Friday, April 27, 2018

Where and why is a 277Vac input used in the USA?

The AC power supplied and used by household and commercial buildings in the USA is primarily 120Vac.  This is suitable for most electrical and electronic equipment in the home or office, but for the higher power requirements of electric driers, air conditioning and electric ovens, 208Vac or 240Vac is available. 


The 208Vac is derived by connecting across two phases of a “Y” or “WYE” three phase supply as shown in Figure 1.

 Figure 1
The 240Vac is supplied from a distribution transformer as shown in Figure 2.
Figure 2
In larger industrial properties, in addition to 208 and 120Vac, 480Vac three-phase is supplied to the building in a "WYE" (or Y) configuration as shown in Figure 3.



Figure 3 480Vac three-phase
The 480Vac three-phase is used to power larger electrical equipment like fork lift battery chargers, and heavy machinery.  Less current is drawn by the load at this higher voltage, enabling the use of lighter gauge wire, and the current is balanced equally in each phase.  This improves both power distribution and generation efficiency.


Measuring across each line to neutral we see 277Vac (480Vac divided by the square root of 3)  This voltage is used in the USA for powering lighting and HVAC (Heating, Ventilation, and Air Conditioning).  Using 277Vac instead of 120Vac reduces the current drawn by lighting fixtures by over 50%, enabling smaller gauge (lighter) and lower cost wiring.  Alternately more lighting fixtures can be connected to a 277Vac feed than with a 120Vac feed.  Power losses in the wiring is also reduced (P=I2 x R).
The awareness of the existence of 277Vac feeds in commercial buildings was very limited until the introduction of LED lighting in the late 2000s.  The AC-DC power supply industry until then had focused on products that operated between 85 – 265Vac.  As LEDs require low voltage DC current, LED “drivers” came on to the market, capable of operating from input voltages of 90 to 305Vac.  This voltage range covers 100V, 120V, 230V and 277Vac nominal inputs (with a regulatory +/-10% tolerance for line variations) to enable use world-wide.


Some of the control infrastructure for smart buildings is now using 277Vac for automatic window sun filtering, intelligent thermal monitoring and security.  Often the location of the controllers and monitors are in places where 120Vac is not available and 277Vac can be easily dropped down from above the ceiling tiles.

TDK-Lambda is one manufacturer that is providing industrial power supplies capable of operating from 90-305Vac.  These are an alternative to using constant voltage LED drivers that potentially can have short product lifecycles.  Engineering resources then have to be diverted to find alternative products.  Power supplies developed for the industrial market tend to be produced for ten years or more.

Below are two such products; the 2 to 4W rated pcb mount KAS series and the 40 to 65W rated chassis / DIN rail mount CSW65 series


Tuesday, February 27, 2018

Synchronizing the i6A DC-DC Converter Switching Frequencies

When multiple DC to DC converters are used to power sensitive circuitry, input and output noise can cause system issues, particularly when measuring very low signal voltages.  The problem is compounded if the converters’ switching frequencies vary with input voltage or output load. 

Even when the converter’s operating frequency is fixed, there will be a tolerance on the switching frequency timing circuit between the converters and just a few Hertz differences between the converters can cause sub-harmonic beat frequencies.

Filtering the inputs and outputs is one solution, but this can be complex over a wide frequency bandwidth.  If the DC-DC converters had a fixed switching frequency and could be synchronized to together or tied to a master clock, any board and/or system EMI filtering would be simpler.

Although somewhat uncommon, there are some DC-DC converters that have synchronization capability.  When the “full feature” option is specified on TDK-Lambda’s low cost, fixed frequency, i6A series of 250W non-isolated DC-DC converters, multiple units can have their operating frequency synchronized.

There are four ways to connect the modules:

1. Master / Slave, with no phase shift.  One i6A module is the “master” and the other modules will operate at the master’s switching frequency.  All the modules draw input current at the same time.

 


2. Master / Slave, with 180 degree phase shift.  One i6A module is the “master” and other modules will operate at the master’s switching frequency, but directly out of phase.  This can reduce the peak input ripple current from the supply, requiring less input capacitance.  Any module with a jumper between pins 4 and 33 will have the phase shift function activated.
 

3. An external clock is used with no phase shift.  All the i6A modules operate at the same frequency as the external clock (no master / slave). All the modules draw input current at the same time.
 
