What type of LED driver or power supply do I need?

Conventional AC-DC power supplies and DC-DC converters provide an output that is regulated to provide a “constant-voltage.”  However, LEDs work most efficiently and safest with a “constant-current” drive.  As a result, many new devices have been developed to provide this type of LED drive.  LED power sources that provide a “constant-current” output have typically been referred to as LED drivers.  In the past, AC-DC power supplies that provided a regulated “constant-voltage” to LEDs were referred to as LED power supplies.  Today, the terms “LED driver” and “LED Power Supply” are used interchangeably.  The important thing to keep in mind is whether the output of the power device provides a “constant-voltage” or a “constant-current.” 

When do I need a “constant-voltage” LED driver?
Most commercially available LED “light modules” are constructed by connecting a number of LEDs in series or parallel to form cluster or string configurations.  In cases where these light modules include a “constant-current” driver as part of the assembly, an external “constant-voltage” driver or power supply is required.  Some LED circuits control the current flowing through the LED with a simple resistor.  This is another case where a constant-voltage power source is required.  Other examples where external “constant-voltage” supplies have been employed include backlit ad signs, traffic information signs and large screen high definition LED displays, such as those described in this article: http://www.ledsmagazine.com/products/20877.  Constant-voltage drivers come in many different forms.  They can look like a conventional power supply or they can be enclosed for moisture/environmental protection. 

When do I need a “constant-current” LED driver?
In cases where a manufactured cluster or string of LEDs does not include an internal “constant-current” driver, an external LED driver or power supply that provides a “constant-current” is required.  Constant current LED drivers are available in many different package configurations, ranging from integrated circuits to enclosed moisture-proof packages, depending on the application and the required output power.  

Series and Parallel LED Configurations
Depending on the application, LEDs can be connected in series and/or parallel configurations.  Obviously, when LEDs are connected in series the forward voltage drop of each LED in the string are additive.  For example, if you put 15 LEDs in series and each one has a voltage drop of 3V (at its nominal current), you need to provide a voltage source of 45V (15 x 3V = 45V) to drive the required current.  This is why “constant-current” drivers always include in their specs the output voltage range that it is capable of providing to overcome the LED voltage drops.  In order to limit the drive voltage to reasonable levels, multiple strings of series-connected LEDs can be placed in parallel and driven by multi-output constant-current drivers.

Below is an excerpt from the datasheet for TDK-Lambda’s ALD6 series of LED drivers. As you can see from the diagram, this driver contains up to 6 independent “constant-current” LED drivers.  The 38V output corresponds to combined forward voltage drop of 10 typical white LEDs connected in series.  For high-current applications, up to 300mA is available to power one series-string of high brightness LEDs.  For applications where the LEDs require up to 50mA, this device can power up to 6 strings of LEDs via its multi-output drivers.  These drivers are ideal for LCD display backlighting and general LED lighting applications.

Click to enlarge

How is LED dimming accomplished?
The light output of LEDs can be controlled by varying the amount of current flowing through the LED (within defined limits) or by turning the LED on and off via pulse width modulation (PWM).  LED drivers like the ALD6 series have the capability of providing “dimming” by both of these popular methods.

The drawing above shows the two methods of light dimming that are included in the ALD6 LED driver.  It is permitted to use a combination of both of these methods simultaneously.

The “Rbr” is an external variable 10kohm resistor input.  By varying this potentiometer from 1k to 10kohms, an analog dimming control is achieved. In this case, the maximum LED brightness occurs when the pot is set to 10k ohms.  This same input can operate with variable analog voltage ranging from 1.6 to 3.8-volts.  In some applications this input can be connected to a temperature sensing device which could reduce the current flow through the LEDs as the temperature rises, thus providing a means for temperature compensation.
The “Vpwm” is a “Pulse Width Modulation” input that controls the LED brightness by varying the duty-cycle of the input signal from 1% to 100%.  Typical PWM frequencies can range from 180 to 270 Hz.

More information about LED drivers/supplies can be found at these web links:
http://us.tdk-lambda.com/lp/products/ledsigns.htm
http://us.tdk-lambda.com/lp/products/ald-series.htm
http://power-topics.blogspot.com/search/label/LED%20lights

What does a BF rating on a power supply mean?

