WHAT IS MPS AND WHY DO WE NEED IT?

Before applying working voltage to the cable, a compliant PSE has to determine that a compliant powered device (PD) is connected to the port (see detection). However, even after applying power, a PSE needs to make sure the PD remains connected. It does it by continuously monitoring the current draw. Obviously we don't want to have voltage in the unplugged cable. Therefore, an IEEE-compliant PD must draw certain current to avoid being disconnected. This minimum current is called Maintain Power Signature (MPS). The MPS value is 10 mA for Classes 1 to 4, and 16 mA for Classes 5 to 8. A PSE must turn off the port if MPS is not detected for at least 400 ms. This way we assure that no voltage is applied to a disconnected cable.

HOW TO IMPLEMENT MPS

Many isolated DC-DC converters would consume the required minimum current even at no load. However, some converters with a sleep mode, as well as synchronous buck converters may need some means to draw additional current to prevent port disconnection. Also, PDs in redundant applications or PDs with auxiliary power source (such as a wall adapter) need an automatic pre-load to keep the PSE port active when a given channel is in standby mode. Otherwise, the application will reset when currently active power source is removed.

There are two ways of providing MPS: with constant current and with pulsed current. The constant current method is the simplest one. It can be implemented with a 5k/1W resistor in series with a 100V signal FET. The FET would connect this resistor across PoE line to draw additional current when necessary. A bit more complex but a more efficient method is with pulsed current. It would need an additional timer (such as 555). With such a method the timer would periodically drive the FET with low duty cycle (see the Table below for timing requirements). Such a method significantly reduce power wasted on MPS.

PD TYPE	PD CLASS	PSE TYPE	MPS	MIN. ON-TIME	MAX. OFF-TIME
1,2	1-4	1-4	10 mA	75 ms	250 ms
3,4	1-4	1,2	10 mA	75 ms	250 ms
3,4	1-4	3,4	10 mA	7 ms	310 ms
3,4	5-8	3,4	16 mA	7 ms	310 ms

Some PD controllers include MPS function. Others need MPS implemented externally.