LED Driver Selection

An LED driver is a type of power supply which is used to deliver the required power to an LED or an array of LEDs. LED drivers are available with both AC and DC as input sources. The primary goal is to have a regulated DC output power for LED applications irrespective of the input source. This article will help you to understand the classification of LED drivers, their output characteristics and safety requirements to enable the right LED selection for the desired application.


Input Source

LED drivers, based on their primary input source, are of two types: AC/DC and DC/DC. The driver’s circuits are either isolated or non-isolated. In an isolated driver, the input and output are separated using an isolation transformer. Isolation reduces electrical shocks. Non-isolated drivers lack isolation circuits. They share a common ground for both the input source terminal and the output terminal and thus, increase the incidence of shocks.

  • AC/DC driver: AC/DC drivers consist of a rectifier and a filter circuit. The rectifier converts the AC input to DC voltage. The filter circuit removes the AC component of the rectified output and helps to obtain a high power factor. These drivers are available in isolated and non-isolated form.
  • DC/DC driver: DC/DC drivers either up-convert or down-convert to the supplied DC input to the desired DC voltage level. These drivers use different DC to DC converter topologies, like buck, boost, fly-back and forward converters. Fly-back and forward converters are used to provide isolation between the DC input and LEDs. Buck and boost drivers do not provide any isolation.

Driving Modes

LED drivers are classified by output characteristics and power consumption by the LED module.

  • Constant Current Driver (CC): In these drivers, the current does not exceed the maximum current rating even in the short-circuiting. The output voltage may vary, but the output current remains constant. Constant current drivers are suitable for high power LEDs and series LED circuits. They maintain a consistent brightness across all the LEDs in series.
  • Constant Voltage Driver (CV): These drivers provide a fixed DC voltage to the LEDs. The flow of current varies depending on the load resistance. Hence, resistors or internal current limiting circuits are used in series with each LED. Most LED strips are designed with a group of LEDs in series with a current-limiting resistor in line with them. CV drivers are used to power parallel connected LEDs, such as LED strips and moving signs.

Dimming Methods

Dimming circuits are used to vary the driver output. These circuits help to control LED light intensity. Different types of dimming methods are used in drivers, of which TRIAC, analog and PWM dimming are the most common dimming methods.

  • TRIAC Dimmer: These drivers function by controlling the AC power on the input side of the circuit. In this type of dimmer, AC power is controlled by using a series-connected TRIAC. An AC to DC converter then produces the proportional DC power to change luminosity of the LEDs.
  • Analog Dimmer (0 to 10 Volt): The analog dimming circuit uses a rotary potentiometer or slider switch to provide step input (0 to 10 DC volts) to the LEDs. Depending on the step input, the driver produces output current to change the luminosity of LED light.
  • PWM Dimmer: The PWM dimmer is an advanced dimming method. It provides high-frequency pulses (ON-OFF signal) to the LEDs. The average PWM signal current helps to change the luminosity in LEDs. If a PWM pulse period is less than the 20ms, the human eye can distinguish individual pulses. It follows that the PWM frequency must be higher than 100 Hz. The LED light brightness depends on the PWM duty cycle resolution. The intensity of the LED can be controlled by modulating the duty cycle ({On time / on time+ Off time} x 100). The following graph shows the PWM signal waveform:

Important Driver Parameters

Driver performance depends on how well output characteristics are matched to LEDs. When we select an LED driver, the focus should be on driver output characteristics like output voltage, output current, power factor, harmonic distortion and efficiency of the driver.

  • Multi-channel Output: A single driver can provide one or multiple independent output paths to drive the LEDs. This can be easily controlled. Multiple outputs allow connection of different LED parallel strips.
  • Output Current: The operating current range of the driver must not exceed than that of LEDs. The life expectancy of an LED driver can be extended if it runs less than rated current. The output current value in the driver datasheet is specified in milliamperes (mA) or Amperes (A).
  • Output Voltage: The driver operating voltage should be equal to the product of forward-voltage drops and the number of LEDs used in an application. In the driver datasheet, the output voltage value is specified in volts (V).
  • Output Power: The driver output power must be higher than the chosen LED module. If a driver is continuously operating at full power, driver life may be reduced. The driver out power is specified in watts (W) in the datasheet.
  • Power Factor (PF): This is a measure of how efficiently the load current is being converted into useful work. Power factor is defined as the ratio of real power consumed by load divided by apparent power. The power factor varies from 0 to 1. PF = Real Power / Apparent Power
  • Power Factor Correction (PFC): This is used to increases the power factor of a driver. The driver circuits are crafted using reactive components, like capacitors and inductors. The capacitors and inductors are energy stores that lead to increased power consumption in the circuit. Power factor correction circuits, in such cases, are used to reduce power consumption from the source.
  • Total Harmonic Distortion (THD): This refers to the measurement of harmonic distortion due to the power of fundamental frequency existing in an LED driver component. The harmonics in circuits get generated due to the fast switching actions that spikes up excess current in the driver output. This may lead to high temperature in that particular circuit’s active components. The THD is expressed in percentage (%). If the harmonic components are less than 10% then their effects on systems are negligible. It follows that the THD must be kept as low as possible.
  • Efficiency: The efficiency is defined as useful power output divided by total power consumed. The following formula is used to calculate the efficiency of a driver:

