Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

Leakage Current Considerations

Leakage current specifications for all IEC60601-1 approved power supplies are more stringent than for non-medical units. The specifications define several different kinds of leakage current, but the most important with respect to power supply design are:

• Earth leakage current: the current flowing along the earth conductor
• Enclosure leakage current: the current flowing from the enclosure to earth via the patient

Maximum leakage current is defined for three main types of application with respect to IEC60601-1 approved power supplies:

• Type B: equipment where there is no physical contact with the patient e.g, laser treatment systems
• Type BF: equipment where there is intentional physical contact with the patient e.g. ultrasound, monitors of various kinds including ECG equipment, and operating tables
• Type CF: equipment where there is intentional cardiac physical contact with the patient e.g. invasive heart monitors

A common misunderstanding is that leakage current specifications vary within these types of application. In fact, in all of these classes of application, the allowable leakage current is the same.  Table 4 shows the permissible leakage current levels for EN60601-1, the European implementation of IEC60601-1. North American specifications are tighter. For example, where 0.5mA is allowable in Europe, just 0.3mA is permissible in the US and Canada, hence the need for medical equipment designers to appreciate where the systems they develop may be sold.

Current Type B             Type BF             Type CF

NC     SFC      NC     SFC         NC     SFC

Earth leakage 0.5     1.0         0.5       1.0           0.5     1.0

Enclosure leakage       0.1     0.5         0.1      0.5            0.1   0.5

Note: NC = normal conditions; SFC = single fault condition

Reducing leakage current within a power supply usually means eliminating or limiting the value of Class Y filter capacitors from live-to-earth and neutral-to-earth. It also demands that stray capacitance to earth is minimised through careful design. Unfortunately, the overall effect of these measures tends to compromise EMC performance, although minimising stray capacitance can reduce common mode noise.  As a result, IEC60601-1 approved power sup- plies often meet EN55022/11 Class A instead of the more demanding Class B EMC specifications. They can be designed to meet Class B, but only through the addition of more complex filtering and screening arrangements, leading to increased power supply size and cost. In applications falling into the BF or CF classifications, often termed ‘patient connect’, an additional level of isolation is required to isolate the patient from earth, signal ports and power supply output.  This is needed to protect the patient against single fault conditions and to maintain the level of patient leakage current within the limits outlined in the standard.  This isolation may be provided by other items that form part of the end equipment, for example plastic probes or tubing that have sufficient insulation.  Where an electrical connection to the patient is needed one approach is to use an IEC60601-1 approved AC/DC power supply to feed one or more isolated DC/DC converters to provide the secondary isolation.  Careful selection of the DC/DC converter is needed to ensure the isolation requirements are met.

Case Study: A Power Supply for a Ventilator

A manufacturer of medical ventilators recently had a Type BF application requiring a power supply. They approached SRS to provide a solution tailored to the application but without using a full-custom design due to the delays and costs involved.

The mains-connected power supply clearly had to meet IEC60601-1 but in this instance advised the customer that there was no requirement for secondary isolation within the power circuit because protection was provided by the ventilator tubing and its associated connection to the equip- ment housing. In other words, there was no electrical con- nection to the patient.

An ECM Series AC/DC medical power supply a single output formed the basis of the power supply system. A 13.8VDC output feeds a back-up battery charging and current monitoring circuit and a DC/DC converter made up of discrete components. The three final outputs are 5, 12 and 24VDC.

Signals for AC failure, battery charging and low battery voltage protection were incorporated into the design. Figure 1 shows the block diagram of the final unit. Earth leakage current was limited to 0.27mA in this application at 264VAC, 60Hz. The power supply meets EN60601-1, UL60601, and CSA-C22.2 No 60601.1 safety specifications and EN55011 Class A with respect to conducted emissions. It also complies with EN60601-1-2 (clause 36.22) and EN61000-3-2 with respect to EMC immunity.

Most importantly, samples of the power supply were available within eight weeks of the agreed specification and the typical time to full production release for this type of

solution is just three months. Because the AC/DC element of the power supply was already IEC60601-1 approved, there was no need for the ventilator manufacturer to seek further detailed safety approvals with respect to the power system.

As custom medical power supplies can often take between one and two years to develop and approve, this ‘semi-custom’ approach is very attractive to medical equipment makers. In 2005 the 3rd edition, IEC60601-1 :2005 was introduced. This standard is more comprehensive and has aligned many areas with IEC60950.  It now makes distinction between circuits that provide Means Of Operator Protection (MOOP) and circuits providing Mean Of Patient Protection (MOPP). The more relaxed requirements of MOOP can be used if, through a risk assessment process,  the patient will not make contact with the medical device.  This will over time provide more flexibility of the type of power supply used within specific types of equipment.  Medical device manufacture are beginning to use this new standard although it will be some time before it is universally used and the previous edition withdrawn.