Wednesday, December 5, 2007

What do they mean by Output Power Derating?

All power supplies have a specified “Operating Temperature Range”. For example, TDK-Lambda’s AC-DC switch-mode SWS600L series of 600 watt, single output power supplies have an operating temperature range from “-20°C to +74°C”. However, the spec also states: “…derating linearly to 50% load above 50°C”. What does this mean?

Please refer to Figure 1 below. Most power supply manufacturers provide this type of curve to make it easier for the end user to determine the maximum output power that can provided by a power supply at various operating or ambient temperatures. Ta = Temperature of the Ambient Air, or, the temperature of the air surrounding the power supply, especially the air at the intake of a fan-cooled supply. By comparing the “Operating Temperature Range” specification listed above to the derating curve, the following information can be seen:
  • The supply can deliver 100% of its rated output power load (600 watts) from -20°C to +50°C ambient temperatures
  • Above 50°C ambient, the supply can deliver a reduced amount of power
  • At 60°C ambient, the supply can provide about 80% of its max. rated power (0.80 x 600 = 480 watts)
  • At 74°C ambient, the supply can provide 50% of its max. rated power load (0.50 x 600 = 300 watts)

Figure 1: SWS600L Output Power Derating Curve

In addition to the supply’s normal “operating temperature range” and output derating-curve, some supplies like this one, have a specified low-temperature “start-up” capability (i.e., -40°C). This means that the supply can “start-up” or be “turned-on” with an ambient temperature as low as -40°C (below the -20°C spec) and deliver 100% of its rated power, however, the supply’s output regulation, hold-up time, ripple & noise, and other specifications cannot be fully guaranteed until the supply warms up to at least -20°C. This cold temperature start-up is a nice feature to have, especially for outdoor-mounted applications. Once the supply is turned-on it will usually self-heat due to the heat generated by its internal electronic power components.

Monday, November 26, 2007

What are the differences between Conduction, Convection and Radiant Cooling of Power Devices?

All power devices generate heat. This is due to the unavoidable internal losses of all power circuits due to their inefficiencies. The higher the efficiency rating of the power device, the less internal heat is generated within it. If we could achieve 100% efficiency, there would be no heat generated within the power device and no cooling required.

There are three methods of transferring or removing heat from power devices: These are conduction, convection and radiant. In all cases, the heat is being transferred from the power device to another medium that is at a lower temperature. Heat is constantly seeking to move to any object or medium that is cooler.

Conduction Cooling: This is defined as the transfer of heat from one hot part to another cooler part by direct contact. For example, many DC-DC converters have a flat surface that is designed to mount directly to an external heat sink or cold plate that will conduct the heat away from the power device by direct contact, thereby cooling it. Conduction is the most widely used method of heat transfer. All power supplies use internal heatsinks to help conduct the heat away from the hot devices.

Convection Cooling: This involves the transfer of heat from a power device by the action of the natural air flow (a low density fluid) surrounding and contacting the device. Many power devices are rated for natural convection cooling as long as the air surrounding the unit remains within a limited temperature range that is cooler than the device. The advantage of this method of cooling is that no electromechanical fans are required.

Another type of convection cooling requires forced-air-flow via fans or blowers across the power device. Many power supplies come with a build-in fan to provide this forced air type of convection cooling. Other types of power supplies specify the amount of air flow that must pass through or around the device (in cubic-feet-per-minute) in order for the supply to provide its maximum rated output power.

Some power devices with heat sinks depend on convection cooling (with or without forced air) to assist in transferring the heat away from the power devices to the cooler air.

Radiant Cooling: This is the transfer of heat by means of electromagnetic radiation (energy waves) that flow from a hot object (power device) to a cooler object. True radiant heat transfer can take place in a vacuum and does not require air. It should be noted that conduction cooled power devices also give off radiant heat; however, radiant heat transfer is less effective as a means to cool a power device than are conduction or convection cooling described above.

Monday, November 5, 2007

Guide to EMC Standards for Power Supplies

EMC refers to ElectroMagnetic Compatibility. Electrical equipment that takes power from a distributed AC or DC source which is connected to other equipment, such as the AC mains in a building, has to have minimal influence on that source. It also has to have minimal influence on other equipment through electromagnetic radiation. A power converter which incorporates switching devices operating at high frequency needs to employ special means to keep the electromagnetic interference within internationally agreed upon limits. In general, electrical equipment has to operate in its environment with minimal disturbance to its environment. The limits to disturbances are defined by the international standards described below.

Types of Standards:
1) Generic Standards:
A top level standard for a type of equipment which encompasses specific basic standards in their references. The current relevant standard for power supplies is EN61204-3: 2000. This covers the EMC requirements for power supply units with DC output(s) of up to 200V, at power levels up to 30kW, and operating from AC or DC. source voltages of up to 600V. The EN refers to Euro Norm or European standard. Europe has led the field in establishing standards for EMC and many other areas which have been adopted worldwide, with some local deviations.

2) Basic Standards List:
The relevant basic standards called up in EN61204-3 are:
EN55022 and EN55011. Conducted and radiated electromagnetic interference emitted by the power supply. This is also known as CISPR22. The FCC has similar standards in the USA. There are two levels for the emission limits, Class A and Class B. Class B is normally required which puts a lower limit on allowed emissions.
EN61000-4-2. Immunity to electrostatic discharge.
EN61000-4-3. Immunity to radiated radio frequencies.
EN61000-4-4. Immunity to fast transient voltages on the input lines.
EN61000-4-5. Immunity to lightning surges on the input lines.
EN61000-4-6. Immunity to conducted radio frequencies.
EN61000-4-8. Immunity to power frequency magnetic fields.
EN61000-4-11. Immunity to damage from input line voltage reductions.
EN61000-3-2. Limits to the harmonic currents that can be taken from the input line.
EN61000-3-3. Limits to the voltage fluctuations that the power supply can cause to the line input voltage.

