The dominant path for the current of the new system is now through C4 and C5, the interwinding capacitances of both transformers. The new patient leakage current is calculated as I leakage = 2.π.f.C4 and C5 in series. V mains (as shown in the capacitance model in Figure 4) and is greatly reduced overall.
The capacitance model after the DC-DC converter is added shows the output capacitance is reduced to within acceptable levels.
As an example, recent work on a customer’s medical system by XP Power reduced an AC-DC power supply’s patient leakage current from 11 µA to 6 µA by adding an additional DC-DC converter as described above. Although this is an easy way to reduce the output capacitance, and therefore the patient leakage current, it does have its downsides. Adding another DC-DC converter to the system is always going to add cost and complexity, as well as increase the footprint of the power subsystem.
An alternative approach taken by XP is to work with the customer to reduce the leakage current of the AC-DC power supply, to try to avoid having to use an additional DC-DC converter. There are two options – either a completely custom power supply can be built, or more commonly, a standard medical power supply can be modified to reduce its leakage current.
The problem with reducing the leakage current of the AC-DC power supply is that because this is dependent on reducing the input to output capacitance, it can have major ramifications for the power supply’s EMI (electromagnetic interference) performance, which is also tightly specified for medical applications. When designing a custom power supply, the task is approached with the trade-off between leakage current and EMI