In many communication systems, remote devices are connected to main units through cable assemblies. The data and control commands are exchanged between each remote device and the main units. Very often these devices are placed in locations with limited power access. Camera modules used in automotive safety (backup cameras) and security surveillance applications are good examples of remote devices. The most convenient way to deliver power to them is through the cable assembly. This article outlines the different methods to send power through a cable assembly, and discusses an implementation to combine both power and data over a shared transmission medium.
Power feed with copper wires
Many transmission standards have incorporated the power distribution capability in the transmission medium. Universal Serial Bus (USB) is an example in which 5V is distributed through a pair of copper wires in the USB cable assembly. It is widely used for providing power to remote peripherals in PC and consumer applications. Using copper wires is the most cost-effective and robust method for delivering power to remote devices.
The figure below illustrates the use of a dedicated pair of copper wires to provide power to a remote camera, while the captured image is serialized by the DS90UB901Q and sent through a separate twisted pair to the DS90UB902Q deserializer at the main processing unit. One pair of wires is used to transport the video data and control information through the FPD (Flat Panel Display Link)-Link III serializer and deserialzer (SerDes), while the other pair is used to carry power.
Power feed with coaxial cable
Sharing a single transmission medium between signal and power eliminates the need for a second pair of wires. Often a second pair may be deemed inconvenient, or cannot be used for certain applications. The use of power and signal over a shared medium is not a new concept and has been widely used in television signal amplifiers located in roof-top antennas (see figure below).
Power is delivered by the same coaxial cable that carries the antenna signal to the TV receiver located in the house. As illustrated in the figure, DC power is injected into the coaxial cable through an inductor L1 at the source, and extracted at the destination with inductor L2. The power return path runs through the braided shield of the coaxial cable. As long as L1 and L2 provide sufficiently high impedance, they appear as an open circuit to the high frequency TV signals, and do not affect signal fidelity.Power feed with shielded twisted pair cable
The concept of sharing power and signal over one transmission medium can be applied to high-speed serial links such as TI’s FPD-Link II/III and Channel Link
II/III SerDes families. The figure below shows a remote camera implemented with the DS90UB901Q serializer and DS90UB902Q deserializer, with power and data transmitted over one shielded twisted pair cable.
The power feed is applied equally to the two wires of the differential pair through a pair of inductors at L3. Power is extracted equally from the two wires at the destination through L4. Any switching noise from the power supply is applied equally at the two wires and they appear as common-mode signals. The differential receiver of the deserializer is capable of rejecting common-mode signals and minimizing the effect from the power supply’s switching noise. The DC return path is carried by the cable’s outside braided shield.
In applications where the braided shield cannot be used as the DC return path, power and return can be applied to the two wires of the differential pair. The following figure illustrates the arrangement with a differential power feed. Any switching noise from the power supply will appear differentially and superimposed onto the high-speed differential signals. In the presence of a large amount of supply noise, this approach is more vulnerable to performance degradation due to the potential for signal-to-noise erosion. Requirements of power feed network
The figure below illustrates a conceptual design for power and serial data over a shared data pair. The power feed inductors L3 and L4 are shunting components on both ends of the cable. They are designed to present sufficiently high AC impedance (ZDIFF
) so they do not materially lower the cable impedance, and do not adversely affect its ability to transmit the differential signal. The inductors must also be able to withstand the DC current load without causing magnetic saturation, whereby the inductance collapses and significantly lowers the inductor’s impedance and negatively impacts the cable’s impedance.
L3 can be built with two inductors in separate packages. However, it is advantageous to use a pair of mutually coupled inductors with both coils wound on a common ferrite core. In the above figure, L3 and L4 are differential chokes with a center tap where the DC power is injected or extracted. The DC currents through the two windings of L3 create magnetic fluxes of opposite polarity. The magnetic fluxes cancel each other and avoid magnetic saturation. The use of mutually coupled inductors helps to reduce the physical size of the ferrite core.