Power Supply Basics

Key points to consider when managing EMI in medical equipment

Posted by Dulcie on November 15, 2017
Posted in: Power Supply Basics

When ensuring EMI compliance of medical equipment, care must be taken not to contravene earth leakage current and touch current regulations. The limit for earth leakage current was increased in the IEC 60601-1 3rd edition standard to 5mA for a Normal Condition (NC) and 10mA for a Single Fault Condition (SFC). Unfortunately manufacturers of medical equipment still have to meet the limits for Touch Current which remains at 100µA for a NC and 300µA (US limit which is lower than Europe) for a SFC.

Medical equipment earth leakage current is primarily generated by the AC-DC power supply’s internal EMI filtering.  Manufacturers of medically certified power supplies design to their products to have less than the 300µA limit so the equipment manufacturer can meet that specification.

For the IEC 60601-1-2 section of the standard, there is a requirement to perform testing for radiated and conducted noise (EMI) on the end product.  Although the power supply should meet that standard when installed correctly, other electronic devices may generate additional noise. The system may still pass if the selected power supply has a good EMI margin but otherwise an additional EMI filter in the AC line will need adding.

For a 150W input power supply, like the CUS150M, the input current would be 2.2A at 100Vac input.  Allowing for some derating, a 3A filter would be acceptable.  TDK-Lambda’s RSEN-2003 filter series would be one choice.  Care has to be taken to ensure that the filter does not add earth leakage current to a point where the 300µA limit is exceeded.

Two versions of that filter are available, the standard leakage RSEN-2003 and a low leakage RSEN-2003L.  From the circuit diagrams in Figure 1, it can be seen that the ‘L’ model does not have any ‘Y’ capacitors.  These capacitors enhance the performance of the filter, but introduce additional leakage current.

Figure 1: Circuit Diagrams

The leakage current for the RSEN-2003 is 1mA at 250Vac, which would not be suitable for a medical application.  The RSEN-2003L however, has a leakage current of just 10µA.

As previously mentioned, the ‘Y’ capacitors improve the filter performance, so it is important to use the correct attenuation vs frequency characteristics.

Figure 2: Attenuation vs Frequency characteristics

When used in conjunction with a well-designed power supply with sufficient EMI margin, the reduction in attenuation of the low leakage filter should not be an issue.

TDK-Lambda UK