Tuesday, November 7, 2017

Power Supply Overvoltage Category (OVC)

The IEC standards class Overvoltage Categories, which are sometimes referred to as Installation Categories, 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

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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