3) Performance Criteria:
In immunity testing, there are four classes by which passing or failure are assessed.
Class A. No loss of function or performance due to the testing.
Class B. Temporary loss of function or performance, self recoverable.
Class C. Loss of function or performance which needs intervention to restore.
Class D. Permanent loss of function or performance due to damage. This would always represent a failure.

Basic Emissions Standards

EN55022 (IT equipment), EN55011 (Industrial equipment), and FCC Class A or B (in the USA):
Conducted and radiated emission limits.
Conducted EMI (electromagnetic interference) is radio frequency energy that the power supply couples into the input power lines. The power supply input incorporates filtering to reduce the conducted emissions as necessary. The radio frequency noise is measured between 150kHz and 30 MHz using a spectrum analyzer or special receiver.

Radiated EMI is radio frequency energy emitted from the enclosure and input and output wiring of the power supply and is measured in the 30MHz to 1,000MHz frequency range. The measurement is usually performed at an “open” site which is an open air location selected to be in a radio frequency quiet zone where television and radio transmissions are weaker. The unit to be tested is placed on a wooden table above a large ground plane 10 meters away from a suitable receiving antenna connected to a spectrum analyzer.

Puts limits on the harmonic currents that the power supply is allowed to take from the AC mains source. The standard applies to power supplies with rated power between 75W and input line current of up to 16 amps per phase.

A power supply which is not power factor corrected will take a current from the source which is not the same shape as the voltage waveform. This is because the input storage capacitors can only charge when the input voltage is higher than the capacitor voltage. Thus the input current flows for only part of the cycle, and has a high peak value which causes currents which are harmonics of the line frequency. With three phase power distribution the absence of harmonic currents ensures that the neutral current is zero. This was not the case when large numbers of personal computers without power factor correction began to be used in office buildings, and the neutral wire would burn out. Most power supplies now incorporate power factor correction circuitry to ensure that the harmonic currents are low.

Limits voltage changes that the unit under test can impose upon the input power source. This is referred to as the flicker test.

Although this is not normally a problem with power supplies, some types of electrical equipment, especially in process control, can load the power source at regular or semi-random intervals. This can cause voltage changes that can affect the brightness of electric lighting and cause flicker. A survey was performed to determine what rates of flicker were the most disturbing to human subjects, and a curve of maximum percentage voltage variation at various frequencies was established. The most disturbing rate was just over 1,000 changes per minute, and the curve reflects the smallest percentage change at this frequency. Above 1,800 changes per minute the flicker is not noticed.

Basic Immunity Standards

Tests immunity to electrostatic discharge from a simulated human body capacitance of 150pF. By walking across a carpet of artificial fiber in a low humidity condition, a person can build up a charge of several thousand volts. This can be discharged to electrical/electronic equipment, and so it is important that the equipment is immune to these discharges. The test is performed at a voltage of up to 8kV by discharging a probe to the chassis at various locations by direct contact, and at up to 15kV through the air, with the power supply operating. Test levels of 4kV and 8kV are common. Class B performance criterion applies.

Checks immunity to incident radio frequency energy in the frequency range of 80MHz to 1,000MHz, and a separate test at 800 MHz to 960MHz to simulate the effect of digital cellular telephone transmissions. The test is performed in an anechoic chamber which is a shielded room with cone shaped plastic moldings on the inside wall surfaces which absorb radio frequency energy, so there are no echoes. The field strength is 10V/m for the carrier. Class A performance criterion applies.

Tests the effect of a fast voltage transient or burst applied between each input line and ground in turn. The applied voltage has a peak level of 2kV, and rises to maximum in 5 nanoseconds, and falls back to zero in 50 nanoseconds. It is applied at a repetition rate of 5kHz. Class B performance criterion applies.

Simulates the effect of a lightning surge voltage applied to the input power lines. Surge voltages are applied between each line and ground, and also between lines. The line to ground peak voltage is normally twice that applied from line to line. 4kV and 2kV are typical test voltages. The voltage has a rise time of 1.2 microseconds, and a fall time of 50 microseconds. Class B performance criterion applies.

Tests the effect of conducted radio frequency energy which is inductively coupled into the input cables with a ground return. The frequency range is 150kHz to 80MHz at 10Vrms amplitude, and the frequency is increased in 1% steps. The carrier is 80% amplitude modulated at 1 kHz. Class A performance criterion applies.

Electromagnetic compatibility, testing and measurement techniques for power frequency magnetic fields. Criterion A, using Helmholtz coil at 50 Hz, to 30 amps (rms) per meter.

Checks the effect of input voltage dips on A.C. input power supplies only.
There are three different degrees of test severity, a 30% reduction of input voltage for 0.5 period, a 60% reduction for 5 periods and a 95% reduction for 250 periods. For the first test, the unit should continue working with no change of output voltage because most units have a hold-up time of one period, which corresponds to 20 milliseconds at 50Hz. The other two tests will cause reduction or loss of output voltage, and intervention may be needed to restore the output. The unit should not be damaged by the testing. Class B and C performance criteria apply.

Friday, October 26, 2007

Isolated & Non-Isolated DC-DC Converters

There are two frequently used terms for types of DC-DC converters; non-isolated and isolated. This “isolation” refers to the existence of an electrical barrier between the input and output of the DC-DC converter.

The simplest example of a non isolated “converter” is the popular LM317 three terminal linear regulator. One terminal for unregulated input, one for the regulated output and one for the common.

