Medical Device Link.

Originally Published MEM Fall 2003

POWER SUPPLIES

Specifying the right power supply for the application is critical to ensuring the safety of medical electronics.

Peter Blyth

Medical equipment—used to diagnose, treat, or monitor—is designed to come into contact with the patient. Patients may be unconscious or hooked up to several different pieces of equipment. Patients may also have open wounds, and so the risk of electric shock can be great. Power supplies play an important role in protecting against electric shock in medical electronics. Because of this role, they are subject to much more stringent design constraints and legislation than if the end application were communications or office equipment. This article examines the issues involved in specifying power products for medical applications.

All power supplies sold into medical applications that are intended for use in Europe, North America, and most of the rest of the world must comply with the IEC 60601 safety directive series of standards. The basic standard containing the core requirements for all electrical medical equipment is "Medical Electrical Equipment—Part 1: General Requirements for Safety," IEC 60601-1, 2nd ed., (1988-12), with Amendments 1 (1991-11) and 2 (1995-03).

Technology-related safety requirements addressing medical systems, electromagnetic compatibility (EMC), x-ray radiation, and programmable systems are covered by supplementary standards. A further group of standards covers device-specific safety and performance requirements for devices such as radio-frequency (RF) surgical devices, electrocardiograph (ECG) monitors, infusion pumps, and hospital beds.

When designing equipment for a specific geographic territory, the international IEC 60601 standard must be considered alongside any standards that cover local installation codes and safety expectations. In Europe, regional deviations to the core safety requirements are contained in EN 60601-1 (1991-01) with Amendments 1 (1994-06), 2 (1996-03), and 13 (1996-07). This standard is known as BS EN 60601-1. In the United States, regional deviations are covered by UL 60601-1. For Canada, they are covered by CSA C22.2 No. 601.1-M90. IEC 60601-1 covers every aspect of medical equipment design in great detail. When designing in power supplies, three main areas are paramount. These are separation, leakage current, and dielectric strength.

Separation

Separation is defined as the physical distance, either through air or across a material, between two components. It is more commonly termed creepage and clearance distances. The actual distances are directly proportional to the working voltage used. They apply to both primary and secondary circuits, but in a power supply, distances are larger in the primary side due to the peak mains working voltage. In the IEC 60601-1 standard, the creepage and clearance distances are split into the following three categories:

• Basic insulation between parts of opposite polarity; i.e., live and neutral lines.
• Basic insulation or supplementary insulation; i.e., between live and earth and neutral and earth.
• Double insulation or reinforced insulation; i.e., between primary and secondary circuits.

Figure 1. This diagram shows the insulation for a typical switch-mode power supply operating from maximum input voltage. (click to enlarge)

Figure 1 shows the insulation diagram for a typical switch-mode power supply operating from its maximum input voltage of 260 V ac. The actual working voltage used will be 450 V dc, due to the peak-rectified mains used in a switch-mode power supply. In the following example, the output voltage is assumed to be 48 V dc. Distances are as follows:

• (a)=Between opposite polarity: 2.4 mm air, 4 mm creepage.
• (b)=Basic/supplementary: 3.5 mm air, 6 mm creepage.
• (c)=Reinforced insulation: 7 mm air, 12 mm creepage.
• (d)=Basic/supplementary: 1.2 mm air, 2.3 mm creepage.

It is important to understand the difference between the types of insulation. Here are some basic definitions:

• Air clearance: the shortest path in air between two conductive parts.
• Basic insulation: insulation applied to live parts to provide basic protection against electric shock.
• Double insulation: insulation comprising both basic insulation and supplementary insulation.
• Supplementary insulation: independent insulation applied in addition to basic insulation in order to provide protection against electric shock in the event of a failure of basic insulation.
• Reinforced insulation: single insulation system applied to live parts. This provides a degree of protection against electric shock.
• Live part (LP): a part in a state that, when connection is made to that part, can cause a current exceeding the allowable leakage current.
• Applied part (AP): a part that is isolated from all other parts of the equipment to such a degree that the patient leakage current allowable in a single-fault condition is not exceeded when a voltage equal to 1.1 times the highest rated main voltage is applied between the applied part and earth. (In this case, LP is treated as a secondary circuit.)

Leakage Current

A good definition of leakage current is unintended electric current passing through the human body. Before looking at actual values of leakage current, it is important to understand the nature of the expected leakage current and the levels that are allowable.

IEC60601-1 details various types of leakage current, including earth leakage, enclosure leakage, patient leakage, and patient auxiliary. Earth leakage is defined as the current flowing in the earth conductor. Enclosure leakage is the current flowing to earth via the patient from the enclosure. Patient leakage is the current flowing to earth via the patient from an applied part (equipment connected to a patient). Patient auxiliary is the current flowing between two applied parts (this is mainly concerned with signal input/output [I/O] ports).

The standard also defines three classes—B, BF, and CF—of equipment types (finished goods). This classification determines the maximum level of leakage current allowed. Type B designates that there is no direct contact between the patient and the equipment, such as a CAT scan. The suffix F applies to cases involving an isolated (floating) applied part.