TDK-Lambda recently launched the EFE-M series, a medically BF rated power supply.  It immediately sparked the question from my colleagues – “What is a BF rating?”  To answer this question we need to start with the term “Applied Part.”

IEC 60601-1 is the international medical electric safety standard that uses the term “Applied Part” to refer to a part of a medical device which may come in physical contact with the patient during its normal operation.

Applied Parts fall into three classifications according to the nature of the medical device and the type of contact.  Each classification must have a different protection level against electrical shock.

Type CF (“Cardiac Floating”) is the most stringent classification, and is used for applied parts that may come in direct contact with the heart, such as dialysis machines.

Type BF (“Body Floating”) is less stringent than Type CF, and is generally used for applied parts that have conductive contact with the patient, or having medium or long term contact with the patient.  Examples of this type of equipment are blood pressure monitors, incubators and ultrasound equipment.

Type B (“Body”) is the least stringent classification, and is used for applied parts that are normally not conductive and can be immediately released from the patient.  Examples of that would be LED operating lighting, medical lasers, MRI body scanners, hospital beds and phototherapy equipment.

Type B applied parts may be connected to earth ground, but Type BF & CF are separated from earth – hence the term “floating”.

Power supply Isolation Voltages vary according to the type rating.

Type	Input to Output Isolation	Input to Ground Isolation	Output to Ground Isolation
B rated	4000VAC	1500VAC	500VAC
BF/CF rated	4000VAC	1500VAC	1500VAC

Please note: power supplies are not medical devices or applied parts, and the outputs of power supplies should never be connected directly to a patient.

Many medical devices contain medical-rated power supplies. However, only the part of these “medical devices” that may come in contact with a patient during normal operation is classified as an “Applied Part.”

Power Supply Losses and the Impact of Rising Efficiencies

When comparing two power supply efficiency specifications, for example, one with a 90% efficiency and the other with a 94% efficiency, there is a tendency to think “there’s only a 4% difference.”

However, reviewing the wasted power (as heat) between the two supplies reveals a more dramatic difference.

As a reminder, the formula for the efficiency of a power supply is:

(Output Power ÷ Input Power) x 100 = Efficiency (%)

                         

And the wasted or lost power within a power supply, due to its inefficiencies, is calculated as follows:

(Output Power ÷ Efficiency) - Output Power = Wasted Power (Watts)

A. Let’s see what the wasted power or losses would be within a 400W power supply that is 90% efficient:

(400W ÷ 0.90) - 400W = 44.4W (wasted power)

B. Now, let’s compare the same 400W power supply if it’s 94% efficient:

(400W ÷ 0.94) - 400W = 25.5W (wasted power)

The above calculations (A & B) demonstrate that a 90% efficient power dissipates or wastes an additional 19W internally compared to a 94% efficient unit (44.4W – 25.5W = 18.9W).  Imagine that this extra 19-watts is a large power resistor within the power supply, radiating heat and negatively affecting thermal management, component derating, and the resultant MTBF and actual field life for the power supply. The payback for employing high-efficiency power supplies now becomes readily apparent.

Chart 1 below shows the internal power losses (wasted power) versus efficiency for the 400W power supplies described above and for efficiencies between those mentioned. From this chart you can see that if a company claims (exaggerates) a 94% efficiency rating, but in reality only achieves 92% they have to ensure that their internal components can operate correctly with an extra 9.3W of heat dissipation (34.8W – 25.5W = 9.3W).  And, as mentioned previously the actual field life of the supply will be compromised. 

Chart 1

Chart 2 below shows the percentage Energy Savings of a 94% efficient unit compared to a baseline of a 90% efficient unit.  By improving the Efficiency by just 4%, it results in nearly a 43% energy savings!  The math from above calculations: [1 - (25.5W ÷ 44.4W)] = 42.5%!

Chart 2

The takeaway is, purchase power supplies from a reputable power supply company that employs conservative component deratings and states realistic efficiency ratings.

Class 2 or Class II power supplies?

One question I am frequently asked is: “The customer is looking for a Class two power supply; what can you offer him?”

