This solution employs MOVs across the AC supply power
lines to divert AC power electrical surges away from sensitive
components and an ESD suppressor mounted in front of the
DC-to-DC converter to mitigate ESD transients. Each of the
PoE implementation's power and data lines is protected by a
transient current suppressor and each twisted pair is protected
by clamping diodes. Finally, all four twisted pairs are protected
isolating transformers.
It is good design practice to use surface mount devices
for suppressors with four-terminal connections to minimize
parasitic inductance. Another good design tip is to use ceramic
capacitors in parallel and close proximity with TVS diodes
particularly when the devices are connected to a power rail.
High-current suppressors must be located in the I/O area.
Finally, line-to-ground capacitors that could be subject to
electrical transients should be rated for voltages above 2kV.
These capacitors must also have low ESR at high frequency and
low parasitic inductance.
The designer has a wide choice of proven circuit protection
products from component vendors. Bourns, for example, is
a specialist in the sector and offers a portfolio of fuses, TVS
diodes, MOVs and specialized proprietary components such as
the company's TCS
™
High-Speed Protectors (HSPs). Among the
company's latest products are the SM91074AL and SM13100EL
transformers with common mode chokes for noise rejection and
circuit protection in PoE applications.
| 8 |
Figure 4: PoE circuit protection implementation suitable for
withstanding severe lightning surges. (Source: Bourns, Inc.)
PoE's simplicity and convenience has resulted
in its use in a range of applications far
beyond that envisaged by the specification's
designers. But, while transmitting both power
and data on a single cable is convenient, it
increases the danger of damage to sensitive
components from external and power supply
electrical transient events. That makes
circuit protection in critical installations
vital. Component suppliers offer a wide
range of blocking, suppression and isolating
technology and associated expertise to help
the designer implement a suitable degree of
protection dependent on their application and
PoE specification employed.
Conclusion
Application example
Figure 4 shows a comprehensive PoE protection
implementation. This implementation is, for example, capable
of mitigating severe lightning surges such as 4kV, 10/700μs
transients. Importantly, such an implementation will have minimal
impact on signal integrity.