The standard states that "no current higher than the patient leakage current allowable in single-fault condition flows if an unintended voltage originating from an external source is connected to the patient, and thereby applied between the applied part and earth." Type BF provides a higher degree of protection against electric shock than is required for Type B applied parts. However, even Type BF is not suitable for cardiac equipment, which demands the highest (CF) protection levels.

When evaluating the leakage current from a switch-mode power supply, earth and enclosure are the two most important types of leakage current to consider. These are interrelated. Under single-fault conditions, the earth leakage current will have an effect on the enclosure leakage current.

Table I gives the maximum allowable values of leakage current by equipment classification types. UL 60601-1 dictates that the earth leakage current is measured at 264 V ac (nominal input voltage the equipment is designed to be used at plus 10%; i.e., 240 V ac + 10% = 264 V ac) and 60 Hz.

From the table it is clear that the maximum earth leakage current that a switch-mode power supply can have is 300 µA. However, it is better to design the unit to have a maximum earth leakage current of 100 µA because 300 µA is the maximum allowable in the whole system. If the power supply were at 300 µA, that would not allow for any other ac components and does not give the OEM any margin for tolerances. Having the limit at 100 µA gives the engineer more flexibility.

Dielectric Strength

The dielectric strength determines the maximum withstand voltage across the insulation used. The level of withstand voltage is proportional to the working voltage used. Table II details the test voltages used to determine where the insulation has the necessary dielectric strength.

Power Supplies

Finding a standard power supply that fulfills all normal demands of different output configurations, while still meeting the stringent regulations as laid down by IEC 60601-1, may seem impossible. However, companies have developed a range of products that meet most needs.

Standard, semicustom, and fully custom power products are available for both external and internal power supplies, covering 25–150, 150–450, and 450–600-W ranges, all approved to EN 60601-1, UL 60601-1, and CSA C22.2 No. 601.1-M90 for a variety of medical applications.

Figure 2. A standard ac-dc unit provides the isolation barrier between the mains part and the live part. (click to enlarge)

For example, a standard ac-dc unit can be used to provide the necessary physical isolation barrier (minimum withstand voltage 4 kV ac) between the mains part (MP) and the live part (LP), as detailed in Figure 2. Similarly, approved dc/dc convertors provide an isolation barrier with a withstand voltage of 6 kV dc between applied parts (equipment that is in contact with the patient) and the live part and the chassis. Both products are standard offerings, which also deliver advantages in terms of size, price, and availability that one would expect from a standard product.

Power supply developers can also advise on battery-backed power systems for applications such as ventilators and blood pressure monitors. In such systems, the most important factors are the type of battery, the battery voltage, and the required system run time. These parameters dictate the size, complexity, and cost of the system.

Additional factors include size, weight, and ambient temperature. In determining the load profile, it is also important to factor in the functions that actually need to be backed up. Selecting the correct battery voltage determines the current and ultimately the size of the system. The following general voltage and power levels are recommended:

• Up to 150 W: system voltage should be 12 V dc.
• 150–500 W: system voltage should be 24 V dc.
• Above 500 W: system voltage should be 48 V dc.

In some very-low-powered systems, 8 V dc might be preferred. These systems, such as those used in an ambulance, require recharge or operation from 12-V vehicle power.

With nickel-cadmium (NiCd) systems, the dc-dc convertor needs to operate over a wider voltage range than in lead-acid systems. For example, in a 12-V system, NiCd battery voltages range from 9.5 to 16 V, while lead-acid systems range from 10.5 to 13.7 V.

Battery-backup medical power supplies require a variety of safety and protective features. Many of these features, such as low leakage current and dual fusing, are required by the medical regulatory agencies. Some are often desirable or required for a given application. Common protective features include:

• A battery fuse, which prevents damage from inadvertent short circuits or reverse polarity.
• Dc-dc convertor outputs that are overload- and short-circuit protected. Overvoltage protection should be used on critical outputs.
• Output voltage lockouts that remove critical voltages in the event of a logic voltage failure.
• A thermal cutout or fan-fail signal for systems incorporating a cooling fan.
• Dc-dc convertor logic that includes deep-discharge cutoff if lead-acid batteries are used. This prevents the battery from being deep discharged and protects against shortened battery life. (Deep-discharge protection is unnecessary in NiCd systems).

Additional protective circuits for the charger and dc output should include current limiting, short-circuit protection, and noncrowbar overvoltage protection to prevent overcharging the battery.

Conclusion

Because power supplies must meet the IEC 60601 safety directive series of standards, they play an important role in protecting against electric shock in medical electronics. The issues involved in specifying power products for medical applications do not have to be complicated. However, when designing in power supplies, it is critical to consider separation, leakage current, and dielectric strength necessary for the particular application.

Selecting the right power supply for the application is key to getting products to market on time and to preventing problems during agency testing, which could result in a redesign.

Peter Blyth is director of sales for the medical market for XPiQ (Holliston, MA). He can be reached at pblyth@xp-iq.com.

Copyright ©2003 Medical Electronics Manufacturing