My response is always “Class 2” or “Class II (with Roman numerals)”, or both?  The pause on the end of the phone signifies an explanation is in order.

Class 2 is a classification referring to the NEC – National Electric Code.  To avoid potential cable overheating due to excessive currents and electric shock, the output of the power supply is limited to 60VDC or 100VA, (100W when used with an AC-DC power supply).  You will often see 24V output DIN rail power supplies or LED drivers rated at 91W rather than 100W because if the power supply is overloaded, any tolerance in the over current protection has to be accounted for.

Often these products will be certified to UL1310 and will list this in the datasheet.  An example of this is TDK-Lambda’s DSP series.  You can see from the model selector list on page 2 of the DSP datasheet that output currents of 4.2A or greater are not approved to UL1310.

Class II (with Roman numerals) refers to power supplies with either a double or reinforced insulation barrier between the input and the output. Class II supplies do not rely on an earth connection to protect against shock hazard. Many cell phone chargers and laptop power supplies are Class II.  TDK-Lambda’s DSP series also are Class II, having just a Line and Neutral AC input without a ground connection.

A Class II power supply rating label will show this symbol:

One advantage of Class II is better surge protection between input and ground and usually a lower earth leakage current.

For more information about leakage current, please see another article about power supply leakage current testing.

Where’s the CSA logo on my power supply?

As I sat here at the end of March pondering what my April blog article was going to be about, I had an email from one of our sales people.  Her customer had purchased one of our SWS series of power supplies based on our data sheet, and could not see the CSA certification mark on the product label, just the CE, TUV (the triangle) and the UL (recognized) marks.

Looking at our data sheet though, it clearly claimed the product had CSA 60950.

(Click to enlarge) You can see the safety approvals

This prompted the email to me!

In 2003, UL & CSA drafted a bi-national agreement to recognize each other’s testing and certifications.  UL can now cross certify to CSA 60950 and likewise CSA can certify to UL 60950.  This avoids manufacturers from having to pay and maintain two separate certifications.

If the product was certified by UL for both countries it would have this mark (often referred to as “cUL”.

If the product was certified by CSA for both countries it would have this mark  

As the SWS power supply has the “cUL” mark, it is certified to CSA 60950 (or to be fully correct CSA C22.2 No. 60950-1-07) and that is stated on the UL test report.

How does the AC Fail signal work in a power supply?

I was recently talking to one of our Design Engineers about my blog and he suggested a clarification of the operation of the AC Fail signal would be a good topic.  He stated that he was often asked “at what input voltage does the AC Fail signal operate?”

A power supply’s AC Fail signal is used to provide a warning to the user that the AC input power has either been lost, or is dropping in voltage to a point that the power supply will soon no longer be able to regulate or provide power.

Customers using such a signal will then have a short period of time (typically 5 to 10ms) in which to store any data or start an orderly shutdown of their system.

Internal to the power supply, the AC Fail circuit is usually a simple circuit comparing a reference with the voltage of a primary side housekeeping supply.  In the event that voltage drops, drive is removed from an opto-coupler and the user provided with an AC Fail signal state change.

Before the widespread use of Power Factor Correction (PFC), the AC Fail did indeed operate at a set input voltage.  I remember as a Test Technician reducing the input voltage with a variable transformer (variac) to check the function.

On those non PFC power supplies, the AC input is peak rectified as shown below.  The main switching converter operates off that unregulated high voltage buss, the value of which is a direct function of the AC input voltage – between 120 and 375VDC.

Click to enlarge

On power supplies with PFC though, that high voltage buss is regulated using a boost circuit – to around 360VDC.  Now any change to the AC input voltage (within the normal operating range) is not reflected in a change in the DC buss; hence a different test method must be used.

Use a storage oscilloscope to monitor the output voltage, the AC Fail signal and if desired, an isolation transformer to display the AC input voltage.

Turn off the input voltage and measure the time between the AC Fail signal going low and the output voltage starting to drop.  This is the amount of warning time you will have.

Unlike with a non PFC power supply, this warning time will not be related to input voltage.

Inrush Currents & External Fusing on Power Supplies

Most power supplies have some form of an internal inrush current limiting circuit.  This avoids a large current being drawn when AC is first applied, causing a circuit breaker to trip or an external fuse to blow.