Source National Semiconductor

Note there is no isolation between the input and output.

Today, non-isolated switching regulators are very common, or Point of Load (POL) converters.

Although low cost and simple, these converters suffer from one disadvantage in that there is an electrical connection between the input and output. Many safety agency bodies and/or customers require a separation from the applied input voltage and the output voltage which is often user accessible.

An isolated DC-DC converter will have a high frequency transformer providing that barrier. This barrier can withstand anything from a few hundred volts to several thousand volts, as is required for medical application.

A second advantage of an isolated converter is that the output can be configured to be either positive or negative.

Where many users get confused concerns how to connect the input up, particularly with the differences between a datacom system (input negative connected to chassis) and a telecom system (input positive connected to chassis).

Below are four scenarios, be aware - figures 3 & 4 will result in failed converters! Most DC-DC converters cannot withstand reversed input connections.

Monday, October 8, 2007

Why is my power supply input only rated from 100-240VAC?

Most power supplies have a rating label that looks something like this:

However, a close look at the power supply’s datasheet will usually show the absolute AC input voltage range, from minimum to maximum. This is usually 90-264VAC, or occasionally 85-264VAC if the power supply has been designed for Japanese use.

Japan uses the lowest AC mains voltage, which is 100VAC nominal; however, short duration AC line droops or brown-out conditions often mandate a rating down to 85VAC. The UK is among the countries that use the highest AC mains, with a nominal rating of 240VAC.

The safety certification bodies (UL, CSA, TUV, etc.) mandate that a rating of 100-240VAC be listed on the power supply’s label. However, they factor in a +/-10% tolerance for the power generation and transmission utilities. -10% of 100VAC is 90VAC, and +10% of 240VAC is 264VAC. All safety testing is performed at the high and low limits as listed on the power supply’s datasheet.

So, if the power supply label states 100-240VAC, it can usually operate over a wider AC operating input range. However, always check with the manufacturer’s datasheet to confirm this. Continuous operation of the power supply over the datasheet’s specified AC input range will not normally cause any problems. In some cases, however, the maximum output power (total watts) of the power supply may need to be derated if the supply is operating off an input voltage that is on the low-end of the specified range. Always check the power supply’s datasheet for the specified minimum AC input voltage with various output load levels. Deratings may also apply depending upon the power supply’s operating ambient temperatures.

Should a label state 100/240VAC (note the slash) it “may” indicate that there is a voltage select switch or jumper that is required to be set for the correct operating input voltage range. Newer products tend to not have an AC select switch or jumper.

Worldwide, the AC mains power has a nominal frequency of either 50 or 60 Hz (cycles per second). However, these frequencies are subject to variations by the power generators in different countries (especially third world) and so the typical AC frequency range for power supplies is 47-63Hz.

Tuesday, September 18, 2007

A “Beginner’s Guide” to Fault Tolerant Power Supplies

The effectiveness of having a fault tolerant power strategy was demonstrated after hurricane Katrina hit the Gulf Coast in 2005. A financial news television station interviewed the heads of two telecom carriers to find out when their telephone services would be operational again. The interview was very short – “we never lost service” they replied.

The telephone systems we take for granted have expensive and complex back up systems. Fault tolerant power supplies are supported by battery banks, generators and uninterruptible power supplies. Large Industrial complexes have also implemented similar systems - having an oil refinery stop production can result in enormous sums of money being lost!

For those with less extensive budgets, this brief article will explain the benefits, terminology and tips on how to implement a relatively low cost, but effective system.

Why have redundant power supplies?
Imagine a 24VDC 10A power supply driving motors and sensors on a conveyor based production line. For two or three years everything works fine, then one Friday (always at the end of the month), the power supply fails causing the conveyor to stop. Even if a spare part is in stock, it could still result in 30 minutes of expensive lost production.

If two identical power supplies had been installed in a fault tolerant, redundant mode, the remaining (good) unit would have continued to power the production line. The failed power supply could then be replaced at a more convenient time during routine maintenance.

Frequently Used Terminology

An expression where N is the number of power supplies needed to run the system. The simple two power supply system mentioned above would be considered 1+1. A triple redundant system (where two failures would have to occur to shut the system down) would be designated 1+2.

Some equipment is operated 24 hours a day, 7 days a week, allowing no time to bring the system down for maintenance. In this case the failed power supply must be “swapped” out and a new one inserted without disruption to equipment operation.

ORing diodes
In the rare event of a power supply failing with a shorted output, low voltage-drop ORing diodes block that short from bringing down the system power.

Current share
Some power systems employ a method of balancing the current between the power supplies to increase field life. This can be an electronic signal wire that links the power supplies together or a switch* on the power supply that initiates a slight drop in the output voltage as more current is drawn. (*Common on high power DIN rail units)

Two Ways of Implementing Fault Tolerance

DIN Rail mount
For the example listed above, the simplest off-the-shelf solution is to use a diode “ORing” module and two power supplies. Here we are using Lambda’s DIN rail mount DLP-PU module and two 24V 10A DLP240-24-1/E power supplies.

Tip: When wiring the system, ensure that the cable lengths from the output of the power supplies to the ORing module are equal. This will help optimize the performance and life of the power supplies.
Inside the diode ORing module are two diodes and two alarm relays. Even in the event of one power supply failing with an internal short circuit, the remaining unit will continue to deliver power. See below.

Tip: - It is important to identify power supply failure using the relay alarms to flag the need for maintenance. Engineers sometimes overlook this which can result in a second failure unexpectedly bringing the system down!

