Q: Are traditional industrial-grade power supplies suited for LED applications?
A: There are three main reasons why industrial-grade power supplies (such as the Xgen modular power supply) are unsuitable for many LED applications.
Firstly, LEDs are current driven devices whose brightness is proportional to their forward current. In order to attain a constant, predictable light output, a driver which is designed to output a constant current over a wide load range is required (i.e. the voltage will vary in order to maintain a constant current). Traditional power supplies are constant voltage supplies, which output a constant voltage over a wide load (the current output will depend upon the load). Constant voltage supplies require additional circuitry in order to drive LEDs reliably.
Secondly, many supplies are designed to operate up to a certain maximum temperature, at which point their power rating is decreased (derating) in order to ensure a safe operating environment for the supplies internal components. LED drivers are often required to operate at much higher ambient temperatures (sometimes greater than 60°C) without any temperature derating. Many LED drivers utilise potting or encapsulant which aids in heat transmission away from vital components to facilitate this high temperature operation.
Finally, traditional power supplies do not have waterproof and lightning protection features for outdoor operation. The potting used in many LED drivers facilitates the waterproof requirements, while LED drivers designed for outdoor use will be designed with surge protection for lightening requirements.
Q: What are the critical factors in selecting an LED driver?
A: There are a number of factors that must be considered when selecting an LED driver:
- Does the application demand a constant current or constant voltage (if the LEDs are to be driven directly, a constant current is required, if a constant voltage driver is to be used, additional circuitry will be needed).
- If a constant current driver is to be used, the operating current must be decided upon, and the voltage requirement calculated (this information can be calculated from the information given in the LEDs datasheet). If a constant voltage driver is to be used, the operating voltage must be decided upon, and the current requirement calculated. This information can then be used to select the power rating of the suitable driver.
- The input voltage must also be determined. The vast majority of Excelsys power supplies are designed to be operated with a wide input voltage range of 90-305 Vac.
- The IP (ingress protection) rating required by the application must be determined, as well as the requirement for lightning protection (needed in outdoor applications).
- Compliance to safety and electromagnetic compatibility standards for the entire fixture should be evaluated.
- Minimum requirements for other driver specifications should be defined, such as efficiency, power factor over load (etc).
Q: Which are the most common configurations for connecting multiple LEDs and what are the advantages and limitations of each configuration?
A: The majority of LED fixtures must be constructed of more than a single LED if the desired brightness is to be achieved. This means that a decision must be made on how to wire the LEDs together in a single fixture. There are three different configurations usually used to do this, (Series, Parallel and Matrix), each with its own advantages and disadvantages.
This is the most straightforward way to connect your LEDs is in a series configuration in which the cathode of each driver is connected to the anode of the next (see figure below).
- The main advantage of this implementation is the fact that all possible LED current imbalances are avoided through using a single string (the same current must flow though each LED).
- Since no external circuitry is required to control the current level through each LED, it is also the most efficient configuration that can be used.
- Any failure which blows an LED short will have no effect upon the remainder of the LEDs.
- In order to generate more light in a single fixture, more LEDs must be added to the string. Since the string voltage is proportional to the number of LEDs in the string, long strings can generate very high voltages which can translate into a safety issue (for example, a string of 48, 3.5 V LEDs would have a string voltage of 168 V).
- Any failure which blows an LED open will break the circuit of the string, and will cause the lamp to stop functioning.
To decrease the potential high string voltages of the series configuration, another option is to use multiple parallel strings of LEDs. These parallel strings will be made up of less LEDs in series, and so the string voltage would be reduced by a factor equal to the number of parallel strings (for example, 48 LEDs can be wired in 8 parallel strings of 6 LEDs each, which would have a string voltage of 21 V).
- The output voltage of the driver used can be kept relatively low (important for Class 2 requirements).
- Small differences in the forward voltages of various LEDs can cause significant imbalances in current due to the non-linear relationship between voltage and current due to the non-linear relationship between voltage and current in LEDs. This difference in forward voltage can be caused by inherent differences in the LEDs physical makeup, as well as voltage shifts due to temperature differences.
- A resistor placed in each string makes the voltage – current relationship of the string more linear thus balancing the current. This however decreases the efficiency of the system.
- Any failure of an LED (open or short) can cause stress in the remaining LEDs.
