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Posted on 27 May 2019

Driver and LED Companies must Work Together to Achieve High-Efficiency, Practical LED Lighting

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The story of a PAR 38 LED spotlight design

Delivering real-world LED lighting systems that live up to their billing – reduced power consumption, increased life, improved efficiency – has not proved to be a simple task. Of course, LED lighting is being increasingly used in many applications, but there have been horror stories too.

By Andrew Smith, Product Marketing Manager, Power Integrations

Partly, the challenge is to cope with legacy bulb formats that are far from optimal for LED technology. Another issue is that control systems which were developed for incandescent lighting, and which are installed in countless houses, offices, work spaces and public spaces are not ideal for simple LED driver power supplies.

In an ideal world, we would all – for domestic, commercial, public and industrial applications – take the decision to move to LED lighting systems and install brand new products developed specifically and optimised for solid state technology. However, this would mean a complete rethink and more than likely a rewiring of the building. To change one blown bulb would mean a total new system installation – and this is clearly not going to happen.

Gradually though, the teething problems which first affected LED lighting systems are being addressed. Design engineers have become very adept in fitting quite complex power systems into traditional light bulb fittings. Dimming, however, can still pose problems, so manufacturers of LEDs and power systems have come together to deliver reference designs that provide efficient, workable and reliable demonstrations that illustrate exactly what can be achieved.

One such design details a PAR 38 spotlight based on Cree’s Easy- White LEDs and powered by a circuit based on a device within Power Integrations’ LYTSwitchTM LED-driver IC family. Explains Mark Youmans, an Applications Engineer based at Cree’s Santa Barbara Technology Center: “The key challenge with this design is really thermal management because the application is a spotlight and as smooth dimming is also a requirement, the power supply needs to be quite complex with very high efficiency.

Cree’s application note CLDA P117 REV0 details a 150-watt equivalent, narrow beam PAR38 replacement lamp using the company’s 36 volt XLamp MT-G2 EasyWhite LED array. It is the first LED array of this type to be built on Cree’s SC³ Technology™, a next-generation silicon carbide LED platform. The MT-G2 array delivers up to 25% more lumens than previous-generation devices, while occupying the same footprint and operating with the same drive conditions, so the new design is a drop-in retrofit replacement for existing products.

Continues Youmans: “Our team set out to create a lamp with a 50,000 hour L70 lifetime (after 50,000 hours of operation, the LED will still deliver at least 70% of its initial luminous flux) which conforms to the latest ENERGY STAR requirements. We used an elegant, commercially-available, lightweight parabolic aluminized reflector (PAR) form factor heat sink design, and worked closely with industry-leading driver and optic partners to create an integrated, optimized system.”

Heat is a killer in such space-restricted, high-luminance applications, and efficiency is the key to reducing heat. Although Cree evaluated several LED driver systems, the PAR 38 spotlight design is based on the LYT4317E IC, a member of Power Integrations’ recently announced LYTSwitchTM IC family which delivers tight-regulation and high-efficiency for tube replacements and high-bay lighting, while providing exceptional performance in TRIAC-dimmable bulb applications. Power Integrations’ DER 350 describes a 20 watt, isolated flyback, LED Driver with a power factor of above 0.98.

LYTSwitch ICs combine the controller with an integrated 650 V power MOSFET for use in LED driver applications. They are configured for use in a single-stage flyback topology which provides a primary side regulated constant current output while maintaining high power factor from the AC input. As well as high-efficiency, the topology also delivers low THD, and low component count, and LYTSwitch ICs also provide a sophisticated range of protection features including autorestart for open control loop and output short-circuit conditions. Line overvoltage provides extended line fault and surge withstand, and accurate hysteretic thermal shutdown that ensures safe average PCB temperatures under all conditions.

Schematic TRIAC dimmable LED driver

The circuit shown in figure 1 (figure 4 in DER 350) shows the schematic diagram for an isolated, high power factor (PF) TRIACdimmable LED driver intended to power a nominal LED string voltage of 36 V at 550 mA typical from an input voltage range of 90 VAC to 132 VAC. The requirement to provide output dimming with low cost, TRIAC based, leading edge phase dimmers introduced a number of trade-offs in the design.

Because LED lighting systems consume so much less power than traditional technology solutions, the current drawn by the lamp can fall below the holding current of the TRIAC within the dimmer. This causes undesirable behaviour, such as the lamp turning off before the end of the dimmer control range and/or flickering as the TRIAC fires inconsistently. The relatively large impedance that the LED lamp presents to the line allows significant ringing to occur due to the inrush current charging the input capacitance when the TRIAC turns on. This too can cause similar undesirable behaviour as ringing may cause the TRIAC current to fall to zero.

To overcome these issues, active damper and passive bleeder circuits were added. The drawback of these circuits is increased dissipation and therefore reduced efficiency of the supply. For non-dimming applications these components can simply be omitted.

The active damper consists of components R6, R28, R29, D10, Q1, Q3, C3, VR5, in conjunction with R8. This circuit limits the inrush current that flows to charge input capacitors C2 and C4 when the TRIAC turns on by placing resistor R8 in series for the first ~0.5 ms of the conduction period. After approximately 0.5 ms, transistor Q1 turns on and shorts resistor R8. This keeps the power dissipation on R8 low and allows a larger value during current limiting. Resistors R6, R29, and capacitor C3 provide the 0.5 ms delay after the TRIAC conducts. Transistor Q3 discharges capacitor C3 when the TRIAC is not conducting; VR5 clamps the gate voltage of Q1 to 15 V while R28 prevents MOSFET oscillation.

The passive bleeder circuit is comprised of C1 and R1. This keeps the input current above the TRIAC holding current while the driver input current increases during each AC half-cycle preventing the TRIAC switch from oscillating at the start (and end) of each conduction angle period.

Cree Services provide a comprehensive suite of Thermal, Electrical, Mechanical, Photometric and Optical tests (TEMPO) for LED luminaires (http://www.cree.com/tempo). The company takes the implementation of its LED arrays very seriously, running a driver compatibility program' (http://www.cree.com/led-components-and-modules/tools-and-support/dcp). This is specifically tailored to compatibility at the company’s range of modules, but also shows some of the testing it performs in conjunction with reference design drivers including the PI PAR38.

 

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One Response

  1. avatar sandeep says:

    pls send some sample

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