AN315 Datasheet, PDF (1/8 Pages) Silicon Laboratories – ROBUST ELECTRICAL SURGE IMMUNITY FOR POE PDS THROUGH INTEGRATED PROTECTION

AN315
ROBUST ELECTRICAL SURGE IMMUNITY
FOR POE PDS THROUGH INTEGRATED PROTECTION
1. Introduction
The Si3400 and Si3401 Power over Ethernet (PoE) powered device (PD) controllers are designed in an efficient
Silicon on Insulator (SOI) process technology, which enables the integration of a robust surge protection function.
Competitive PoE PD devices require external transient voltage suppressors (TVS), such as an SMAJ58A, to
provide surge protection. However, due to their inherently wider range of clamping voltage specifications, external
TVS-based protectors typically necessitate the use of 100 V processes for the PD controller device. This note
discusses some of the detailed considerations regarding the Si3400's proprietary surge protection circuitry that
enable the Si3400 and Si3401 devices to yield robust and standards-compliant electrical surge immunity when
used in Power over Ethernet powered device applications.
2. Traditional PD Surge Protection Considerations
As will be discussed later, surge currents of up to 5 A must be tolerated in PoE applications. The surge protector
most often used with PD controllers is an external SMAJ58A TVS diode (D1 in Figure 1).
To
RJ 45 &
Magnetics
B1
D1
B2
VSS
VSS
VPOS
RDET
802.3af
PD I/F
+
Switching
Regulator
VCC
VSS
SWO
DET
FB
EROUT
RCLASS
VNEG
VSS
M1
Opto
Figure 1. Traditional PoE PD Interface and Switching Regulator
(without High Voltage Device Integration)
For example, a standard SMAJ58A TVS from Diodes Incorporated has a maximum operating voltage of 58 V,
25 °C ratings of between 64.4 and 71.2 V at 1 mA, and a clamping voltage under 93.6 V at 4.3 A.
While it is possible to sort individual devices for tighter clamp voltage ratings, the typical maximum clamping
voltage (including temperature variations) when using this type of protector is close to 100 V. Consequently, most
PoE PD controllers are implemented in processes providing a 100 V breakdown voltage.
Rev. 0.2 10/06
Copyright © 2006 by Silicon Laboratories
AN315