In order to improve upon the Parallel String configuration, it is possible to make additional connections to form a Matrix. In a Matrix configuration, multiple LEDs are connected in parallel in ‘bands’ and each of these ‘bands’ is connected in series to form a Matrix.
Using the example used above in the Parallel String configuration, (48 LEDs wired in 8 parallel strings of 6 LEDs each), the Matrix configuration would be 6 ‘bands’ of 8 parallel LEDs connected in series. In this configuration, the string voltage is still 21 V, so the same voltage is applied across each LED as the parallel configuration, and the same drive current flows through each LED.
- The output voltage of the driver used can be kept low (important for Class 2 requirements).
- A single LED failing short would take out the band of LEDs, but the remaining LEDs will operate without too much additional stress.
- Balancing (or load sharing) of the LEDs is still an issue.
- It is more complex than the other configurations.
The most robust solution is to use a driver with multiple outputs, each one regulated with independent constant current control. This would give all the load balancing advantages of series connected LEDs, while limiting the forward voltage of the strings of LEDs (the advantage of the Parallel configuration)
This type of driver will be significantly more expensive due to the requirement of multiple independently regulated outputs.
Q: What are the benefits of using high efficiency LED drivers?
A: There are a number of reasons why it is advisable to use LED drivers with high efficiencies.
The first, of course, is the cost benefits of high efficiency drivers. The less energy that is required to provide the desired luminous output, the less it costs to power the fixture. Since one of the main reasons for switching to LED lighting is the accompanying cost saving, it makes sense to use a driver with a high efficiency to maximise these cost savings.
Secondly, the energy that is dissipated in a power supply is dissipated as heat. This heat can reduce the lifespan of the critical components of the supply. A more efficient driver will therefore have an improved product life and MTBF. For example, the power dissipated in a 90% efficient driver is half that dissipated in a 80% efficient driver. Doubling the heat dissipated in the driver will greatly increase component temperature. This is of particular concern with regard to electrolytic capacitors, where a 10 degree increase in temperature can lead to a 50% decrease in component life. These electrolytic capacitors are generally the weakest link in LED drivers, and any improvement in their lifespan will also improve the driver’s lifespan.
Excelsys LED drivers have been designed with long-life capacitors as well as to be as efficient as possible in order to maximise their lifespan.
Q: What are the differences between MTBF and Lifetime?
A: There is a certain degree of confusion when it comes to the MTBF and Lifetimes of power supplies. The Mean Time Between Failure (MTBF) of a unit is the statistical approximation of the cumulative hours a number of units can be expected to operate before a failure occurs. For example, if 10,000 units operated in the field 10,000 hours with an MTBF of 1,000,000 hours, 10 failures would be expected (failure rate = (10,000 x 1,000) / 1,000,000)). The MTBF is not the length of time a supply can be expected to operate without failure.
The lifetime of a power supply is the period of time expected between starting the supply and the beginning of its wear-out phase. This is calculated by determining the life expectancy of each component of the supply is finding the component with the shortest lifespan, or the weakest link. The life expectancy of this component determines the life expectancy of the supply itself. In power supplies, the weakest link is usually the electrolytic capacitors. Excelsys uses long life capacitors in order to maximise the life expectancy of our supplies.
Q: What are PF and PFC and why are they important in specifying LED drivers ?
A: The power factor of a power supply is a ratio of the real power to apparent power of the power consumed by the supply. It is expressed either as a number between 0 and 1 or as a percentage between 0 and 100%.
Real power is the actual power drawn by the supply, whereas apparent power is the product of the input current and the input voltage. Since voltage and current may be out of phase in non-linear loads such as switch mode power supplies, this product can be much greater than real power.
In order to maintain a high power factor, many power supplies (including LED power supplies) must employ some form of power factor correction (PFC) whose role is to ensure that the input current waveform matches in the input voltage waveform as closely as possible in both waveform shape and phase.
A high power factor is required because a power supply with low power factor will draw more current for a given power consumption than a supply with high power factor. That means that a low power factor supply will result in greater power losses in all transmission lines and a large number of low power factor loads may even require a resizing of these utility lines. There are a number of standards now in effect requiring certain minimum levels of power factor in power supplies, including LED drivers.
Q: How are waterproof levels specified for LED drivers?
A: The waterproof level of LED drivers is specified by two numbers known as Ingress Protection (IP) ratings, which are a measure of the environmental protection that the driver provides.
The first number of the IP rating specifies the driver’s protection against solid materials, whereas the second figure specifies protection from liquids. A description of the levels of each of these two numbers is shown in the table below.
