Active Bridge Rectifiers Reduce Heat Dissipation within PoE
Security Cameras – Design Note 519
Ryan Huff
Introduction
Power over Ethernet (PoE) has been embraced by the
video surveillance industry as a solution to an age-old
problem: complicated cabling. For instance, a basic,
traditional fixed-view security camera requires two
cables: one for power (10W to 15W from a 24V AC
or 12V DC), and a separate, coax cable for the video
signal. With PoE, a single Ethernet cable carries both
video data and power. Everything is simplified. Right?
Not quite. To meet compatibility with existing systems,
camera manufacturers must produce PoE-enabled
cameras that are also compatible with legacy power
sources—they must accept PoE 37V to 57V DC from
an RJ-45 jack or 24V AC, +12V DC, or –12V DC from
an auxiliary power connector.
The Old Way Loses Power
Figure 1 shows the power architecture used by many
PoE camera manufacturers to solve this problem. A
full-bridge diode rectifier after the auxiliary (old-school)
input produces positive DC power from either 24V AC, …