Linear LED Driver IC Design Reduces Losses (Magazine)

Linear LED Driver IC Design Reduces Losses (Magazine)

In a feature article published 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 a number of advantages that linear LED drivers can present in SSL product improvement. Advantages of linear LED driver architecture include decrease price, simple scalability of the output channels, no magnetics required, vital ripple suppression, better electromagnetic interference (EMI) performance, and the topological capability to place a number of circuits in parallel with out a cumbersome collection of LED gadgets.

However, there is a reluctance to employ linear LED approaches, led linear light which is justified by an anticipated low general system efficiency. If you are you looking for more in regards to led wall washer (http://www.4mark.net) check out our own website. Designs for a dedicated working vary optimize the effectivity of a linear circuit, however exterior this vary, the losses are nonetheless larger than for a change-mode topology. A novel method for the control loop suggestions reduces this disadvantage considerably, enabling comparable efficiencies to these of change-mode circuits. Measurement outcomes present the brand new driver IC design achieves these design goals.

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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 external transistor overlaying a wide power vary. The LED current is adjustable by an external resistor. Fig. 1 reveals a typical utility schematic with the gadget.

A singular feature of the IC strategy is the feedback to the primary side controller by regulating the output voltage of the first stage converter. The integration within the control loop minimizes the voltage overhead and, in turn, power losses. This feedback loop makes use of frequency compensation to avoid interference with the power issue correction (PFC) operate.

The suggestions to the first aspect controls the headroom for the external transistor. This voltage margin guarantees the intended current circulate. An exterior resistor at pin VDROP determines the worth of the overhead. Once the preliminary headroom is arrange, the system will dynamically regulate the input voltage by offering an appropriate feedback signal to the first stage. In consequence, a 30W LED association with 18 LEDs and 0.56A working present achieves higher than 94% power effectivity (see the desk for information on output voltage elements evaluated). This is comparable to the power effectivity of a median buck controller.

This suggestions characteristic differentiates the BCR601 from other linear controllers. Also, it allowed the the brand new linear controller IC to be relocated to the printed-circuit board (PCB) of the LED driver, slightly than on the identical board as the LEDs, as is found with widespread linear regulators.

Ripple suppression

Linear drivers have an intrinsic property that, with correct dimensioning, permits them to keep away from fluctuations in the LED present. This property eliminates variations in brightness levels. Changing mild levels would possibly trigger a number of unfavourable effects for SSL system end customers. The usual terms for such results are flicker or momentary light artifacts (TLA). In numerous international locations, regulation or laws to limit TLA is in place or underway. The most popular topology is a single-stage flyback with built-in PFC driving a continuing present on the secondary aspect. For proper PFC functioning, some AC voltage after the transformer is critical, as only then can the input current comply with the road voltage and minimize reactive energy use (good PF). Thus, a secondary stage has to deal with that. Alternatively, a large and expensive capacitor might smooth the voltage after the rectifier diode.

The main operate of a linear regulator is the control of the output present by regulating the output voltage to the specified worth. The respective driver transistor blocks the surplus voltage degree. As lengthy as the minimum input voltage to the linear driver has a ample stage – greater than the output (LED) voltage – the output current will probably be clean without the ripple from the input voltage, thereby avoiding any TLA. A task for the product developer is to consider the losses of the output transistor (VDROP). Power losses are proportional to the RMS (root mean sq.) value of the voltage over the transistor.

Dimming

The inherent capability of linear LED drivers to take away output ripple enhances the sunshine high quality considerably. This supports human-centric lighting necessities, resembling the most recent tips and Light Concepts outlined in the Well Building Standard, since fluctuations in light ranges at inopportune times may need a significant impact on the wellbeing of occupants in a space. The standard methodology to dim configurations comparable to LED strips or channel letters is a pulsewidth-modulated provide voltage. This means the availability is turned on for a sure duration of a cycle. Turned off for the remainder of the cycle. Depending on the time, the supply voltage is turned on relative to the cycle time (responsibility ratio), then the common brightness of the LED scales with the identical ratio relative to full current. For electromagnetic compatibility (EMC) reasons, 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 higher the impact on people.

To avoid this gentle flicker, the newly developed linear driver IC structure makes use of a unique methodology for dimming. The dimming input is analog and may modulate the output current proportionally down to about 3% of the nominal current. Applying a voltage between 0.1V and led wall washer 3.3V ends in an output present between 3% and 100% of the nominal present; the mapping to the respective output present is linear. The continuous output present is analog adapted to the respective dimming sign. The technique offers a stable gentle output over a large brightness range.

Lifetime considerations

LEDs sometimes have a really lengthy lifetime of 50,000 hr, with some even promising up to 100,000-hr operating time. However, the definition of this lifetime shouldn’t be dictated by widespread logic. Instead, a lifetime assertion for an LED assumes that the LED will provide, after the end of the interval, greater than 70% of the light output from the first working hours (known as L70). Furthermore, a common idea in the lighting trade ensures the lifetime promise just for 90% of the entire inhabitants, so 10% might fail earlier.

Most common lighting purposes require constant, uniform illumination. There are two methods to ensure that. One option is to drive the LED with a higher electrical current initially to compensate for the 30% loss – which also requires no less than 30% extra vitality over the lifetime of the SSL system. An alternative methodology is the dynamic improve of the LED present proportional to the deteriorating efficacy. That requires suggestions from a mild sensor circuit, reducing the extra lifetime efficiency loss by half. With the continued price reductions for LEDs, one other suggestion is to alternate the LEDs as soon as the luminous flux drops beneath a threshold. For the last approach, it is useful to trade the LED module with supply voltage utilized; this provides instant feedback if the brand new module is working. Thus, the costly contractor on the ladder saves time not having to climb down, change the light on, and go up once more to complete. A scorching-plug function noticeably reduces the cost of the substitute.

The novel features of the newest linear LED driver ICs open up new options for energy 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 working mode offers an extra advantage because it has no high-frequency switching, which reduces the complexity of assembly EMC requirements. The omitted output power switching additionally eliminates the radiated vitality on LED wires, facilitating longer connection to the LEDs. KURT MARQUARDT is senior director of LED lighting programs. Product marketing at Infineon Technologies.

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