Monday, December 5, 2011

What is a Power Supply’s IP Rating?

The popularity of outdoor electronics has brought the subject of a power supply’s IP rating from almost obscurity to an everyday question.  I frequently get asked about it by our sales people now, so I thought it would be nice subject to cover in our blog. In researching this blog article I even discovered something new myself.

IP is the acronym for Ingress Protection and for power supplies the IP Rating Code consists of the letters “IP” and two numbers as defined below.

The first number indicates the power supply’s protection level against the ingress of solid objects or dust.


First Number for Solids or Dust

LevelSize of ObjectType of Object
XTest not madeTest not made
0N/ANo protection
150mm or largerLarge body surfaces*
212.5mm or largerFingers
32.5mm or largerSmall tools
41mm or largerScrews
5Dust protected-
6Dust tight-
* Does not include deliberate body part contact

The second number indicates the power supply’s protection against the ingress of water or other liquids.


Second Number for Liquids

LevelProtected against
XTest not made
0No protection
1Water dripping vertically
2Water dripping at an angle
3Spray water up to 60° from vertical
4Splashing water from any angle
5Low pressure water jets
6Strong spray jets, heavy seas (ship decks)
7Temporary immersion (up to 1m)
8Permanent immersion (deeper than 1m)

Most recently LED power supplies, or drivers as they are often referred to, have ratings of IP66 or higher.  Referring to the charts above, an IP66 rating means the unit has ingress protection from Dust and Strong Jet Sprays of Water.

These IP ratings also apply to the end system of course, and many of our customers utilize a NEMA enclosure to make their products meet a higher rating.


The IP rating is usually stated in the Safety Certification reports and the installation manual


Thursday, November 3, 2011

Mounting Precautions for Power Supplies

Before mounting your power supply, be sure to read its installation manual if you intend to mount it in an orientation other than along the horizontal plane (Fig. A). Many power supplies have restrictions regarding mounting. For example, since heat rises, if you mount some power supplies on a vertical plane (Fig. B, C, & D), the heat from the lower section of the power supply will rise and further heat the upper part of the supply, which may cause over heating problems. Likewise, with some power supplies you are not allowed to mount them upside down (Fig. E) because this traps the heat and restricts the normal convection air cooling around the power supply.
In some cases, vertical mounting of power supplies is permitted as long as you reduce the amount of power that will be drawn from the supply. This is referred to as “Derating” the power supply. Below are the derating curves for the TDK-Lambda’s model LS150-12, a convection cooled 150-Watt, 12V output, AC-DC power supply.

This graph shows the percentage of rated output power on the vertical axis and the operating ambient temperatures on the horizontal axis. Notice when mounting this power supply on the horizontal plane (Fig. A), the power supply is rated at 100% output power from -25°C up to +50°C. However, if you mount this supply on a vertical plane (Fig. B, C, & D), the maximum ambient temperature is reduced to +40°C before the power must be derated.

It is worth mentioning that many low cost competitors do not mention the preferred mounting orientation, and some do not even have an installation manual on their website!

An incorrectly mounted power supply will get too hot resulting in premature electrolytic capacitor degradation, catastrophic semiconductor failure or even a fire due to transformers overheating.

Other general power supply mounting considerations include the following:
  • Make sure there is adequate space around the power supply to allow air to circulate.
  • Do not block off vent holes on convection cooled supplies or restrict air inlet or outlet ports on fan cooled supplies.
  • In the event fans are employed within power supply, a system, or an enclosure make sure the airflow direction for all fans are the same

Tuesday, October 4, 2011

How to safely power LEDs

For well over 25 years, LEDs (Light Emitting Diodes) have been used in TV remote controls.  These specific LEDs emit invisible light pulses in the infrared (IR) light spectrum.  Because the LED can be turned on and off very rapidly, it easily transmits pulses of binary-coded messages to the receiver built into the TV.  


In addition, early applications of LEDs included red-segment clocks, calculators and even digital watches that have now been replaced by more modern display technologies, such as LCDs (Liquid Crystal Displays).
       
Today, white or multi-colored LEDs are rapidly being employed in modern home/street lighting, signage, traffic signals, large screen displays and backlit LCD monitors, etc.  In these applications, multiple LEDs are placed in either clusters or connected as strings to provide the required light intensity or light distribution. 

