The Challenging Power Demands of a Bus WiFi System

June 12, 2018

The expectations of public transportation are increasing rapidly, particularly when it comes to on-board facilities. Although almost every traveler will carry a cell phone, they expect on-board WiFi to provide a better connection and avoid expensive cell phone bills.

Today many buses, trains and even aircraft have on-board WiFi, and in this application a manufacturer wanted to adapt a railway WiFi router to provide connectivity on buses. As the demands of the rail industry are similar to those of bus operators (particularly the need for rugged components, high efficiency, security, reliability and speed), the system met many of the requirements “out-of-the-box”. Although the router couldn’t be used without modification. In particular, the manufacturer needed to cut the system cost to succeed in a more competitive market and reduce the size to simplify the after-market installation.

The electrical environment proved to be particularly demanding as in a bus, power is supplied from a 12V battery. But during engine cranking spikes will be generated on the power rail, and the output voltage can fall below the nominal 12V. The power system was, therefore, specified to deliver a regulated output of 12V over a very wide 8 to 36V input range.

The Need for Wide Input Range

There are many reasons why systems might need wide input range power systems: see 5 Reasons You Need a Wide Input Range to discover some of them. In this case the spikes generated during engine cranking were a particular problem.

ZVS Buck and Buck-Boost Regulators Meet the Requirements

Vicor ZVS Buck Regulators and ZVS Buck-Boost Regulators are designed to operate over a wide input range without a significant drop in efficiency. This is due to the use of the Zero Voltage Switching (ZVS) Topology that enables higher power throughput and reduces losses than conventional Regulators when using larger step-up or step-down ratios. This is particularly the case at higher output currents, where efficiency matters the most (see figure 1). As the size of the router needed to be reduced, the customer needed to ensure high efficiency to avoid thermal design problems.

Figure 1 – Efficiency of Vicor ZVS Buck Regulator at Different Input Voltages

Figure 1 – Figure 1 – Efficiency of Vicor ZVS Buck Regulator at Different Input Voltages

Other benefits of using Vicor ZVS Buck and Buck-Boost Regulators are the compact LGA SIP package and the wide operating temperature of -40°C to +125°C, enabling operation in all weathers.

The Solution

The customer chose a ZVS Buck Regulator to drive the 3.3V rail in the system and a ZVS Buck-Boost Regulator to provide a regulated 12V rail from the 8 to 36V delivered from the battery.

Final Power System for WiFi Router

Figure 2 – Final Power System for WiFi Router

This solution allowed the customer to meet the performance objectives in a footprint of just 2.8cm2. With an overall system efficiency of 91.5%, even when operating at high ambient temperatures no moving air cooling was required, reducing the size and cost of the router while increasing its reliability.

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