Harmonized Two-Stage LED Driver Systems

Article By : Michael Weirich and Matthew Reynolds, Infineon Technologies AG

It has become problematic to satisfy all design and regulatory specifications of modern lighting systems with a single-stage power-conversion architecture.

It has become problematic to satisfy all design and regulatory specifications of modern lighting systems with a single-stage power-conversion architecture. Today’s hardware electronics engineers are resolving the requirements by developing multiple-stage driver designs.

The transformation from fluorescent and incandescent lighting to solid-state lighting (SSL) in a relatively short timeline is not something you see often. Fast-paced disruptive technologies occur only when the need is obvious and the economics make sense for adoption. An additional force on the transition to SSL was the fact that SSL offers a better quality product than the incumbent, which sealed the deal.

The pace of better quality (dimming, color-point, consistency, etc.) and cost have slowed due to full adoption, but there is still plenty of room for improvement. Today’s lighting suppliers focus on exceptional light quality, meeting regulatory needs for today’s standards and the future while maintaining a reasonable cost position.

Quality lighting encompasses many categories, a few of which are described here. One of them is human-centric lighting (HCL). HCL focuses on artificial lighting in such a way as to not disrupt or confuse your body’s natural rhythm, and in some cases, it can enhance your performance (sleep or work) by adjusting the light type.

Dimming range, the ability of the fixture to dim precisely from 100% to less than 1%, is another important metric. And, as important as the range is, the uniformity between fixtures is vital. There is nothing more noticeable to a lighting engineer than walking into a room with dozens of fixtures that are all illuminated at slightly different outputs from one another.

Light modulation is defined as a repetitive change in magnitude over time, or modulation, of the luminous flux of a light source. The modulation’s magnitude and frequency are the two variables describing the severity of the modulation — more significant changes (high-low) and lower frequencies being the most detrimental to neurological problems, fatigue, headaches, etc. One may not visibly see the light source’s modulation, but its effects may be felt. IEEE-1789, a standard that addresses light modulation, is becoming the dominant standard for light-modulation limitations within lighting products. It will be a requirement in some form over the next couple of years.

Finally, no article on lighting is complete without the mention of intelligent lighting systems. The adoption of intelligent lighting systems has not matched the pace of SSL adoption, but there are signs of life as the market settles on common ground.

Considering the challenges, it has become problematic to satisfy all design and regulatory specifications with a single-stage power- conversion architecture. Today’s hardware electronics engineers are resolving the requirements by developing multiple-stage driver designs. With two power-conversion stages, the design engineer can split the performance and regulatory requirements between the two conversion stages and optimize accordingly.

Let’s review a few common LED driver architectures for LED drivers with sub-100-W loads and rate them on the discussed metrics:

Single-stage architecture (PFC/flyback)
+++ Cost

  • Dimming performance

  • Efficiency

  • Ease of regulatory compliance

Dual-stage architecture: PFC boost followed by a constant-current flyback or LLC converter

  • Cost
    ++ Dimming performance
    ++ Efficiency
    ++ Ease of regulatory compliance

(Source: Infineon Technologies)

Dual-stage architecture: PFC/flyback with a constant-voltage output followed by a DC/DC secondary-side post regulator (constant current)

++ Cost
+++ Dimming performance
++ Efficiency
+++ Ease of regulatory compliance

(Source: Infineon Technologies)

For designs with output power less than 100 W, the topology that satisfies all requirements is the single-stage PFC/flyback followed by a DC/DC constant-current (CC) buck regulator. The AC/DC PFC/flyback focuses on regulatory requirements such as PFC/iTHD and on delivering a well-regulated constant voltage to the output. The DC/DC CC buck regulator’s sole function is to deliver a well-regulated DC current to the LED array. This system approach satisfies the designer requirements with a minor penalty of cost and efficiency — if done correctly.

Infineon Technologies has released two AC/DC PFC/flyback controllers targeting these applications. The output of these devices is a constant voltage (CV) but intended to be paired with DC/DC CC regulators for the lighting market.

The lighting market has unique regulator needs, including exceptional PFC and low iTHD over wide operating conditions. Fast startup time and protection schemes are required in SSL systems.

Infineon has developed secondary-side regulators such as the ILD8150 and BCR601/602 to ensure that best-in-class dimming and light-quality standards are achieved. They include dimming below 1% and meet IEEE 1789 light-modulation regulations.

The company has designed the ILD8150 and BCR linear regulators to optimize the system performance by working in harmony with the AC/DC controllers — by dynamically controlling the magnitude of the PFC/flyback’s output voltage depending on LED array voltage requirements. The buck converters can dynamically optimize the system for the best efficiency and best dimming possible. The traditional two-stage design would pay the penalty in efficiency; this inefficiency is easily recaptured — and the efficiency of the traditional design is often surpassed — with harmonized two-stage approaches.

Linear regulators with active headroom control

Linear regulators have benefits such as ease of use, cost, and not creating further EMI noise in an LED driver. Of course, they can be an inefficient solution if used traditionally. A linear regulator is an efficient method as an LED regulator as long as the input voltage is greater in magnitude than the LED array voltage. Infineon has developed a series of LED linear regulators that can adjust/control the regulation of the AC/DC converter. The BCR601 adjusts the feedback loop to ensure there is always just enough headroom across the pass element to regulate its desired current.

Using linear regulators, traditional LED drivers would require the design engineer to set the AC/DC converter’s output voltage to be greater than the worst-case LED array voltage. Worst-case (greatest) voltage requirements on the LED array are present when the LEDs are cold, when they have high current density through them, and when tolerance of the forward voltage is considered. Once the worst case is determined, the output voltage is set, but typical LED array voltages would be lower than worst-case ones, and an inefficient system is realized.

The BCR601’s active headroom control feature dynamically adjusts the AC/DC converter’s output voltage regardless of LED count, LED temperature, and LED current density, all of which affect LED array voltage; the output voltage will continuously be optimized for LED current regulation and efficiency.

Simplified Illustration of XDPL8219 + BCR601 (Source: Infineon Technologies)

Adjusting the AC/DC voltage to be slightly greater in magnitude than the LED array requires for proper regulation has benefits with DC/DC CC switching regulators. Minimizing the duty cycle regardless of the topology often results in higher-efficiency systems. Infineon has developed devices, circuits, and evaluation platforms that take a system approach to LED driver design.

Infineon Technologies’ ILD8150 is a 1.50-A 80-V hysteretic DC/DC CC regulator designed to meet IEEE1789 standards.

By pairing the ILD8150 with the XDPL8219 or the ICL8800 AC/DC converter and allowing the DC/DC converter to control the feedback loop, system efficiency for a two-stage design can be greater than 92% with exceptional dimming performance.

Simplified illustration of Infineon’s XDPL8219 controller + ILD8150 DC/DC converter IC (Source: Infineon Technologies)

Easily configurable digital AC/DC controllers, paired with DC/DC CC regulators designed to work in harmony, are required to meet today’s and tomorrow’s quality of light and regulatory requirements.

This article was originally published on EE Times.

 

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