Rack MountSystem Engineers requiring more power are turning to the communications style racks. These sophisticated low cost systems allow power supplies to be hot-swapped and come completely self contained. An example of such a product is Lambda’s FPS series.
Advantages of this solution include:
  • Easy mounting into a standard 19” rack
  • All in one solution
  • Hotswap capable (ORing diodes or MOSFET switches built-in)
  • No tools are required for replacement of a supply
  • High density, low profile (1.75”)
  • Off the shelf parts
  • Fully safety approved
  • All necessary warning signals included
  • 12V, 24V, 32-36V and 48V outputs

Click on for an animated example.

Finally, one important note
A company wanted to ensure that in the event of a power supply failure their system would continue to operate. A battery was installed across the power supply output to give 24 hours uptime in the event of a power supply failure.

Unfortunately no thought was given to how anyone would know that the system needed maintenance! The power supply did eventually fail and the battery kept the system up for 24 hours before it discharged resulting in a system shutdown. A simple alarm circuit could have prevented that.

If you take Lambda’s recommendation to invest a little extra money up front to make your power system more secure, test your system to make sure you have it right!

Friday, September 7, 2007

Advances in Power Supplies for Automated Electrochemical Mini-Plants

On-site and on-demand production of disinfectants, biocides and water purification chemicals including sodium hypochlorite, chlorine dioxide has been substantially improved via the use of advanced switch-mode power supplies that provide the power for automated electrochemical generators.

Many municipal water, food processing, and wastewater treatment plants are switching over from the use of chlorine disinfectants and biocides to safer and more environmentally friendly point-of-use and on-demand generated chemicals. The primary reasons for this change are that conventional chlorine agents require transport by tankers on accident-prone highways or railroads, ever increasing safety and environmental regulations regarding toxic gases and chemical spills, and the required bulk storage of these hazardous materials at the sites where they are used. Safer and in many cases more effective chemicals have been developed that can replace chlorine. For example, after many trial and error attempts to find a way to effectively control Legionnaires’ disease, it was found that chlorine dioxide (CIO2) was one the few chemical agents that could consistently and safely disinfect Legionella bacteria (see References). Add to this the ability to manufacture these safer chemicals at the locations that use them, and only when needed, and the advantages in total become obvious.

Two popular substitute chemicals for chlorine are sodium hypochlorite (NaOCI) and chlorine dioxide (CIO2) both of which can be manufactured via mini-plants (aka, generators) that are delivered to the end users’ site as a complete package and provide the disinfectants on-demand and as needed. In many cases, these mini-plants operate automatically and can be employed in unmanned locations such as municipal water treatment sites.

These electrochemical generators use the process of electrolysis as the basis for the production of these disinfecting and biocide chemicals. Recalling our science classes, electrolysis is a common method of separating bonded elements and compounds by passing an electric current through them. It involves applying a voltage between two electrodes (anode and cathode) which are submerged in a conductive solution (electrolyte). When a voltage is applied to the electrodes, electric current flows and in turn breaks down the molecules within the solution into its components (Figure 1).

Figure 1 shows part of the process that is used to produce sodium hypochlorite (NaOCI), which is more commonly known as household bleach when sold as a solution containing 5-6% of NaOCI. However, instead of a static vessel as shown in Figure 1, modern electrochemical generators pump the electrolyte solution continuously through one or more tubes that have the electrodes mounted within them. As the electrolytic solution flows through these tubes (electrolytic cells), the electrolysis process continuously separates the molecular components. In some instances, the solution is run through the dual-electrode electrolytic cells more than once to further refine and separate the resulting chemicals. (Note: Batteries operate by a reverse process from electrolysis; they generate electricity by means of galvanic or voltaic cells that contain anode and cathode electrodes that are in contact with an electrolyte solution or gel.)

Historically, the power supplies that provide the driving force for electrochemical generators have evolved from basic transformer and diode rectifiers, to transformer and SCR (silicon-controlled-rectifier) power sources, to modern and more sophisticated power sources. The development of the switchmode power supply greatly reduced the size and substantially improved the efficiency of these power sources. In addition, switchmode power supplies have the ability to provide electronic signals for status information (volts, amps, temperature, etc.), remote control, and communications to and from a PLC (Programmable Logic Controller) or a local/remote computerized controller.

The vast majority of switchmode power supplies are designed to operate as regulated voltage power sources. These supplies regulate the output voltage very precisely regardless of the amount of current drawn from the supply, up to its design limit. For example, a 1500-watt supply can provide a 12-volts output while providing from 0 to 125 amperes of current. Once the maximum current of 125-amps is reached, the supply is designed to go into a current-limit mode (where the output voltage is automatically reduced or the supply shuts down).

Figure 2 above shows the loop diagram of a typical sodium hypochlorite generator. The part of the system shown above that is called the Electrolyzer consists of multiple electrolytic cells (tubes containing electrodes), connected in series, through which the electrolytic solution is pumped and in turn separated into its primary chemical components (e.g., sodium hypochlorite solution and hydrogen gas) via electrolysis. The electrochemical process for manufacturing chlorine dioxide is similar to the above except it starts with a solution of sodium chlorite.

It has been found that in many electrochemical processes, including the production of disinfecting agents, that standard voltage-regulated power supplies do not always provide the ideal power profile for these processes. In fact, in many instances, the power supplies are being forced to operate at a fixed voltage and at close to their maximum current rating. If these operating conditions are maintained for long periods of time, the supply will internally heat-up and prematurely fail, thus shutting down the production of the disinfecting agents.

