Monday, April 29, 2019

How do I balance the current between two DIN rail power supplies used in parallel?

Before economically priced DIN rail power supplies gained popularity, many 100W or higher power rated units provided users with a parallel operation function in the form of switch fitted on the front panel.  This switch allowed the user to select between “droop mode” current share between two or more supplies and single unit operation. Droop mode allowed for balancing of the output currents between the units.

If power supplies without a parallel operation function are being used and additional current is required for the system load, a second power supply could be connected in parallel, although many manufacturers do not recommend this.  Unless the output voltages of both power supplies are set to the same voltage, the unit with the greatest output voltage may deliver the majority of the current and operate in an over current condition. This will reduce the unit’s field life due to excessive internal heating.  Ideally the output current of each power supply should be routinely measured during preventative maintenance to ensure they are balanced, which is both time consuming and cumbersome.

One alternative is to use a “redundancy” module with a load balance option, like TDK-Lambda’s DRM40.  Figure 1 shows a DRM40 used to connect two 20A power supplies to deliver up to 40A and Figure 2 the DRM40 with its current balancing LED.

Figure 1: Two units connected in parallel with the DRM40

Figure 2: DRM current balance LED

If the LED is not illuminated, measure the output voltage on both power supplies. Adjust the voltage of the power supply with the lowest voltage higher until the LED turn on.  Alternatively, one can adjust the voltage of the power supply with the highest voltage lower.  The current balance LED will be illuminated when the difference between the voltages is less than 50mV and the output currents are balanced.

The DRM40 has internal MOSFETs, used to block reverse currents in the event of one power supply failing short circuit.  They also allow the measurement of each power supply’s output voltage without disconnecting any load cables.

The DRM40 can also be used in redundant power systems, where two power supplies are used in a 1+1 configuration as shown in Figure 3.  If one power supply fails the other will continue to provide current to the load.  The load balancing function can be used in same manner.

Figure 3: DRM40 used in a 1+1 redundant configuration

Power Guy

Friday, January 25, 2019

What is the difference between efficiency and average efficiency?


Power supply datasheets include product efficiency in a percentage format for each voltage and output power model, as a guidance to how much power is lost in wasted heat when the product is running.   As the actual operating efficiency varies with input voltage, output load, ambient temperature and component tolerance, usually there is a test condition noted. 
Phrases like “up to 95%” or “typically 93% at 230Vac input, 100% load and 25oC ambient” are widely used.

If the selection of the power supply is being made purely on efficiency, then the manufacturer’s evaluation data has to be studied in order to determine the measured efficiency at the user’s load condition.  Figure 1 shows the efficiency vs. output current plot for TDK-Lambda’s 600W rated 24V output GXE600-24 for different input voltages.  At 60% load, 230Vac input one could expect the efficiency to be 94%.

Figure 1: GXE600-24 Efficiency vs Output Current

Average efficiency

External power supplies complying with the DoE (Department of Energy) and EU efficiency regulations will sometimes only state the standard (and its revision) they comply with.  TDK-Lambda’s DTM110PW240C8 datasheet for example, states compliance with the latest DoE Level VI & EU Tier 2 Efficiency standards and also includes that the average efficiency is >89%. The average efficiency for an external power supply rated between 49-250W has to be at least 89% to comply with the current and proposed standards.

Is “Average Efficiency” the same as “Efficiency”?  No.

Average efficiency is calculated by measuring the efficiency at 25%, 50%, 75% and 100% loads.  These four values are added together and the total is divided by four to obtain the average. Measurements are taken at 115Vac and 230Vac inputs.

Using the measurements from Figure 2 for the DTM110PW240C8, the calculated average efficiency at 115Vac is 90% and 90.5% at 230Vac.

Load (%)
Vin: 115V/50Hz
Vin: 230V/50Hz

Figure 2: DTM110PW240C8 Efficiency Measurements

Efficiency readings are also taken at 10% to check compliance to the EU Tier 2 Efficiency standard.  For a power supply rated at 49-250W it must have a minimum efficiency of 79%.  At 10% load the DTM250-D has an efficiency of 89%.

