Only When They Are Handled Properly Can We Obtain the Power Supply Meeting the EMI Standard

A small amount of parasitic capacitance (100 millipico farads) from the switch node to the input lead will make you unable to meet the requirements of electromagnetic interference (EMI). What does 100ff capacitor look like? There are few such capacitors in digi key. Even if there are, they will provide a wide tolerance due to parasitic problems.

However, it is easy to find 100ff capacitors as parasitic components in your power supply. Only by dealing with them can you obtain an EMI compliant power supply.

Figure 1 is an example of these unplanned capacitors. On the right side of the figure is a vertically mounted FET with switching nodes and clamping circuits extending to the top of the picture. The input connection enters from the left to a position within 1cm from the drain connection. This is the fault point, where the switching voltage waveform of the FET can be coupled to the input by bypassing the EMI filter.

Figure 1. Proximity of switch nodes to input connections reduces EMI performance

Note that there is some shielding provided by the input capacitor between the drain connection and the input lead. The shell of the capacitor is connected to the main ground to provide a path for the common mode current to return to the main ground. As shown in Figure 2, this small capacitance will cause the power supply EMI signature to exceed the specification requirements.

Figure 2. EMI performance beyond specification due to parasitic drain capacitance

This is an interesting curve because it reflects several problems: lower frequency radiation that obviously exceeds the specification requirements, 1MHz to 2MHz components with common mode problems, and the attenuated sinusoidal (x) / X distribution of higher frequency components.

Measures need to be taken to keep the radiation within the specification. We use the general capacitance formula to reduce it:

C = ε ˙ A/d

We can't change the permittivity( ε), Moreover, the area (a) is already the smallest. However, we can change the spacing (d). As shown in Figure 3, we extend the distance between the component and the input by three times. Finally, we use a larger ground plane to increase the shielding.

Figure 3. This modified layout not only increases spacing, but also brings shielding performance

Figure 4 is the modified effect diagram. We have obtained about 6dB margin for EMI specification at the fault point. In addition, we have significantly reduced the overall EMI signature. All these improvements are only due to the adjustment of layout and do not change the circuit. If your circuit has high voltage switch and uses shielding distance, you need to control it very carefully.

Figure 4. EMI performance is improved by shielding and increased spacing

In conclusion, the 100ff capacitance from the off-line switching power supply switching node will lead to EMI signatures that exceed the specification requirements. This capacitance can be easily realized by parasitic components, such as routing the drain connection close to the input lead. This problem can usually be solved by improving the spacing or shielding. In order to obtain greater attenuation, it is necessary to increase filtering or slow down the circuit waveform.

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