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

Introduction

There can be few more critical applications for power supplies in electronic equipment than in the field of medical electronics. Medical equipment will often have lives dependent upon its reliable operation. Power supplies must not only deliver the specified electrical performance but must also meet stringent specifications with respect to isolation in order that patients and medical staff are protected from the risk of electric shock. EMC is a critical issue too, both in terms of immunity and emissions.

As a result, the design of power supplies for use in the medical industry is driven as much by legislation as it is by the technical requirements of powering the end equipment. System designers therefore need an understanding of this
legislation, and of the markets into which their products will be sold if power solutions are not to be over-specified, over-engineered and excessively expensive as a result of building in too much safety margin when it comes to meeting legislative requirements.

In practice, there is often some confusion over the relevant requirements and a tendency for companies to adopt custom power solutions involving long development times, long approval times, high costs, and high risks of ending up with a less than optimal power system. The use of standard commercial power supplies, with or without input and output conditioning, is often possible and can be the most cost-effective solution without compromising equipment safety or performance.
EN60950 is the safety specification internationally applicable to the majority of electronic equipment connected to the mains. Medical power supplies also need to meet the minimum requirements set out in this specification. The international safety standard for medical equipment is IEC60601-1 and there are three regional variants: EN60601-1 in Europe, UL60601-1 in the US, and CSA22.2 No 60601.1 in Canada. In all cases, the legislation covers protection against electric shock, protection against fire and mechanical hazards.

The degree of protection needed in any particular medical application is related to the proximity of equipment to the patient, equipment that is directly applied to patients needing the highest specification with respect to isolation. We therefore have three progressive safety levels to consider regarding isolation and protection when designing medical electronic equipment:

• The basic safety requirements of EN60950 that apply to all mains-connected electronic equipment
• The more rigorous IEC60601-1 standard for equipment used in patient vicinity
• The requirement for an additional isolation barrier in equipment which is in intentional physical contact with patients

We now consider each of these requirements in more detail.

EN60950 and Levels of Protection (LOP)

‘Levels of Protection’, or LOPs, are used to define safety specifications for all lectronic equipment. An LOP is provided by insulation or by a protective earth and fuse. Insulation is defined as one of five types (See Table 1) with varying LOP ratings. Similarly, an earth can be classified as functional or protective, with no protection provided by a functional earth and one level of protection by a protective, or fused, earth.  The basic principle is to provide two LOPs against electric shock.

Reference     Earth type            LOP     
FE                   Functional              0       
PE                  Protective               1       
Reference     Insulation Type     LOP   
OP                 Operational             0      
B                    Basic                       1      
S                    Supplementary       1     
D                    Double                    2     
R                    Reinforced              2

Each of the insulation types is defined in terms of air clearance and creepage distances. Creepage limits include specifying the minimum spacing of components on the printed circuit board for a given AC and DC applied voltage.

Leakage current specifications are also a key part of safety regulations, as scussed later.

Where IEC60601-1 differs from EN60950

The main differences between IEC60601-1 specifications, the standards for equipment used in the patient vicinity, and those of EN60950 relate to increased air clearance, creepage distances and test voltages, as shown in Tables 2 and 3. In addition, the allowable leakage current is also much lower.

Air        Creepage            Air           Creepage

clearance   distance           clearance   distance

Basic or supplementary 2.0           3.2                        2.5            4.0

Double or reinforced 4.0            6.0                       5.0            8.0

* All distances in mm.

In addition, the test voltages applied to insulation are greater for IEC60601-1 than for EN60950, as below:

Insulation Type                EN60950              IEC60601-1

Test voltage             Test Voltage

Basic 1500                     1500

Supplementary 1500                     2500

Double or reinforced           3000                    4000

A 25% - 33% increase in component clearances and more substantial insulation requirements means that IEC60601-1 approved power supplies are often larger than their non- approved counterparts.