Why does this happen and how can it be avoided? As described above, during the electrochemical process, in order to keep up with the continuous electrolysis process with constantly flowing electrolyte solutions, the power supply must provide a high enough voltage to overcome the impedance between the two electrodes and the solution surrounding them, and, more importantly, to provide a high enough current density (amperes) to effectively separate the molecules during the short time (determined by the flow rate) that the solution comes in contact with the electrodes. By using a switchmode power supply that is designed to operate in a “constant-current” mode (instead of constant-voltage, as is the norm) the electrochemical process has been found to produce chemicals much faster, with consistent high quality, without forcing the power supply into an overload state.

There are a number of ways of providing current-mode power supplies for enhanced electrochemical applications. One method is to use Programmable Power Supplies. These supplies are designed to be manually or remotely programmed to operate in a voltage-mode and/or a current mode, at a specific voltage and current range, along with other specified parameters. As an added bonus these supplies usually include a serial digital communications port that allows it “talk” to local or remote computer controllers. Additionally, these supplies can be connected in parallel to the electrodes, or to groups of electrodes, when an electrochemical process requires more current than one supply can provide. For example, Lambda Americas’ model ZUP10-80/U programmable power supplies is adjustable from 0 to 10-volts with 0 to 80-amps (800 watts total). This type of supply is being used in its “constant-current” mode to efficiently produce disinfectant and biocide chemicals at many unmanned, non-air conditioned, municipal water treatment sites. In some applications, two or more ZUP supplies are connected in parallel to provide the necessary amount of current for the electrochemical process.

Another method of providing a “constant-current” mode power supply is to modify the design of a voltage regulated supply. This can be done by adding circuits that monitor the supply’s output current to prevent an overload, yet maintain a “constant-current” profile from the supply. For example, Lambda Americas has produced modified versions its HWS-CC 1500-watt supply to do exactly this. In electrochemical applications that produce disinfectant and biocide chemicals, a number of these “current-mode” supplies are connected to different sets of electrodes, and/or, in parallel, to support different output current requirements for various models of electrochemical generators. Obviously, generators that produce higher output rates of chemicals require higher current levels.

Lambda’s HWS-CC Series Power Supplies

This paper has focused on the techniques and benefits related to advanced switchmode power supplies for mini-electrochemical generators (self-contained plants) that produce disinfecting and biocide chemicals on-site. It should be noted that electrolysis processes are used extensively in many other chemical and industrial areas, some of which are listed below.
  • Production of aluminum, copper, sodium
  • Anodizing
  • Production of hydrogen (e.g., for the cars and fuel cells of the future)
  • Electroplating and polishing
  • Large waste water treatment plants
  • Factory and power plant cooling towers - recirculating water treatments
Many of these electrochemical processes require power levels that far exceed the range of the switchmode power supplies described above. These high power rectifier systems (ranging from 300 to 30,000 kW) are very specialized, large, heavy, and are usually comprised of huge transformers, rectifiers, thyristors, SCRs, capacitors, regulating controllers, and water cooling systems. Some of these high power sources are as large as a typical bathroom, kitchen, and larger. There is no doubt that as technologies advance, these huge power sources will see reductions in size and improvements in efficiencies.

In summary, the application of switchmode power supplies operating in a “constant-current” mode has been shown to provide significant improvements in electrochemical self contained mini-plant generators that are used to produce disinfecting and biocide chemicals. These benefits include:
  • Improved current-density control for consistent electrolysis
  • Enhanced quality of the resulting chemicals
  • Higher efficiencies and improved regulation of the power sources
  • Reduced space and weight
  • Power Supplies meet international Safety and Power Factor Correction (PFC) standards
  • Availability of digital communications, remote control, and status signals
  • Substantial reduction of downtime


Thursday, September 6, 2007

What’s all this stuff about "Digital Power"?

It seems that every 2-3 weeks an article or news announcement about “Digital Power” appears in electronic design periodicals or online news links. In fact, one industry newsletter seems to be having a love affair with digital power, as it mentions it in just about every issue.

So what is all this fuss about Digital Power and what is it anyway? Well, the simple answer is that there are two basic types of digital power. These are Digital Control (used internal to the power devices) and Digital Power Management (provides external control and communications between power devices and a master controller).

Digital Control
The majority of switchmode AC-DC and DC-DC power supplies/converters use analog techniques to regulate/control the output voltage, current, and power factor correction circuits, etc. The closest that most of these devices come to looking a bit digital (On/Off states) in nature is by employing Pulse Width Modulation (PWM) in their switching regulator circuits; but even that is a bit of a stretch.

In recent years, new integrated circuits (ICs) have been developed that can replace “analog” control ICs and discrete circuits, which are used extensively in all power devices, with those that are, at least in part, “digital” in nature. These internal ICs and circuits perform such control functions as: voltage regulation (VR), power factor correction (PFC), pulse width modulation (PWM) control, internal monitoring/alarms, and external communications.

The advantage of these digital ICs is that they can be programmed by engineers with digital or analog electronics training. And, since the Universities are pumping out more digital (e.g., computer science) than analog engineers these days, these digital ICs are becoming attractive. However, at present the cost of these digital ICs (along with NRE for the equipment needed to program the devices) is still higher than for the mature analog ICs. Nonetheless, some predict that these IC costs will become equal within the next 12 months or so. A potential disadvantage of these digital ICs is that, by their nature, they require a high speed clock to operate, which can add to the radiated and conducted noise coming from the power supply or converter. However, advanced functions such as fault diagnostics/prevention and improved power efficiencies are among the promises of the new digital control ICs.

Digital Power Management
As mentioned above, Digital Power Management (DPM) involves the external control and communications between power supplies (or converters) and a master controller. Currently, many analog-based power solutions already have the ability to communicate with an external computer or controller via digital communications links (e.g., RS232, RS485, GPIB, or I2C bus).

