Input to Output Capacitance
Let’s consider the input to output capacitance of a typical medical power supply. Figure 1 is a simple representation of such a device, with a leakage current path shown by the dotted line. The capacitance between the primary side and the secondary side of the transformer comprises the bridging capacitance (C4) and the transformer’s interwinding capacitance. If C4 increases, the impedance of this current path decreases; if the impedance decreases, more current will flow through point G2. This current constitutes the patient leakage current. Note that C1 and C3 also create a path for current leakage – this is the earth leakage current rather than the patient leakage current.
Figure 2 is a capacitance model of figure 1. The patient leakage current, I leakage, is calculated as 2.π.f.C4.V mains. That is, the patient leakage current is directly proportional to the bridging capacitance.
Popular techniques for reducing the patient leakage current therefore focus on reducing the input to output capacitance of the power supply. Probably the most common method of doing this is to add an additional DC-DC converter between the power supply and the applied part. This adds an extra layer of isolation as shown in Figure 3.