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Posted on 23 October 2019

How Digital Isolation Technology Drives System-Level Efficiency

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Power supply efficiency continues to be an area of focus in the power electronics industry. Advances in power electronics components and topologies are driving individual power supply efficiencies to higher performance levels. The improvement in the efficiency of each power supply is an important – and essential – element in addressing the rapid growth of power consumption. Another, perhaps greater, opportunity to dramatically reduce power consumption is the system-level, intelligent management of energy.

By Atefeh Mirbagheri, Faisal Ahmad and John Camagna Akros Silicon, Inc., United States

This article describes implementation of high-speed, digital isolation as an enabler for system-level power management in enterprise networks utilizing Power over Ethernet (PoE) technology. With this approach, the network switch and all of the powered devices on the network become part of an intelligently managed system that offers the potential for energy savings that go far beyond the capabilities of individual power supplies.

Network-Level Power Management

Digital power control provides the opportunity to go far beyond the full-load efficiencies of individual power supplies by actively controlling, in real time, the power consumed at each node in a system. For example, in a networked building, devices in unoccupied rooms can be placed on standby or can be turned off. Further, device power levels can be adjusted based on environmental conditions or according to real-time demands for computing power and bandwidth.

All of this control requires a system-level approach into the management power in the network. The opportunity for energy savings is growing as PoE advances are now pushing the power levels at individual nodes from 30 to 90 Watts. This changes the landscape by allowing higher powered devices to be controlled from the network switch.

Enabling intelligent power management requires real-time data. Since PoE is an isolated power system [1], gathering this data requires access to information across an isolation barrier at data rates that go beyond the capabilities of traditional isolation methods. An approach using high-speed, digital isolation is able to overcome this limitation, thereby allowing for the full realization of system-level energy management.

Network-level power management requires real-time telemetry and control of all regulators in the system. As an Ethernet network is a distributed system via CAT-5 or higher cables, galvanic isolation is mandated for ground loop suppression and safety concerns [2]. Figure 1 shows a typical PoE implementation with the connection between a PoE Powered Device (PD) and the network switch acting as the PoE Power Sourcing Equipment (PSE).

Network System Diagram with PSE & PD

The system-level power management protocols are implemented in a processor in the network switch, which commands the power system through an isolated I2C interface to the PSE controller. The PSE controller controls the power delivered to each individual PD. The PSE controller also reports voltage and current information back to the processor through the same I2C interface, so that it has the power consumption information needed to manage energy usage.

A challenge to implement this level of energy management at the system level is transferring data across the isolation barrier at a frequency that enables optimized network power management. To enable both real-time monitoring and control of multiple Ethernet ports from a single processor, data transmission speeds as high as 3.4MHz on the I2C bus are needed. Not only does the communication from the processor to the individual PSE need to be faster, but with high-end switches available in the market having as many as 96 ports, the need for high communication bandwidth has never been greater. Compounding this system challenge, PSE controllers are also moving from 4 channels to as high as 12 channels per IC, also requiring three times as much communication between a processor and a single PSE controller. The traditional solutions, optocouplers, have long propagation delays, which limit their ability to support these higher communication speeds [3]. In contrast, high-speed digital isolators can enable these higher data rates.

Isolating a High-Speed I2C Bus

The I2C bus has been a commonly used interface for bidirectional data transfer in many applications (Figure 2).

I2C Bus Data Transfer Diagram

For decades, its popularity has been due to its simplicity and flexibility [4]. The bus is controlled by one device; the master, which can communicate with one or more slave devices. The clock (SCLK) and data lines (SDATA) are shared by all slaves. Each bidirectional data transfer happens during one clock cycle.

The timing is very critical for accurate data transfer, especially in high speed applications. All propagation delays through transmission path need to be carefully accounted for, in order to ensure accurate data transfer. As shown in Figure 3, the falling edge of MCLK is propagated across the isolation boundary before it reaches SCLK with a delay time referred to as M-SCLK_DELAY.

Propagation delay from Master to Slave

The PSE response time to receiving the falling edge on SCLK is SRESPONSE_DELAY. When the PSE responds with a falling edge on SDATA, it is propagated back to the processor to reach MDATA with a time of S-MDATA_DELAY. Adding these delays together results in:

Total_{DELAY} = M-S_{CLK_DELAY} + S_{RESPONSE_DELAY} + S-M_{DATA_DELAY}

For correct data transmission, TotalDELAY must be less than the time that MCLK is low for a 3.4MHz operating frequency or 150ns period with a 50% clock duty cycle.

Considering the challenges, managing the propagation delay through the isolation boundary can be done very accurately using high-speed digital isolators. For instance, compared to optocouplers, the propagation delay through a digital isolator is much shorter. A typical propagation delay through optocoupler is greater than 100ns, while typically 40ns [5] for the high-speed digital isolator.

Digital Isolators Vs. Optocouplers

In order to prevent ground loops that could damage the equipment and to meet industry safety requirements, 1500Vrms galvanic isolation is required in PoE. While both optocouplers and digital isolators provide this level of isolation, digital isolators allow high-speed data transmission (Figure 4).

Comparison of Optocouplers Vs. Digital Isolators

Digital isolators are implemented in standard CMOS technology that provides benefits of lower propagation delays, reduced power consumption and improved reliability. Optocouplers, on the other hand, use photons to cross the isolation barrier and the process of integrating the photons to create an electrical signal is slow. As the industry steadily adds higher bandwidth requirements to their systems, to either allow for control of multiple systems or for faster monitoring of individual channels, digital isolators uniquely enable the requisite communication.

Conclusion

With the ongoing industry trend of enabling system power management to reduce energy consumption, network-level power control is needed to achieve system-wide efficiency gains. All distributed systems mandate galvanic isolation, and implementing cost-effective, reliable data communication across this boundary is necessary to successfully implement network-level power management. Using digitally isolated power management ICs allows for system-level energy management that enables the trend of improving system-wide energy efficiency. In contrast to optocouplers, digital isolators provide a unique solution that can be built using reliable and low-power CMOS technology to accommodate high-speed data transfer up to 3.4MHz.

References
[1] IEEE Standard for Ethernet, IEEE Standard 802.3, 2012.
[2] IEEE Criteria for Class IE Electric Systems, IEEE Standard 308, 1969.
[3] Avago Technologies, “High Speed LVTTL Compatible 3.3 Volt Optocouplers.” HCPL-260L/060L/263L/063L datasheet, May. 2013.
[4] I2C-bus specification and user manual, UM10204, 2012
[5] Akros Silicon, “AS174x datasheet, High Speed I2C Bus Digital Isolator.” Datasheet, 2014.

 

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