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Power converter environments are electrically and magnetically noisy so communications links in and around them must be robust and reliable. How many times have you seen the communications drop out or errors increase when the converter switching is running. It happens regularly that the power converter operates meets EMC but the internal communications link has errors. It is often the case that the converter noise is increasing the bit error rate of the communication link. And the converter noise is largest right where the communication link operates.
Typically for power converter communication links the information needs to arrive accurately and at a certain time. A synchronisation packet or pulse for the phase reference in parallel connected converters needs to arrive at a set time without jitter for the PLL to stay locked. This locked phase reference is necessary for the parallel converter operation. And the timing accuracy of any gate pulse needs to have really well controlled jitter.
Comms links at the heart
Communications links are at the heart of modern digitally controlled power converters. Often the communications is what makes the converter work or is the most required feature that you really need. The internet of things is a wave that will roll over everything. Without the link there is no product to ship. The stress of trying to hunt down an error in a communication link with the R and D Manager and VP of Engineering breathing down you neck is difficult. Being that VP of Engineering or the R and D manger and watching over the shoulder of your team is truly devastating personally, to the bottom line and maybe the career. Potentially the product will not be safe because the safety critical communications link is unreliable.
Communication link failure in the field annoys customers creates trouble with your manager and the sales guys. The brand damage is enormous when the communications do not work.
ELMG PowerComms Communication links
Powercomms links are custom and semi-custom communication links design specifically for use in power converter high EM noise situations. High noise immunity digital signals with large signal integrity and high noise margins ensure the signal layer is robust. Differential signalling, matched impedances and good signal encoding are all deployed. Then robust clock recovery and phase lock loops ensure timing recovery and synchronisation of time signals with controlled jitter. Technologies such as LVDS provide high data speeds and good data integrity. Multidrop differential signal systems can be used as back planes if required. The ubiquitous Ethernet is useful as a physical layer. CANBus and Modbus are also alternatives with Ethernet versions also.
Transmission control and error correction are handled with the link end state machines implemented on FPGAs. These ensure the links can operate with high fidelity and safety. For lower speed links state machines can be implemented in software on DSP or microprocessors.
For high reliability systems in power converters
In high voltage and medium voltage power electronics the EM noise is large. In high frequency switching power supplies the EM noise can also be very large. And when a single bit error might cause the destruction of the power converter or even worse an embarrassing delay for the customer data update reliability is paramount. These high noise medium voltage systems also have the problem where in a fault the ground potential can rise leading to large currents flowing in the wires. Preventing this requires isolated links.
Data over the AC power line
AC power converters are connected to the AC network. And this AC network is great as a communications link. Using its own line connection the power converter can communicate with other power converters through the AC network.
Ripple control, power line communication (PLC) and ethernet over the in home wiring are all alternates for networking with low cost.
In power electronics the fibres are plastic. The ultra-fast links for the telecom industry run down glass fibres. The difference is speed, cost and distance. Plastic is slower but still fast enough, cheaper and shorter with a distance limit of maybe 50 meters. These three characteristics are just right for a power converter. And the big difference with glass links is that in order to go so fast they are AC coupled. This means that fast glass fibre systems cannot be used to do on/off gate drive signalling.
Often it seems that plastic fibre optics are too expensive. When you look at cost of the connectors and the cable plastic fibre optic links are very competitive with wire connections in terms of cost for the same data rate. The advantage of the fiber is the electrical or galvanic isolation. Clearance and creepage distances (spacings) of meters are possible. Common mode interference is completely avoided.
When cables can safely be used ELMG PowerComms links are easily able to meet typical power electronic converter communication link requirements. Depending on the speed features could include DC offset management through symbol encoding, a reasonable high clock rate and a really good clock recovery system to add to the fidelity of the link. The data rates over cables depend on the cable.
Powerlink, Ethercat, ProfiNet and EtherNet/IP all use Ethernet Cables to provide fast networks for industrial and power automation. All of these network systems can be integrated directly into the ELMG FPGA Digital controller.
Communication links over PCBs run between ICs. Using differential tracking and good coding techniques allows excellent noise rejection, EMC compatibility and high data rates.
We have spent the last twenty five years designing, developing and building power electronic converters. And in that time almost every communication link that we saw had problems with the electric field, the magnetic field, the displacement current of the power converter or just the general EM noisiness. Sorting out the communications link has fallen to us power guys because the noise is from the converter. Typically “The communications works if the power converter is off so it is the power converter causing the problem.” And this is probably true. And the communication links designers never really delivered the guaranteed latency links. So we decided that we’d make links that give us what we need for power converters.
There are off the shelf communications links that go at 100Gb/s. None of them really suit power electronics well as they miss on price, often do not operate at zero frequency DC which is needed for on/off signalling and most problematically guarantee throughput not constant latency or delay. The flexible semi-custom combination of extreme noise and high fidelity requirements for extreme timing accuracy with low jitter are what ELMG PowerComms link provide.
Download this report to learn more about rugged communications links specifically for power
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