IP First number – Protection against solid objects
|0||No special protection|
|1||Protected against solid objects over 50 mm, e.g. accidental touch by a persons hands.|
|2||Protected against solid objects over 12 mm, e.g. persons fingers.|
|3||Protected against solid objects over 2.5 mm (tools and wires).|
|4||Protected against solid objects over 1 mm (tools, wires, and small wires).|
|5||Protected against dust limited ingress (no harmful deposit).|
|6||Totally protected against dust.|
IP Second number – Protection against liquids
|1||Protection against vertically falling drops of water e.g. condensation|
|2||Protection against direct sprays of water up to 15o from the vertical|
|3||Protected against direct sprays of water up to 60o from the vertical|
|4||Protection against water sprayed from all directions – limited ingress permitted|
|5||Protected against low pressure jets of water from all directions – limited ingress|
|6||Protected against temporary flooding of water, e.g. for use on ship decks – limited ingress permitted|
|7||Protected against the effect of immersion between 15 cm and 1 m|
|8||Protects against long periods of immersion under pressure|
Q: How are lightning protection levels specified for LED drivers?
A: The IEC 61000-4-5 standard and test-method establishes a common reference for evaluating the immunity of electrical and electronic equipment when subjected to surges such as lightning.
Surge test standards for lightning protection as below, IEC61000-4-5 ranks:
Level Open-circuit test voltage
1 0.5 kV
2 1.0 kV
3 2.0 kV
4 4.0 kV
NOTE X can be any level, above, below or in between the other levels. This level can be specified in the product standard.
Q: What is the difference between efficacy and efficiency?
A: Efficiency generally relates to one of two ratios in regards to lighting fixtures. It can refer to the ratio input power to output power of the LED driver (driver efficiency) or it can refer to the ratio of lumens exiting the bare lamp and the light exiting the total fixture (exiting light can be reduced due to lamp shades, reflectors etc.)
Efficacy is a term that is generally used when the input units and output units are different. For lighting, we are concerned with the conversion of electrical energy (watts) into light (lumens). The efficacy of a lighting fixture is a measure of the amount of light produced per unit of electricity and will take account of energy lost by the power supply (AC/DC conversion), energy lost converting the DC energy into light (LED), and the energy lost in directing the light to where it is needed. It is measured in lumens per watt.
Q:What is junction temperature ?
A: A metric that is often listed on LED datasheets is a maximum junction temperature. The junction temperature of a diode is the temperature at the point where a diode connects to its base. There are a number of reasons why this junction temperature should be kept as low and stable as feasibly possible.
- Lumen output decreases with increasing junction temperature, decreasing the fixtures efficiency.
- The forward voltage of an LED can vary with junction temperature. This can be problematic for parallel strings, where strings with differing forward voltages can lead to current imbalances, and in worst case scenarios, thermal runaway.
- The colour output of the light emitted from an LED can vary with changing junction temperature.
- The junction temperature is also proportional to the lifetime of the diode. The lower the junction temperature, the longer the lifetime.
The three metrics affecting junction temperature are forward current, thermal resistance to ambient and ambient temperature itself. Therefore, the current that be driven through an LED while maintaining expected lifespan, light output and colour depends how much heat can be removed from the junction, which in turn depends on both ambient temperature and thermal resistance to output. Careful design is important to maximise the heat transfer to ambient.
Q:Why is it important to use long-life LED drivers? How are they different ?
The main justification for moving to LED lighting from other lighting technologies is monetary savings. This is due to the reduced energy requirement to provide similar luminous output.
However, it should be remembered that energy consumption is not the only factor to be considered when judging the monetary saving that can be achieved by switching to LED lighting. Other factors include:
- Initial cost of fixture – LED lighting fixtures tend to have a higher initial cost than rival technologies due to the cost of the LED die itself.
- Lifespan of the fixture – LED’s are very durable devices, and are capable of operation for many thousands of hours.
- Maintenance costs – Some lighting fixtures are in very inaccessible locations (e.g. street lighting), and the cost of maintenance may be greater than that of the fixture itself.
Looking at these factors, it is obvious that in order to maximise the monetary savings from LED lighting, it is important to maximise the lifespans of the fixture itself. As mentioned previously, the LED’s themselves are very durable, so much so that the driver itself is generally considered to be the weak link of the fixture.