Power Guy

Wednesday, November 28, 2018

Can a dual output DC-DC converter provide a single output?

Yes, many dual output DC-DC converters can actually be used to provide a higher voltage, single output.
Figure 1 shows a simplified block diagram of the TDK-Lambda’s CCG +/-12V dual output converter.  Two transformer secondary windings are rectified and filtered to provide two 12Vdc voltages and are connected together, effectively in series.  Internally the 0V of the upper circuit is connected to the +12V of the lower circuit and this point is supplied to the user as the 0V or common connection.  This provides a +12V and a -12V output to the user.

Figure 1: A dual output DC-DC converter providing a +/-12V output

If a single 24V output voltage is needed, a dual output converter can be used as shown in Figure 2.

Figure 2: A dual output DC-DC converter providing a 24V output

Internally nothing has changed, there are still two 12Vdc voltages connected in series.  If the common terminal is not connected to the user’s circuit, the converter can now provide a 24V single output.
Note the maximum available output current remains the same as the maximum output current of the dual output converter.  For example the 30W CCG30-24-12D is rated at +/-12V +/-1.25A, meaning it is capable of supplying +12V at 1.25A and -12V at 1.25A or if connected as a single output, 24V at 1.25A (30W).

Likewise a dual output +/-15V converter can be configured to supply 30V.
Always confirm with the manufacturer if their datasheet does not state it can be used as a single output.

Power Guy

Friday, October 26, 2018

Z+ - Generating Arbitrary Waveforms

Arbitrary waveform generators are used to test electrical and electronic equipment to ensure that the product operates properly, or to pin point a particular fault.  These can be used either repetitively or as a once only (single-shot).  The waveforms can be triggered to run by an external event, a signal from another piece of equipment for example, manually using the front panel controls or by using the GUI interface.  An arbitrary waveform generator differs to that of a function generator in that specific points in the waveform can be programmed to create custom waveforms.

The Z+ series of programmable power supplies allows the storage of up to four arbitrary waveforms in internal non-volatile memory cells to control the output voltage or current.  Profiles can contain up to 12 steps and be triggered to operate using the communication interfaces or via the front panel.  Additional waveforms can be stored on a computer.

These arbitrary waveforms can be easily created by using the “Z+ Waveform Creator” application provided on the CD-ROM.

There are two programmable modes; LIST and WAVE. 
LIST allows a step function to be entered and run.  The example in Figure 1 sets the output from 0V to 2V after an external trigger. After a 0.5s delay the output is increased to 4V and back down to 2V 0.5s later.  After 1s the output is increased to 8V for 1s before reducing to 5V. 1s later the output is set to 4V where it remains for another 1s.
 Figure 1: List example
WAVE also allows gradual output voltage or current changes.  In Figure 2 the output is again set to 2V for 1s after an external trigger.  This time it is gradually increased to 4V over a 0.5s time period. It remains at 4V for 0.5s before being programmed to gradually increase to 9V over 0.5s where it remains for 0.5s, before decreasing to 3V over 1.5s period, staying at 3V for 1s.
Figure 2: Waveform
The Graphical User Interface (GUI), which can also be downloaded from the website, contains a Waveform Profile Generator which can be used for more complex waveforms, including sine, triangle and saw tooth. See Figure 3.
Figure 3: Waveform Profile Generator
The arbitrary waveforms can be used for a variety of applications including vehicle battery starting profiles to test automotive components and assemblies.  See Figure 4.
Figure 4: Examples of arbitrary waveforms
For additional assistance, please contact your local TDK-Lambda sales office.




Wednesday, August 22, 2018

External Power Supply Efficiency Standards

Note: As the last EU implementation date has slipped, this is the current situation as of August 24, 2018.  A draft EU amendment dated 2018 indicates that the legislation may mirror more closely the DoE Level VI limits effective April, 2020.

