Recent considerations have shown the usefulness of linear LED drivers for several applications. There are many obvious advantages of such driver architecture like lower cost, simple scalability of the output channels, no magnetics required, significant ripple suppression, better EMI performance or the topological advantage to put several circuits in parallel without a cumbersome selection of LED devices. There is a reluctance against this architecture justified by an anticipated low efficiency. Designs for a dedicated operating range optimize the efficiency of a linear circuit, but outside this range, the losses are still higher than for a switch-mode topology. An innovative approach for the control loop feedback reduces this drawback significantly. It enables comparable efficiencies as switch-mode circuits. Measurement results with Infineon’s BCR601 that implements this method, demonstrate that impressively.
BCR601 is a linear LED controller IC regulating the LED current with an external driver transistor. It controls an external transistor covering a wide power range. The LED current is adjustable by an external resistor. Figure 1 shows a typical application schematic with the device. A unique feature of BCR601 is the feedback to the primary side controller by regulating the output voltage of the primary stage converter. The integration in the control loop minimizes the voltage overhead and, therefore, the power losses.
This feedback loop uses a frequency compensation to avoid interference with the PFC function. The feedback to the primary side is controlling the headroom for the external transistor. This voltage margin guarantees the intended current flow (Table 1: Output voltage components). An external resistor at pin VDROP determines the value of the overhead. Once the initial headroom is set up, the system will dynamically regulate the input voltage by providing an appropriate feedback signal to the primary stage. As a result, a 30 W LED arrangement with 18 LEDs and 0.56 A operating current (Table 1) achieves the power efficiency of more than 94 %. This is comparable to the power efficiency of an average buck controller.
This feedback feature differentiates BCR601 from other linear controllers. Also, it relocates BCR601 – in contrast to the common linear regulators usually located on the same board as the LEDs – to the PCB of the LED driver.
Ripple suppression Linear drivers have an intrinsic property which, with proper dimensioning, enables them to remove fluctuations in the LED current. This property avoids variations in brightness. As several negative effects for the users of the illumination are caused by the changing light levels, it is of significant benefit to avoid it. The usual terms for such effects are flicker or temporary light artifacts (TLA). In different countries, regulation or legislation to limit the TLA is under way. The popular topology is that of a single stage flyback with integrated Power Factor Correction (PFC) and a constant current stage on the secondary side. As a PFC needs some AC voltage residual at its output, the following secondary stage needs to minimize its influence on the LED current. Alternatively, a large and expensive capacitor could smooth the voltage.
Figure 2: Waveform of secondary side voltage and corresponding LED current with BCR601
LEDs typically have a very long lifetime of 50 000 h, with some even going to 100 000 h. However, the definition of this lifetime is not as expected from by common sense. Instead, a lifetime statement for a LED assumes that the LED will provide, after the end of the period, more than 70 % of the light output from the first operating hours. In addition, a common term in the lighting industry guarantees the lifetime promise just for 90 % of the whole population, so 10% might fail earlier.
Some applications require a constant illumination strength. There are two methods to ensure that. One option is to drive the LED with a higher electrical current at the beginning to compensate for the 30% loss – which also requires at least 30% more energy over lifetime. An alternative method is the dynamic increase of the LED current proportional to the deteriorating efficacy. This needs a feedback from a light sensor circuit, cutting the additional lifetime efficiency loss by half. With the continuous price reductions for LEDs, another suggestion is to exchange the LEDs once the light flux is below a threshold. For the last approach, it is beneficial to exchange the LED module undervoltage, as the expensive man on the ladder saves time to climb down and up reducing the cost for the replacement.
The novel features of the latest linear LED driver ICs open new options for the power conversion. For example, linear LED drivers support smaller LED drivers as they need fewer external components and especially no heavy and bulky magnetics. The linear operating mode provides an extra advantage as it has no high frequency switching. This saves many efforts to meet EMC requirements. The omitted output power switching eliminates also the radiated energy on LED wires facilitating longer connection to the LEDs.
The inherent feature of linear LED drivers to remove output ripple, enhances the light quality considerably. This is beneficial for the trend of human-centric lighting. The usual method to dim configurations such as LED stripes or channel letters by PWM-modulating the supply voltage would cancel this benefit. Hence, BCR601 uses a different methodology for dimming. The dimming input is analog and can modulate the output current proportionally down to about 3% of the nominal current. This technology provides stable light output over a wide dynamic range.