Customers call TDK-Lambda wanting their
medical product to meet the strict IEC 60601-1-2:2015 4th edition
immunity standard, and ask us if our medically certified power supplies fully comply. In particular, concerns are raised about meeting
the section dealing with voltage dips and short interruptions to the AC supply.
IEC 60601-1-2 is derived from the IEC
61000-4 standard, which covers Electromagnetic compatibility (EMC). The testing and measurement methods are very
similar, but some of the test levels for dips and interruptions in the section
based on IEC 61000-4-11 are much tougher.
The interruption test of removing the AC supply for 5 seconds, without
the loss of the output, is almost impossible without a custom solution with
some form of battery back-up. One may
well question why are standard medical power-supplies being sold if they do not
meet that standard.
Firstly, power supplies are not classified
as medical devices, it is the customer’s product or system that is the
medical device.
Secondly the term “essential performance” used
in the standard has to be examined. In
the 3rd edition of IEC 60601-1 it is defined as “the performance
necessary to achieve freedom from unacceptable risk”. To clarify, the designer/manufacturer has to
determine if a loss of performance or functionality of their medical device
product or system will result in an acceptable risk or an unacceptable
risk. That risk is the potential to harm
a patient, operator or the environment.
Analysis must be made of the probability or the frequency of an event
happening compared to the severity of that event.
Let’s give a simple example. Diabetics check their blood glucose level on
a regular basis and most use a handheld battery-operated meter that accepts
disposable test strips. If that meter was
to stop working, say due to a faulty display, it would be classified as an
acceptable risk. Replacement meters are
readily available from supermarkets and pharmacies and a short delay in testing
would not normally cause harm. An
unacceptable risk would be if the internal sensor measuring the blood glucose
level was to produce incorrect readings and the diabetic administered too much
or too little insulin.
Power supplies, although not classified as
medical devices, can have an impact on the IEC 60601-1-2 immunity performance
of the device they are powering. For the
voltage dips and interruptions section of the standard, there are five tests
performed. Table 1 below shows the input
voltage dip and the duration. 100Vac
input and 50Hz conditions are shown as they could represent the worst case.
Test results are judged against four
performance criteria levels:
Performance Criteria A – ‘Performance
within specification limits’
This is the best
result. A very slight drop in output of
a few milli-volts (within the regulation limits) should not cause the end
device to malfunction.
Performance Criteria B – ‘Temporary
degradation which is self-recoverable’
Criteria B is
usually acceptable in the majority of cases.
Performance Criteria C – ‘Temporary
degradation which requires operator intervention’
This would be
classified as unacceptable from a user point of view, without even considering
a risk analysis. If the AC power was
interrupted and the power supply had to be reset by a patient or operator, it
would be much too inconvenient.
Performance Criteria D – ‘Loss of function
which is not recoverable’
Criteria D is really
a “fail” test result. If a power supply is
damaged and needs replacing after the test, it is very unlikely that a product
with this performance level would be placed on the market.
AC Input Voltage
|
Actual Voltage Dip for
100Vac nominal
|
Voltage Dip by AC
Input Cycle
(50/60Hz)
|
Voltage Dip Time Period
for 50Hz
|
Suggested Performance
Criteria Level
|
Dip down to 0%
|
0Vac
|
0.5 of a cycle
|
10ms
|
A
|
Dip down to 0%
|
0Vac
|
1 cycle
|
20ms
|
A
|
Dip down to 40%
|
40Vac
|
10/12 cycles
|
200ms
|
B
|
Dip down to 70%
|
70Vac
|
25/30 cycles
|
500ms
|
A
|
Dip down to 0%
|
0Vac
|
250/300 cycles
|
5000ms (5s)
|
B
|
Table
1: Test Levels
Referring to Table 1, most power supplies will
pass the first two tests with a Performance Criteria level A with some output
derating to increase the hold-up time.
The
third and fourth tests requires the power supply to continue to operate for
200ms when the input drops to 40% of nominal or for 500ms at 70% of
nominal. Criteria A could be achieved by
having the power supply’s low voltage input protection circuitry modified to allow
the power supply to operate at the lower input voltage for a short time. As the AC input current will be higher, it is
best to ensure that the power supply is not operated at full load. As hold-up time is related to the actual
output power drawn, operating the power supply at 50% load will result in a
significant “ride through” capability during the interruption.
The fifth test of a 5 second interruption
to the AC supply is usually met with the installation of battery back-up or a UPS
(Uninterruptible Power Supply). Adding
sufficiently large energy storage inside the power supply would result in a
significant increase in size.
In summary, the medical device
designer/manufacturer must decide which performance criteria is needed, based
on their risk analysis to meet IEC 60601-1.
Unless continuous performance is critical, most manufacturers will opt
for the criteria in Table 1.
