IEEE Power Electronics Magazine - June 2015 - 33

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other industries, manufacturers
are challenged to develop medical
equipment that is smaller, more
reliable, has greater functionality,
higher efficiency, and is competitively priced.
However, one area has not
changed: the safety of the patient
and the operator remains of paramount importance. This means that
the nature of medical devices leaves
no room for compromise in terms
of reliability, quality, and, above
all, safety. The reliability issues
involved with medical equipment
are very different than from those
of industrial equipment in that
problems may literally be a matter of life or death. Power supplies
are a critical component in medical
equipment. Connected directly to
the ac power grid, they are often in
the first line of defense to ensure
patient and operator safety.
It is the responsibility of power
supply manufacturers to conduct,
record, and monitor a thorough preproduction (component) inspection, in-production (process and test) inspection, and a final (burn-in and test) inspection of their power
supplies. The combination of testing, characterization, and
continuous monitoring of changes in regulations and legislation continually challenge power supply manufacturers.
They must define, modify, and add to their testing protocols to ensure that their power supplies comply with the
relevant standards and deliver the expected performance
and reliability to the customer.

Safety First: A Matter of First Principles
The first steps to ensuring the safety of medical equipment
takes place months, and often years, in advance of a product
coming to market. Understanding of and adherence to the
required safety and electromagnetic interference (EMI) standards need to be part of the power supply design process and
must be adopted from the first principles of product design.
IEC 60601-1 (Third Edition) identifies the primary
objective as the reliable and effective isolation between
the ac input to the power supply, the internal working
voltages, and the dc output. Adequate spacing between
conductors and components is critical to achieving effective isolation. Reliable insulation design also contributes
to effective isolation, ensuring that when a power supply
is stressed to a higher voltage that it normally operates, it
does not fail or breakdown in a manner that could harm
the patient or the operator.
EMI must also be considered as part of the power supply
design. For example, imaging equipment, clinical chemis-

try, and patient monitors have detection and signaling components that are very sensitive to EMI.

Isolation, Insulation, Means of Patient Protection,
and Means of Operator Protection
The third edition of the IEC 60601-1 standard now recognizes that the potential risks to the operator are very different from the risks to the patient. Being in good health, an
operator is generally less susceptible to shocks and is
trained in the use of the equipment. To address the two levels of safety needed, IEC 60601-1 has defined the different
requirements for the protection of the operator as means of
operator protection (MOOP) and the more stringent protection of the patient as means of patient protection (MOPP).
This distinction can result in different insulation and
isolation safety requirements for circuits with which operators and patients come into contact. Specifically, anything
that falls within the limits of operator protection only has to
meet the creepage and clearance requirements of IEC 60950
(safety for information technology equipment). By contrast,
circuitry that falls within the realm of patient protection
must meet the more stringent requirements of IEC 60601.
Medical equipment must incorporate one or more means
of protection (MOP) to isolate patients and operators from
the risk of electric shock. This ensures that in the event of
one MOP failing, there is always another MOP to protect the
patient and operator. Any insulation, creepage distance, air
clearances, or protective impedance will contribute to MOP.
These measures must be designed into the power supply.
The printed circuit board (PCB) layout, isolation barriers,
and component selection must take all of these factors into
account in the product design stage. Due to these requirements, MOOP and MOPP are becoming commonly used
when describing creepage and clearance levels and isolation.
Table 1 shows the requirements of the insulation level
of each MOP as well as the creepage, clearance, and test
voltage required to attain the insulation level. These are for
typical medically approved power supplies for use in Pollution Class II environments.
To achieve 2 # MOPP levels, the very demanding isolation test of 4,000 Vac requires that the creepage distance is
>8 mm (double what is required for 1 # MOPP). These levels

Table 1. The MOOP/MOPP requirements.

Insulation Insulation Isolation Creepage
Level
Type
Voltage
(mm)
1 # MOOP

Basic

1,500 Vac

2.5

2 # MOOP

Double or
reinforced
Basic

3,000 Vac

5.0

1,500 Vac

4.0

Double or
reinforced

4,000 Vac

8.0

1 # MOPP
2 # MOPP

June 2015

Test
Voltage
1,500 Vac or
2,121 Vdc
3,000 Vac or
4,242 Vdc
1,500 Vac or
2,121 Vdc
4,000 Vac or
5,656 Vdc

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