STARTUP PROCEDURE IN PoE SYSTEMS


PoE STARTUP PROCEDURE



A PoE system containing a IEEE 802.3 complaint PSE and PD goes through a number of steps before the powered network device (PD) becomes operational:

Below I will go over the details of each of these steps.

DETECTION



A compliant PSE periodically probes one or both pairsets (depending on its design) fishing for a compliant PD. Power can be applied only when a valid PD detection signature is found, or at least when a pairset does not exhibit an invalid detection signature. The probing is done by sending a series of low-voltage pulses of different amplitudes within the detection range (2.8 V to 10.1 V), and by measuring certain characteristics of the impedance curve. When a compliant PD device detects voltage at its input connector in the above range, it has to present a specific so-called valid detection signature per IEEE802.3. See detection procedure for more details.

CONNECTION CHECK



A PSE capable of supplying power over four pairs should also do a so-called connection check prior to the classification. The purpose of this check is to find out if PD has a single-signature or a dual-signature configuration. The requirements for connection check are provided in paragraph 145.2.6.1 of IEEE802.3bt. The standard however does not tell you how to determine the PD configuration- it is left up to the designers. Some PSE may perform connection check prior to detection.

CLASSIFICATION



After PSE found an IEEE compliant PD and determined its configuration, it performs classification. It is basically a process of mutual identification of PSE and PD. During this process PSE is querying a PD in order to determine its power requirements, while at the same time broadcasting its own type. The mutual identification function is intended for use with advanced power management. Classification can be done in two ways: Physical Layer classification and Data Link Layer (DLL) classification. During physical layer classification PSE generates a series of voltage pulses VCLASS called “class events”. A class event by definition occurs when a PSE applies voltage pulse 15.5V to 20.5V. When a PD detects voltage in this range, it can provide its class signature by drawing specific current level representing its class (for the details see classification process).

POWER UP


After classification, PSE can apply full operational voltage to the PoE line. The following diagram shows an example of voltage waveforms during detection, classification and startup (not to scale). It is important to note that the rise time should be longer than 15 μs. Note that non-standard (passive) midspans and injectors often has shorter rise time. Their high dV/dt can damage your PD controller. That's why I don't recommend using a non-IEEE compliant PSE. PoE startup diagram

After startup the PSE and PD can negotiate power allocation. It is performed via so-called Data Link Layer (DLL) classification. The main purpose of DLL is to negotiate power allocation to the PD. For example, a PD that did not receive its full wattage (power demotion) can send another request for higher power. Alternatively, a PD can request lower power than can be used for its class. This would allow PSE to re-allocate wattage to other devices. DLL performs via so-called Link Layer Discovery (LLDP) protocol. Its latest version is defined in clause 79 of the IEEE 802.3bt standard. This standard requires all PD classes between 4 and 8 to support DLL classification. However, for PSEs the DLL classification support remains optional. Note that DLL is performed by the application’s MCU rather than by PD controller, which just passes power to the application after initial start up.

DC-DC converters within PD normally have large bulk capacitors. To limit inrush current that charges these capacitors an IEEE-compliant PSE for the first 50 to 75 ms operates in current limiting mode. In this mode PSE limits both the total current and the current in each pairset according to the PD class. Then after initial inrush state is over, PSE switches to full operational current. Until this moment, PD should not draw current (both total and in each pairset) exceeding PSE inrush current. Otherwise the PSE may not exit inrush state and will shut down. High initial current of the application is the main reason why PoE system fails to start.