4. An external clock is used with a 180 degrees phase shift.  All the i6A modules operate at the same frequency as the external clock (no master / slave), but any module with a jumper between pins 4 and 33 will have the phase shift function activated.  This again reduces the peak input ripple current from the supply, requiring less input capacitance.

 

The i6A non-isolated DC-DC converters are a series of step down converters (the input voltage has to be higher than the output) in the industry standard 1/16th brick footprint.  All models feature wide range input voltages, as high as 9 to 53V and have wide range output adjustment from 3.3 – 15V to 3.3 – 40V.  Operating efficiencies can be as high as 98%.

Monday, January 8, 2018

Factors to consider when using 1U high power supplies in industrial environments

The height of rack mounted equipment is specified in multiples of “U”, where 1U is defined as 1.752” or 44.5mm in accordance with the Electronic Industries Association EIA-310-D standard.  The actual rack, or shelf as it is sometimes referred to, is slightly lower in height to allow easy insertion and removal without binding on adjacent shelves.  Commonly 1.721 inches (43.7 mm) tall is used.  The most common rack width is 19”, but is also available with a 23” width.  A wide range of standard electronic and electrical equipment is designed to be mounted into a 19” enclosure, including instrumentation, computers and power supplies.
Embedded (chassis mount) power supply height is often referred to as being “suitable for 1U applications” even though it is not offered with a rack enclosure.  In this case the product height will normally be less than 41mm (1.61”) to allow for the rack sheet metal thickness.

The communications industry has standardized on 1U high power supply racks to power datacenters and other datacom related equipment for networking.  These power supplies are frequently configured for redundancy so if one fails, it can be replaced without disturbing equipment operation.   The amount of power that can be delivered from a 1U rack can be 10kW to 14kW with four plug-in power supplies.
Why have these high density, cost driven products not found a great deal of popularity in the industrial segment?  There are several reasons for this:

Audible noise


To cool a 2kW to 3kW 1U power supply requires fan cooling in the form of two, high speed 40mm fans.  These emit very high audible noise levels at frequencies quite annoying to the human ear.  As datacenters rarely have human operators present for long periods of time, this is not an issue.  For industrial use where operators are constantly monitoring and using equipment, like test and measurement and analyzers, such levels of noise would not be considered acceptable.
Industrial mid power (600-2000W) power supplies tend to be 63mm in height for single phase inputs and use lower speed 60mm fan(s).  Three phase mid to high power (3000W+) 2U high units may be fitted with a low speed 80mm fan.  Temperature controlled fan speeds extend field life and dramatically reduce audible noise levels.
Fan life and contaminants

The faster a fan rotates, the quicker the life of the fan is reduced due to mechanical bearing wear.
The higher the airflow speed, the more contaminants (dust and dirt) will be drawn into the power supply.  Eventually this will block airflow and/or cause product failure due to circuit shorts if the contaminant is conductive.  Datacom equipment is usually situated in fairly sterile conditions, unlike industrial equipment particularly in harsh environments.
Operating environment

Datacenters have a controlled operating environment, usually air-conditioned.  This is not always the case in the industrial sector.
Redundant configuration of the power supply is usually mandated with datacom products.  In this mode, the power supplies are not run at 100% load in normal operation.  For example, with in a 3+1 2500W power supply configuration, where the maximum load drawn is 7500W, each of the four power supplies will provide 1875W and be running at 75% capacity.  Only if a unit fails will they operate at full load until the faulty unit is replaced.  Such derating of a power supply reduces internal temperatures, particularly those of the electrolytic capacitors, and improves its operating life.
The ability to operate power supplies in a redundant mode can be very important to some industrial users, to minimize production downtime.
The input supply to a datacenter is usually two dedicated feeds (again for redundancy), protected from AC line transients by switchgear and backed up by uninterruptible power supplies.  Industrial power supplies are subjected to input transients when large neighboring inductive equipment is switched on and off.
Datacom equipment power supply loading is usually well defined and relatively static with few severe load changes.  Industrial power supplies can be driving inductive DC motors, relays and capacitive loads, all of which can stress a power supply.
Product availability

The technology in the communications industry is constantly evolving and that includes the power supply architecture.  Datacom systems initially required 48V output power supplies and migrated to 12V to drive non isolated DC-DC converters.  Now datacenters are being run using high voltage 380VDC outputs.  Due to the vast amount of electricity that is consumed by these centers, an increase in efficiency warrants a change in power architecture.  This can result in early power supply obsolescence.
Industrial equipment is expected to be in service for 10 or more years.  Having to rework a system when a spare power supply is not available can lead to long downtimes.  Industrial power supplies are often produced for 15-20 years.
Summary

1U high datacom power supplies are used in some industrial applications where space is extremely limited, audible noise is not an issue and the environment is controlled.  Industrial power supplies are more likely to be subjected to uncontrolled environments and noisy AC inputs.  Product lifecycle and field reliability are key considerations to industrial users, who value long term availability and maintenance free operation.

