EMI filters

The DC filter choke is used for ripple voltage and current reduction on the output of the switching power supply immediately following the existing filters. It is also used as an EMI filter on switching power supplies that have a single power line on the input, such as battery and distributed power systems. 

Actually the design of the dc EMI filter inductor is an easy matter. The core manufacturer provides a graph entitled Normal Magnetization Curves for the MPP cores, as seen in Figure 3-22. A permeability of 60 or below is recommended. 

The input reetifier/filter eireuit plays a largely unappreeiated role within the switehing power supply. The typieal input reetifier/filter eireuit is eomposed of three to five major subseetions an EMI filter, possibly a start-up surge eurrent... 

Figure 3-54 Typical ac and dc input filter circuits (a) ac input filter circuit for a single or universal input power supply (common-mode EMI filter shown) (b) a voltage doubling ac input circuit for 110V and 220V ac inputs (c) single dc bus input filter.

Designing the input EMI filter seetion (refer to Appendix E)... 

I will be using a seeond-order, eommon-mode filter. The main purpose of an EMI filter is to filter the switehing noise and its harmonies from the input power line. One starts by estimating the amount of attenuation needed at the switehing frequency. 

Some comments on the design of PFC circuits. First, the EMI filter is an integral part of any PFC eireuit. It filters out the switehing harmonies from the input eurrent waveform. Without an EMI filter, your produet will fail the EMI/RFI tests whieh are in addition to the power faetor tests. Please refer to Appendix E for information on how to design an EMI filter. Seeondly, using a variae during the measurements will affeet the input line impedanee and thus affeets... 

Leads that enter or exit the enclosure ideally should have their associated EMI filters at the point of entry or exit from the enclosure. Any unfiltered leadlengths that run within the enclosure will inductively pick-up noise within the case and allow it to exit the case, thus making any EMI filtering less effective. Likewise, any unfiltered leads within the case will radiate any transients from outside the case into the case, which may affect the static discharge behavior of the contained circuits to external static events. 

There are two types of input power buses. DC power buses are single-wire power connections such as found in automobiles and aircraft. The ground connection forms the other leg of the power system. The other form of input connection is the ac, or two or three-wire feed systems as found in ac power systems. The design of the EMI filter for dc systems is covered in Section 3.12 and takes the form of a simple L-C filter. All the noise is common-mode between the single power wire and the ground return. The dc filter is much more complicated, because of the parasitic behavior of the components involved. 

The purpose of an input conducted EMI filter is to keep the high-frequency conducted noise inside the case. The main noise source is the switching power supply. Filtering on any of the input/output (I/O) lines is also important to keep noise from any internal circuit, like microprocessors, inside the case. 

Figure E-4 A complete third-order, input EMI filter (common-mode and differential-mode).

Sometimes the high-frequency attenuation is insufficient to meet the specifications and a third pole needs to be added to the EMI filter. This filter is typically a differential-mode filter and will share the Y capacitors from the common-mode filter. Its corner frequency is typically the same as the commonmode filter. This filter is made up of a separate choke on each power line, and is placed between the input rectifiers and the common-mode filter. 

One of the problems we faced was a mounting standoff on the primary side that we just couldn t get around satisfactorily. We had EMI filters and other items taking up all remaining space. The only way out was to position the bulk capacitor and the switch on opposite sides of the standoff, as shown in Figure 7-2. Unfortunately, these long traces were... 

Now, where could the problem lie Is it within the power supply stage itself, or is it external If you have peeled the onion as I suggested previously, you are hopefully sure the efficiency is not being lost within the EMI filter of your AC-DC power supply. Bypass or disconnect everything external and confirm you have a problem. 

Fig. 10.4. Mounting of electromagnetic interference (EMI) filters inside a shielding box.

The last filter that will be looked at in this chapter is the EMI filter. This filter is commonly used on the input of a power circuit to reduce conducted and reflected emissions. For instance, a flyback converter can draw current from the bus that looks like a sawtooth waveform with... 

They are several concerns when designing an EMI filter. The parameters of the EMI filter examined in this book reflect these concerns. If the EMI filter is to be used on a converter, the input impedance of the converter must be greater than the output impedance of the filter at all frequencies. It is good practice to allow 6 dB of margin for this parameter. If the output impedance of the filter gets too close to the input impedance of the converter, there can be problems with the stability of the converter. It may be important to note here that this output impedance is sensitive to the effective series resistance (ESR) of the output capacitors. For the hardware data taken for this unit, tantalum capacitors, which have unspecified ESR, were used. The ESR of a similar capacitor was measured for the simulations. 

Other important characteristics of the converter are the reflected ripple attenuation and the turn-on characteristics. It is expected that the turn-on characteristics will be difficult to simulate because of the nonlinear characteristics of a saturating core. A nonsaturating core is simply described by Faraday s law, and it can be easily modeled by any of the SPICE simulators. The model used for the EMI filter is shown in Fig. 3.66, and the results of each of the simulators output and the measured impedance plots are shown in Figs. 3.67 to 3.70. 

The inrush current of an EMI filter is usually examined to ensure that no parts are overstressed during power-up. If the inductor does not saturate, the inrush current is described by Faraday s law and can easily be modeled by mathematics or a simple SPICE model. It is also not too difficult to determine if a core is saturated during turn-on. A slightly more difficult calculation is to determine what the maximum current will be under a given turn-on condition. The hardware used for measurements used a transformer made of two stacked 55025 cores... 

Note the saturating core model is available in both PSpice and IsSpice. In PSpice the core is available as part of the AEi Systems Power IC Model Library for PSpice. This figure is shown to give a visual representation of the effects of an EMI filter. 

FIGURE 7.12 Typical electromagnetic interference (EMI) filter schematic and outline the filter yields 60 dB common-mode attenuation and 50 dB transverse mode attenuation between 100 kHz and 1 Mhz. 

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