What is Remote Sensing?

Most medium to high power AC-DC power supplies and DC-DC converters have “Remote Sense” connection points (+/- Sense) that are used to regulate the supply’s output voltage at the load. Since the cables that connect a power supply’s output to its load have some resistance, as current flows it will cause a voltage drop in the cables. Since it is best to regulate the voltage at the load site, the use of the two Remote Sense wires connected from the supply to the load will compensate for these voltage drops.

Typical remote sensing circuits are capable of correcting from 0.3V to 1.0V of voltage-drop in the output cables. However, to be sure, always check your power supply’s instruction manual to determine the maximum remote sense compensating range. If the voltage drop across the cables exceed the range of the remote sense circuits, this can be remedied by either reducing the length of the cables or increasing the size of the cable’s conductors. The remote sense leads carry very little current, so light gauge wires can be used. Steps should be taken to ensure the remote sense wires do not pick up noise by either twisting the +/- Sense wires together and/or shielding the wires from noise. It is important to observe the correct polarities, i.e., the +Sense wire should connect at the load to the +V output cable and the –Sense wire should connect at the load to the –V output cable. Refer to Figure 1.

Fig. 1: Power Supply with Remote Sense Wires Connected at the Load

When not using the remote sense feature, Local Sense (LS) connections should be used. In this case the +/-Sense points should be connected to their corresponding output or local sense terminals at the power supply (+Sense to +V output or +LS and, –Sense to –V output or -LS). Most power supplies are shipped from the factory with these “Local Sense” connections in place.
Refer to Figure 2.

Fig. 2: Power Supply with Local Sense Jumpers Installed

Choosing an Input EMI / EMC Filter for a Power Supply

There are two primary functions that an input EMI filter can perform:

1. Minimize outgoing electrical noise to avoid interfering with neighboring equipment
2. Attenuate (reduce) incoming electrical noise that could damage the system

Regarding outgoing noise, although most power supplies meet the governmental regulations for EMI, noise is additive and if there are multiple power supplies or high speed processor boards, it can result in a failing grade.

If the noise is only slightly out of specification, then a (lower cost) single stage filter may suffice. If the noise is considerably out of specification then a higher performance two stage filter will be required.

An example of these would be Lambda’s RSEN (single stage) and RSHN (two stage) filters. Look for the terms “wideband” or “low frequency attenuation” in the features.

Incoming electrical noise is usually in the form of a spike or burst of energy. It can be generated from natural causes such as a lightning storm or man made by a large piece of industrial equipment.

This type of filter may have “high pulse attenuation” listed as a feature and will have internal values optimized to reduce these potentially harmful spikes from reaching the power supply. The filter will also have some outgoing noise attenuation, but may not be as effective. An example would be Lambda’s RSMN series of filters.

Droop Mode Current Share

If two power supplies are to be connected together to produce more power or share the load, then a parallel capable model should be selected. TDK-Lambda’s DPP100, 120, 240 and 480 models are all parallel capable. On the front of each power supply is a small black switch. For parallel operation this switch should be set to “parallel” (Fig. 1).


In single mode the load regulation (the amount the output voltages changes with load) is minimal, the difference being less than 0.24V from zero load to full load for a 24V output power supply.

In parallel mode that load regulation is artificially increased to 1.2V using internal circuitry (Fig. 2).

The extra voltage drop or “droop” is proportional to the load drawn, so that when two or more power supplies are connected in parallel the output load is shared between the power supplies. If one of the paralleled power supplies tries to provide more current, its output will droop slightly and the other supplies will balance.

For optimal performance, all power supplies should have their outputs set to the same voltage.

What is PFC and why do I need it?

Switchmode power supplies without Power Factor Correction (PFC) tend to draw the AC input current in short bursts or spikes relative to the line voltage, as shown in Fig. 1. The Power Factor of a power supply is technically the ratio of the real power consumed to the apparent power (Voltsrms x Ampsrms) and is a decimal between 0 and 1.0. If left uncorrected the Power Factor (PF) of switchmode supplies will generally be around 0.65 or less.

Figure 1. Input of switchmode power supplies without PFC. The voltage waveform is a sinewave and the current waveform is a pulse or spike. PF<1 span="">

The Power Factor can be improved by using PFC circuits. These circuits “smooth out” the pulsating AC current, improving the PF, and reducing the chances of a circuit breaker tripping prematurely. There are two basic types of PFC, passive and active. Passive PFC circuits are less expensive and typically can correct the PF to about 0.85. Active PFC circuits are the most popular, are built into the switchmode power supply and can increase the PF to 0.98 or higher. The closer the PF comes to being 1.0, the better the performance of the power supply. Ideally, we want to end up with the input voltage and current waveforms being sinusoidal and in phase with each other as shown in Fig. 2.