The power supply inrush circuit usually consists of a thermistor in series with the AC line.  This thermistor has a high resistance when cold, but once the power supply has turned on; its self heating effect drops the resistance to reduce losses (increasing the power supply efficiency).

See my previous post at http://power-topics.blogspot.com/2010/04/cold-temperature-start-up-of-low-cost.html

 A typical inrush current plot for 115VAC input looks like this:

Click to enlarge

You can see the AC is applied at the peak of the AC input voltage to measure worse case conditions.  The peak inrush current is 25.65A for a period of 2-3ms with what we call a cold start, in that the inrush thermistor is initially at room temperature (and in a high resistance state).

If we were to expand the time scale, on top of that peak would be a larger spike of current with a pulse width of less than 200μs, generated by the “X capacitors” charging up.  X capacitors are fitted across the input to reduce electrical high frequency noise from exiting the power supply.  As this is a low energy spike, most power supply manufacturers exclude it from the inrush current specification.  The energy drawn is so small it will not trip a circuit breaker, or blow a fuse.

Many customers are confused with the external fuse rating suggestion found in the installation manual.  They see from the power supply datasheet that the inrush current is say 30A, but then read from the application note that the recommended external fuse is only 4A (which corresponds to a steady state input current draw by the power supply of around 2A).

This prompts a call to our technical support group saying that they believe there is an error in the application note.

Most of those application notes specify the use of a time delay, or "slo-blo" fuse.

Looking at Littlefuse®'s datasheet for such a 4A fuse we can see from the graph that the average time for the fuse to open varies with the length of time the current passes through the fuse.

Click to enlarge

Going back to the power supply evaluation data, one can see that the inrush current is a maximum of 25.65A and that the time for that pulse is say 3ms (worst case).  From the above graph, even at 10ms (0.01s) the current would have to be some 70A for the 4A fuse to blow, giving an adequate design margin.

If a fast acting fuse (type F) had been chosen, the pulse current for the fuse to open would be approximately 30A, which is why we recommend that slo-blo (type T) fuses be used.

Power Supply Efficiency – How Important is it?

ENERGY STAR®’s decision to “sunset” programs for EPSs (External Power Supplies), and applications using them, has raised the question “Does this mean that power supply efficiency is no longer a concern?”

In a mainly cost driven power supply market, product marketing is often challenged during a new product business plan review, regarding the demand for high efficiency on the product specification.

The biggest requestor of high efficiency power supplies are the manufacturers of large data centers, primarily because of the huge amount of electricity they consume.  In 2007 the EPA estimated that the national annual electricity cost for servers and data centers could be a staggering $7.4 billion in 2011.

Power supply manufacturers have been told by data center producers that they are willing to pay (a little) more for higher efficiency products, because the operators are aware that the ROI is quite short.  TDK-Lambda also is participating in the evaluation and use of DC power systems. 

DC power systems for data centers use up to 15% less power by removing one or more conversion stage by supplying ~380VDC to the servers rather than high voltage AC.

Current UPS Backed-up System

DC Bus System

The US government has had the 80 Plus program in place for servers and computers for a number of years with a variety of efficiency levels ranging from a basic level to “platinum”.

In consumer electronics where although the power draw is quite small (particularly in standby), there are 100s of millions of devices deployed (laptop power supplies, phone & camera chargers & LCD TVs to mention a few) and the overall energy usage is considerable.  The main drivers for efficiency improvement are the state and utility companies, because of the cost and time to build new power stations.  The average consumer is not that concerned as it will not dramatically affect their electricity bill.

As an industrial power supply manufacturer, TDK-Lambda is being asked by some customers for higher efficiency power supplies.  These customers are selling to end users like hospitals or large retailers who are going "green".  Where the purchaser is not being mandated to reduce energy consumption or is driven by price, the efficiency “sell” usually fails.

TDK-Lambda's R&D is very focused on developing higher efficiency power supplies, driven by what we see as a future market and TDK's initiative on environmental consciousness.  Current high efficiency products include the new EFE series of power supplies that feature 90% efficiency using digital control.  http://us.tdk-lambda.com/lp/products/efe-series.htm.

