After clarifying the interference sources and dry winding paths of each circuit in the solar ship inverter control system, specific EMC designs were carried out for each circuit. The EMC design circuit and parameters mentioned in this article have been demonstrated through engineering product testing. The overall principle of design is to bypass interference at the interface input or ensure that the interference level at the pins of the integrated circuit or device in the interference conduction control system is within its voltage resistance and other capabilities, or to reduce the interference intensity of the interference signal to a safe level before entering control, to ensure the normal operation of the control system.
1. EMC design of power supply interface
The design of the power supply for the control system is one of the most important key technologies to ensure the reliability of marine electronic equipment, with a voltage input of 24 VDC.
As shown in Figure 1, through the design of a switching power supply, transformer isolation technology generates four voltage levels: 1) 5 V voltage, powered by a digital circuit; 2) 3.3 V voltage, generated by a 5 V to 3.3 V voltage stabilizing circuit, powered by a digital circuit; 3) ± 15 V voltage, mainly powered by analog circuits; 4) 24 VIO power supply, powered by digital input or output, generated by a transformer at a ratio of 1:1.
The design of power EMC is shown in Figure 1, and the design measures mainly include the following aspects:
1) Transformer isolation technology separates the external power supply from the internal power supply of the control system, while the internal power supply separates the analog power supply, digital power supply, and IO power supply.
2) The voltage power supply layers of each level on the printed circuit board are designed independently, and the analog ground and digital ground are separated.
3) The design of magnetic beads for pulse signal lines in switching power supplies reduces high-frequency harmonic secondary interference.
4) Input power polarity and overvoltage protection.
2. EMC design of signal interface
The signal interface EMC design is mainly aimed at DI (Digital Input) signals (digital acquisition), DO (Digital Output) signals (digital output), AI (Analog Input) signals (analog acquisition), and AO (Analog Output) signals (analog output).
2.1 EMC Design of DI Signal Interface
As shown in Figure 2, EMC design for DI signals can achieve protection in three aspects: 1) overvoltage protection. In addition to polarity protection, the input diode can also protect against overvoltage; 2) Optoelectronic isolation design, using optoelectronic technology to achieve physical isolation between external signals and internal signals; 3) Design a bypass capacitor that can bypass common mode interference signals.
2.2 EMC design of DO signal interface
Optoelectronic isolation DO design can be used to isolate the digital output control signal of the control system from external signals. For situations with higher isolation requirements, a relay dry node output DO design can be used, and the output signal power supply adopts an independent power signal, which can completely isolate the power supply, but the cost is relatively high, as shown in Figure 3.
2.3 EMC Design of AI Signal Interface
AI signal protection mainly adopts the design of digital power and analog power isolation technology and hardware filtering technology (Figure 4), as well as overvoltage protection measures. AI analog signal acquisition adopts integrated circuits with high common differential mode suppression capability.
2.4 AO signal interface EMC design
Interference bypass design can be applied to AO signals, and operational amplifiers with high common mode suppression ratio can be selected. In addition, photoelectric isolation technology is adopted for digital monitoring circuits, polarity protection and overvoltage protection are adopted for output interfaces, and measures such as separating digital and analog ground protection are taken. The specific design circuit diagram is shown in Figure 5.
3. Other electrical performance EMC design
In addition to the main signal interface, the solar ship inverter control system should also comprehensively consider the following four aspects of EMC design:
1) Communication interface. Integrated circuits with high common differential mode suppression capability are usually used, and communication data jumping or communication crashes are often found during experiments. The main consideration for communication data jitter is contamination of communication cables or power sources, and shielded twisted pair and coaxial cables should be used. Fiber optic cables can be used when the distance is far or the interference is particularly severe. The phenomenon of system crashes should consider the overall EMC design of the control system, with grounding design being particularly important. Identify the circuits that affect communication jitter or crashes by investigating them one by one, and modify the EMC component parameters of the circuits to eliminate defects.
2) Lightning protection and electrostatic protection. Electrostatic protection mainly adopts reliable grounding and dust removal measures to prevent the accumulation of static electricity. For human contact with static electricity, it is necessary to conduct an EMC static electricity test on the human contact area in the control system structure. The metal screws in the control system structure should be at least 8 mm away from the surface of the structure. For the control system, lightning protection mainly considers secondary lightning protection measures, namely induction lightning. Surge protectors such as varistors and air discharge tubes can be designed to ground at the signal interface input, or EMC components such as varistors and voltage regulators can be added between the positive and negative poles of the signal.
3) EMC software design. For digital signals, signal jitter is particularly common in components that are sensitive to electromagnetic interference. Solving this problem entirely through hardware is often costly and difficult. EMC software design can solve the instantaneous jitter of digital signals caused by instantaneous interference through software digital filtering, provided that the filtering time cannot exceed the minimum signal response speed required by the system. The filtering of solar inverter control systems below 100 ms can mostly meet the requirements.
4) Control system external interference EMC design. External EMC design mainly refers to the electronic products specified in the EMC standard that the external radiation interference cannot exceed the range specified in the standard. For solar inverter control systems, the external interference is basically the same as the affected circuit. During the test, the circuit that does not meet the requirements can be found through troubleshooting, and then the EMC components in the circuit can be adjusted to achieve the purpose of passing the test.
4. EMC process design of control system
The EMC process of the solar inverter control system for solar powered ships includes design process and usage process requirements, mainly including grounding, overlap, cable laying, and shielding. EMC process design requires special attention to material selection and process feasibility.
The EMC design process for circuit boards mainly considers 7 aspects:
1) Ground design. The digital ground is separated from the analog ground and adopts an independent geological design. The control system casing is reliably grounded, and the safety grounding wire length of the control system cannot exceed 0.5 m and the diameter cannot be less than 35 mm.
2) The printed circuit board under the photoelectric isolation chip needs to be hollowed out.
3) Place protective components near the interface input.
4) Interface terminal design for earth connection nodes.
5) The digital power supply and analog power supply should be separated, and the distance between the power line and the signal line should be ensured to be at least 3mm.
6) In each signal interface circuit, capacitors, resistors, and other EMC components at the signal should be arranged nearby near the input and output of the connectors.
7) The control system uses power separately from IO power, contactor power, and circuit breaker power.
In terms of cable EMC process design, shielded cables are used to connect the cables from the outside to the solar inverter control system, and the shielding layer of the shielded cables is ensured to be grounded at both ends. Meanwhile, inside the photovoltaic inverter, shielded cables and coaxial cables are used to connect to the control system, and the shielding layer is reliably grounded.
In terms of the process requirements for layout and wiring, the main considerations are: 1) avoiding parallel wiring of control system cables with paths passing through strong currents; 2) The shielding layer of the shielded cable must be grounded; 3) The control system should be kept away from high-power devices such as circuit breakers and contactors.