Developers of switched-mode power supplies face the challenge of developing their circuit to be EMC-compliant circuit board. Conducted interference circuit board particularly occurs in the input circuit whereby other electrical equipment can be disrupted. Therefore the conducted interferences are first checked at the beginning of an EMC test of power supplies. The developer can already perform measurements accompanying the development during the development phase to obtain an impression circuit board of possible interference. This Application Note describes the procedure for examining differential mode interference in the input circuit of DC/DC switching regulators. Cause and development of the interference voltage circuit board A distinction is made between differential and common mode interference for conducted interference. circuit board The triangular input current of the switching regulator is the active current and initially pure differential mode interference. However, this differential mode interference can transform into common mode interference in the case of unsymmetrical supply lines and results in increased electromagnetic radiation over the supply lines. circuit board The differential mode interference can be connected to ground using parasitic capacities and results in common mode interferences circuit board. Figure shows the typical input current of a DC/DC switching regulator with a pulse frequency of 2 MHz. The input current flows with the pulse frequency of the switching regulator and resembles the coil current in the illustration flowing through the storage inductance of the switching regulator circuit board.
The AC component (approx. 260 mA in this example) of the input current is circuit board decisive for the differential mode interference. It is imperative to minimise this using an input filter. Due to parasitic effects of supply lines and components in the input circuit, high frequency oscillations in the MHz range occur during rise and fall of the input current. The input current, via the ESR of the input capacitor and the impedances of the supply lines of the switching regulator, results in voltage drop, circuit board the so-called interference voltage. It must not pass the supply lines of the power supply and must be suppressed to a minimum. The unwanted AC component at the input capacitor of the switching circuit board regulator can be determined using measurement with an oscilloscope. This measurement enables a first statement about the interference voltage.
In this example, there is an electrolyte capacitor with unknown but relatively high ESR which causes a peak-to-peak value of the unwanted AC component of approx. 2.8 V. circuit board Oscillations in the high frequency range circuit board caused by parasitic effects are also present. In this example, the frequency of this oscillation is approx. 71 MHz circuit board.
