Figure 3.39: Typical EMI/RFI Noise
Noise ?lters add noise to the active layer or to the selection. Noise is de?ned as a highfrequency electrical distortion of the voltage waveform. It is comprised of unwanted and interfering voltages and currents generated by motors, of?ce equipment, industrial equipment, etc. It is commonly referred to as conducted (EMI/RFI). Noise can be further broken down into three types: Line to Neutral, Line to Ground, and Neutral to Ground. Line to Neutral noise is called normal mode, and Line to Ground/Neutral to Ground is called common mode.
Figure 3.40: Typical Noise Paths
• Filter capacitor Capacitors are reactive elements, which make them suitable for use in analog electronic ?lters. The reason for this is that the impedance of a capacitor is a function of frequency, as explained in the article about impedance and reactance. This means that the effect of a capacitor on a signal is frequency-dependent, a property that is extensively used in ?lter design.Analog electronic ?lters are used to perform a prede?ned signal processing function. An example of such a function is a low-pass ?lter (LPF), which passes through low frequencies, but blocks high frequencies. Another example is the high-pass ?lter (HPF), which passes through high frequencies but blocks low frequencies. These are some basic ?lter types which can be combined
to create other more complicated ?lters, such as band-pass or notch ?lters. Electronic ?lters can be realized in many different ways. They can be made using analog components only, such as capacitors, inductors, resistors, transistors, and operational ampli?ers. They can also be realized using digital technology digital signal processing circuits that consist of a specialized computer or microcontroller and software appropriate for the application. Analog ?lters are further divided into passive and active ?lters. Active ?lters use amplifying circuits and components such as transistors and opamps, while passive ?lters use resistors, inductors and capacitors exclusively. The advantage of passive ?lters is that no power source is needed apart from the processed signal itself, while the advantage of active ?lters is reduced size and cost. Thecapacitancevalueneededtosupplythepowersuppliesoutputcurrent(I)withthespeci?ed amount ofripple current (Vrms) with full wave recti?cation is:
C = I Where Vrms = V(p-p) for f = 60Hz Vrms x 4f 2 C = 2.4 I I = DC Load current of power supply in milliamps Vrms A more general formula is: C I x Vdc Where Vdc = Vm – Vp-p VrmsVm 4 2 Vm = max. voltage of input waveform Vp-p = peak to peak ripple voltage