QPI-8LZ-01【PDF 4/17 页】IC HOT SWAP EMI FILTER 16LGA

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Picor Corporation ?picorpower.com

QPI-8

Rev 1.5, Page 4 of 17

QPI-8

QUIETPOWER

?/DIV>

EMI Management

The more effectively EMI is managed at the source, namely

the power converter, the less EMI attenuation the filter will

have to do. The addition of Y capacitors to the input and

output power nodes of the converter will help to limit the

amount of EMI that will propagate to the input source.

Figure 4 An unfiltered converters response to open-frame

(light blue) and base-plate (purple) EMI configurations.

There are two basic topologies for the connection of the re-

circulating Y capacitors, referred to as open-frame and

base-plate. Figure 4 illustrates how a converter can

favor one topology versus another. The EMI generated by

the base-plate configuration is much greater than that

generated by the open-frame. Selecting the right topology

will greatly reduce the amount of EMI signal that needs to be

filtered.

Figure 2 shows the base-plate topology of re-circulating Y

caps. Here, CY1 to CY4 are connected to each power node of

the dc-dc converter, and then are commoned together on a

copper shield plane created under the converter. The

addition of the copper shield plane helps in the containment

of the radiated EMI, converting it back to conducted EMI and

shunting it back to its source.

The RY resistor, connected between the shield plane and the

QPIs shield pin, provides an impedance that makes the QPIs

common mode noise cancelation signal more effective at

removing the common mode noise that would normally

return to the shield/earth connection. It is important when

laying out the QPI that the RY resistor connects to the QPIs

shield pin before making the connection to earth ground.

In Figure 5, the open-frame topology is shown where the Y

capacitors (CY1 and CY2) re-circulate the EMI signals between

the positive input and output, and the negative input and

output nodes of the power conversion stage.

Figure 5 - Typical 'open-frame" application using a converter with low enable.

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