Power Supply Overvoltage Category (OVC)

The IEC standards class Overvoltage Categories, which are sometimes referred to as Installation Categories, are as follows (most stringent to the least stringent):

Category IV

Used at the origin of the installation.  Examples are utility transformers, electricity meters, fusing and distribution panels.  High transient voltages are very likely.

Category III

Used in fixed installations and for cases where the reliability and the availability of the equipment is subject to special requirements.  These installations will have permanent connection to the distribution panel (hard wired).  Wiring impedance, fuses and circuit breakers somewhat reduce the level of voltage transients.

Category II

This covers energy-consuming equipment supplied from the fixed installation.  These would be items normally plugged in to a regular wall outlet or other plug in fixture requiring 115 or 230Vac.  The impedance of wiring circuits further reduces transient voltages to a lower level. Outlets, lighting switches and building connections more than 10m from a Category III source are classed at a Category II.

Category I

These are circuits requiring low voltage, which limit over voltage conditions to the appropriate level, i.e. protected electronic circuits.

Each category has to withstand a different level of voltage transient depending upon the nominal input voltage as shown in Table 1

Table 1: Tolerated transient voltage

The standard IEC 60204-1:2016 governs the safety of machinery. The general requirements apply to electrical, electronic and programmable electronic equipment that are fixed in their location.  The equipment covered by this part of the standard commences at the point of connection of the supply to the electrical equipment of the machine.

An industrial robot, or material forming machine, wired to the distribution panel would need to comply with Overvoltage Category III, as shown in Figure 1. If the AC-DC power supply inside of the robot controller was only OVC II, then an isolating transformer (not necessarily a step down type) would have to be fitted inside or between the controller and the distribution panel.  The impedance of the transformer would be enough to reduce the transient voltage level.

Figure 1: Levels of Overvoltage Category inside of a factory or facility

Some OVC III industrial AC-DC power supplies are available, like TDK-Lambda’s 240W rated 24V output ZWS240RC-24 power supply.

Although based on TDK-Lambda’s ZWS300BAF-24 which is OVC II, it has increased spacing from the Line and Neutral to ground, including additional spacing on the input connector.  It is also certified to EN 62477-1 - safety requirements for power electronic converter systems and equipment.  It is said to be a more suitable specification than EN 50178 which covers electronic equipment for use in power installations.

The use of such a power supply can eliminate the isolation transformer, saving both cost and space.

Power supply fan noise reduction

Unwanted audible noise is now part of modern human life, whether it is produced by people or machinery.  Although low level noise is not necessarily harmful, prolonged exposure can cause fatigue and stress related health issues.  This can also apply to the workplace where the use of fans to cool electronic equipment is becoming more widespread in order to make product size smaller.

Industrial, communications and medical equipment frequently use fan cooled AC-DC power supplies because of their small size.  Many equipment manufacturers though are being asked by their customers to reduce the amount of audible noise generated by their products.  Fans in medical equipment used in the proximity of the patient can delay or complicate recovery.  Engineers using laboratory test equipment or technicians operating analyzers do not want to be distracted by the noise of an irritating fan.

Some applications, like datacenters, have little human presence and equipment size takes priority. Here it is preferred to use 1U high (44.4mm) power supply racks and one or two high speed 40mm fans to keep the power supplies cool.  Although these fans have high airflow, they emit a great deal of acoustic noise at frequencies very annoying to the human ear.

The amount of noise a fan generates is related to their size, rotation speed, construction and how the air is travelling over or through the components it is cooling.  For the same airflow requirement, a smaller fan is much noisier than a larger one because it has to rotate at higher speeds to produce the same cooling effect.  Table 1 below compares a 40mm and a 60mm fan.  It shows that the 40mm version has 66% of the cross sectional area blocked due to the fan hub, whereas the 60mm has only 51%.

Table 1

The fan bearing type also will affect acoustic noise.  The quietest is the sleeve bearing, but is less reliable long term than a ball bearing type, especially at higher temperatures and care has to be taken with the mounting orientation.  Bearings do create vibration which can affect the system performance of say a digital microscope or scanner.  A lower speed fan can reduce this.

Fan blades are often designed to create turbulence in the airflow, which increases acoustic noise as it passes over the components and heat sinks inside the power supply. When high velocity turbulent air comes into contact with a physical object, it can create a highly annoying audible tone and increase the noise by up to 10 dBa which equates to a doubling of the perceived loudness.

