Linear LED Driver IC Design Reduces Losses (Magazine)

In a feature article printed during 2020, we summarized a number of methodologies for successfully driving LEDs in strong-state lighting (SSL) designs. The takeaway from that article was that there are multiple advantages that linear LED drivers can present in SSL product improvement. Advantages of linear LED driver structure embrace lower cost, easy scalability of the output channels, no magnetics required, important ripple suppression, better electromagnetic interference (EMI) performance, and the topological functionality to place several circuits in parallel and not using a cumbersome choice of LED gadgets.

However, there is a reluctance to employ linear LED approaches, which is justified by an anticipated low overall system efficiency. Designs for a dedicated operating range optimize the efficiency of a linear circuit, however exterior this range, the losses are still higher than for a switch-mode topology. A novel approach for the management loop feedback reduces this downside considerably, enabling comparable efficiencies to these of swap-mode circuits. Measurement results present the new driver IC design achieves these design objectives.

Inside a brand new IC structure

The BCR601 is a linear LED controller IC regulating the LED current with an exterior driver transistor. It controls an exterior transistor overlaying a large power vary. The LED present is adjustable by an external resistor. Fig. 1 exhibits a typical application schematic with the device.

A novel function of the IC method is the suggestions to the first side controller by regulating the output voltage of the first stage converter. The integration within the management loop minimizes the voltage overhead and, in turn, power losses. This feedback loop makes use of frequency compensation to avoid interference with the ability issue correction (PFC) perform.

The suggestions to the primary aspect controls the headroom for the exterior transistor. This voltage margin guarantees the intended present flow. An exterior 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 suggestions sign to the primary stage. In consequence, a 30W LED association with 18 LEDs and 0.56A operating present achieves larger than 94% power efficiency (see the table for data on output voltage elements evaluated). That is comparable to the ability effectivity of an average buck controller.

This feedback characteristic differentiates the BCR601 from other linear controllers. Also, it allowed the the new linear controller IC to be relocated to the printed-circuit board (PCB) of the LED driver, reasonably than on the same board as the LEDs, as is discovered with widespread linear led light regulators.

Ripple suppression

Linear drivers have an intrinsic property that, with correct dimensioning, enables them to keep away from fluctuations in the LED present. This property eliminates variations in brightness ranges. Changing mild ranges would possibly cause a number of damaging results for SSL system end users. The usual terms for such results are flicker or non permanent mild artifacts (TLA). In several nations, regulation or laws to restrict TLA is in place or underway. The most well-liked topology is a single-stage flyback with integrated PFC driving a relentless current on the secondary facet. For proper PFC functioning, some AC voltage after the transformer is important, as solely then can the input current follow the road voltage and decrease reactive power use (good PF). Thus, a secondary stage has to deal with that. Alternatively, a large and expensive capacitor might easy the voltage after the rectifier diode.

The main perform of a linear regulator is the control of the output present by regulating the output voltage to the desired worth. The respective driver transistor blocks the surplus voltage degree. As lengthy because the minimal enter voltage to the linear driver has a ample stage – larger than the output (LED) voltage – the output present will be smooth 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) value of the voltage over the transistor.

Dimming

The inherent skill of linear LED drivers to remove output ripple enhances the light high quality considerably. This supports human-centric lighting requirements, comparable to the newest pointers and Light Concepts outlined in the Well Building Standard, since fluctuations in gentle ranges at inopportune occasions might have a significant influence on the wellbeing of occupants in a space. The same old methodology to dim configurations equivalent to LED strips or channel letters is a pulsewidth-modulated provide voltage. This implies the availability is turned on for led wall washer a sure duration of a cycle. Turned off for the remainder of the cycle. Depending on the time, the availability voltage is turned on relative to the cycle time (duty ratio), then the average brightness of the LED scales with the identical ratio relative to full current. For electromagnetic compatibility (EMC) causes, the cycle time usually lies between 10 ms and 1 ms. The longer the cycle time is for this digital modulation (full-on, full-off), the better the effect on folks.

To keep away from this gentle flicker, the newly developed linear driver IC structure uses a different methodology for dimming. The dimming enter is analog and may modulate the output current proportionally all the way down to about 3% of the nominal present. Applying a voltage between 0.1V and 3.3V results in an output present between 3% and 100% of the nominal current; the mapping to the respective output present is linear. The continuous output current is analog adapted to the respective dimming signal. The technique supplies a stable mild output over a wide brightness vary.

Lifetime issues

LEDs usually have a really long lifetime of 50,000 hr, with some even promising up to 100,000-hr working time. However, the definition of this lifetime is not dictated by frequent logic. Instead, a lifetime assertion for an LED assumes that the LED will present, led neon Flex news after the top of the period, more than 70% of the sunshine output from the first operating hours (known as L70). Furthermore, a common concept within the lighting business guarantees the lifetime promise just for 90% of the whole inhabitants, so 10% would possibly fail earlier.

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Most general lighting applications require fixed, uniform illumination. There are two methods to ensure that. Here is more about led neon flex news take a look at our site. One choice is to drive the LED with the next electrical current at first to compensate for the 30% loss – which also requires no less than 30% extra power over the lifetime of the SSL system. Another method is the dynamic increase of the LED current proportional to the deteriorating efficacy. That requires suggestions from a gentle sensor circuit, cutting the extra lifetime efficiency loss by half. With the continued worth reductions for LEDs, another suggestion is to trade the LEDs as soon as the luminous flux drops beneath a threshold. For the last approach, it is useful to exchange the LED module with provide voltage utilized; 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, swap the sunshine on, and go up once more to complete. A hot-plug feature noticeably reduces the cost of the replacement.

The novel features of the latest linear LED driver ICs open up new choices for power conversion. For instance, linear LED drivers assist smaller LED drivers as they want fewer exterior parts and particularly no heavy and bulky magnetics. The linear operating mode provides an additional advantage because it has no high-frequency switching, which reduces the complexity of meeting EMC necessities. The omitted output power switching additionally eliminates the radiated power on LED wires, facilitating longer connection to the LEDs. KURT MARQUARDT is senior director of LED lighting systems. Product marketing at Infineon Technologies.

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