TDK-Lambda also supplies two of the major automotive manufacturers the DC-DC converters for hybrid electric vehicles and is developing new products for a host of upcoming electric cars.  Efficiency standards of automobiles is something that the purchaser understands, primarily because of the cost of running a vehicle is significant.

Using the Inhibit or Enable function on Power Supplies

The inhibit or enable function allows the user to electronically turn on or off the output voltage of a power supply without having to interrupt the input AC or DC voltage with a relay or switch. This is useful during initial set up of the system, during maintenance or for saving energy during periods of non operation.

An easy way of remembering the difference between the two types is that “Inhibit” requires that the user has to do an action to turn off the output voltage, where as “Enable”, the user has to do an action to turn on the output voltage.

It is standard with most DC-DC converters for example, to utilize an enable type function that requires the remote on / off pin to be connected to the negative input (primary side) to activate the output voltage. This is often referred to as “negative logic”. First time users of DC-DC converters often forget to pull that pin low and call Tech Support to complain about a non functioning power supply.

AC-DC power supplies usually have a remote on / off referenced to the secondary side for safety reasons. An “Inhibit” type function, requiring an external voltage, is usually more popular on simple power supplies because once the output voltage is turned off, any auxiliary voltages driving the secondary control circuit is also turned off.

One way power supply designers overcome this is to have an integrated independent “stand-by” auxiliary output like the one used on pc power supplies. This is also used to power the secondary control circuit and allow a closed contact to the 0V terminal to enable the output.

If the system uses several power supplies, the remote on /off can be used to sequence the voltages. I know of thermal printer applications where if the 5V supply driving the control circuit goes faulty, they require the 24V motor drive be inhibited to avoid embarrassing amounts of paper shooting out!

Where several different voltages are used to drive processors, sequencing the output voltages is often critical to avoid damaging those devices. The 3.3V output is rarely allowed to be applied before the 5V is present.

The industry standard I²C based PMBus is now gaining popularity. Power supplies such as TDK-Lambda’s HFE series can be remotely turned on or off using the PMBus software, either as a group or individually for load shedding to save energy.

Damaging Power Supplies with Repetitive Peak Current Draws

There are many devices that require peak currents when first turned on including print heads, motors, disk drives and pumps.

Many users often do not measure the actual peak current and rely on an empirical method whereby they try a power supply in the application to see if it will work.  If the power supply cannot provide enough peak current, capacitors are added to the output.  Those capacitors will act as temporary energy storage, enough to deliver load current for a few hundred micro seconds.  If that works then the power system solution is deemed as working, and the Engineer moves on to the next phase of their project.

Many power supplies have the ability to supply high peak currents, even though the datasheet does not mention it.  In fact some of the cheapest power supplies on the market can deliver very large currents for a short period of time because the output current limit is very crude, and is primarily there to protect the unit against a short circuit on the output.

In discussion with TDK-Lambda Engineering, I learned that this can lead to field failures.  Let me explain further.

Below is a schematic of a forward converter, the power FET is shown as a switch for simplicity.  That “switch” operates at a rate usually in the hundreds of kHz, energy is transferred from the secondary side to the output rectifiers and then is smoothed by the output LC filter.

When a pulsed (peak) load is applied to the power supply in excess of its rated current, the energy is first drawn from the output capacitor.  This can add to the capacitor ripple current, raising the temperature and reducing the component’s life.  Heat, as I explained in earlier blogs, dries out the capacitor’s electrolyte.

When the energy stored in the capacitor starts to deplete, the power supply will then try to continue to provide the peak current from the main switching circuit.  This in turn leads to repetitive surge currents in both the output diodes which is then reflected by the transformer to the power FET.  Often this peak current exceeds the maximum rating of the semiconductors leading to latent and erratic field failures.

Additional heating in the transformer, inductors and printed circuit board traces is also experienced because, although the average power drawn from the power supply is less than the continuous rating, we are dealing with the formula I2R and the peak current is now squared.

TDK-Lambda recommends using a power supply that has a specified peak power rating like our HWS-P series or working closely with the power supply manufacturer to determine if the product is suitable for the application.