Newer DPM control and communications formats have evolved that are designed to operate with the new digitally-controlled power devices. These include DPM technologies such as PMBus (Power Management Bus) and Z-One. Sadly, these technologies are not compatible or interchangeable. In fact, currently there are lawsuits between the backers of both of these technologies.

If I were a potential user of these Digital Power Management schemes, I would stay clear of them until the lawsuits are settled (expected to occur within the next 12 months), rather than find out later that the cost of these DPM ICs or controllers have substantially increased due to royalties that must now be paid to the company that won the lawsuit.

The potential advantages of the DPM and digital power technologies in general, include enhanced bidirectional communications, fault diagnostics, remote programming of the linked power supplies/converters, automatic compensation of dynamic input and output load changes, and overall improvements in efficiencies that relate to green-power.

In Summary
Although “Digital Power” is a popular buzzword these days, especially by those companies who have developed or adopted the technologies, the bottom line will always be: “What do I get for my money?” At present, there are hardly any power supply or converter applications that “must have” digital power when compared to the many lower cost and field-proven analog solutions that exist.

For example, during their new power-product design and development process, Lambda has designed in-parallel devices that employed both analog along with those that use digital control ICs and technologies. In all cases, the final decision on which technology ultimately goes into production has been based on comparative price/performance factors; which is the dominant decision factor for their customers.

When the time comes that digital power products offer the same or better performance and reliability, along with the ”needed features”, at the same or lower price as analog-based products, that is when Digital Power will become the winning technology. Realistically, someday digital power will provide a price/performance advantage over purely analog power devices. Who knows for sure when that time will come.

Tuesday, September 4, 2007

Can I Operate my AC-DC Power Supply with a DC Input?

The answer is yes, sometimes.

Many standard AC-DC switch mode power supplies (most of Lambda’s products) specify a high voltage DC input range in addition to the more common AC input range of 90-264VAC. We receive many questions about how and where to connect the DC input to an AC-DC supply that is spec’d to operate off of DC as well as an AC inputs.

Where and why is high voltage DC power used? It turns out that many power generation facilities provide a high voltage DC to power the plant’s equipment rather than the regular 115VAC or 208VAC power grid. This high voltage DC (typically 120 or 130-330VDC) can be easily used with batteries to provide a secure source of power rather than using expensive centralized or local UPS systems.

Now back to the subject. The topology of many switch-mode power supplies actually lends itself to operation from either AC or DC input. Important Note: Always check your power supply’s Operations Manual or spec sheet to confirm that it is designed to operate from either an AC or DC input.

Referring to the simplified power supply schematic below:

When powered by an AC sine wave, during the first half cycle the current flows from the Line terminal through the input filter and charges capacitor C1 through diodes D1 and D3. During the second (negative) cycle, current flows from the Neutral terminal and capacitor C1 is charged through diodes D2 and D4.

When powered from a high voltage DC source, the polarity of the connection is not critical as far as the operation of the power supply is concerned. If the positive connection is made to the Line terminal, C1 is charged through diodes D1 and D3. If the positive connection is made to the Neutral terminal, then C1 is charged through diodes D2 and D4.
An important note of caution to insert here is about the protective fusing of the power supply. Internally most power supplies have a fast acting AC rated fuse in series with the Line terminal. It is recommended that a DC rated fuse be installed external to the power supply. If one side of the high voltage DC buss is connected to ground, then the fuse is usually positioned in series with the “hot” side (the ungrounded side). It is recommended that you consult with your local safety engineer to be sure.

Friday, August 17, 2007

What size and type of output wires should I use?

There are two main considerations for sizing DC wiring from the output of a power supply to its load. They are ampacity (fancy term for the number of Amps) and voltage-drop (remember ohms law: V = I x R). Ampacity refers to a safe current carrying level as specified by safety organizations such as Underwriters Laboratories and the National Fire Prevention Association, which publishes the National Electric Code (NEC).

AWG stands for American Wire Gauge and defines the diameter and cross sectional area of the wire. The smaller the AWG number, the larger the diameter, cross-sectional area, and current carrying capacity of the wire. Always use insulated wires with solid or stranded pure copper conductors (do not use aluminum or copper-clad steel wires). The voltage-drop is simply the amount of voltage lost in a length of wire due to the resistance of the conductor.

DC wires may be sized for either ampacity or voltage drop depending on the wire length and conductor heating. In general, ampacity considerations will drive wire selection for short wire lengths (less than 50 feet) and voltage drop will drive wire selection for longer lengths (greater than 50 feet). Note: If you are using the Remote Sense feature of the power supply, remember to stay within the maximum voltage drop across the cables that the Remote Sense is designed to compensate for, which can range from 0.3V to 1.0V (check the power supply’s user-manual for details).

The National Electric Code table 310.16 provides ampacity values for various sizes, bundles, and insulation temperature rated wires. ALWAYS FOLLOW THE NEC RULES, LOCAL CODES, AND YOUR COMPANY’S PRACTICES WHEN SELECTING DC WIRING.

Table 1 shows the MINIMUM recommended wire sizes for different load currents. The use of larger diameter wires (with a smaller AWG number) would reduce the voltage drop (and heat generated) across the wires. The current ratings in Table 1 are based upon using 90° C rated insulated wire. If using a lower temperature rated insulated wire (e.g., 60° C), the wire diameter would need to be larger. Refer to the following web site for more information about wire gauges: .

For example, per Table 1 below, a load current of 200 Amps would require a minimum of two # 2 AWG wires connected in parallel for each of the output connections (one pair or wires for the positive (+) and one pair for negative (-) output connections to the load). Again, larger diameter wires would decrease the voltage drops across these wires.

