Disturbance power test
  • Disturbance power test

Disturbance power test


Harassment power test is one of the most common EMC tests in the certification of AV products, home appliances and power tools. This paper starts from the test principle of harassment power, discusses the main points of the test, as well as the analysis of the reasons for the failure of the test, and the countermeasures that can be taken. After giving a practical rectification case to illustrate.

Disturbance power test

Test method for disturbance power

Generally, when the frequency exceeds 30MHz, the disturbance energy generated by the equipment is transmitted to the disturbed receiver by radiation. Experience has shown that most of the energy is radiated from power lines and other connections close to the equipment. Therefore, the disturbance level can be defined by the disturbance power on the power line and other wires of the device under test.

The disturbance power test needs to be carried out in the shielded room to eliminate all kinds of electromagnetic disturbance from the outside. During the test, the tested equipment is placed on a non-metallic table with a height of 0.8m, at least 0.8m away from other metal objects or the human body. The feeder to be tested shall be spread out flat on the table, and its length shall be sufficient to place the absorption clamp and adjust the position of the absorption clamp if necessary to obtain large radiation values. The absorption clamp shall be sleeved on the feeder to be tested, and one end of the current converter shall face the equipment to be tested, so as to measure the disturbance power on the feeder.

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Cause analysis and countermeasures of unqualified disturbance power test:

Because the frequency of the disturbance power test is high (30~300MHz), and the test is aimed at the connection lines of various terminals, the main reasons for the failure of this test are generally as follows:

1) For the metal terminal, it is not connected to the ground of the whole machine and is in a suspended state. When there is interference on the ground of the metal terminal, it is easy to "conduct" through the external terminal line.

2) The overlap between the metal terminal and the metal casing is not ideal, and the overlap produces a larger impedance. This impedance drives the interference current to be emitted outward. This is a common mode interference model.

3) The internal PCB design of the prototype is unreasonable, so that the high-frequency signal cannot be reflowed in the circuit board.

4) Due to the unreasonable layout, "crosstalk" occurs between the interface cables ".

5) There is no filter circuit on the signal cable, or the characteristics of the filter circuit do not meet the requirements.

6) The interference noise generated by the brushes of the motor is too large.

There are many similarities between the failure of the disturbance power test and the failure of the radiation test. In terms of countermeasures, the two also have something in common.

Disturbance power test

Disturbance power test

The countermeasures for the failure of the harassment power test are generally:

1) In the PCB design stage, comply with EMC design specifications and reasonable wiring. Try not to leave too long traces or loops with too large areas.

2) For metal terminals, we must pay attention to good grounding, so that the grounding impedance will be as low as possible.

3) Reasonably handle the interconnection lines between the circuit boards within the product, especially the connection lines of the terminal boards, to avoid the occurrence of "crosstalk. For a port disturbance power test exceeds the standard, a ferrite magnetic ring can be added to the internal cable of the port to eliminate high-frequency interference.

4) Add magnetic beads/resistors/inductors and capacitors on the signal line to form a filter circuit. The purpose is to filter out unwanted signals.

5) For the interference generated by the brush, it can be filtered in the form of a combination of capacitance and inductance to avoid interference noise from being transmitted to the outside world.

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