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

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.

 

 

 

 

 

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.

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 20^th, 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).            

Person	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

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

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 1/16^th 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%.

See http://www.us.tdk-lambda.com/lp/products/i6a-series.htm for further information.

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.