    Efficiency = Output power / (Output power + Total power loss in the circuit)

    If an LED driver has more than 80% efficiency it is a better choice for LED applications.

Safety Requirements

Safety considerations are considered essential to increase performance and reduce maintenance in LED applications. The LED driver designs are different from one another. They have a variety of protection circuits, like over-voltage, over-current, short-circuit, over temperature protection, harmonic reduction circuits and enclosed, which are represented by different standards. When selecting a driver, important factors like IP rating, electromagnetic compatibility and safety approvals should be taken into account.

IP rating:

Ingress-Protection (IP) rating indicates the level of protection offered against external forces and weather conditions. Each LED driver has a two-digit IP code. The first digit, between 0 and 6, defines the level of protection against physical force and dust. The second digit, between 0 and 8, indicates the level of protection against moisture and water sprinkles on LEDs and driver. A higher number corresponds to greater protection against solids and liquids.

Drivers with IP20 or IP40 coded LED lights are protected against solid objects, but they are vulnerable to water. These are used for indoor lighting, especially in living rooms.

If an LED driver comes in contact with water/dust, such as in cases of garden lighting, street lighting or outdoor LED strip lights, you can use drivers with an IP rating from IP54 to IP65. These drivers are protected against dust and temporary immersion in water.

Drivers with ratings of IP67 or IP68 are protected from ingress of dust and continuous submersion in water. These drivers can be used in industrial applications. IP68 rated enclosures keep products safe from high temperature and high-pressure water.

Electromagnetic Compatibility (EMC):

Electromagnetic compatibility refers to radio interference caused by cables, heat sinks and the enclosure. They may function as antennas and may increase radiated emission which could possibly affect nearby electronic equipment, like communications devices, TVs and radios. All LED drivers should be tested and official results obtained from an EMC test lab.

These drivers satisfy the EMC standards to ensure that the products do not cause any radio interference.

Safety Approvals:

Organizations like the Underwriters Laboratories (UL), the National Fire Protection Association (NFPA), the American National Standards Institute (ANSI), and Canadian Standards Association (CSA) individually publish product safety standards. They are authorized to evaluate and certify products. While selecting, ensure that the driver has at least one of these authorized laboratory certificates.

The Lifetime of an LED Driver:

LED drivers enjoy a life expectancy time known as MTBF (mean time before failure). The lifetime of an LED driver is calculated based on the life expectancy of components used in it (capacitors, inductors, switching devices and transformers). Their performance may vary depending on operating temperature, current and voltage.


Examples

LED circuits can be connected in multiple combinations, with LED series, parallel and series-parallel combinations being the most common.

Series Application

In this circuit, LEDs in a series application circuit are connected back to back. The first LED anode is connected to the power supply and its cathode end is connected to the anode of the next LED. The final cathode is connected to the negative of the power supply. Hence, the flow of current starts from the first LED and ends with the last LED. The following figure shows the series connection of LEDs:

The flow of current in the above series circuit must be constant through all LEDs, and the voltage will be vary with the forward voltage drops in LEDs. The following equation is used to calculate the total required power.

Total required power = (Forward voltage of all LEDs *LED Current)

Current and forward voltage are specified in the LED data sheet.

Parallel Application:

In this circuit, the anode end of each LED gets connected to a positive source of the power supply, through a resistor and the cathode end is connected to the negative end of the power supply. The constant voltage drivers are used to power the parallel LEDs.

In a parallel circuit, the same voltage gets applied to each LED, but the current may vary on the series connected resistor. The LEDs require a current limiting resistor to protect from overcurrent. The following equation is used to calculate the limiting resistance value:

Limiting resistance = (Power supply voltage - LED dropout voltage) ÷ Forward current

Where LED dropout voltage = (Number of LEDs x voltage drop across one LED)

Series-parallel Application:

This circuit is a combination of the series, and parallel LEDs are connected to a driver. The series part of the circuit should draw the same current, and the parallel part should maintain the same voltage. Hence, the total required current must be calculated first by adding the parallel paths current.

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