Figure 2. Voltage and Current waveforms are sinusoidal and in-phase. PF=1.

PFC is Required by International Regulations
An important reason to have PFC in your power supply is to comply with international regulations, especially if you intend to sell your equipment in Europe. Since 2001, the European Union (EU) established limits on harmonic currents that can appear on the mains (AC line) of switchmode power supplies. Today, the most important regulation is the “European Norm” EN61000-3-2. This regulation applies to power supplies with input power of 75 watts or greater, and that pull up to 16 amps off the mains. It sets severe limits on the harmonic currents up to the 39th, when measured at the input of switchmode power supplies.

For example, the first harmonic is the primary input frequency, typically 50 Hz for the EU countries. The third harmonic is 150 Hz, and the 39th harmonic is 1,950 Hz. These unwanted harmonic currents have a direct relationship to the Power Factor of switchmode power supplies. Therefore, power supplies that meet EN61000-3-2 inherently have high power factors that are typically 0.97 or higher.

PFC Increases the Supply’s Output Power Capability
The PF, much like the supply’s efficiency rating, determines the amount of useful power a switchmode power supply can draw from the AC line and then deliver to its output load. Specifically, the formula that determines this is:

VLrms x ILrms x PF x Eff = Pout

As an example, if a power supply is operating off of 120VAC line, which is protected by 15A circuit breaker, UL guidelines say you should not draw more than 12A. So, using the formula above, we can compare two power supply examples with different Power Factors, as follows:

Example A: No PFC, PF = 0.65, 85% Efficiency, 120VAC input, 12A max. current:
Therefore: 120VAC x 12A x 0.65 x 0.85 = 796 Watts Output Power

Example B: PFC used, PF=0.98, 85% Efficiency, 120VAC input, 12A max. current:
Therefore: 120VAC x 12A x 0.98 x 0.85 = 1200 Watts Output Power

As can be seen above, the power supply in Example B (with PFC) can deliver 404 Watts or 51% more power to its output load than the non-PFC supply, a significant increase.

Why do I need PFC?
A power supply with PFC can supply higher output load currents than those without PFC. PFC significantly reduces the AC current harmonics, leaving mainly the “fundamental” current frequency that is in-phase with the voltage waveform (Fig. 2). International regulations dictate the substantial reduction of harmonic currents. The vast majority of AC-DC power supplies manufactured by Lambda Americas has active PFC, is in accordance with EN61000-3-2 and provides typical power factors in the range of 0.97 to 0.99.

What does 1U, 2U or 3U mean?

Many rack-mounted power systems are specified as being 1U, 2U, 3U, etc. What does this mean? For electronic equipment racks (e.g., 19 or 23 inches wide), the term 1U is used to define one rack unit of height.

1U equals 1.75-inches (44.45mm) of rack height. Therefore, a 2U rack mount height would be 2 x 1.75", which equals 3.5-inches high. A 3U height would be 3 x 1.75" = 5.25-inches.

It should be noted that the 1U, 2U, 3U, etc., heights are maximum dimensions. In order to allow for mechanical tolerances and to provide some space between panels, typically, for each 1U of height manufacturers may deduct about 0.03" (see Photo #1). For example, a 2U panel, which has a nominal height of 3.50" may be only 3.44" high [3.50" – (2 x 0.03") = 3.44"].

Individual power supplies are sometimes mounted within rack-mounted enclosures that require integral power. In these cases, the power supply needs to be a bit shorter than the equipment’s overall height to allow for the top and bottom covers. So a 1U high enclosure-mountable power supply needs to be shorter than 1.75-inches; a 2U enclosure-mountable supply needs to be shorter than 3.5-inches, and so forth (see Photo #2).

Examples

Photo #1: This 19" rack-mountable power system can hold up to 3 plug-in, hot-swap and redundant power supplies. The enclosure with mounting ears is 1.72" high (= 1.75" minus 0.03") and is therefore considered 1U high.

Photo #2: This 1000-watt switch-mode power supply is 3.25” high and, therefore, can be mounted in a 2U rack-mountable enclosure, which can vary between 3.44" to 3.50" high.

Since we still live in an English and Metric measurement world, here are a couple of handy conversion factors: 1 inch = 25.4 millimeters (mm), 1 mm = 0.03937 inch

As a side note, Lambda ran a clever ad campaign that those who understand what “1U” or “2U” really means would appreciate. Here is a copy of that ad, which hopefully you will find humorous.

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.

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.

Guide to EMC Standards for Power Supplies

Introduction:
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.

EN61000-3-2
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.

EN6100-3-3
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

EN61000-4-2
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.

EN61000-4-3
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.

EN61000-4-4
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.

EN61000-4-5
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.

EN61000-4-6
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.

EN61000-4-8
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.

EN6100-4-11
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.

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.

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.