What is a Limited Power Source?

There are many technical details regarding Limited Power Sources (often referred to as an LPS) covered in the IEC60950-1 safety standard, involving a variety of applications, but I will cover just the basic aspects regarding AC-DC power supplies in this blog article.


Several safety standards refer to IEC60950-1 for a wide range of applications, and one of those referrals involves the use of Limited Power Sources.

What is, and why are Limited Power Sources important? Simply put, if a piece of electrical or electronic equipment supplying DC power to external devices is to be installed by a third party, such as an electrician, the risk of wiring fires & electrical shock needs to be minimized. That electrician will not be expected to know all the potential fault scenarios and use the appropriate cable thicknesses and insulation to cover those hazards. By using a Limited Power Source, the system wiring can also be reduced, saving cost.

If a Limited Power Source is used, then the electrician’s job is simplified, even if there is a (single) fault inside of the power supply.

Some examples of testing that the power supply manufacturer will do, to determine the maximum output current and power meet the “Limited Power Source” requirements are:

1. Check the power supply at maximum rated load
2. Increase the load of the power supply until it is on the verge of overload
3. Simulate an internal fault by shorting the current-limiting resistor
4. Short out the opto-coupler that provides feedback to the control loop

The conditions for a “Limited Power Source” AC-DC power supply are:

1. For a power supply rated at 30V or less, the following must be met even with a single fault condition:
    a. The output current must not exceed 8A
    b. The output power must not exceed 100W
2. For a power supply rated above 30V, but not exceeding 60V:
    a. The output current must not exceed 150 ÷ Vout
    b. The output power must not exceed 100W

This graph shows the limits for Limited Power Sources:

Taking some examples from TDK-Lambda’s DSP series of low profile DIN rail power supplies:

 

The DSP60-5 is rated at 5V 7A – That would meet (1b), but when put into overload it would certainly not meet the (1a) limit of 8A. This model is not listed as being a Limited Power Source.

The DSP60-12 is rated at 12V 4.5A – Overload current would be around 7A which meets (1a) and the maximum power is ~ 84W (12V x 7A) which also meets (1b). This model is listed as being a Limited Power Source.

The DSP100-24 is rated at 24V 4.2A – Right out of gate this unit exceeds 100W, and so it is not listed as a Limited Power Source.

The DSP100-24/C2 is rated at 24V 3.8A – This model actually has a special current limit & over power circuit which strictly limits the output current and power under a fault condition, so it meets the <8A requirement of (b) and the limit of <100W. Interestingly though, a single fault on the control circuit made the output rise to 30.8V and so it now falls under the 30-60V limit of 150 ÷ Vout = 4.8A maximum current (2b), which it passed. Therefore, this model is listed as a Limited Power Source.

 

Test results performed by the safety bodies, such as UL, CSA or TUV, are normally found in the CB report for the power supply.

 

Minimum loads and cross-regulation on multiple output power supplies

One subject that our Technical Support team frequently gets asked about is minimum loads on multiple output power supplies, so I thought this would be a good subject to write about.


On a low cost, low power, multiple output power supply, the datasheet will often state that to maintain regulation, a minimum load has to be applied to one or more of the outputs.

To explain why, here is a block diagram of one such simple triple output power supply.

On the middle right hand side of the diagram are the three output windings from the transformer.

On Channel 1 (+5V), the output from the transformer is rectified and filtered to provide a smooth, DC output. If that output voltage is not at the set voltage, say due to a load change, the power supply will automatically correct itself. It does this by sensing the output voltage, comparing it to an internal reference, and feeding back a signal to the control circuit via the opto-coupler. The control circuit will then adjust the pulse width of the converter accordingly. The regulation on this output is typically 1 to 2%.

On Channels 2 & 3 (+V and –V) though, it can be seen that there is no feed back to the control circuit. These outputs are referred to as “semi-regulated”. If Channel 2’s load were to increase for example, the output would drop slightly, but there would not be any automatic correction. That voltage drop is specified by the load regulation specification, typically 3 to 5%.

With respect to minimum loading, if there is little or no load on Channel 1, the output will still be at the set voltage, but the switching converter pulse width will be very narrow. The output voltage on Channels 2 & 3 drops fairly dramatically at those narrow pulse widths, particularly if the outputs are supplying their full rated load. An output voltage of 12V may drop to 8V.

Conversely, if the full rated load is applied to Channel 1, but Channels 2 & 3 are not loaded, the voltages on 2 & 3 will rise, and a 12V output could deliver over 14V.

The effect that varying loads on Channel 1 has on the “semi-regulated,” Channels 2 & 3, is many times referred to as the “cross regulation” specification.

