It's pretty easy to find 100-fF capacitors as a parasitic element in your power supply. You need to recognize that you'll have to address them in order to have an EMI-compliant power supply.
As we discussed in Power Tip 40, a small amount of parasitic capacitance (100 femtoFarad) from the switch node to the input leads can cause you to fail electromagnetic interference (EMI) requirements. So what does a 100-fF capacitor look like? Chances are you are not going to find many in Digi-Key. Those that you do find are going to have very wide tolerances due to parasitics.
But it's actually pretty easy to find 100-fF capacitors as a parasitic element in your power supply. You need to recognize that you will have to address them to have an EMI-compliant power supply.
Figure 1 shows an example of one of these unintended capacitances. On the right side of the picture is the vertically-mounted FET with the switch-node and clamping circuitry extending toward the top of the image. Input connections enter on the left and come within 1 cm of the drain connection. This is the trouble spot where the switching voltage waveform of the FET can couple into the input, bypassing the EMI filter.
Note that there is some shielding between the drain connection and the input lead provided by the input capacitor. The capacitor's can is connected to primary ground and provides a path for common-mode currents to return to the primary ground. This tiny amount of unintended capacitance causes the power supply EMI signature to be out of specification as shown in Figure 2.
This is an interesting curve as it shows several things: the lower-frequency emissions, which are clearly out of spec; the 1-MHz to 2-MHz components where common-mode problems are usually evident; and the decaying sin(x)/x distribution of the higher-frequency components.
Work was required to bring the emissions within specification. We took advantage of the general equation for capacitance in order to decrease it:
C = ε • A/d
We could not do any anything about the permittivity (ε), and the area (A) was already minimized. However, we could change the distance (d). As shown in Figure 3, we extended the separation between the components and input by a factor of three. Finally, we provided additional shielding with a larger ground plane.
Figure 4 shows the results of these efforts. We have achieved about 6 dB of margin to our EMI specs at the trouble spot. In addition, we have significantly reduced the total EMI signature. All this improvement was due just to layout changes -- not circuit changes. You want to be very mindful of your circuits that have high-voltage switching and use shielding distance to keep them in control.
To summarize, 100 fF of capacitance from the switch node of an offline switching power supply can cause an out-of-spec EMI signature. This amount of capacitance is easy to realize with just parasitic elements such as routing the drain connections close to the input leads. The problem often can be fixed with improved spacing or shielding. To obtain additional attenuation, additional filtering or slowing the circuit waveforms would be required.
Please join us for the next Power Tip where we will look at weighting the feedback divider in multi-output switching power supplies. You can find all Power Tips articles in the Power Tips Index.
Check out TI Power Lab Notes for a designer's prospective on his power supply designs.
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