Already accompanying the development, circuit board the measurement of the interference voltage of differential mode interference can be performed using an LISN (Line Impedance Stabilisation Network) and a spectrum analyser. The circuit ground forms the reference potential in this test set-up circuit board.
This defined test set-up enables performing reproducible measurements circuit board and generating defined impedance both for the spectrum analyser as well as for the switching regulator. circuit board Figure 4 shows the internal design of a DC-LISN according to CISPR 25.
The CISPR 25 is used as the basis for testing vehicle electrical systems and can be used for EMC testing accompanying development of DC/DC circuit board switching regulators. The function of the LISN is decoupling of the interference voltage as pure alternating quantity VDiff. It is measured using the internal 1 k resistance. circuit boardThe internal low pass filter of the LISN prevents interference with other electrical equipment connected to the supply network. The voltage source VSupp is connected to the input terminals of the LISN. The test item, in this case the switching regulator, is connected to the output terminals and also supplied with the voltage VDC/DC. circuit board Figure 5 shows the result of a peak value measurement of the interference voltage with a spectrum analyser circuit board.
As test item, a DC/DC step-down (buck) converter without input filter was examined with a switching frequency of 2 MHz, input voltage of 10 V and input effective current of 0.7 A. The interference voltage is determined in different frequency ranges depending on the standardisation. In this example, circuit board the measurement was performed according to the test regulation CISPR 25 with start frequency of 150 kHz and stop frequency of 108 MHz circuit board. The basic oscillation corresponding to the switching frequency is clearly visible. The harmonics reaching into the high MHz range drop in amplitude. The amplitude of the basic oscillation is at its greatest with 128 dBµV. The interference level VDiff is generally defined using the following expression (Equation 1) in dBµV: Developers of switched-mode power circuit board supplies face the challenge of developing their circuit to be EMC-compliant.
If Equation 1 is transformed, circuit board the measured interference voltage VRipple can be approximately determined using Formula 2: Developers of switched-mode power supplies face the challenge of developing their circuit to be EMC-compliant circuit board.
Using this, we obtain a value for the interference voltage VRipple of 2.6 V. circuit board This approximately corresponds to the peak-to-peak value of the previously measured voltage at the input capacitor (see Figure 2). An input filter is obviously required. For example, the CISPR 24 Class 1 defines a limit value of the peak value of 66 dBµV for narrow band interference and a frequency of 2.0 MHz circuit board. However, the limit values according to CISPR 25 and other standards are not applicable for such measurements accompanying the development as the measurement of the differential mode interference is not defined in the EMC standards. However, they can be included to make a rough assessment of the interference emission in power supplies. circuit board The worst case is usually considered to be able to determine the maximum interference emission. Therefore, the measurements for switching regulators are performed at full load and smallest input voltage. circuit board In order to illustrate this, the same test item, in contrast to the first measurement, was loaded with a load current of only 50 mA circuit board.
The measurement in Figure 6 shows that the interference emission at low load is lower in the high frequency range. circuit board In this case, the basic oscillation shows a value of 112 dBµV and in comparison with the full load is only 16 dB lower, however not negligible circuit board.
