When deciding upon power architecture for CPE applications, a question which commonly arises is whether to use 5V or 12V DC input voltage for low power applications (≤15W). In this article, we will be exploring the pros, cons and drivers behind each approach.
Application Versus Output Voltage
CPE gateways, routers and higher power set-top boxes typically use 12V PSUs. However, lower power compact IP STBs such as OTT dongles type use 5V so that they can be powered via a standard USB cable.
12V Architecture Benefits
With a 12V DC output, power dissipation related to the PSU is lower, as are the costs associated with managing the thermal solution.
Higher efficiency is achieved due to the lower forward voltage drop which is developed across the output rectifier e.g. in the case of a Schottky Diode or through the drain source impedance, when a synchronous rectifier is utilised. The power loss in the DC cable and connector is also lower due to the lower output currents for a given power level.
From a cost and thermal perspective, the case size, component ratings, heat-sinking, E-cap life and voltage drop in the DC cable will all benefit from a lower output currents due to the higher output voltage at a given power level – Ohms law applies!
Whilst the power supply efficiency and costs may seem optimised with a 12V architecture we also need to consider the total cost of power for a given application. To do this we must also take account of the efficiency and costs of the additional power regulation in the host CPE device.
5V Architecture Benefits and Challenges
The adoption of 5V DC architecture can be very beneficial in compact IP set-top boxes and OTT dongles because there are no parts that require a voltage higher than 5V. This simplifies the DC-DC circuit design.
The duty cycle of the internal DC-DC converters can also be optimised leading to very high efficiency and low component count. This keeps the design compact and minimises heat dissipation.
A 5V output is most commonly used when the end application needs powering via a USB connector, for example a mini/micro B or Type-C plug. Or, if space is at a premium, the DC cable and USB plug can be removed altogether and replaced with a female socket on the case like on phone chargers.
Typically for 5V PSU designs with USB connectors the PSU output would be 5V 1.0A (5W) or 5V 3.0A (15W) to follow USB standards. NetBit has previously designed both types of PSUs with several different USB plug and connector options.
One challenge when using 5V DC with video devices is that the HDMI port must have a 5V output per the HDMI standard. Typically, there is a voltage drop between the DC input and the HDMI output which requires a step-up regulator to ensure that there is 5V on the HDMI output port.
In conclusion, it can be stated that the power level and application area will have the biggest influence on the selection of the optimum DC output voltage from the power supply. Whilst 12V may seem to be more optimised for cost and efficiency than 5V, careful consideration should be given on how to balance power dissipation, voltage drops and cost trade-offs between the external power supply and the CPE itself.
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