Open Circuit Inductance (OCL)

And DC Current Imbalance

Inductance

Inductance is the property of a conductor by which a change in current

in the conductor "induces" (creates) a voltage (electromotive force) in

both the conductor itself (self-inductance) and any nearby conductors

(mutual inductance). This effect derives from two fundamental

observations of physics: First, that a steady current creates a steady

magnetic field (Oersted’s Law) and second, that a time-varying magnetic

field induces a Voltage in a nearby conductor (Faraday’s Law of

induction). From Lenz’s law, in an electric circuit, a changing electric

current through a circuit that has inductance induces a proportional

voltage which opposes the change in current.

To add inductance to a circuit, electronic components called inductors used, typically consisting of coils of wire to concentrate the magnetic field and so that the magnetic field is linked into the circuit more than once.

The relationship between the self-inductance L of an electrical circuit in henries, voltage, and current is

where v denotes the voltage in volts and i the current in amperes. The voltage across an inductor is equal to the product of its inductance and the time rate of change of the current through it.

Current Imbalance

• In a POE LAN interface, Current imbalance in a PoE LAN interface occurs due to the unequal resistance of various segments of the DC current path associated with transferring power from the PSE to the PD. • Bias current is the net effect of this current imbalance In the interface transformers associated with the current path.

LAN Transformers Specification • The number of turns chosen to yield a transformer and the ferrite Core are the determining factors for the Bandwidth required to meet all IEEE 802.3 requirements. • Wire gauge is 39 QPN. (Min to Nom). • Wire cross sectional area tolerance from Min to Nom. Diameter is 6.5 %.

The Standard Cable Specification

• 3 Meters of 24 AWG CAT 5e Cable • 9 ohms/100 Meter • DCR Imbalance 3.5%

Specification Assumptions At Non-

Peak levels • All Resistances are arranged in the circuit to yield the highest current imbalance i.e. All Minimum Resistances are in series and all Maximum Resistances are in Series. •The interconnect cable is assumed to be minimum

length (3 Meters)

D.C. Current Imbalance POE Initial Standard: • DC Current Imbalance at a Load Current of 350 mA is 9.2 mA. POE Plus Standard: • DC Current Imbalance at a Load Current of 850 mA is 22.5 mA.

Negative Effects of current imbalance

A graph showing the relationship between OCL of a typical Interface transformer and DC Bias current will

review that the Effective Inductance vs D.C. Bias @ 0, 20, AND 70 C. (uH vs mA)

Negative Effects of current imbalance The minimum transformer inductance spec. of 350 uH cannot be maintained above a 16 mA DC Bias which the previous graph shows. As an 8mA DC Bias is an IEEE 802.3 requirement, the remaining overhead for imbalance is 8 mA.

Negative Effects of current imbalance POE Initial Standard: • The OCL of the transformer is 325 uH with a DC Current Imbalance 0f 9.2 mA + 8 mA Bias. POE Plus Standard: • The OCL of the transformer is 156 uH with a DC Current Imbalance 0f 22.5 mA + 8 mA Bias.

Negative Effects of current imbalance POE Initial Standard: • With a DC Current Imbalance 0f 9.2 mA + 8 mA Bias the Waveform Droop of the transformer is at 1000 ns is 9.9 %. POE Plus Standard: • With a DC Current Imbalance 0f 22.5 mA + 8 mA Bias the Waveform Droop of the transformer at 1000 ns is 15%.

Summary

• The POE scheme has a marginal problem in terms of transformer inductance (OCL) and waveform Droop. • The expanded 850 mA load with POE Plus has a increase effect on the interface transformers used in today's LAN interface.