IEEE Power Electronics Magazine - June 2015 - 35

damage the power supply. Testing protocols can be implemented to mitigate this issue while ensuring safety.
Type testing is carried out by the safety test agency to
ensure compliance, as mentioned previously. However, as
stated, this is not a guarantee of compliance in production.
Product testing by the power supply vendor can be carried out during the manufacturing process. Reinforced
insulation testing can be done separately at the isolation
barriers, such as transformers, optocouplers, or PCBs,
independently and prior to insertion to ensure their compliance. To ensure the appropriate levels of insulation are
present when the power supply is assembled, 100% testing
of these components is considered the best practice.

Leakage Current
Leakage current is specified to ensure that, in the event of
direct contact with the medical equipment, the operator or
patient is unlikely to experience electric shock. Patients may
be in a weak condition, and even exposure to small leakage
currents may have detrimental effects on their health. The
allowable leakage current from medical equipment can vary
depending on the application and ranges from few microamperes to 500 nA. Leakage tests are designed to simulate a
human body coming into contact with different parts of the
equipment, and these measurements must be below certain
limits, as defined in the IEC 60601-1 standards. For worldwide approval, the maximum permissible earth leakage current is 300 nA. This applies to the equipment and not just to
the power supply. There may be other leakage components
in the system that contribute to the earth leakage, and these
must be considered for system testing.
There are a number of factors that affect leakage current:
■■The parasitic capacitance of the transformer and other
components.
■■Input line voltage: The earth leakage is directly proportional to the input line voltage. Therefore, the same
power supply tested at 110 Vac will have lower leakage
current than when tested at 230 Vac.
■■The line-to-ground capacitance value of the Y capacitors:
The lower the Y-capacitance, the lower the leakage current. However, as will be described later, there is a tradeoff in EMI performance in power supplies and the leakage current that must be managed.
■■Input line frequency: The earth leakage is directly proportional to the line frequency.
As part of the product design, capacitor tolerances must be
considered to ensure that even in the worst-case scenario
(capacitor values +20%), the leakage current will still be below
the maximum allowable limits. This is verified as part of the
safety agency testing of the power supply to IEC 60601-1.
Production testing should be carried out on 100% of
power supplies. Leakage current can be measured as part
of the final testing of a power supply using dedicated testers that have been set up to simulate the worst-case conditions of operation, i.e., the input voltage set to the highest
rated input voltage.

Electromagnetic Interference
Typically, there are two types of power supplies used in
medical equipment. Power supplies that are built into the
end equipment and are not accessible without opening the
equipment are defined as component power supplies.
Power supplies that are external to the equipment, such as
adaptors or lump-in-cord power supplies, are defined as
stand-alone power supplies. Component power supplies
are not stand-alone parts and, as such, are not required to
meet EMI standards from a regulatory perspective. However, switch mode power supplies are active parts in medical equipment, and their EMI characteristics can have a
direct impact on the equipment EMI performance. As mentioned previously, there is a fine line between designing and
manufacturing power supplies to meet low-leakage requirements and also ensuring compliance with the international
standards for EMI. Leakage current is a critical safety issue,
whereas EMI can have detrimental effect on the equipment
in the vicinity of the system.

Emissions: conducted and radiated
By their very nature, power supplies create noise. Any
source of changing voltage or current, with respect to time,
will result in ringing. Any switch mode power supply will
have a number of these events during every switching cycle.
EMI must be factored into the product design from the
initial stages and monitored throughout the design process,
paying particular attention to the PCB layout by
■■minimizing ground loops

iac

(a)
X

(b)
fig 2 (a) High di/dt loops and (b) high dv/dt areas on a forward
converter.

June 2015

z IEEE PowEr ElEctronIcs MagazInE

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - June 2015