In a feature article published during 2020, we summarized a number of methodologies for effectively driving LEDs in stable-state lighting (SSL) designs. The takeaway from that article was that there are a number of advantages that linear LED drivers can present in SSL product development. Advantages of linear LED driver architecture include lower value, easy scalability of the output channels, no magnetics required, important ripple suppression, higher electromagnetic interference (EMI) performance, and the topological capability to place several circuits in parallel with no cumbersome selection of LED units.
However, there is a reluctance to employ linear LED approaches, which is justified by an anticipated low general system efficiency. Designs for a devoted working vary optimize the effectivity of a linear circuit, but outdoors this range, the losses are still increased than for a change-mode topology. A novel method for the control loop feedback reduces this drawback significantly, enabling comparable efficiencies to those of switch-mode circuits. Measurement outcomes show the brand new driver IC design achieves these design goals.
Inside a new IC structure
The BCR601 is a linear LED controller IC regulating the LED present with an exterior driver transistor. It controls an exterior transistor masking a wide power range. The LED current is adjustable by an external resistor. Fig. 1 reveals a typical software schematic with the system.
A singular function of the IC method is the suggestions to the primary side controller by regulating the output voltage of the first stage converter. The combination within the control loop minimizes the voltage overhead and, in flip, energy losses. If you liked this article and you also would like to be given more info with regards to linear led light cost generously visit the web page. This suggestions loop makes use of frequency compensation to avoid interference with the power factor correction (PFC) function.
The feedback to the first facet controls the headroom for the exterior transistor. This voltage margin ensures the supposed current circulation. An external resistor at pin VDROP determines the worth of the overhead. Once the preliminary headroom is set up, the system will dynamically regulate the input voltage by offering an acceptable feedback signal to the first stage. As a result, a 30W LED association with 18 LEDs and 0.56A working present achieves higher than 94% power efficiency (see the desk for information on output voltage components evaluated). That is comparable to the ability efficiency of a mean buck controller.
This suggestions characteristic differentiates the BCR601 from different linear controllers. Also, it allowed the the new linear controller IC to be relocated to the printed-circuit board (PCB) of the LED driver, fairly than on the same board as the LEDs, as is found with widespread linear regulators.
Ripple suppression
Linear drivers have an intrinsic property that, with proper dimensioning, allows them to keep away from fluctuations within the LED current. This property eliminates variations in brightness ranges. Changing gentle ranges might cause several adverse results for SSL system finish users. The same old terms for such effects are flicker or non permanent gentle artifacts (TLA). In numerous countries, regulation or legislation to limit TLA is in place or underway. The preferred topology is a single-stage flyback with integrated PFC driving a relentless current on the secondary facet. For correct PFC functioning, linear led light cost some AC voltage after the transformer is important, as solely then can the enter present follow the road voltage and reduce reactive energy use (good PF). Thus, a secondary stage has to deal with that. Alternatively, a large and expensive capacitor may easy the voltage after the rectifier diode.
The primary perform of a linear regulator is the control of the output present by regulating the output voltage to the specified value. The respective driver transistor blocks the surplus voltage level. As lengthy as the minimum input voltage to the linear driver has a adequate stage – increased than the output (LED) voltage – the output current will likely be easy without the ripple from the enter voltage, thereby avoiding any TLA. A task for the product developer is to think about the losses of the output transistor (VDROP). Power losses are proportional to the RMS (root mean square) worth of the voltage over the transistor.
Dimming
The inherent ability of linear LED drivers to take away output ripple enhances the sunshine high quality significantly. This helps human-centric lighting necessities, comparable to the newest guidelines and Light Concepts outlined in the Well Building Standard, since fluctuations in mild levels at inopportune occasions might need a major affect on the wellbeing of occupants in a space. The standard technique to dim configurations equivalent to LED strips or channel letters is a pulsewidth-modulated provide voltage. This means the provision is turned on for a certain duration of a cycle. Turned off for the rest of the cycle. Depending on the time, the provision voltage is turned on relative to the cycle time (duty ratio), then the average brightness of the LED scales with the same ratio relative to full present. For electromagnetic compatibility (EMC) causes, the cycle time often lies between 10 ms and 1 ms. The longer the cycle time is for this digital modulation (full-on, full-off), the greater the effect on people.
To keep away from this gentle flicker, the newly developed linear driver IC structure makes use of a distinct methodology for dimming. The dimming enter is analog and might modulate the output current proportionally all the way down to about 3% of the nominal current. Applying a voltage between 0.1V and 3.3V results in an output present between 3% and 100% of the nominal present; the mapping to the respective output present is linear. The steady output present is analog tailored to the respective dimming sign. The technique gives a stable mild output over a large brightness vary.
Lifetime issues
LEDs sometimes have a very long lifetime of 50,000 hr, with some even promising as much as 100,000-hr operating time. However, the definition of this lifetime will not be dictated by common logic. Instead, a lifetime assertion for an LED assumes that the LED will provide, after the top of the interval, greater than 70% of the light output from the primary working hours (often known as L70). Furthermore, a standard concept in the lighting trade ensures the lifetime promise only for 90% of the entire inhabitants, so 10% may fail earlier.
Most basic lighting applications require fixed, uniform illumination. There are two strategies to make sure that. One choice is to drive the LED with a higher electrical present in the beginning to compensate for the 30% loss – which additionally requires no less than 30% more energy over the lifetime of the SSL system. An alternate methodology is the dynamic enhance of the LED current proportional to the deteriorating efficacy. That requires suggestions from a light sensor circuit, reducing the additional lifetime effectivity loss by half. With the continued worth reductions for LEDs, one other suggestion is to change the LEDs once the luminous flux drops below a threshold. For the final method, it is helpful to exchange the LED module with provide voltage applied; this gives immediate feedback if the brand new module is working. Thus, the expensive contractor on the ladder saves time not having to climb down, change the light on, and go up once more to finish. A sizzling-plug function noticeably reduces the price of the alternative.
The novel features of the most recent linear LED driver ICs open up new options for power conversion. For instance, linear neon led flex drivers help smaller LED drivers as they need fewer exterior components and especially no heavy and bulky magnetics. The linear working mode gives an extra advantage as it has no high-frequency switching, which reduces the complexity of assembly EMC requirements. The omitted output power switching additionally eliminates the radiated energy on LED wires, facilitating longer connection to the LEDs. KURT MARQUARDT is senior director of LED lighting techniques. Product advertising at Infineon Technologies.
