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How to do ESD Protection for RF Module Products in Hardware Design?

Views: 2661     Author: James Wu     Publish Time: 2021-06-16      Origin: Original

Static electricity is harmful to most electronic products. When using wireless module, the antenna's radio frequency interface should avoid static electricity directly shock. Even if you use your hand to hold the RF module, you should wear anti-static gloves. That is to say, the RF output of the wireless module is very sensitive to static electricity, which may cause permanent damage to the wireless module if it exceeds a certain static level. Therefore, ESD protection is an important consideration for hardware engineers when designing products with our RF modules. 

With regard to ESD immunity level, different products and different industries have different standards. The IEC61000-4-2 standard promulgated by the International Electrotechnical Commission is suitable for testing the electromagnetic compatibility of various electrical and electronic devices. The ESD immunity level of the product needs to be defined before product design, either according to the standard or according to the actual needs of the product. In this way, product design and testing can be carried out on a basis.

ESD Protection

The implementation methods of ESD immunity level mainly include shell design, hardware design, PCB layout, component selection, software debugging, etc. In hardware design, an important method is to add ESD protection devices at key circuit nodes of input or output. Transient Voltage Suppressor (TVS) diodes, commonly referred to by hardware engineers, are used in ESD protection devices. The key feature of TVS diodes is that they have high impedance at normal operating voltage and become low impedance when the voltage exceeds normal operating voltage, and the current is directed directly from the sensitive parts to the system or the earth (enclosure), which can suppress the reverse transient high voltage between the two poles of the TVS diode. Generally speaking, two-way TVS is recommended, but not one-way TVS.

For the IOT industry, wireless products are an important component, and wireless modules are an important part of wireless products. Generally speaking, the wireless module product can be designed with ESD protection devices, while the base board using the wireless module needs to be designed with a reliable and effective electrostatic discharge path at the antenna interface to ensure that the electrostatic discharge path does not pass-through sensitive devices and sensitive lines.

An ESD design circuit structure for the RF output of a wireless module is recommended here, as shown in the following figure:

ESD design of wireless module RF output

Although the TVS diode can improve the ESD performance of the wireless module, there is a hidden danger that harmonic noise may be generated and the receiving sensitivity of the wireless module will be reduced. This puts forward a requirement for the selection of TVS diode, which can be avoided by optimizing the TVS diode. Therefore, the key index of harmonic noise should be paid attention to in the selection of electronic components. 

For a wireless module with an external antenna, the antenna interfaces (such as SMA interfaces) need to be placed on the base board of the applied wireless module. Meanwhile, we need to take full account of the discharge path of static electricity on the base board, to ensure that static electricity does not enter the wireless module product itself through the radio frequency adapter, and that static electricity can be quickly discharged to the earth through the discharge path we designed, even if there is static electricity into the antenna interface, so as to effectively improve the reliability of the product. 

SMA interface & SMB interface

In general, it is recommended to use the resistance-capacitance network shown in Figure as below to conduct static electricity. Please note the PCB layout ensures that the electrostatic discharge path is shortest and does not pass through electrostatic sensitive devices, radio frequency circuits and sensitive lines (such as data lines, clock lines, etc.), where the capacitance needs to be high voltage resistant.

Electrostatic discharge resistance-capacitance network

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