Power Electronic Communications Links

Specialised digital links for power electronics service

When comparing communications links these days, most people see only the throughput: “My Gigabit Ethernet is faster than your 100Mbps”; “4G is faster than 3G”. However, in power electronic comms links, throughput is often at the bottom of the list. You can go as fast as you want on average, but it’s the speed off the line which is more important. This is latency. The reason latency is so important is that nearly all power comms links are part of a control loop. Latency=delay=phase shift=stability issues. Any decent power comms link has minimal and fixed latency. If we consider Ethernet (using TCP/IP), the latency can be in milliseconds due to the protocol ensuring all packets arrive reliability, with minimal concerns on when they arrive.

Noise Immunity

The next key design point is noise immunity. Power converters, especially high or medium voltage converters are electrically noisy. Common-mode noise is flying around and you have to make sure that it does not affect the comms link. At worst, it could mean that a single bit error could cause the converter to blow up, meaning thousands of dollars in repairs or replacement. And lots of apologizing. In order to control this risk at ELMG we design power comms links with the noise environment in mind and mitigate its effects so that catastrophe doesn’t occur. In very safety critical cases, redundancy can be used to ensure that the chance the comms link fails is infinitesimal.

Synchronisation means controlled latency

If your design has multiple converters then synchronization becomes a very significant issue. Imagine if two power converters become 180 degrees out of phase to each other because of a lack of synchronization. In the best case, you may lose some of your efficiency as current goes out converter and in another, in the worst case instability and oscillations could occur leading to catastrophe. This is why you a reliable comms link to ensure that parallel converters are always in phase with each other. Synchronization pulses or packets need to arrive at a set time in order for the PLL to stay locked. At ELMG we can design synchronization schemes into comms links as well as the PLLs to maintain constant locked phase references for each converter.

Hardware choices make the difference

The most basic type of power comms link is the most important. This is the signalling between the controller and gate drivers. For this PWM signalling the key requirements are latency and DC signalling. Why DC signalling you ask? “My switching is at 10kHz, I should have no problem with an AC coupled line.” This may be the case for the majority of the time but there exist situations (such as faults) where you may want to turn on (or off) a switch for a long period of time. AC coupling will not work for these situations. The other issue is that if you are using a physical layer from a fast comms link (such as Ethernet), these are optimised to provide communications at or near their data rate. Switching at 10kHz may well be too slow and be affected by the AC coupled hardware.

Reliability without question

The final trait that a good power comms link should have is reliability. With latency being the driving requirement, techniques such as acknowledgement and retransmission are unsuitable. Therefore the receive must be able to at least know if a message has an error or even better be able to tolerate some bit errors and still decode the correct message. This can be very effective as the chance to get two errors is much much less than the chance to get one. Using the right code can make the error rate infinitesimally small. If coding is not enough or takes too much overhead, techniques such as redundant links can be used in highly safety critical environments.

Protocols

There are thousands of communications protocols that exist today. These range from your simple onboard protocols such as SPI to field buses like Modbus to TCP/IP and gigabit Ethernet. A subset of these are suited to being suitable for certain power electronics links.

The first suitable group is the field bus. These include Modbus, Profibus and the newer Ethernet based ones such as Profinet and Ethercat. Field buses generally have had latency in mind in their designs with some like Ethercat achieving nanosecond latency. The other benefit they have is that they are standard and thus compatible with other products using the same bus. However they are an off-the-shelf solution which may have unwanted features that you still pay for and may be missing others. Field buses are generally suitable for links which carry data such as control setpoints. They are not suitable for PWM signalling. ELMG has experience in designing and using field buses.

Download our research report to learn more about rugged communications links specifically for power electronics.

Solutions for Short Links

If the comms link is short or even across the PCB from one control chip to another then standard protocols such as SPI or PCM links (like TI’s McBSP) are suitable. Their speed is settable and thus their latency can be fixed. The software or firmware protocols on top of these links can be designed to ensure they meet the requirements of the system. At ELMG we have used these extensively and they are likely to be in most power controllers in some shape or form.

Off the shelf is not good enough

Sometimes standard does not cut it. Want to meet strict performance requirements that an off-the-shelf solution can’t handle? Want to transmit data over a single pair of wires or fibers? A custom protocol may be the solution. With the FPGA controllers we design (on FPGAs such as Xilinx Zynq and Artix), adding in a protocol which can define custom packets, custom data streams and provide reliability is possible with direct logic access to the I/O pins at high speed. This would not be possible in a MCU. At ELMG we have designed power comms links which have minimal fixed latency, synchronization ability and reliability across a pair of fibers. We can design one for any application.

Physical Layer

In power comms links choosing the physical layer is a very important decision. With all the common-mode noise coming from your own converter, a poor physical layer choice can spell havoc with your communications. Two main choices of physical layer remain each with its own advantages and disadvantages.

Differential signalling

A very common physical layer is the differential pair of wires. It is already used in a number of systems such as RS485, CAN and Ethernet. It is also used in the very high-speed buses of PCIe and other high-speed serial links. The key benefit to the differential pair is its common mode immunity as the data is held in the differential mode only. However, this immunity is only valid if its signal conditioning circuits and receivers are up to the challenge. Addition of appropriate common-mode filtering is a must with these circuits. At ELMG, we use the benefits of the FPGA’s natural ability to use differential pairs directly from silicon. This ensures that there is no area on the board which has weak single-ended signals which allow common mode noise to corrupt your power comms link.

Fiber for best noise immunity

Although differential pairs are very good, in terms of noise immunity, the best physical layer is fiber optics. In most cases you will envisage high speed internet connections carried along a bundle of glass fibers. In power comms links this is not the case. The main fiber used is plastic optical fiber and although considerably slower than its glass equivalents it is fast enough for most power comms link applications and also is a lot cheaper. The other advantage of plastic is that there are transmitters and receivers designed for power electronic applications and have the critical DC signalling needed by some power comms links. At ELMG, we have vast experience in using plastic fiber optic systems from the system level down to the receiver design. We have qualified a large number of suppliers and have options to second-source your fiber optic needs.

Powerline comms

For convenience, using the AC line as your physical layer is a good choice. Doing this involves superimposing a power comms link above the standard 50/60Hz AC line. It can allow you to communicate over existing wiring which can be a positive in certain situations. At ELMG, we are able to create a power over AC line system which meets your needs, whether they need to be compliant to a standard or not.