The life of an LED driver can be determined by the life of the electrolytic capacitors used. The life of electrolytic capacitors reduces by 50% for every 10 degree rise in the operating temperature of these components. Therefore, to maximise the life of LED drivers, it is critical to select long-life, quality electrolytic capacitors, as well as practicing good thermal management of these components.
As well as selecting high quality capacitors, Excelsys Technologies LED drivers are designed with the highest possible efficiencies, and are potted in order to transmit any heat generated away from the critical components of the drivers.
Q: What is THD?
A: THD stands for Total Harmonic Distortion. The term “harmonics” originated in the field of acoustics, where it was related to the vibration of a string or an air column at a frequency that is a multiple of the base frequency. A harmonic component in an AC power system is defined as a sinusoidal component of a periodic waveform that has a frequency equal to an integer multiple of the fundamental frequency of the system, i.e.
fh = (h) x fundamental frequency
where h is an integer. For example, the frequency of the fifth harmonic of a 50 Hz power supply can be calculated as:
f5 = (5) x 50Hz = 250Hz
The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. It is used as a measure of the power quality of electric power systems.
Q: Can LED lighting be dimmed?
A: The light output of an LED is directly proportional to the current flowing through it. Therefore, the light output can be controlled and dimmed by controlling the net current flowing through the LED. This can be done in two ways.
Current Reduction (CR)
Current reduction (also known as analog or linear dimming) works by driving the LED at the lowest possible forward current and forward voltage in order to achieve a target light output. Current reduction requires a very accurate current control to be effective, and so unless you choose the right controller it can be somewhat difficult to implement. Excelsys LED drivers utilise accurate Current Reduction.
Pulse Reduction (PR)
Pulse reduction is achieved by switching LEDs fully-on and fully-off at a frequency greater than the human eye can perceive. For a stationary light source this is 100 to 120 Hz, a moving light source requires a higher frequency in order to avoid a strobing effect. As long as the frequency is kept above this ‘flicker fusion threshold’ the eye integrates the pulses and so perceives a steady but dimmer light source which is roughly equal to the pulsed brightness averaged over time. If the frequency is any lower the eye will be able to see the individual pulses. Pulse reduction is the most common approach to dimming as it is simpler to implement than current reduction, but it is less efficient.
- Within pulse reduction there are three different methods of pulse reduction:
Pulse width modulation (PWM): A constant current pulsed at variable pulse duration and at a constant frequency.
- Pulse frequency modulation (PFM): A constant current pulsed at fixed pulse duration and at a variable frequency.
- Pulse code modulation (PCM): A constant current pulsed at random pulse durations.
The luminous output of an LED is directly proportional to the duty cycle (or ‘on time’) that the LED is driven at. Therefore by varying the duty cycle from 0 to 100%, the luminous output is also scaled from 0 to 100%. PWM is by far the most common method currently used for LED pulse dimming.
Q: Why are my LEDs flickering or changing in brightness?
The most likely cause of this flickering is when an LED driver is operating outside its recommended operating range. It usually signifies that the driver is in a protection mode such as short circuit protection which occurs when a string of LEDs has a forward voltage less than the lower limit of a constant current drivers operating voltage range.
To avoid this occurring, it is important to calculate the forward voltage drop of your LED load at your desired operating current (taking account of any forward voltage drop that may occur due to increased temperature). This information can be calculated from the V-I and temperature-current curves on the LED datasheet.
Another factor that should be taken into account is when using a dimmable driver is that the voltage drop of the LED load must remain in the operating voltage range of the driver throughout the dimming range (from the lowest to the highest operating current values).
Q: What level of THD and harmonic content is required for lighting applications?
In relation to harmonic content electrical equipment is classified as follows:
- Balanced three-phase equipment
- Household appliances, excluding equipment identified by Class D
- Tools excluding portable tools
- Dimmers for incandescent lamps
- Audio equipment
- Everything else that is not classified as B, C or D
- Portable tools
- Arc welding equipment which is not professional equipment
- Lighting equipment
- Personal computers and personal computer monitors
- Television receivers
- Note: Class D equipment must have power level 75W up to and not exceeding 600W
The limits imposed on the harmonic content of Class C equipment (lighting equipment) are shown in the table below:
Harmonic order (n)
Maximum permissible harmonic current expressed as a percentage of the input current at the fundamental frequency (%)
11?n?39 (odd only)
* ? is the power factor