Thursday, August 16, 2007

FAQ Modular power supplies

I often get asked the same questions by our customers concerning Modular Power Supplies that I thought it would be helpful to list them here:

Q) What is a Modular Power Supply?
A) A Modular Power Supply is one that is assembled using a variety of pre-built building blocks (modules). This gives the customer a maximum choice for his voltage, current, signals & case style.

Q) What is the minimum quantity I have to buy?
A) At Lambda you can buy as few as one. Their modular power supplies are built using sophisticated manufacturing methods. The bills of materials are automatically generated, fed down to the shop floor and then quickly assembled using pre-built assemblies. Stock of the pre-built assemblies is managed by Kan Ban methods that trigger replenishment orders from the various workcells in the facility.

Q) Is there a set up charge for production?
A) None

Q) How long does it take to get all the safety approvals?
A) A good modular power supply series has all the safety certifications in-place to cover all of the potential configurations. Often these include EN61010 (test & measurement) and EN60601 (medical) certifications.

Q) How soon can I get a unit?
A) Because the modular units use pre-built assemblies, lead-time can be as short as 48 hours. Several US distributors have arrangements with power supply manufacturers to do the assembly. These are often called VARs (Value Added Resellers). Lambda has several VARs who can quickly build supplies to your needs.

Q) Are these units expensive?
A) Not really. Prices have dropped substantially since 1980 when Lambda first introduced modular supplies. It is not unheard of for our customers to be using in excess of 1000 units/year.

Q) Why should I use a modular over a modified standard or custom?
A) As these modular products are sold to a huge number of different customers, who are using them over a wide range of applications, the likelihood of a problem is minimal. With a custom or modified standard that risk goes up dramatically as you will be the "beta" site.

Q) Can I get odd output voltages?
A) Yes, many output "modules" feature a wide adjustment range, often 2:1. Recently, we had a customer ask for five (5) floating 18V outputs and we were able to meet their needs with a modular power supply.

If you have any questions regarding Modular Power Supplies, please let us know.

Thursday, August 9, 2007

Types of Distributed Power Architectures

Compact DC-DC converters have made their way into millions of electronic products and systems. The vast majority of these depend upon an AC front-end-box to convert the AC power source into a DC voltage from which the converters operate. In addition, international regulations have mandated that these front-end-boxes include Power Factor & Harmonic Correction (PFHC) to maximize the available power from the power grid.

Traditional Distributed Power Solutions

Traditional designs that employ distributed power architecture place DC-DC converters on PC boards very close to the point-of-load to maximize system speeds and efficiencies. To power the DC-DC converters, the required AC-DC power supply with PFHC is typically mounted somewhere in the system’s enclosure, external to the main pc-board (Figure 1).

This technique is quite reasonable for most applications. However, when it comes to equipment that must be mounted outdoors and occupy the smallest possible volume, there are now improved power products available.

Improved Power Distribution Methods

Typical medium power (400-700 watts) PCB mounted DC-DC converters are packaged in “full brick” sizes (e.g., 2.4” W x 4.6” L x 0.5” H). A number of major manufacturers of DC-DC converters have seen the need for, and are now providing AC input PFHC front ends in brick-formats that are PCB mountable near to the DC-DC converter(s). This has the advantage of placing all the power components on the same pc-board thus reducing the end products size and eliminating the power interconnect wires (Figure 2).

These AC-DC w/PFHC front-end bricks require some external components (capacitors, resistors, etc.), but the space required for these items is small in comparison to the elimination of the external “metal boxed AC front end”. And, these external components can be robotically inserted during the production of the pc-board. An added benefit of utilizing these brick packages is that they can be cooled without fans, by means of heat sinks or cold plates (e.g., mounting the brick bases against the system’s metal enclosure).

The Latest AC-DC Power “Brick” Solutions

Power Supply manufacturers have not stopped developing smaller and better power solutions. In fact, in recent times the AC/PFHC brick mentioned above has been merged with a DC-DC converter to form the ultimate power solution; an AC/PFHC/DC integrated brick. These 2-in-1 devices accept wide range 85 to 265 VAC inputs, correct the power factor, and provide the DC output(s) to the system. All this is accomplished within the same size constraints of a single “full brick” package measuring only 2.4” W x 4.6” L x 0.5” H, thus providing a 50% board space savings (Figure 3).

These integrated 2-in-1 pcb-mounted Power Bricks are ideal for Distributed Power Architectures where POL (Point of Load) Converters are needed. Since the 2-in-1 Power Bricks provide the conversion from AC to DC (with PFHC) along with the needed isolation, and the Intermediate Bus Voltage, the use of multiple low-cost, non-isolated POL converters becomes quite practical (Figure 4).

Recent advances in components and power design technologies have made these new
2-in-1 pcb-mount power bricks possible. In order to increase power densities, special Permalloy cores have been developed and employed in the inductors. New substrates and innovative transformer winding techniques have facilitated component height compressions and improved thermal management. And, of course, advances in integrated and hybrid circuits have contributed greatly to this next generation of power products.

Applications of 2-in-1 AC-DC Power Bricks

These new “2-in-1” AC-DC power bricks are ideal for many outdoor and indoor applications including:
  • Custom Power Supplies
  • PCB Mounted Bulk Power for Multiple DC-DC or POL Converters
  • Large LED & Liquid Crystal Displays
  • Traffic Information, Control, & Signaling Equipment
  • Toll Devices
  • Pico & Cell Phone Repeaters
  • WiFi, Telecom Sub-Stations
  • Underwater Surveying Devices
  • Automatic Pass-Reading-Devices for FastTrac Car Lanes
  • Oil Pumping & Pipeline Monitoring Devices
  • Security Systems
New 2-in-1 AC-DC Power Bricks

Lambda, a unit of TDK Corp., is currently one of the manufacturers of a new range of integrated “single-brick” AC-DC power bricks. These “2-in-1” pcb-mount devices are so innovative, they have seven patents pending.

