Categories Lighting

Reducing LED Die Counts and Light Costs Through Better Thermal Management

The LED market is sitting tight on its upward curve at the moment, with huge predictions in terms of revenue expected in the coming years. However, this trend could easily be derailed if the initial purchase price of LED lights remains high or consumers perceive there is a problem with reliability.

We’re seeing fierce price competition across all sectors of the LED market. In many instances this is forcing designers to adopt cheaper components. This is a mistake. Using cheaper components brings with it the inherent risk of compromised products; lower light quality, reduced lifespan, and the potential for reputational damage to a young industry.

A better approach to using cheaper components to save money, especially in the HB-LED (High Brightness LED) market, lies in better thermal management. Clever design incorporating innovative, emerging thermal management technologies holds the potential to radically lower the prices of certain classes of LED lighting products.

In an LED light, the semiconductor chips can account for as much as 60 percent of the BOM (Bill of Materials). If a business could halve the number of LEDs, running the remaining LEDs at higher power to achieve the same Lumens output, that business could obviously make a significant saving.

In many designs of HB-LED lights the semiconductor chips reach a thermal limit before they reach their electrical limit. If the thermal demands can be met, the economics are very attractive. Even if the improved thermal management involved a small added cost, say 5 percent, a business would still save 25 percent on its overall BOM in this scenario – a highly desirable figure in the current environment.

The traditional approach to conducting heat away from LEDs is to use Metal Clad Printed Circuit Boards (MCPCBs).  These are essentially an aluminium panel coated with about 75 to 300 microns of epoxy resin to act as a dielectric layer. The resin is filled with ceramic particles to improve its through-thickness thermal conductivity. Unfortunately this approach is reaching its technological limit. Epoxy resins cannot currently be made any thinner without losing workability, or filled with more ceramic as this reduces fracture toughness and causes loss of adhesion.

In other words, we’re at an impasse. The shortcomings of available MCPCBs are limiting the industry’s ability to achieve significant cost reduction through cutting the number of LEDs in a design.

However, a radical new approach to tackling the LED heat extraction impasse has been discovered that makes use of nanoceramics.

A process has been developed to convert the surface of aluminium panels to a dense, adherent layer of  aluminium oxide, possessing a nanocrystalline grain structure (an alumina nanoceramic). This process creates a dielectric layer as thin as 3 microns with thermal conductivity in the region of 7W/mK. The combination of the thin dielectric, with good thermal conductivity, gives a 20 percent thermal improvement over any commercially available MCPCB. The thickness of the nanoceramic dielectric layer can be set to meet industry breakdown voltage requirements. Because the boards are essentially panels of aluminium they can be slotted into existing PCB fabrication lines, so there are no additional costs associated with changing the manufacturing process.

In recent tests HB-LEDs mounted on a nanoceramic MCPCB ran 34°C cooler than on a standard MCPCB, resulting in slightly more Lumens being produced for the same electrical input. More importantly, this result opens up the design space and the possibility of decreasing the number of LED die by running each closer to its maximum electrical rating. This is the kind of radical improvement that is required if the LED industry is going to continue to push the limits of High Brightness LEDs.

Categories Lighting

Comparing Critical Conduction Mode and Discontinuous Mode PFC Designs for LED Lighting Applications

Engineers building LED lighting applications have to be constantly aware of evolving regulatory requirements for power factor (PF), Total Harmonic Distortion (THD) and efficiency. Today regulatory agencies generally require PFC to be >.9 for designs > 5W at the nominal line voltage for that design. But the trend is for regulatory agencies to continually push the requirements for PFC down to lower and lower power levels.

A similar movement is underway in THD requirements. Currently regulatory agencies require <25 percent THD for applications > 25W. But we are already beginning to see agencies worldwide push that requirement down to < 10 percent in many applications. While there are no specific THD requirements for light bulb applications currently, it seems clear designers can expect them in the near future.  Given these trends it appears clear that designers building LED lighting applications today need to achieve as high a PF as they can and a THD as low as they can over as wide an operating voltage as possible.

In terms of efficiency, regulatory agencies have set some standards for power levels <20W.  A non-dimmable power supply for that voltage needs to be at least 85 percent efficient across nominal line voltages. Regulatory agencies reduce that requirement for lighting applications using phase-modulated dimming because the dimming circuitry requires external components that run at a fixed loss. But, again, regulatory requirements are clearly moving toward higher efficiency requirements.

Most AC-input and isolated power supply designs for lighting applications use a flyback circuit to meet PFC requirements. This single stage flyback PFC solution typically incorporates a Critical Conduction Mode (CrCM) control IC, which operates with a fixed on time, variable off time. This approach offers excellent constant current or constant voltage regulation at the output. But it was originally intended for boost PFC applications, not single stage flyback applications. With circuit modifications for load control and the addition of a fast start circuit, designers can achieve passable efficiencies and reasonable PF and THD. But the CrCM approach has some inherent characteristics that impose upper limits on how well it can perform in those applications. Furthermore, the CrCM architecture adds a number of additional components that impact reliability and cost.

Another popular option is the Discontinuous Mode (DCM) PFC approach. In this topology the power supply operates with a fixed on-time and fixed frequency for any given line load configuration. By doing this, it eliminates some of the limiting factors that occur in a CrCM approach and allows designers to achieve higher PF and lower THD.

Recently Fairchild took two evaluation boards and built two comparable 50W LED lighting solutions using both the CrCM and DCM PFC architectures. Both solutions worked well and each offered distinct advantages. But the CrCM approach offered PF, THD and efficiency performance that barely met current requirements for a class C lighting application. In contrast, the DCM board, based on Fairchild’s new FL7733A single stage, primary-side-regulated LED driver, significantly outperformed the first design in all three categories with a significantly simpler design. Given the trend in regulatory requirements, the DCM PFC approach may offer a better fit for LED lighting designers looking for more performance headroom in future applications.