External power supplies (EPS) are widely used in households around the world to charge phones or operate tablets, laptops, game consoles and a variety of consumer electronic and electrical items.

Studies on European energy use of these power supplies indicate that reducing off-load power consumption and improving product efficiency will save nearly 10TWh of power a year.  A Terawatt hour (TWh) is 1,000,000,000,000 Watts used in an hour.  Reducing energy usage saves money, reduces the requirement to add new power generating capacity and reduces environmental pollution.

Two parameters were originally specified to regulate waste energy reduction, which when multiplied by a billion products becomes very significant:

Maximum off-load power consumption

Many power supplies are left plugged in to the AC supply and fully operational; even when the device it was charging has been removed.  Although the power supply is not powering anything, it still consumes power.  The maximum off load power consumption varies with the output power of the EPS and is stated in the specification.

Minimum average efficiency level

With loads less than the full rating of the EPS, the efficiency of the power supply can decrease significantly.  Low load conditions occur as a battery becomes partially charged, or when a device goes into an inactive (sleep) mode.  External power supplies rarely remain in a 75 to 100% load condition.

First legislation was announced in 2004, starting with the California Energy Commission’s (CEC) intent to restrict the sale of non-efficient external power supplies.  The USA, EU, China and a host of other countries and regions followed over the next ten years with increasingly tighter legislation.

In 2014, the EU issued a voluntary Code of Conduct (CoC) version 4, running in parallel with the mandatory Ecodesign Directive 278/2009.  Also in 2014, the Department of Energy (DoE) published their Energy Efficiency Level VI standard. 

In February 2016, the Level VI standard became law in the US.  The EU issued the more stringent version 5 CoC Tier 2 voluntary standard that was supposed to go into effect January 2018. That has been delayed and mandatory implementation was expected early in 2018.

Table 1 shows some of the differences for a single output, basic voltage power supply between these two newest standards. Efficiency limits have been given for a power supply rated between 49 to 250W.  Please consult the relevant US and EU websites for the full regulations.

Table 1: A comparison between the latest US and EU regulations

Average efficiency measurements are required at four load conditions; 25, 50, 75 and 100%.  These are added together and divided by four to produce an average efficiency figure.

Note, the EU has added a minimum efficiency level at 10% load.

The USA now includes power supplies that are rated at greater than 250W.

The term external power supply is defined by the US Department of Energy as one that is used to convert household electric current into DC current or lower-voltage AC current to operate a consumer product.  Currently the US also grants exemptions for supplies powering FDA approved medical devices.

Modifying a power supply to meet a decrease in off-load power consumption is not a simple component change.  The control IC often has to be changed to one that draws less power and energy saving techniques like pulse skipping implemented.  Changes to primary side circuitry requires resubmitting the products to have the safety certifications updated.  Both the redesign and submittal process are expensive and older products are often obsoleted as a result of this.  As the energy standards are directed at the ultra-high volume, short production life consumer EPSs, this gives some relief to the industrial market which prefers long production lifecycles.

TDK-Lambda has a number of external power supplies in the DT series that meet all the latest requirements, with ratings up to 300W.

Wednesday, June 27, 2018

An update to the transition from IEC 60950-1 to IEC 62368-1

On a blog post back in March 2015, I mentioned that a Hazard-Based Safety Engineering (HBSE) IEC 62368-1 standard will be replacing IEC 60950-1 and IEC 60065, covering hazards and hazard prevention for ITE (Information Technology Equipment) and audio / visual equipment.

The transition date to EN 62368-1:2014 has now been extended 18 months to December 20th, 2020 for new submittals (or a significant modification of the existing 60950/60065 file) in the EU.  UL has also announced that the UL 62368-1 Edition No. 2 date is now December 20th, 2020. A number of other countries have adopted the standard, including Australia, Singapore, Malaysia, Japan and Russia.  No exact date has been published for other countries like China, Taiwan, South Korea and Argentina.