LEDs are similar to conventional diodes in that they are designed to conduct current in one direction and when doing so, in most cases, they emit visible light.  A basic LED circuit consists of a voltage source, a current limiting resistor and the LED as shown below.

The current limiting resistor (R) is required to maintain the current flowing through the LED at a safe operating level.  When conducting current LEDs have an inherent “voltage drop” that can vary from 1.2V to 4.0V, depending upon the model.  Referring the circuit diagram, if the LED has a voltage drop of say 2V (Vd) with a safe operating current of 20mA (I), and the voltage source (Vs) is 5VDC, the value of the current limiting resistor can be calculated as follows:

R = (Vs –Vd) ÷ I, therefore, R = (5V – 2V) ÷ 0.02A = 150 ohms

The voltage drop across an LED and its light output will vary with the current flowing through it.  Below are curves that show the forward voltage drop (Vd) versus the current (I) flowing through two sample LEDs.
 


In viewing the white curve above, it’s important to notice that the forward voltage drop across the LED between 3.2V and 3.6V (a 0.4V change), results in a current increase of over five times (from 10mA to 60mA).  In this example, if the maximum allowable LED current is 40mA and if 60mA or more current is allowed to flow through it, the LED could be destroyed or its operational life substantially reduced. 

As current flows through an LED its forward voltage drop times the current results in wasted power (e.g., 3.3V x 40mA = 132mW).  This wasted power, in the form of heat, becomes a real problem when high brightness LEDs is employed in lighting applications.  The internal LED heat must be dissipated by either its design, the substrate it’s mounted on, or via added heat sinks.  As the internal junction of an LED gets warmer, the current through it at a given voltage increases.  If not controlled, this can result in thermal runaway, where the LED self-destructs.

The main point here is that LEDs are “current driven” devices and that this current must be carefully controlled.  In the circuit above, the resistor is used to control the current though the LED.  However, the resistor also causes a voltage drop which contributes further to wasted power.  As a result “constant-current” LED drivers have been developed that maintain the current flowing through the LED (or strings/clusters of multiple LEDs) at a safe level with improved efficiency.

For more information about selecting power supplies and drivers for LEDs, see the article at this web link:  http://power-topics.blogspot.com/search/label/LED%20lights

References:

Friday, September 9, 2011

Using Power Supplies with Brushed and Brushless DC Motors

There is often confusion regarding the use of external diodes when power supplies are used to power DC motors.  Most people know that a diode has to be used, but are unsure where to place them or what their purpose is.  
From a power supply concern there are two types of DC motors; a brushed DC motor and a brushless DC motor.

Brushed DC motors
With this type of motor, the magnets are stationary and the coil spins.  Electricity is transferred to the spinning coil by the use of “brushes”.
The advantages of this type of motor are low initial cost and easy speed control.
When the power is interrupted, the motor coil will act like an inductor and will try to continue to produce current, effectively becoming an inverted voltage source.  This will apply a reverse polarity to the power supply and can cause damage.  (Back EMF – Electro-Magnetic Flux)
By using a diode, as shown below, the diode provides a current path for the reverse motor current and will clamp the reverse voltage to a level no greater than the forward voltage drop of the diode.  This protects the power supply’s output capacitors and other components from being stressed by the reverse voltage.
Brushless DC motors
Brushless DC motors, often referred to as BDCMs or BLDC motors, have permanent magnets that rotate and the armature is fixed.
Although more expensive, they are more reliable in the long term as there is no brush or commutator wear and position control is more accurate.
When the motor is turned off or reversed, it will act as a generator and produce a high voltage spike.  This spike can cause the power supply’s overvoltage protection to trip, shutting down the unit.
By using a diode in series with the output, as shown below, the spike will be blocked from interfering with the power supply.

In both cases a general purpose diode can be used, providing that the voltage and current ratings for the diode are correctly calculated.

Tuesday, August 2, 2011

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

Wednesday, July 6, 2011

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.

TypeInput to Output Isolation Input to Ground IsolationOutput 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.”

Tuesday, June 7, 2011

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.

Monday, May 23, 2011

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.

Friday, April 1, 2011

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.

Wednesday, March 2, 2011

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.

Monday, February 7, 2011

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.

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