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LED lighting with its extraordinary visual performance (users’ viewing experience) and eyecatching energy savings is an excellent lighting option when compared to traditional lighting options.

Today, LEDs have become even more efficient than earlier generations, providing 25-30 per cent energy savings compared to CFLs and up to 80 per cent savings compared to incandescent bulbs. Along with energy savings, the other reason driving the switch to LEDbased lighting is the significantly longer lifespan of the LEDs.

Worldwide, LED lighting market is growing at accelerated pace. These efficient LEDs need compatible LED drivers. The market expects highly reliable, energy efficient, protective and differentiated LED driver products at competitive prices.

Keeping these tough design challenges in mind, Power Nucleus has worked with Infineon to effectively meet and exceed the market expectations from LED drivers.

In the beginning, the lighting industry had promised consumers a new, affordable light source (LED) with low energy consumption and a long life. Unfortunately, the industry’s focus on cutting cost to the extent of sacrificing quality meant that the market was soon flooded with poor quality and low-cost products. To fulfil the initial promise to consumers, a design revolution for a high-quality product at low costs needs to be the top priority for the LED lighting industry.

The strongest need for efficient and reliable LED drivers comes from customers serving the street lighting segment, where the cost of lamp replacement is sometimes higher than the cost of LED drivers itself. They also need to provide 5 to 7 years of warranty on their products. These factors are considered during the design stage and selection of components and topology. ICL5101 from Infineon is a resonant controller which offers great features to develop efficient LED drivers. Few of its key features include:

 Universal input from 80 to 305V
 Ultra-fast time to light <200ms
 Secondary side-controlled PFC+LLC or PFC+LCC (CV/CC)
 Fully integrated 650V HB driver stage
 Supported outdoor use by extended temperature range (-) 40 to (+) 125-egree C
 High power quality with PFC >0.99 / THD <10 per cent
 Highest efficiency up to 94 per cent in resonant topology
 Protection: OCP / OVP / OLP / short output
 Saturation control during start-up
 Adjustable ext. temperature protection.

Figure 1: Constant current performance of 150W LED Driver

Constant current or constant voltage or both

LEDs are driven with DC voltage or current. Almost all LED drivers are simple AC-DC power supplies. That being the case, why is it important to term a power supply as LED driver? Let’s understand a little bit of background of the LEDs.

You can drive the LEDs with constant voltage (CV). There are many CV drivers available in the market, but without some kind of output current limit. With CV drivers, the problem lies with LEDs heating up, whereby the forward voltage drops and the current passing through the LED increases. Higher current leads to more heat generated from the LED. Without the forward current limit, the LED will fail due to the high heat at its junction. This type of failure is known as thermal runaway.

The LED light output is proportional to the forward current flowing through it. This means, any variation in the LED current will vary the light output as well. This is why driving the LEDs in CV mode results in LED lifetime issues. The learning from this is that the LEDs should be driven with constant current (CC) which guarantees a constant light output from the LEDs. Accurate and ripple-free constant LED current is recommended to produce excellent, flicker-free light output.

Let’s assume that the LED load is disconnected for some reason, or in case of installation, wherein the input power to the LED driver is on and no LED current flows; since the driver is configured in the constant current mode, it will drive up the output voltage, high enough to damage the output circuits including output capacitors.

This is known as open load condition, when driver output voltage must be limited to a safe value when the LED current is lower or when there is no load. This requires the driver to operate at constant voltage.

Figure 2: System efficiency of 150W LED driver

Drive towards high efficiency (efficacy matters)

Many system integrators, LED lighting contractors and even the governments have started to demand for more lumens per watt (>120 lm/W). More lumens per watt means more energy savings.

There are several ways to achieve this. For example, one can use very high efficiency LEDs at a higher cost. The better way is to build highly efficient LED driver, so that:

 It meets one’s efficacy demands easily.
 Relaxes thermal management requirements which helps to meet more serviceable life from the driver.

Now, let’s take a look as to what it takes to develop high efficiency LED drivers:

 Right power conversion topology selection for the given power rating of the LED driver.
 Knowledge about the selection of power semiconductors incl. MOSFETs, input-output diodes with optimum derating, without impacting the cost of the product.
 Deep knowledge about the magnetic components design for various power topologies (LLC converter, PFC converter, Flyback converters, interleaved converters). Many a times, this is the real cause of overall poor efficiency of the LED driver. A power designer expert will be able to handle with great care and do a wonderful job.
 Control the converter in such a way that it operates at minimum losses. The best way of doing it is to have zero voltage switching (LLC) or quasi resonant operation which helps to maximise the product life and at a lower cost.
 Also, the selection of aluminium electrolytic capacitors. These must be rated for high load life (>8,000 hours) at maximum operating temperature.

Figure 3: Input power factor w.r.t. input voltage of 150W LED driver

Advantages of selecting LLC topology

 Zero voltage switching (soft switching) helps to lower switching losses and improve system efficiency.
 Due to lower switching losses, the design can operate at higher frequency which can result into smaller magnetic components and reduction in board size i.e., increase in power density.
 Soft switching over wide load range – high efficiency at lower loads.
 Soft switching enables less EMI – Lower EMI component cost.
 Half bridge switching enables usage of 600V MOSFETs, unlike 800V MOSFETs in Flyback topology.
 Lower VRRM (reverse voltage) of output diode due to lower stress in soft switching topologies.

High efficiency in LED drivers is achieved by using Infineon’s power semiconductors and combo – LLC Controller IC ICL5101.

Figure 4: Input current THD performance of 150W LED driver

Thermal management

While high power LEDs are running, the lighting fixture temperature can be quite high under some conditions. As the drivers are generally enclosed within the fixtures, the LED driver’s electronics is most likely to be exposed to the same temperature. If the LED drivers continue to operate at such high temperatures, their serviceable lifetimes will be degraded significantly. The high-quality drivers enable intelligent adaptive temperature protection. The output current of the LED driver starts to drop when the thermal limit is exceeded until the thermal is within operating range again and the driver delivers the full current.

To achieve this excellent requirement, the thermal foldback feature is implemented in all LED drivers designed by Power Nucleus LED Drivers.

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