A transformer is naturally used in isolated type of DC-DC converters. The transformer is actually a mass of stray capacitance.
Figure 1 shows the state that the DC-DC conveter has allowed the noise to pass through.
The C1, C2, C3 and C4 are the common mode noise reduction capacitors that are highly effective in removing common mode noise, however, as the area between the primary side and the secondary side of the transformer is a mass of stray capacitance, the isolation transformer works as a noise propagation route and the noise passes through there.
Figure 2 is a concept image of the capacitive coupling of an isolation transformer. It is important that the isolation transformer of DC-DC converters has good isolation performance even for AC.
Figures 2-3 and 2-4 show that an electrostatic shield has been placed between the primary and secondary windings to block the noise passing through.
In this section, the existence of stray capacitance can be studied by seeing figures rather than reading textual explanations.
Written by Shotaro Suzuki