Manufacturers of power supplies specify a minimum load requirement on Channel 1, usually 10%, to warn the user. Minimum loads may also be specified on Channels 2 & 3 to promote a better regulation specification.

Operating without a minimum load will not normally cause a power supply to fail, but can stress the user’s equipment.

Some products like TDK-Lambda’s MTW series employ two converters to improve power supply regulation, one to supply Channel 1 & one to supply Channels 2 & 3. Note that both V2 and V3 are sensed by the control circuit.

Fig. 2: Two converters (click to enlarge image)

Here is a link for more details about the MTW series: http://www.us.tdk-lambda.com/lp/products/mtw-series.htm

Other products like the NV175 series employ post regulators on each output, completely eliminating the minimum load requirement. Although this does add cost to the power supply, it removes any concern for the user and helps with system flexibility. Here is a link to the datasheet for NV175 series: http://www.us.tdk-lambda.com/lp/ftp/Specs/nv175.pdf

Wide range adjustable power supplies

SWS1000L Series

We had a customer that wanted a 1000-watt power supply to drive a medical centrifuge.  The only catch was that the speed of the DC motor that drives the centrifuge had to be varied and they were going to do this by using a step down dc-dc converter to change the voltage applied to the motor from 12V to 60V.

We suggested TDK-Lambda’s SWS1000L-60 power supply, which has, as standard, a Programming Voltage (PV) Input.  By applying from 1 to 5.5 volts to this input, the output of the power supply can be varied from 12 to 66V.

The connections are very simple, using the PV input and common terminals on the front panel of the power supply (see diagram).

Alternatively, the Programming Voltage could have been derived from the SWS1000L’s 12V auxiliary output, which in this case could be connected to an external potentiometer to provide the variable dc input.

This simple solution saved the customer both design time and money.  More information about the SWS1000L power supplies can be found at this web link http://www.us.tdk-lambda.com/lp/ftp/specs/sws600_1000l.pdf

The PV function is also available as on option on TDK-Lambda’s HWS series 300-600W models and is standard on the 1000-1500W models. More info can be found here http://www.us.tdk-lambda.com/lp/ftp/specs/hws1500.pdf

Advantages of Conduction-Cooled Power Supplies

Most mid- to high-power supplies use fans to help dissipate the internal heat that is generated as a result of imperfect AC to DC conversion efficiencies.  Since fans are electromechanical devices, they reduce the system’s MTBF and add to the required maintenance expenses.

Attached is a photo of a power supply that operated for many years at a postal depot where mail is handled and sorted automatically.  As can be seen (after the fan was removed) paper fragments and airborne dust contaminants were pulled into the supply by the fan and eventually caused a blown fuse. 

As might be expected, the proper maintenance program for any fan-cooled power supply calls for the periodic inspections of the supply, with the fan removed, and the replacement of the fan with a new one.

A new breed of conduction-cooled power supplies has been developed that do not depend on fans for cooling.  Instead, the required cooling is accomplished by conducting the internal heat loads to an external metal structure or enclosure, which act as a large heat sink surface.


The second attached photo shows TDK-Lambda’s new CPFE1000F series, which are conduction-cooled, 1,000 watt AC-DC power supplies.  (A 500 watt version is also available.)  All heat is conducted to the supply’s aluminum plate, which is designed to easily mount to a metal enclosure or cold plate for cooling.  More details and specifications for these power supplies are at this web link: http://www.us.tdk-lambda.com/lp/products/cpfe-series.htm

In some applications, these conduction-cooled devices are mounted to liquid cooled cold plates that are made of metal with internal serpentine channels through which a liquid circulates while removing the unwanted heat.  The net result is that the system operates with a substantial reduction in audible noise, reduced maintenance costs (no dust build-up and fan wear-out), and an enhanced MTBF.

Recently, I visited a Television Broadcasting Station that consumes about 100 kilowatts of power.  At this location, in separate areas, was a traditional fan-cooled system as well as the latest generation system, which uses conduction-cooled power supplies and RF amplifiers that are cooled via liquid flow cold plates. During the operation of the traditional system with fan cooling, the audible noise was so loud that personnel within 100 feet of the system had to wear hearing protection devices.  By comparison, in the other area where the new system with liquid cooling was operating, the noise level was so low (similar to an office environment) that no hearing protection was required.

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

Level	Size of Object	Type of Object
X	Test not made	Test not made
0	N/A	No protection
1	50mm or larger	Large body surfaces*
2	12.5mm or larger	Fingers
3	2.5mm or larger	Small tools
4	1mm or larger	Screws
5	Dust protected	-
6	Dust 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

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

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

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.

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.