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The “life-supporting equipment” category has been removed from the standard, and it has been replaced by electromagnetic environments of “intended use”.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As far as timing for the update, EN 60601-1-2:2007 3rd Edition is scheduled to be withdrawn on December 31st, 2018, and will be replaced with the 2015 version of EN 60601-1-2.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

Thursday, May 26, 2016

Constant Voltage, Constant Current Battery Charging


There are three common methods of charging a battery; constant voltage, constant current and a combination of constant voltage/constant current with or without a smart charging circuit.

Constant voltage allows the full current of the charger to flow into the battery until the power supply reaches its pre-set voltage.  The current will then taper down to a minimum value once that voltage level is reached.  The battery can be left connected to the charger until ready for use and will remain at that “float voltage”, trickle charging to compensate for normal battery self-discharge.  A typical example would a low cost auto battery charger for home use or basic back up power systems.  This method enables fast charging rates and is suitable for lead acid types, but not for Nickel Metal Hydride (Ni-MH) or Lithium-Ion (Li-ion) types.

Constant current is a simple form of charging batteries, with the current level set at approximately 10% of the maximum battery rating.  Charge times are relatively long with the disadvantage that the battery may overheat if it is over-charged, leading to premature battery replacement.  This method is suitable for Ni-MH type of batteries.  The battery must be disconnected or a timer function used once charged.

Constant voltage / constant current (CVCC) is a combination of the above two methods.  The charger limits the amount of current to a pre-set level until the battery reaches a pre-set voltage level.  The current then reduces as the battery becomes fully charged.  This system allows fast charging without the risk of over-charging and is suitable for Li-ion and other battery types.

Smart charging involves the use of a micro-controller to compensate for temperature rise and adjust the charge current and charge time accordingly to the battery specifications.  This extends battery life and is used with Li-ion battery types.  This battery management circuit or unit can be fitted externally to the charger.  A number of the power semiconductor manufacturers offer control circuits to perform this function.

An example of a CVCC charger is the TDK-Lambda EVS series.  The output voltage and the charge current can be set by two potentiometers and the output characteristics are shown below.  The transition between constant voltage and constant current is automatic.

As an example, consider a 24V battery system (with a maximum float voltage of 28V) and discharged down to 15V.



When the discharged battery (at 15V) is connected to the power supply, the battery will start to charge at the pre-set constant current level.  The current will remain constant until the voltage rises to 28V.  At this point the power supply will transition to constant voltage mode and the current will decay to zero when the battery is fully charged.

The charge current is controlled to avoid overheating and the float voltage limited to avoid over-charging.

A typical application for the EVS being used with a battery management unit is shown below.

 
Under normal conditions, when AC is present, the electronic switch would be closed and AC would be connected directly to the end equipment.  The EVS power supply will charge the battery via the battery management unit and transition to constant voltage mode when complete.  In the event of an AC power interruption, the switch would connect the battery and DC/AC inverter to the end equipment.  If the power interruption was extensive and the battery was to approach a fully discharged condition, the switch would isolate the battery to avoid a damaging deep discharge.

The EVS power supply can be used with the EVS-RP module to avoid the battery discharging into the power supply when the AC supply is not present, or under a fault condition.


EVS300, EVS600 and EVS-RP Module

Monday, April 18, 2016

What is a Power Supply Standby Voltage?


The standby voltage is generated by a power supply circuit within the main power converter.  This became widely used in 1995 when the ATX specification was published to allow a desktop computer to be put into a sleep-mode to save energy.  The standby voltage supplies a small amount of power to the motherboard enabling the computer to quickly restart, rather than performing a full, lengthy, boot cycle.  The term “standby” is often confused with an auxiliary output, which has a different function.

A standby voltage is generated by a separate switching circuit and is not affected by the use of the remote on/off signal or even an overload condition on the main output of the power supply.  A typical block diagram is shown below.



Figure 1 Block diagram of a typical power supply with a standby output

The main and the standby switching converters share the high voltage output voltage (typically around 380Vdc) from the rectifier & PFC circuit.  This saves cost by not duplicating the rectification and filtering components.  It can be seen that they are independent of each other and the remote on/off control is only applied to the main converter.

The auxiliary output is supplied from an additional winding on the main converter transformer.  If the main output is turned off by the remote on/off, the auxiliary output will also turn off.  An auxiliary output is often used to power an external cooling fan if the power supply has a forced air cooling rating.  In this case if the auxiliary output is not present when the power supply output is inhibited, it does not matter as the main converter will not be providing any load and will not require additional cooling.

Figure 2 demonstrates how the various outputs and function interact with each other.  If AC power is removed for any significant length of time (10-50ms), then of course all the outputs on the power supply stop functioning.

 



Figure 2 – Timing diagram

Many power supply designers also use the standby converter to power any “housekeeping” circuitry on the output of the main converter.  This allows an “enable” type remote on/off to be offered, where the signal is pulled low to activate the main converter.  Without a standby circuit, an external voltage has to be applied to the remote on/off to inhibit the power supply.

Manufacturers of mid to high power converters with a standby voltage will often state the off-load power draw, or off-load power consumption, with the remote on/off activated from the standby voltage.

Power Guy

Monday, February 29, 2016

An alternative to isolated DC-DC converters

Traditionally when several voltages (5V to 24V) are required in a system, either a multiple output power supply is used or a single output “bulk” supply with isolated DC-DC converters.  For voltages lower than 5V (0.6 to 3.3V) the electronics industry has migrated to using multiple non-isolated DC-DC converters, often referred to Point of Load or POLs to drive FPGAs powered from a bus voltage between 5V to 12V.
With low power (typically less than 300W) dual, triple or quad requirements in the standard voltages of 5V, 12V, 15V and 24V, a single AC-DC power supply is used.  These are cost effective and readily available.
For medium power requirements (350W to 1500W), often the choice is to use a modular power supply like TDK-Lambda’s NV, Vega or Alpha series.  As the term “modular” implies, they are put together using pre-assembled modules and are available with short lead-times.  All the outputs are conveniently put into one package.


TDK-Lambda’s Vega series

Another choice is to use a single output AC-DC power supply with board mount isolated DC-DC converters to produce additional outputs.  These readily available converters range from around 10W to 60W, can accept input voltages of 12V, 24V or 48V and supply single, dual or triple outputs, with output voltages of 3.3V, 5V, 12V and 15V.


TDK-Lambda’s CCG series of 25mm x 25mm 30W isolated DC-DC converters

When the requirement is for a higher power (100W or greater) second, third or fourth output, the DC-DC converter choice becomes more limited and because of the power involved, heat dissipation is harder to manage.  Cost can also become an issue.  Utilizing technology developed from the low voltage output Point of Load non-isolated converters, higher output voltage non-isolated converters are now being considered.
Without the constraints of input to output isolation, high performance “buck” (step-down) converters with very high efficiencies can be achieved.  With less waste heat, package sizes can be minimized and costs reduced.
TDK-Lambda’s i6A24014A033V, for example has the following specifications:


Input voltage:    +9 to 40Vdc
Output range:    +3.3 to 24Vdc
Output power:   Up to 250W
Output current:  Up to 14A
Efficiency:         Up to 98%
Package size:     33mm x 23mm



As a note, these types of (step-down) buck converters cannot supply a voltage higher than the input.


Although these types of converters have no input to output isolation, the AC-DC power supply will have, in accordance with the safety standards IEC 60950 / 60601.
Below is a typical application using the i6A:



Power Guy
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