Tuesday, November 7, 2017

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.


Friday, September 1, 2017

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.

Wednesday, May 24, 2017



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 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.

Tuesday, January 17, 2017

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

Tuesday, November 22, 2016

How does IEC 60601-1-2 EMC 4th Edition relate to power supplies?

The growing use of wirelessly connected devices like mobile phones, tablets, laptop computers and gaming consoles pose a risk to equipment sensitive to EMI and EMC.  On aircraft, restrictions on the use of these devices have long been in place and in general, the public are aware of that policy.  In the past, many of us have seen notices in hospitals asking visitors to not use their phones in intensive care, critical care pediatric units and where specialized medical equipment is located.  

With the growing popularity of home healthcare, enforcing such a policy is impossible.  The medical regulatory bodies, like the FDA (Food and Drug Administration), are now requiring equipment manufacturers to design and test their products to avoid any potential risk of patient harm.  This also includes electrostatic discharge (ESD), radio interference, voltage surges and power interruptions. 

In 2014 an update to IEC 60601-1-2 was published and it “applies to basic safety and essential performance of medical equipment and systems in the presence of electromagnetic disturbances and to electromagnetic disturbances emitted by that equipment and systems”.  Product categories were added and higher EMC test levels introduced.  Manufacturers must submit risk analysis documentation for both normal and abnormal use of their equipment and systems.  This standard is often referred to as the “4th edition”.

The “life-supporting equipment” category has been removed from the standard, and it has been replaced by electromagnetic environments of “intended use”.  According to IEC 60601-1 (2012) it is defined as “use for which a product, process or service is intended according to the specifications, instructions and information provided by the manufacturer”.  These intended use environments are:

1)    Professional healthcare facilities with attending medical staff, and include hospitals, dental surgeries, surgery rooms and intensive care.

2)    Home healthcare which is defined by IEC 60601-1-11 as dwelling places where patients live or places where patients are present - excluding (1)

3)    “Special” environments are those that exclude (1) and (2), but include heavy industrial plants or medical treatment areas with high powered medical electrical equipment (such as short wave therapy equipment).

As far as timing for the update, EN 60601-1-2:2007 3rd Edition is scheduled to be withdrawn on December 31st, 2018, and will be replaced with the 2015 version of EN 60601-1-2.  This is also the FDA compliance date in the US, after several recent delays from July 2014, aligning it with the European Union Medical Devices Directive 93/42/EEC.  The FDA has urged manufacturers to test for compliance as quickly as possible.

Power supplies are not medical devices and the Medical Device Directive cannot be documented on the CE Declaration of Conformity, even for an external power supply.  It is highly recommended that power supply manufacturers comply with IEC 60601-1-2: 2014, to avoid failures in the end equipment or system.  Most are testing and working to meet the higher levels of susceptibility, as the changes to emissions are relatively minor.

The susceptibility changes are based on the IEC 61000-4 set of standards and include:

IEC 61000-4-2 (Electrostatic Discharge):  Test levels for contact discharge increased from ±6kV to ±8kV and air discharge levels nearly doubled to ±15kV from ±8kV.  This is to cover higher levels of ESD that will occur with home use.

IEC 61000-4-3 (Radiated RF Electromagnetic Fields):  Again this is aimed at home healthcare use where the 3V/m test has been extended to 10V/m. The RF susceptibility test has been extended from 80 MHz to 2.7 GHz, because of potential proximity to wireless communication equipment, including Bluetooth and WLAN.

IEC 61000-4-4 (Electrical Fast Transients):  The pulse repetition frequency rose from 5 kHz to 100 kHz, to reflect real operating environments.

IEC 61000-4-5 (Surge Immunity) + ISO 7637-2 (Electrical transient conduction along supply lines):  Changes here were made to include permanently connected DC input devices, for applications such as ambulances.

