Solutions for high-frequency transient interference of RS-232/485 (Figure)

RS-232/485 is a widely used serial interface in the industrial control industry. RS-232/485 is defined as a single-ended standard that increases the communication distance in low-speed serial communication. However, in actual circuit applications, it is often affected by interference and surges. There are many reasons for this interference. To solve the problem of interference, you must first find out the cause of the interference and then solve the problem. Generally, the method of adding an isolation transformer is used to solve the problem of interference. This method is mainly aimed at the conducted interference from the power supply, which can block most of the conducted interference before the isolation transformer, and also has the function of power supply voltage conversion. MORNSUN has designed a solution for customers to specifically solve the harm caused by this interference to the circuit in response to this situation.

This design example introduces a method to protect the serial interface from damage caused by high-frequency transient interference. This technology is being widely used in corresponding fields to play a role in the safe isolation and protection of the interface circuit and the subsequent circuit.

However, in many cases, the use of an isolation transformer alone cannot achieve the effect of isolation protection. In fact, as long as there is a possibility of surge pulses at the input end of the circuit, or the power supply at the input end is unstable (such as car batteries, etc.), there will still be high-frequency transient interference in the circuit, which will cause great damage to the circuit. Generally, when switching high-power inductive loads, such as motors, transformers, relays, etc. or lightning processes, transient interference with a high amplitude will be generated. If not properly protected, it will damage the RS-422 or RS-485 communication interface, and even damage the subsequent circuit. This transient interference can be protected by isolation or bypass methods. The most common and effective method is to connect a TVS tube or a voltage regulator tube (low voltage drop) to the input end of the isolation converter, as shown in Figure 1. In addition, the selection of various parameters such as the withstand voltage and clamping voltage of the TVS tube should also be appropriate, otherwise it will not play a protective role (too high) or affect the operation of the circuit (too low).

The newly released B0505LM-1W series products from MORNSUN can be better applied in this solution. This ultra-small, highly reliable DC/DC converter with 1000VDC isolation has an input voltage of 5V, an output voltage of 5V, and a power of 1W. Figure 1 shows a typical application circuit of this product. Since 1000VDC isolation is achieved between input and output, and the output short-circuit time can last for 1s, it can effectively resist the interference of high-frequency transient surges.

MORNSUN has improved its process to make the product more reliable. At the same time, quality engineers strictly control the selection of suppliers and the quality control of incoming materials. The internal potting material uses American resin, and the thermal expansion coefficient is well matched with the internal substrate and the shell to improve the reliability of the product and provide guarantee for the quality of the product.

B0505LM-1W can work normally in the temperature range of -40～+85℃. MTBF has been increased from 1 million hours of existing products to more than 3.5 million hours. The product meets the IEC61373 standard in terms of vibration and impact resistance and is suitable for industrial vibration and impact occasions. At the same time, it has realized lead-free production process and meets various RoHS directives.

The volume of B0505LM-1W product is: 16.5*5.9*7.5MM. This high-reliability DC/DC converter is 1/7 shorter in length and 40% smaller in volume than the previous generation. With its small size, light weight and strong anti-tilting ability, the product better meets the more stringent requirements on volume, weight, portability and other aspects in the applicable field. This product is suitable for small current isolation and DC voltage conversion, such as intelligent instruments, industrial control circuits, buses, power monitoring systems and other fields.