Filters, presumably electronic peers are all too familiar with this device, this is a very common and fairly frequently used component.
Especially when it comes to the field of radio frequency, it is no exaggeration to say that to do RF circuits is to learn to use circuits with various filters. Because the functional indicators of the chip have been fixed, it is only how to match the filter in each link needs our attention. It is precisely because the filter is used too frequently, so it has many types and a wide range of processes, which often make engineers not know how to choose, and today I will talk about how to choose the filter.
First, the selection of filters
The selection of filters, according to the application, purpose and requirements, etc., the selection principle is roughly as follows:
1.Working conditions
Current i, voltage v, operating temperature t and volume dimensions.
(1) Rated current
The rated current corresponds to the rated operating temperature, and the rated operating temperature exceeds the derating. If it is lower than the rated working temperature, the working current shall be subject to the rated current.
(2) Rated voltage
In general, the AC filter can be used as a DC filter of the same voltage, but if it is used for a higher DC voltage, you need to consult the manufacturer. For filters, the nominal voltage rating is the maximum allowable operating voltage of the device, as described in UL60939
(3) Other special requirements
For example, low leakage current for medical applications, high-grade insulation requirements, IT power grids, high impulse voltages, excessively long output cables, and so on. This requirement is very important and is directly related to the suitability of the power supply filter. If selected at will, it may lead to failure or safety accidents.
(4) Installation location and requirements
Most of the power filters are used at the input end of the power supply, and the mounting plate of the filter is generally required to be a metal flat plate (no insulation layer covered and anti-oxidation), and it is best to have a large area grounded. If there are no corresponding conditions, the impedance of the grounding wire should be reduced as much as possible and the grounding resistance should be reduced.
2. Requirements for use
(1) Routine application
It is routinely used to filter out general interference and increase the anti-interference ability of the system, and the current interference situation and target requirements are not very clear. Most of these applications include control boards, switching power supplies, or functional devices inside devices or systems. In this case, it is generally enough to use the regular medium function. There is a saying in the industry that 3db is that if the filter attenuates the interference to 3db, the energy of the interference is only half of the original. Generally speaking, the attenuation capacity of the power filter in its working frequency band is tens of dB, so for conventional, the interference energy is relatively small after attenuation, and the impact on the series is correspondingly smaller, and the reliability is better guaranteed.
(2) There are clear standards
Filters used in applications such as household appliances, mechanical equipment, motor controllers, etc., need to pass the corresponding standards in accordance with the equipment. There are clear requirements and standards for these applications, and the electromagnetic interference value (conducted interference) must be limited to a certain standard range. At this time, information should be provided to the filter manufacturer as much as possible, and the manufacturer will select products with similar applications for preliminary testing according to the application situation (if conditions permit, several more types can be provided), and then make adjustments according to the field test situation. For example, the switching frequency of the inverter, the rectification mode, the working frequency of the motor, the cable type and length, whether there is output filtering, etc.
(3) Functional requirements
In addition to the relevant standard requirements of EMI, there are also some functional requirements. For example, the filter used for the output of the inverter needs to protect the motor insulation, cable insulation, reduce the bearing current, and so on. At this time, there are great differences in response plans and products for different requirements.
Second, the installation of the filter
1. Requirements for the installation position of the filter
When a disturbance source affects multiple sensitive devices, the filter is placed close to the disturbance source. Conversely, when there is only one sensitive device and there are multiple sources of nuisance, the filter is placed close to the sensitive device. The advantage of placing the filter close to the disturbance source makes the interference confined to the vicinity of the disturbance source, cuts off the interference path of the disturbance source, and suppresses the emission of the disturbance source.
The filters on the PCB should be installed at the interface, and when multiple filters need to be installed, they should be installed in parallel to prevent the coupling of different signal lines before and after filtering; When there is only one interference source at the signal interface, the filter should be installed as close to the disturbance source as possible.
2. Filter output input wiring requirements
The input and output lines of the filter must be shielded and isolated, and the input and output wirings will be coupled to each other when they are close to each other, reducing the filtering effect of the filter.
Other signal wiring should avoid being placed close to the interfering signal that needs to increase the filter, so as to avoid coupling the interfering signal source to other signal wiring, and reduce the filtering effect of the filter.
The filtered signal cloth should avoid being close to the best disturbance signal wiring, so as to avoid secondary pollution of the signal line after filtering, and reduce the filtering effect of the filter.
3. Grounding requirements for the filter
The filter is grounded through a thin ground wire, and the high-frequency filtering effect is very poor, so that the grounding shell of the filter is well overlapped with the metal structure plane to improve the grounding effect.
The grounding point of the filter should be static (the ground plane without pollution of the disturbance source), the grounding wire should be as short and thick as possible to reduce the parasitic inductance of the grounding, and the grounding point should be selected as the point with the smallest area of the loop of the backflow interference source.