Sifos Technologies PowerSync PSA100, Техническое справочное руководство

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PowerSync Analyzer Reference Manual
March 9, 2010 Sifos Technologies page 12
2.1.5. Power Removal
802.3 compliant PSE’s offer one of two means to determine that a Powered Device has been disconnected, and
therefore DC power should be removed. By implication, the two different mechanisms are mutually exclusive, though
the specification allows for what effectively would be an impractical combination of both methods.
The AC MPS method involves the
superposition of a low level, relatively low
frequency signal on the DC power rail. The
AC signal is sourced from a high output
impedance such that when exposed to a
nominal load resistance of 25 K
Ω
, the
amplitude of the signal is attenuated to well
below 500 mVp-p. Typically it will be well
below 200 mVp-p. When the 25 K
Ω
signature load is removed, the AC signal
amplitude increases and can be detected on
the PSE output. The PSE must then wait for
an interval of at least 300 mSec, but not
longer than 400 mSec to remove power. The
main advantage of the AC MPS method over
the DC MPS method is the ability to tolerate
well below 10mA of load current indefinitely
so long as the AC MPS load impedance of 25
K
Ω
is detected.
The DC MPS method relies on a continuous measurement of DC load current. When the DC load current drops below
10 mA, the PSE has the “right” to remove DC power. When the DC load current drops below 5 mA, the PSE must
remove DC power. As with AC MPS, the timing of the low-load current is such that the PSE must tolerate 300 mSec of
low load, but not longer than 400 mSec of this condition. Additionally, the specification makes allowance for the DC
MPS signature to be intermittent so long as it is present for a continous 60 msec out of every 360 msec interval. The
main advantage of DC MPS is that it does not add any noise onto the power rail.
2.2. Power Sourcing Equipment Characteristics
The 802.3af specification leaves considerable room for implementation dependent behaviors. Additionally, many
vendors of Power Sourcing Equipment (PSE) will choose to go outside the 802.3 specification in ways that will not
affect the ability to power and maintain pure 802.3 Power Devices (PD). This high degree of variation will add a
number of challenges to the generation and performance of PSE specification conformance tests.
2.2.1. Signaling Variations
One area of implementation variation relates to the signaling utilized prior and during power-up. The following table
describes some of the possible
variations
in the area of signaling.
Signal Type Variants
Amplitude: 2 – 30 VDC
Course Detection Range: Any Detection Signature > 33 K
Ω
Course Detection Method: Not strictly specified – may use 802.3af
Δ
V/
Δ
I Steps
Open Circuit Detection
Measurement Timing: No firm requirements, just recommendations
Amplitude: 2.8 – 10 VDC (given valid PD signature from 19 to 26.5 K
Ω
)
Pulse Duration: 20 – 500 mSec Step Magnitude: 1 – 7.2 V
Δ
V/
Δ
I Steps: 1 or more
Pulse End: RZ or NRZ
Connected Detection
Signature
Step Edge: Rising, Falling, Both Pulse Separation: >0 (>2 Midspan) sec
Amplitude: 15.5 – 20.5 VDC Duration: 15 to 70 mSec Classification
Classification Pulse Count: > 1 Pulse End: RZ or NRZ
Waveform: CW or Pulsed AC MPS Signal
Frequency: 20 to 500 Hz
Removal: Before, During, After
Power-Down (or Never ?)
AC MPS Threshold: 27 K
Ω
to 1.98 M
Ω Power-Down
DC MPS Threshold: 5 – 10 mA
Dissipation: PD Load, PSE Shunt, or
Active PSE Discharge
Figure 2.4
802.3af Power-Down