Optimizing MR16 LED bulb designs

By HUBIE NOTOHAMIPRODJO,
Director of Marketing, Solid State Lighting Products,
Dialog Semiconductor,
www.dialog-semiconductor.com

The basic schematic of an electronic transformer (ET) includes a traditional full-bridge rectifier, a self-oscillating chopping circuit, and an output-power transformer. The self-oscillating circuit operates at a relatively high frequency, typically between 20 kHz and 300 kHz, compared to the line voltage. This allows for smaller magnetics on the output stage of the transformer and, therefore, a lower overall cost for an electronic transformer versus a magnetic transformer. The key characteristic of electronic transformers that impacts halogen-replacement LED lighting is their self-oscillating intermediate stage.  

Typical electronic transformer

Fig. 1  shows the schematic of a typical ET. The circuit is composed of three main parts: the self-oscillating half-bridge circuit with the small transformer that drives the bases of the power transistors; the start-up stage, which kick-starts the output section into a self-oscillating mode; and the main power transformer stage. The charge/discharge of the half-bridge circuit is driven by the small transformer in series with the main power transformer. When the current draw through the transformer is not large enough, there is not enough energy to switch the power devices fast enough to sustain a stable oscillation. In the absence of this load, the output stage may exhibit erratic switching behavior or it may not switch at all.  

Fig. 1:  Simplified electronic transformer schematic.  

Halogen bulbs work fine with electronic transformers because they present a minimum 35-W load, with each bulb typically on its own transformer—more than enough load to keep any electronic transformer operating properly. LED bulbs designed to replace 50-W halogen bulbs draw 8 W on average, while 35-W replacement bulbs typically draw 4 W. Due to the minimum loading requirements of electronic transformers, this presents a major compatibility issue when using LED bulbs in legacy installations. Even if multiple bulbs are powered from one transformer, compatibility can still be an issue, resulting in light shimmer due to the unstable operating characteristics of the ET at light loads.  

To make an LED-based MR16 bulb backward-compatible with legacy installations, the controller needs to keep the ET oscillating correctly, both at full output power and when interacting with a dimmer. Dimming compatibility increases the complexity of the LED-driver circuit because there is a limited amount of energy available to provide current to the LEDs, while simultaneously maintaining transformer oscillation. 

Low-voltage electronic transformers are often combined with leading-edge or trailing-edge phase-cut dimmers, which work the same as traditional dimmers do with magnetic transformers. The LED driver needs to measure and interpret this waveform and reduce the output current proportionally. As the phase of the dimmer output reaches low levels, the LED driver needs to compensate for the lower power draw from the ET to ensure stable operation. One simple way to do this is to use a dc bleeder circuit, which draws a high continuous current. But thermal limitations directly affect this approach, not to mention the efficiency impact. MR16 halogen bulbs thrive in hot environments, making thermals a non-issue, but LEDs suffer degraded lifetimes when operating at excessively high temperatures. If a bleeder draws constant current, continuously dissipating heat and raising the temperature inside the LED bulb, the LEDs and even the passive components used to create the LED-driver circuit can face a significantly reduced operating life. Eliminating the dc bleeder circuit can solve this problem.  

LED driver

The iW3662 LED-driver controller from Dialog Semiconductor enables MR16 bulbs with a very broad dimming range, from as low as 5% to 100%, and integrates a controller configurable for either single-stage or two-stage operation. The primary stage is an initial boost stage, and the second stage can either be configured as a buck regulator that doubles as a current regulator or simply a linear current regulator to control the current to the LEDs. This allows the controller to drive either high-voltage chip-on-board (COB) LEDs (single-stage boost-linear mode) or low-voltage LEDs (two-stage boost-buck mode). The key to the device is the digital-control algorithms, which manage the input and output current to ensure that it regulates the LED current according to the detected dimming phase angle, draws sufficient current from the input to keep the electronic transformer regulating properly, and monitors temperature to prevent overheating failures while maintaining good efficiency.  

Fig. 2  shows a typical application diagram using the iW3662 to power a high-voltage, chip-on-board LED. The boost input stage provides all the power conversion, and the second stage becomes a linear current regulator to provide the necessary constant current control to the LEDs.

Fig. 2: iW3662 MR16 LED driver configured in boost-linear mode for high-voltage CoB LEDs.

The iW3662 LED driver integrates two additional circuits that work with the boost-converter stage to keep the dimmer and electronic transformers functioning correctly. During dimming mode, when the requested output current is low, to keep the input circuits latched and operating properly, the controller enables a bleeder circuit that draws current from the input. When the boost converter and the load draw enough current to maintain ET and dimmer compatibility, the bleeder circuit is disabled. This dynamic bleeder circuit provides the dimmer compatibility required for MR16 applications.  

When the input voltage is at its maximum value, any excess energy burned by the controller or the bleeder circuit can be conserved to save energy. This technology, integrated in the main boost-converter stage of the iW3662, improves efficiency and reduces heat generation. The combination of a dynamically loaded bleeder circuit at minimum dimming conditions and hysteretic control of the input boost-converter stage provides an optimal solution for MR16 lighting. The key to this functionality is the core digital algorithms that allow the input circuit to accomplish multiple functions without adding an excessive number of external components. The size aspect is crucial because MR16 bulbs are physically smaller than other traditional bulb types, limiting the available space to create an LED-driver solution. The result is a low-BOM cost solution with good efficiency and the ability to dim MR16 bulbs from 100% to as low as 5%, depending on the dimmer. 

Fig. 3  shows a simplified representation of an MR16-based lighting system from 230 V_AC . The line voltage connects directly to a TRIAC-based dimmer. The output of the dimmer provides the supply voltage to the electronic transformer, which outputs a modulated 12 V_AC to power the MR16 bulb. The LED-driver circuit, in this example designed using the iW3662, is incorporated in the base of the MR16 bulb. The base has a reduced space in which to implement such a complex circuit, but the highly integrated iW3662 solution enables a small enough size to allow the circuit to fit inside the enclosure without sacrificing performance. 

Fig. 3: MR16 application diagram — LED-driver circuit in base of MR16 bulb.  

Table 1  shows tested data for the iW3662 LED driver using three different TRIAC trailing-edge dimmers and five different electronic transformers, with the iW3662 powering a single, 5-W MR16 LED bulb. Trailing-edge dimmers are historically more commonly used in MR16 applications in the general marketplace, and they offer technical advantages over leading-edge dimmers.  But leading-edge dimmers are also used and the overall circuit must be compatible with these dimmer types as well.

Table 1: Tested compatibility of the IW3662 LED driver circuit with trailing-edge dimmer and electronic transformer combinations.

Dimming

Fig. 4  shows the dimming capability of the iW3662 paired with a leading-edge dimmer and an electronic transformer in a 5-W, single MR16 LED bulb application. In this application, the iW3662 achieves a full dimming range with what is a traditionally difficult combination of dimmer/transformer technology. Leading-edge dimmers can cause dimmer compatibility issues when working with electronic transformers in LED applications due to residual voltages. But digital control can actively manage these issues, offering dimming characteristics comparable to those of trailing-edge dimmers. The iW3662 LED driver optimizes MR16 designs for transformer and dimmer compatibility, with good dimming performance, while maintaining a small circuit size and low overall BOM cost.

Fig. 4: Dimming curve of the iW3662 with Druck Wechsel leading-edge dimmer and YT70 electronic transformer.