1 Detailed Descriptions

The following test conditions apply to the results found in this document:

• V_IN = 12 V and 24 V
• V_OUT = 5 V
• Output Current = 0.85A
• 2 layer PCB with 1oz copper
• PCB Dimensions = 1.4 inches x 2 inches (3.6 cm x 5.1 cm)
• CISPR 25 Class 5

The LM5180 EVM was modified as shown in Figure 1-1 and in Table 1-1. First, the 10 μH EMI filer inductance was changed to 4.7 μH. The smaller inductor has a higher self resonant frequency (SRF) and, therefore, less parasitic capacitance. The parasitic capacitance of the inductor will by-pass the high frequency noise around the inductor, thus reducing the effectiveness of the EMI filter. A high SRF is desired for an EMI filter inductance in order to maintain the low-pass characteristic to the highest possible frequency. An additional filter capacitor, C3a, is added to bring the filter cut-off frequency back to near the value obtained with the 10 μH inductance.

In every fly-back transformer, switching noise on the primary side is couple to the secondary by the inter-winding capacitance. Unless a dedicated path is provided for this current, it will flow through any extraneous path to get back to the primary side. This common mode current will flow in an unnecessarily long path back to the primary side and show up as increased EMI. C1 is used to connect the primary and secondary grounds, providing a controlled path for this current while maintaining the D.C. isolation of the secondary output. Selecting, or designing, a transformer with low inter-winding capacitance will also help to reduce this noise contribution. The copper shield shown in Figure 1-2 is also helpful by capturing and shunting any capacitive noise current from the transformer windings back to the primary ground.

Finally, the snubber circuit capacitors, C8 and C10, were increased in value to help damp the ringing that occurs between the parasitic capacitance at the SW node and the transformer inductance. Ringing will also occur on the secondary side when the output diode turns off. This ringing will affect both conducted and radiate EMI, and should be damped as much as possible without affecting the operation of the regulator or unduly reducing the system efficiency.