
β Transmitter: πΌπ»π =
π»ππΆπΌπ»
π»ππ°π΅
β Receiver: πΌπΉπ =
πΉππΆπΌπ»
πΉππ°π΅
AC power measurements enables these
V Power
Transmitter Power
Receiver
Load
ππ₯πΌπ ππ₯πππ π π₯πΌπ π π₯πππ
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How to measure AC power?
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AC power in resistive loads
o Power dissipation occurs in βresistiveβ loads β βresistiveβ means: voltage and current are in phase
β Voltage: 100kHz, 1V amplitude
β Current: 100kHz, 0.5A amplitude
β Double frequency
β Always positive
β Power: β’ 200kHz, 0.5W amplitude β’ Average P(t): 0.25W
β’ RMS: ππ ππ β πΌπ ππ =1
2β0.5
2 =0.25W
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AC power in reactive loads
o Power dissipation in WPT systems? β TxOUT and RxIN are not really βresistiveβ, not really βreactiveβ
β Voltage and current are not exactly in phase β’ Amount of phase shift depends on the impedance
o Power measurements when π β π β π = π΄ππΊ π π‘ β πΌ π‘ still holds true
β But also: π = ππ ππ β πΌπ ππ β πππ π (for sinusoidal signals)
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How to measure AC power?
o AC power comparison
Differences caused by sampling frequency (10MHz)
Ο AVG RMS Diff
0 0.249 0.249 0
18 0.237 0.237 7.6e-5
36 0.201 0.202 2.3e-4
54 0.146 0.147 3.2e-4
72 0.077 0.077 2.3e-4
90 3.5e-17 1.5e-17 2.0e-17
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AC power in reactive loads
o Power dissipation in WPT systems β TxOUT and RxIN are not really βresistiveβ, not really βreactiveβ
β Voltage and current are not exactly in phase β’ Amount of phase shift depends on the impedance
o Power measurements when π β π β π = π΄ππΊ π π‘ β πΌ π‘ still holds true
β But also: π = ππ ππ β πΌπ ππ β πππ π (for sinusoidal signals)
o So we look at the βresistive partβ of a βcomplex loadβ β This can be modeled as a resistor in parallel or series with the power transfer coil
β The phase shift depends on the ratio between resistance and reactance
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What model(s) to use?
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A simple model again
V Power
Transmitter Power
Receiver
Load
ππ₯πΌπ ππ₯πππ π π₯πΌπ π π₯πππ
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Typical WPT model Transmitter parameters
Vin: input voltage
Cp: Series resonance capacitor on primary side
Lp: Primary winding
Rp: ESR of Tx
Receiver parameters Cs: Series resonance capacitor on secondary side
Ls: Secondary winding
ZL: Receiver loading
Rs: ESR of Rx
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What signals to expect?
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First simulation
o Invertor: β Transient: 10V, 160kHz
β Bode: 50kHz-250kHz
o Transmitter: β Qi TPT#2
o Receiver: β Qi TPR#1B
o Loading: β ZL=10Ξ©
o Coupling: β k=0.5
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Nowβ¦analyze the system
o Let the computer run through the complete load range
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Nowβ¦analyze the system
o Now add different couplings to the mix
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Nowβ¦analyze the system
o ..and different frequencies
thank you