Some of the salient features of Lambda’s single-brick AC-DC PFE Series power modules include:
  • Operates from Universal 85 to 265VAC, 47-63Hz Input
  • Power Factor & Harmonic Correction Meets EN61000-3-2
  • Low Profile, Single-Brick Footprint
  • High Power Density (up to 129W/in3) & Efficiency (up to 90%)
  • Regulated and Isolated DC Outputs with Wide Operating Temperatures (at baseplate)
  • PFE500-12: 12VDC Output, 400 Watts, -40 to +85°C
  • PFE500-28: 24 to 28VDC Output, 500 Watts, -40 to +100°C
  • PFE500-48: 48VDC Output, 500 Watts, -40 to +100°C
  • PFE700-48: 51VDC Output (semi-regulated), 714 Watts, -40 to +85°C
  • ±20% Output Voltage Adjustment Range
  • Over Voltage/Current/Temperature Protection
  • Approved to UL/CSA/EN60950-1, CE Marked, & RoHS Compliant
  • Optional Heatsinks & Evaluation Kits Available

Wednesday, August 8, 2007

Linear vs. Switch-mode Power Supplies

The Power Guy blog focuses on modern switch-mode power supplies and converters. However, to provide the newbie (newcomer) with some background information, we have included the following discussion.

Linear power supplies were the mainstay of power conversion until the late 1970’s when the first commercial switch-mode became available. Now apart from very low power wall mount linear power supplies used for powering consumer items like cell phones and toys, switch-mode power supplies are dominant.

What are the differences and how do they work?
Linear power supplies have a bulky steel or iron laminated transformer. It provides a safety barrier between for the high voltage AC input and the low voltage DC output. The transformer also reduces and the AC input from typically 115V or 230VAC to a much lower voltage, perhaps around 30VAC. The lower voltage AC is then rectified by two or four diodes and smoothed into low voltage DC by large electrolytic capacitors. That low voltage DC is then regulated into the output voltage by dropping the difference in voltage across a transistor or IC (the shunt regulator).

Switch-mode supplies are a lot more complicated. The 115V or 230VAC voltage is rectified and smoothed by diodes and capacitors resulting in a high voltage DC. That DC is then converted into a safe, low voltage, high frequency (typically switching at 200kHz to 500kHz) voltage using a much smaller ferrite transformer and FETs or transistors. That voltage is then converted into the DC output voltage of choice by another set of diodes, capacitors and inductors. Corrections to the output voltage due to load or input changes are achieved by adjusting the pulse width of the high frequency waveform.

Comparisons of both technologies
Size: - A 50W linear power supply is typically 3 x 5 x 5.5”, whereas a 50W switch-mode can be as small as 3 x 5 x 1”. That’s a size reduction of 80%.

Weight: - A 50W linear weighs 4lbs; a corresponding switcher is 0.62 or less. As the power level increases, so does the weight. I personally remember a two-man lift needed for a 1000W linear.

Input Voltage Range: - A linear has a very limited input range requiring that the transformer taps be changed between different countries. Normally on the specification you will see 100/120/220/230/240VAC. This is because when the input voltage drops more than 10%, the DC voltage to the shunt regulator drops too low & the power supply cannot deliver the required output voltage. At input voltages greater than 10%, too much voltage is delivered to the regulator resulting in over heating. If a piece of equipment is tested in the US and shipped to Europe, or even to Mexico in some cases, the transformer “taps” have to be manually changed. Forget to set the taps? The power supply will most certainly blow the fuse, or may well be damaged.

Most switch-mode supplies can operate anywhere in the world (85 to 264VAC), from industrial areas in Japan to the outback of Australia without any adjustment. The switch-mode supply is also able to withstand small losses of AC power in the range of 10-20 milliseconds without affecting the outputs. A linear will not. No one will care if the AC goes missing for 1/100th of a second when charging your cell phone, it will take 100 of these interruptions to delay the charge by one second. However, having your computerized equipment shutdown or reboot 100 times a day will cause a great deal of heartburn.

Efficiency: - A linear power supply because of its design will normally operate at around 60% efficiency for 24V outputs, whereas a switch-mode is normally 80% or more. Efficiency is a measure of how much energy the power supply wastes. This has to be removed with fans or heatsinks from the system. For a 100W output linear, that waste would be 67W. A 100W switch-mode would be just 25W. Therefore, 67W – 25W = 42W is the extra power lost by a linear supply. Doesn’t sound much, but don’t try touching a 40W light bulb. If the equipment were running 24 hours a day, then the extra losses would be 367kW hours, at the current average cost of $0.10 per kW hour; that’s an extra $37 a year for a power supply that costs around $80.

As a quick note, in Europe, they are trying to limit those losses of all power supplies used by consumers particularly when operating in the “Off” mode (as many products are left plugged in 24 hours a day). Imagine 250 million power supplies eating up a couple watts. That equates to the output of a whole power station.

About Power Topics

The purpose of this blog is to provide end-users of AC-DC Power Supplies and DC-DC Converters with useful information regarding product applications, helpful hints, news, comments, and answers to your questions. My focus is on modern switch-mode power products in the range of 1 to 3,000-watts.

My goal is to provide OEM designers, who select power products for their end-products, and purchasing agents, who buy these devices, with a valuable resource to assist you in making decisions involving power solutions for your next and/or existing products.

I hope you find this website and blog informative and useful. You are invited to contact me with your questions and comments.

Thank You,
Power Guy

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