To protect operators and service technicians, a finger guard is usually fitted to the fan.  As the guard is in the path of the airflow, it will also create acoustical noise.  Table 2 shows typical levels of noise for two fan guard types when used with a low speed fan.  The wire grill (with circular wire) offers the best balance of protection and acoustic noise, but is more costly and involves manual labor to assemble compared to a machine punched pattern in the power supply enclosure.  Of course, the noise levels rise as the speed of the fan is increased.

Table 2

Careful consideration should be taken when designing the internal construction and layout of the power supply.  Obstructions to airflow will reduce its cooling effect, resulting in the need for a higher speed fan and hence higher noise.

Variable speed fans are growing in popularity, as quite often a power supply is not running in a hot environment, or running at full load. Not only will audible noise be reduced, the fan will last longer. Sensors are used to measure a heatsink temperature or other hot component to determine if more or less air is required. The circuit must have sufficient hysteresis to avoid constantly changing fan speeds, as this can be more annoying that a fixed speed, higher noise fan.  TDK-Lambda’s recently launched QM series of modular power supplies senses the incoming air temperature.  This enables the fan to run slower and quieter at room temperature, but faster at higher temperatures, where humans would not normally be present.

The amount of wasted heat is determined by the efficiency of the power supply. The graph in Figure 1 shows that with a 90% efficient 600W output power supply, only 67W is generated, compared to 115W unit with an efficiency of 85%.  Increasing efficiency enables the use of quieter, slower speed cooling fans.

Figure 1

TDK-Lambda’s 91% efficient 1200 to 1500W rated QM7 series utilizes two slow running 60mm fans to further reduce audible noise.

The QM series design team performed extensive audible noise testing on current TDK-Lambda products, the QM7 and competitive models using the BS EN ISO 3744:2010 standard (Acoustics: Determination of sound power levels and sound energy levels of noise sources using sound pressure).

The results proved that the steps taken in the QM7 design had significantly reduced fan noise, with the 1500W rated QM7 measuring a low 44.3 dBa. Other products measured were as high as 58 dBa. Note again, a 10 dBa which equates to a doubling of the perceived loudness.

Twenty one employees were asked to take part in a “blind” acoustic study, listening to the individual models and rating them on loudness and how annoying they were.  The tests again revealed that the QM7 came out as the best model.

The QM series is the latest in a 37-year legacy of modular power supplies, beginning with the invention of the ML series, a world first in 1979.  Having both medical and industrial safety certifications, the QM is very suitable for a wide range of applications, including BF rated medical equipment, test and measurement, broadcast, communications and renewable energy applications. With a wide range 90-264Vac, 47-440Hz input the QM7 can deliver 1200W, and 1500W with a 150-264Vac high line input.  Up to 16 outputs can be provided, with voltages ranging from 2.8V to 52.8V, with additional higher voltages of up to 105.6V in development.

Medical power supplies meeting IEC 60601-1-2 4th edition voltage dips and interruptions

Customers call TDK-Lambda wanting their medical product to meet the strict IEC 60601-1-2:2015 4^th 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 3^rd 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.

How will the Power Supply Industry be affected by EN 55032 replacing EN 55022?

In 2014 the Hazard-Based Safety Engineering (HBSE) standard IEC 62368-1 was announced combining the Information Technology Equipment (ITE) standard EN 60950-1 and the audio, video and similar electronic apparatus safety standard EN 60065.  This step was taken as there was no longer a clear definition between ITE and multimedia equipment with advent of internet connected TVs, smartphones and other home entertainment products.

Now the EMC standards are also being combined and as of March 5th, 2017, EN 55022, EN 55013 and EN 55103 will be replaced by one unified emission requirements standard called EN 55032.  The current “Electromagnetic compatibility of multimedia equipment” was first published in May 2012 as EN 55032:2012+AC 2013, will be withdrawn on May 5th 2018.  EN 55032:2015+AC:2016, which was announced May 2015 and published February 2016, has already superseded the 2012 standard.

The three standards that are being replaced by EN 55032 are:

EN 55022: Information Technology Equipment, Radio disturbance characteristics. Limits and methods of test.

EN 55013: Sound and Television Broadcast Receivers and Associated Equipment.

EN 55103: Audio, Video and Entertainment Lighting Equipment for Professional Use.

Fortunately for those in the power supply industry serving the ITE market who have relied on EN 55022 as their core standard for many years, there are no changes to the test requirements.  The multimedia equipment (MME) makers will have additional test requirements to interface ports, port type and emissions from cabling.  The individuals that prepare and sign their company’s CE Declaration of Conformity will be kept busy updating their forms though!

Power Guy