Wireless Power Transfer Audio Issue: New form of Middlebrook's theorem
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AUDIO INVESTIGATION REPORT Sanjaya Maniktala Date: 09/24/2013 NOTE (4/5/2017): This was the basis of a subsequent patent application in 2014 (for which the name inventors received $0, despite written assurances! A gift to humanity it is!) https://www.google.com/patents/US20160094042
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Transcript of Wireless Power Transfer Audio Issue: New form of Middlebrook's theorem
AUDIO INVESTIGATION REPORT
Sanjaya Maniktala Date: 09/24/2013
NOTE (4/5/2017): This was the basis of a subsequent patent application in 2014 (for which the name inventors received $0, despite written assurances! A gift to humanity it is!)
https://www.google.com/patents/US20160094042
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The Issue
It was confirmed on EVB v2.0 using Firmware FF05 that a ~ 8 to 9kHz audio emission was occurring.
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Root Cause
There is a resonant action being created between the ceramic output caps (30μF) and the inductance of the Rx coil (10μH)
It will be seen on ALL wireless mat chargers independent of architecture if a Buck regulator is being used off VRECT.
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Middlebrook’s theorem
For deeper analysis see Appendix
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The Peaking of the LC is causing a Violation of the Theorem
● If the peaking is damped, the red oval (intersection) will disappear, and there will be no oscillations, since Middlebrook’s theorem would be satisfied
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Intuitive Understanding of Middlebrook’s Theorem● At resonance of the parallel LC, its impedance can be as high as
infinity if there is no damping. The L and C would have currents exactly 180 degrees out of phase, so the current coming out of the cap would go straight into the inductor and vice versa. In other words, no current would leave a perfect parallel resonant circuit. The two reactive components would be completely satisfied swapping their stored energy back and forth mutually, and no energy would leave the resonant circuit. So VRECT will start collapsing.
● On the other hand, the Buck regulator, being a constant input power circuit will try to ask for more current as its input voltage collapses. So this will make things worse. Eventually there will be severe oscillations.
● The peaking of the LC is responsible. To control the peaking, sufficient damping is required….so typical solutions involve using an Aluminum electrolytic cap in parallel to the input of a Buck converter. IDT’s unique solution instead uses a series resistor of about 5 ohm in series with ceramic caps.
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PRIMARY RECOMMENDATIONS
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PRIMARY RECOMMENDATIONS
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SECONDARY RECOMMENDATIONS
1) New firmware version which increased filtering (count) for reverting back from Synchronous Mode to Non-Synchronous Mode in the Bridge Rectifier. (550-400 version)
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APPENDIX
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A NEW FORM OF INPUT INSTABILITY
Middlebrook’s Instability theorem was originally written with a pure DC input. In wireless mat charging, the same instability appears, but in a more subtle form, based on an applied high-frequency input, which serves as a carrier frequency for the beat frequencies. The equivalent input filter resonant circuit is altered as shown in the following slides.
On a spectrum analyzer, we can see two sidebands appear as soon as the “8 or 9 kHz oscillation” on VRECT appears. These are spaced equidistant on either side of the 230kHz transmitter frequency.
Surprisingly there is no energy peak observed at the audible frequency. What we are therefore hearing by pizeoelectric effect in the ceramic caps is only the beat frequency.
IDT has first discovered the root cause of this unique manifestation of Middlebrook’s theorem and also implemented a proprietary fix : in the form of a series RC snubber (a large output ceramic cap in series with a small resistor of 2 to 5 ohms, as shown in Slide 7).
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SPECTRUM ANALYSIS
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SPECTRUM ANALYSIS CONTINUED
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EQUIVALENT CIRCUIT IN DETAIL
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EQUIVALENT CIRCUIT (EFFECTIVE)
PAGE 16
Thank You