The new standard uses a Hazard Based Safety Engineering (HBSE) science discipline, and operates in four steps:

1.      Identifying energy sources in the product

2.      Classifying the energy for the potential of causing injury or harm

3.      Identify the necessary safeguards needed to protect from those energy sources

4.      Qualify the safeguards as effective

The Energy Source ES classification handles the effect on the body for a number of hazards.  See Table 1 for example of the two most relevant to power supplies – electrical and thermal energy.

Energy Source class
Effect on the Body
Effect on combustible material
Class 1
Detectible, but not painful
Unlikely ignition
Class 2
Painful, but not an injury
Possible ignition, but limited
Class 3
An injury
Likely ignition, growth rapid & swift

Table 1 Energy Source classification

In addition, a three person scenario has been adopted for 62368-1 (see Table 2).            

Description (See standard for full details)
Ordinary Person
A person who is a user or is close by to the equipment
Instructed Person
A person that is trained to identify sources of pain causing energy and avoid them. Must not be exposed to injury causing energy sources, even during a single fault condition
Skilled Person
A person who has the training to recognize & avoid energy sources that could cause pain or injury.  Must be protected against accidental contact with injury causing energy sources

Table 2: Description of types of person

Depending on the Person type and the Energy Class, Safeguards have to be in place to protect that Person.  See Figure 1.

Figure 1: Protection for an Ordinary Person with different Energy Classes

As an example, one small change power supply manufacturers may have to make to their designs to meet IEC 62369-1, concerns “capacitance discharge” of the AC input. 

Figure 2 shows the typical schematic of an EMI filter section of a power supply and the location of a resistor that discharges the Line to Neutral X capacitors after the AC has been removed.  This avoids a user unplugging an AC plug, touching the pins and getting an electric shock.

Fig 2: EMI Filter Schematic

If the line cord was removed from the AC plug at the peak of a 240V AC cycle, then the voltage would be 339Vdc.  The time for that to decay to a safe voltage depends upon the values of the X capacitor(s), the Line to Neutral resistor and any loading of the power supply converter circuitry.

IEC 60950-1 states that after one second that voltage has to be at a maximum of 42.4Vpk.  IEC 62368-1 states that for an X capacitance of 300nF or greater, after two seconds, the limit is 60Vpk in normal condition or 120Vpk in a single fault condition.

A 10W power phone charger will have a very small X capacitance and a high value discharge resistor. Under 62368-1, if that resistor failed (a single fault condition) the power supply would probably still meet the 60Vpk condition after 2 seconds.  From Figure 1, this still complies with no Basic Safeguard is needed, even for an “Ordinary Person”.

A higher wattage power supply on the other hand, will have a much larger X capacitance.  With a single fault condition, the loss of the bleed resistor may result in a voltage of greater than 120Vpk after 2 seconds.  That would then be an ES2 - Class 2 Energy Source - and from Figure 1, a Basic Safeguard would need to be in place.  Instead of just one bleed resistor, a second one would need to be added, in parallel with the first.  The values of the resistor would be sized accordingly to ensure that the loss of one, would still discharge the X capacitor to a maximum of 120Vdc after 2 seconds.

Supplementary Safeguards would be used in other parts of the power supply circuit in the same way as reinforced or double insulated is in 60950-1.

One side effect of the ES1 and ES2 limits is that IEC 62368-1 deems a 48V or even a 36V output power supply in hiccup mode over current as an AC output.  The ES1 limits for DC are 60V, but for up to 1kHz that drops to 30Vrms or 42.2Vpk.  IEC 60950-1 recognizes that as SELV, but under the new standard that would be ES2 and the output should not be made accessible to an Ordinary Person.  Pins and connectors not accessible by a blunt probe are exempt.

Power Guy

Popular Posts

TDK Corporation - Americas | Press Release