Many power control and protection systems seek an integrated MOSFET along with the controlling IC in order to achieve two goals: (1) a smaller solution, (2) a lower cost solution.
As you may know, such things are never as simple as we would wish. In this blog, I will examine the subject of integrating MOSFET's in PoE PSE (Power-over-Ethernet Power Sourcing Equipment) devices in order to achieve these benefits.
There are several tradeoffs to consider when using an integrated MOSFET and controlling IC, such as: power loss which results in cost of energy wasted to the end user, increased heat sinking due to power lost in heat dissipation which translates to increased real estate, and increased operating temperatures which can result in lower reliability.
Taking existing technology as an example, when we compare existing integrated MOSFET PSE vs. external MOSFET solutions, the most dramatic performance difference is the Rdson of the MOSFET. Existing solutions on the market show a clear discrepancy in Rdson:
- “Brand X” uses an integrated MOSFET with Rdson of 300mΩ and requires a current sense resistor of 360mΩ.
- “Brand Y” is an integrated MOSFET PSE providing a 350mΩ Rdson accompanied by "lossless" current sensing.
- “TPS23861” on its 100M EVM board uses a 3.3 x 3.3mm MOSFET with 44mΩ Rdson combined with a 255mΩ shunt.
The table below compares the losses of each approach, and since PSE's are often used in high channel-count systems, the losses are calculated for up to 48 channels.
|
|
MOSFET |
Shunt |
Total R |
Power loss (watts) |
48 Channel Power Loss (watts) |
|
Brand X |
0.3 |
0.36 |
0.66 |
0.19 |
8.9 |
|
Brand Y |
0.35 |
0 |
0.35 |
0.10 |
4.7 |
|
TPS23861 with 44mΩ MOSFET |
0.044 |
0.255 |
0.299 |
0.08 |
4.03 |
In order to achieve the smallest possible, high-performance solution at the lowest possible cost, it is important to remember that it is possible for a discrete solution to come out a winner in this contest. Also, remember that although MOSFET integration is attractive, you can meet all size and cost objectives while simultaneously improving performance through reduced losses, power dissipation, and heat generation using a solution like the TPS23861 100M EVM.
Download the compete reference design with schematic and GUI for the TPS23861 100M EVM.
Read more Power over Ethernet blogs.
