PWM controller addresses power supply concerns for LED lighting

20 March 2015

LED lighting offers opportunities for energy efficiency and innovative lighting design. 

LED lighting promises visually stunning products, with reduced power losses and long device lifetimes, but ultimately it is not enough for an LED to meet those goals; the entire circuit must deliver on that promise. 


In applications with a short lifetime, low-current LEDs can easily be biased with a resistor; but in applications which require high reliability, a long lifetime, high efficiency, or good light quality there are better ways to power and monitor the diode. A power supply acting as a constant current source can compensate for the variability inherent in high-brightness LEDs, ensuring device reliability and a better customer experience. The demand from these applications is creating more capable LED drive solutions: PWM controllers, integrated constant current regulators, and switched-mode power supplies capable of managing the unique needs of LED applications. Microchip’s MCP19114 was designed to address many of these concerns, and can create the visually appealing results desired by consumers.


Consistency and quality


LED production yields wide variation in forward voltage, luminous flux and chromaticity (color quality). LED manufacturers go to great lengths to ensure a homogeneous product. If not matched properly, LEDs placed side by side can generate visibly different outputs at the same current. In many cases, the results can be visually disturbing. While good LED binning is always desirable, a well-designed power supply can also compensate for limitations in the LED manufacturing process.


Ultimately, the goal of the circuit is to deliver a specified light output. Designing a circuit to deliver consistent light output and good colour quality is not trivial. This problem is even more complicated in colour-mixing applications, where multiple LEDs need to be controlled relative to each other. An effective power supply needs to be able to accurately regulate the output currents and selectively dim the lighting with a PWM signal. It needs to compensate for changes in the input and the load, over time and temperature, to ensure good operation. The MCP19114 integrates an 8-bit microcontroller, an A/D converter, and multiple general purpose I/O pins. These can be programmed to monitor and adjust key performance parameters. For example, if the light output is a known function of temperature, and the LED temperature is read by the A/D converter from a remote thermsistor, the controller can reduce the current or modulate the supply output to compensate for the temperature measurement and maintain constant light output. This is a rare feature among power supply controllers and beyond the capability of a simple switching regulator.


Reliability and lifetime


LED light sources have extremely long lifetimes, sometimes quoted as long as 100,000 hours. This introduces two potential problems: the drive circuit must last as long as the LED and the drive circuit must keep the LED operating at acceptable conditions. A well-engineered power supply circuit can do both.


When LEDs are overdriven they suffer multiple negative effects. The luminous efficiency of the device falls to give less light per unit of input power; the colour temperature and chromaticity both shift; and the device’s lifetime is reduced. With higher currents, the total light output can actually fall as the current increases. In extreme cases, electromigration in the LED or fusing of the bond wires can cause early device failure.


Excessive heat also reduces the device lifetime. Some LEDs are rated to operate as high as 100°C, but all have a temperature limit at which the diode should be shut off. As a result, high-brightness LEDs are often mounted on heatsinks, with the heatsink sized according to the worst-case operating conditions. There are other ways to solve this problem. A smarter LED driver can measure the temperature of the LED with a thermistor or temperature sensor, and then the output can be adjusted accordingly. Microchip’s MCP19114 and MCP19115 digitally-enhanced power analogue controllers integrate an A/D converter which can be used to track changes in the LED temperature. Depending on the application, the controller can be programmed to dim the LED if it begins to overheat. At extreme temperatures, the controller can also respond by shutting down the application to prevent damage. Both the threshold and the desired response can be configured in firmware. In some cases, this could allow for smaller heatsinks, secured by the knowledge that the power supply is capable of regulating the LED temperature under extreme conditions. Finally, the controller itself also has a built-in temperature shut off component which protects the power supply and the load. These self-protection features make the end application more reliable, ensuring that both the LEDs and the drive circuit last for the intended product lifetime.


Fault detection and reporting are increasingly desired in power supply applications and an intelligent power supply solution, such as the MCP19114, can support these requirements. In LED-lighting applications, this can include identifying open circuits, short circuits or forward voltage shifts in the LED string, which could correspond to LED failures or performance degradation. Using the integrated I2C interface, the MCP19114 controller can report that data from the LED power supply to the system, identifying current and future problems. In a high-reliability or safety-critical system, this could be valuable information. The system could be configured to constantly monitor circuit performance or it could run diagnostic checks when requested. 


Efficiency


Often, consumers are looking to offset the high initial cost of lighting replacements with a gradual payback in energy savings. Similarly, portable and embedded applications, such as consumer electronics and automotive modules, need to make optimal use of the limited power available in the system. This is only possible with a high-efficiency power supply. 


The MCP19114 supports boost, SEPIC, Cuk and flyback topologies. It allows for flexible compensation selection to match the desired topology, configurable slope compensation, precise selectable dead-time and blanking time. The firmware can be programmed to adjust the switching frequency, with options to select between fixed frequency and quasi-resonant operation. Most importantly, these are all adjustable in the application firmware of the MCP19114. One circuit design can be reprogrammed to accommodate a number of different LED loads, allowing the power-supply design to be reused across different end products, without component or layout changes. In addition, firmware can be updated late in the design cycle to accommodate unexpected problems without requiring board layout or component changes, which reduces development risks. 


High-brightness LEDs come with high up-front costs. The quality of the lighting and longevity of the device can often justify the expense, but only with a high-quality LED drive solution. Robust, versatile LED drive solutions such as the MCP19114 are necessary to deliver the full benefits of high-brightness LEDs to the consumer.


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