Watch your Power over Ethernet OPEX or risk obsolescence!

Other Parts Discussed in Post: TPS23752, TPS23751

We've been tracking an interesting trend in the POE world and it should come as no surprise to anyone who pays an electricity bill… or knows someone who pays an electricity bill: Growing concern with total power consumption over a product’s lifetime.

Decrease operation expenses

Total cost of ownership is generally considered to be the sum of CAPEX (capital expense.. or purchase price) and OPEX (Operating Expenses… or what it costs to run ). Designers and hardware development managers have historically been more concerned with CAPEX than OPEX, but are starting to see a shift toward total cost of ownership as the correct metric by which to judge solutions. This is coming from the end users who actually have to pay to purchase and run such equipment.

A similar shift started in the telecom industry a few years back when we started seeing board/system redesigns expressly for the purpose of increasing efficiency. Until that point all telecom boards were using ORing diodes to merge the redundant -48 V feeds on high and low side. Using FETs and ORing controllers was viewed as far too risky when diodes were cheap, rock solid, and 100% derating was easy. Enter customer demands for higher efficiency and ORing FETs with controllers began appearing in many designs. Now, they are the norm. It’s not often that efficiency can be boosted by 1.5% with a simple change like that.

Today’s example is Power over Ethernet (PoE) solutions; Specifically, PoE cameras.

With many IEEE compliant POE solutions available today there are not many ways to differentiate a solution. More features and gee whiz functions are not necessarily the best way to impress the majority of the IT, security, and infrastructure buyers around the world. What seems to draw the most interest is low cost of ownership, which means low OPEX.

The math is pretty simple, even if some assumptions are used.


  1. Average IP camera life span is 10 years
  2. Average IP camera is powered 24/7.
  3. Average IP camera is idle 22 hours each day. (light load meaning pan, tilt, zoom motors not running )
  4. An idle IP camera designed for high light load efficiency can consume 350 mW less power than a camera with standard light load efficiency.
  5. Incremental cost for light load efficiency is $0.01. (Actually, not even that much.)

Lifetime Savings = Number of Cameras x $/kWh x 22hours/day x 365days/year x 10 years x 1kW/1000W x (PDIS_NORMAL – PDIS_HIGHEFFICIENCY)

Running some numbers through the equation above, a facility with 100 cameras paying $0.14/kWh will save almost $400 in ten years.

This savings will not require extra upfront silicon costs and that $400 goes right to the bottom line. (Note: The assumed electricity costs were based on relatively low US pricing. In Europe, the savings will be 2 to 2.5 times as great, and those costs are only going up.)

But Tex…How can my design reduce POE OPEX ???

A fine question.

TI has recently released two POE PD controllers specifically designed for high efficiency over very broad load ranges. The TPS23751 and TPS23752 operate as standard isolated flyback converters while under load. When the load drops off and the device goes into idle, three features kick in to reduce power consumption:

  1. Synchronous Rectifier (SR) shutoff. Efficiency at normal loads is improved by having a FET perform the SR function. At light loads, the energy required to run the SR is greater than the energy being saved and it is more efficient to rectify using the body diode of the sync FET. At this point it is more efficient to turn off the SR control and let the body diode of the FET do the job. The load level at which SR is turned off can be programmed by the designer.
  2. Variable Frequency Operation (VFO). Adjusts the switching frequency at light loads. During VFO mode, the inductor peak current is fixed and the CTL voltage varies the switching frequency. This minimizes switching losses at light load to further improve efficiency.
  3. Sleep Mode (TPS23752 only). Sleep Mode disables the converter to minimize power consumption while still generating the Maintain Power Signature (MPS) required by IEEE802.3at.

 TPS23751 and TPS23752

Wishing you lower energy costs,
-Jim Bird

Index of all Power House blog posts.

Tell me, how has this helped you in reduce operating expenses?