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Bob Smith Terminations and Active Current …

Bob Smith Terminations and Active Current Balancing for PoE Plus Steve Robbins




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BobSmithTerminationsandActiveCurrentBala ncingforPoEPlusSteveRobbinsIEEE Task Force2Nov 2005 Plenary Vancouver, BCIntroduction In PoE systems, the Bob Smith Termination(BST) is essentially short-circuited in both thePSE and PD. If Active Current Balancing (ACB) were used inPoE Plus, the problem could be partially solved(at one end only) with little additional cost Task Force3Nov 2005 Plenary Vancouver, BCPDWhyBSTDoesn t WorkinPoEDC/DCCONVPSEABCDPower supply is an AC-shortbetween nodes A and and cap form an AC-shortbetween nodes C and Task Force4Nov 2005 Plenary Vancouver, BCPDA SimpleChokeWon t WorkPSERFChokeRFChoke PSE source-impedance must be less than from DC to 100kHz forSource/Load stability per Annex 33D recommendations. (Therefore,L<477nH.) Impedance must be much greater than 75 above 1MHz to have small affecton BST performance. (Therefore, L>60 H.) Contradictory requirements! (A higher-order circuit might work, but would beexpensive.)IEEE Task Force5Nov 2005 Plenary Vancouver, BC This example shows half of a 4P system, with pair-to-pair ACB in the PSE. Impedance requirements from previous slide still apply, but it s easier toimplement the high-order function in silicon than in passives. Problem solved at PSE end, but not at PD end of PartialSolutionACBDC/DCCONVIEEE Task Force6Nov 2005 Plenary Vancouver, To keep the power dissipation in the ACB circuitreasonably low, the voltage drop across it must beless than approximately 250mV. A MOSFET would be in the linear region at such lowvoltage. Drain looks like a low-value voltage-controlled resistance. No way to make it high-Z at high freq with control loop. A bipolar with low VCE(SAT)would still be in its activeregion. Collector looks like a current source. High impedance. Feedback loop would keep impedance low at low Task Force7Nov 2005 Plenary Vancouver, BCA PossibleImplementationLPFLPFLPF = Low Pass FilterFrom Power SupplyPositive PoleTo TransformerCenter-Taps This circuit is used only as anexample. (A more practical circuitwould probably use opamp circuitsinstead of emitter feedback tobalance the DC currents.) At low freq, the filter shortscollector to base. This makes twocurrent mirrors that operate back-to-back for symmetry. Collectorsappear low-Z. At high freq, the bases are AC-grounded. So the collectorsappear high-Z. The key point is, the frequency-shaping components are not in thehigh-current branches, so they canbe ACB chip.(Low-side chip wouldbe complement.)IEEE Task Force8Nov 2005 Plenary Vancouver, BCSummary BST circuits doesn t work with PoE because thepower feeding paths short them out. Fixing the problem with passives would bedifficult and expensive. ACB can provide a partial solution (one end ofthe cable only). Bipolar offers a big advantage over CMOS forthis application. The marginal cost of adding freq compensationto an ACB circuit would probably be small.

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