What is a Filter?

Switching power supply noise is divided into differential mode interference and common mode interference according to its coupling mode. Differential mode interference is defined as symmetrical interference between lines, while common mode interference is defined as interference between lines to a reference point. Common mode interference includes symmetrical common mode interference and asymmetrical common mode interference. Asymmetrical common mode interference is often converted into differential mode interference.

The power filter should filter out both differential mode interference and common mode interference. In fact, the power filter is a differential-common mode hybrid filter. The power filter circuit is scientifically designed based on the amplitude of the noise interference, the bandwidth of the noise interference, the impedance characteristics of the noise interference, and the requirements of the product electromagnetic compatibility test standards.

The figure above is a simple power supply filter circuit, in which the capacitors at positions C1 and C2 are called differential mode capacitors, also called X capacitors. Their main function is to bypass the differential mode interference noise generated inside the switching power supply and prevent it from flowing into the impedance stabilization network (LISN) and causing the power supply end conducted interference test to exceed the standard; the capacitors at positions C3 and C4 are called common mode capacitors, also called Y capacitors. Their main function is to bypass the common mode interference noise generated inside the switching power supply; L is the common mode inductor, also called the common mode choke, which mainly attenuates the common mode current generated by the switching power supply. Due to the leakage inductance of the irrational common mode inductor, the common mode inductor actually has the filtering effect of the differential mode inductor.

2.1. Main circuit forms of single-phase power supply filter

2.2. Main circuit forms of three-phase power supply filter

The main measurement indicators of power supply filters include: filtering performance technical indicators, safety performance indicators, reliability performance indicators, environmental regulations indicators, etc.

3.1 Filter performance indicators

Insertion loss is an important indicator to measure the performance of power filter. It is expressed in dB and refers to the ratio of the power on the load before and after the filter is connected. In principle, the larger the dB number, the better the filter's ability to suppress noise. The insertion loss of power filter is divided into differential mode insertion loss and common mode insertion loss.

As shown in the figure above, the dotted line part represents the differential mode insertion loss curve, and the solid line part represents the common mode insertion loss curve. The insertion loss of the filter is usually tested under 50Ω impedance. In actual applications, the impedance of the noise source end is uncertain, and the impedance of the load end is 50ohm for power supply conduction, which is also uncertain for the load end impedance in actual applications. Therefore, the filter insertion loss is usually only used as a reference for the selection of power supply filters.

The insertion loss curve of the power filter can be obtained by testing with a network analyzer. The specific test method refers to the international standard CISPER17:2011 and the national standard GB/T7343-2017. The insertion loss of the power filter can also be simulated by circuit simulation software. The accuracy of the simulation depends on the accuracy of the circuit modeling.

3.2 Safety performance indicators

The power filter is a safety control component and needs to comply with the corresponding safety regulations and standards. The corresponding international standards are UL1283, IEC/EN60939, etc. The main test phases include insulation withstand voltage, leakage current, LN line residual voltage, temperature rise, and rated current.

Insulation withstand voltage:

The withstand voltage test is the main method to check the overvoltage ability of electrical appliances, electrical equipment, electrical devices, electrical lines, etc. It is to check whether the power filter reaches the rated withstand voltage to meet the requirements for the insulation level of the power filter.

Insulation testing is generally performed using an insulation withstand voltage tester. Insulation resistance testing is a qualitative test that mainly checks the insulation level requirements of the power filter. The main difference between the two is the test voltage.

Leakage Current:

Leakage current test is a specific safety test that detects internal circuit defects and insulation failures to maintain the safe operation of electrical equipment. For power filters, the number and total capacity of Y capacitors are important factors affecting their leakage current.

LN line residual pressure test:

To prevent the risk of electric shock when a person touches the power plug connector when the power cord is unplugged, the safety standards have clear provisions on the LN residual voltage when the power cord is unplugged. For the power filter, the discharge speed of the X capacitor is an important factor affecting the residual voltage.

Rated current:

The rated current of the power filter should be greater than the maximum current of the load, and a margin should be reserved. The wire diameter of the common mode inductor in the power filter should meet the rated current requirements to prevent overcurrent damage. It is recommended that the rated current of the power filter be 1.5 times the load operating current, and at least 1.2 times.

Insulation withstand voltage:

The withstand voltage test is the main method to check the overvoltage ability of electrical appliances, electrical equipment, electrical devices, electrical lines, etc. It is to check whether the power filter reaches the rated withstand voltage to meet the requirements for the insulation level of the power filter.

Insulation testing is generally performed using an insulation withstand voltage tester. Insulation resistance testing is a qualitative test that mainly checks the insulation level requirements of the power filter. The main difference between the two is the test voltage.

3.3 Power filter reliability

In addition to filtering performance indicators and safety indicators, reliability indicators of power filters are more easily overlooked and account for a large proportion of the manufacturing cost of power filters. The factors that affect the reliability of power filters mainly include: welding quality of device solder joints, reliability of connections between devices, reliability of connector connections, plug-in and pull-out tolerance of connectors, reliability of the materials used in the device itself, and long-term life test verification.

3.4 Environmental protection index requirements

The electronic components, connectors, solder, PCB, plastic materials, etc. used in power filters need to comply with the corresponding environmental standards, such as EU RHOS, WEEE, etc. How to ensure that all components in the power filter comply with the requirements of laws and regulations, in addition to regular sampling of finished products for qualified third-party testing, a complete set of incoming material inspection control standards and process control of hazardous substances in the production process of power filters are also crucial.