IEC 61000-4-6 (Conducted RF Immunity):  It is here where the differentiation has been eliminated between life support and industrial, scientific and medical.  Testing has to be made at a potential risk frequency, for example where the equipment might be used in proximity with ham radios.

IEC 61000-4-8 (Power Frequency Magnetic Fields):  Test levels for power frequency magnetic fields have risen from 3 A/m to 10 A/m for all environments, but only for equipment that may be sensitive to magnetic fields, containing relays or hard disc drives for example.

IEC 61000-4-11 (Voltage Dips and Interruptions):  This is where the risk management documentation will be often used.  Although tests must now be made at multiple phase-angles (not just at 0o and 180o) the percentage dip in line voltage, and number of periods, have also been changed for some devices.  The 5 second interruption requirement will need to be met at the equipment level as it is highly unlikely that a standard power supply will continue to operate with the input being removed for 5 seconds.  The equipment manufacturer for a heart rate monitor could document that this will not be a problem, since battery back-up is in place.

Power supply manufacturers will qualify their products as “compliant”, and provide a test report detailing the results.  For example, for the 5 second interruption in IEC 61000-4-11, it will be stated that the power supply will shut down, and automatically recover.

Power Guy

Thursday, September 1, 2016

Comparing Power Supply MTBF Numbers


One subject that confuses specifiers of power supplies is comparing MTBF numbers from different manufacturers, who often use different standards for calculating the number of hours.  There are many well written articles going in to great detail available on the internet on the calculation of MTBF, but this blog article will attempt to simplify things.

MTBF (Mean Time Between Failures) is the mean time between successive failures, and only really applies to a part that will be repaired and returned to service.  So if the up-time of the power supply was a year in each case below, then the MTBF would be ½ x (1 year + 1 year).

A low cost power supply will probably not be repaired and if it is under warranty, it will normally be replaced.  In this case, the numbers to look for would be MTTF (Mean Time To Failure), but that figure is not widely stated.  Usually life testing of a large number (to cut the test time down) of power supplies is used to calculate that. 

The MTBF number is often thought to be the minimum (guaranteed) time before a failure; that is certainly not the case!  Reliability “R” is based on the probability that a piece of equipment, in our case a power supply, will operate satisfactorily for a given time period “t” (based on specified conditions – for example ambient temperature and output load).   

For random failures, the probability that a power supply will survive to its calculated MTBF is just 37%, no matter what the MTBF number is:

R(t) = e –t /  MTBF = e-1  = 0.368 (when t = the MTBF number)

To complicate things further, a variety of methods are used to calculate MTBF.

Prediction Standard
Applications
Disadvantages
MIL-HDBK-217F
Provides failure rate data and stress models for parts count and parts stress predictions. It also provides models for many environments ranging from ground benign to projectile launch
Hasn’t been updated since 1995, gives higher failure rates of commercial parts than is seen in actual product life
Telcordia SR332
Gives three prediction methods based on parts count, lab testing and field life
Narrow ambient temperature range
RCR-9102
Produced by JEITA - Japan Electronics and Information Technology Industries Association.
In each update component failure rates (FIT) have been changed, particularly fans
Issued in 1994, based on MIL-HDBK-217F
RCR-9102A
Issued in 2000, based on MIL-HDBK-217F (Notice 2)
Includes SMT parts & pcbs
RCR-9102B
Issued in 2006

Usually for commercial power supplies, the figures are calculated at 25oC, ground benign or fixed

Taking TDK-Lambda’s RWS150-B series as an example, the calculated numbers are as follows:

RCR-9102             444,089 hours

RCR-9102B          218,172 hours

Telcordia              2,235,743 hours Ta=25

Telcordia              1,063,230 hours Ta=40

It can be seen from the above numbers, that there is a 10-fold difference between RCR-9102B and Telcordia, and more than a 2 fold difference between RCR-9102 and RCR-9102B.  Several customers have asked why our newer products calculated using the JEITA method appeared to be less reliable than older products, but did not know the significant impact of the updated, harsher standard.
Engineers should be more concerned about electrolytic capacitor and fan life (if used) as these are the typical failure modes.  Many manufacturers are showing expected capacitor lifetimes in their reliability reports.  Below are the plots for the RWS150B, which was designed for long capacitor life.  As a note, some manufacturers show similar plots, but state in small print that the convection cooled power supplies had external forced air cooling applied.
 
 

 
Power Guy

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