Driving high brightness LEDs
Apart from manufacturing quality, operating temperature is the main factor when it comes to longevity. This means that careful heatsink design, along with a driver that keeps the current at an optimal level while contending with abnormal conditions (such as cooling loss – which could otherwise result in early failure and expensive replacement costs) will be mandated. To increase the benefit side of the equation, the driver can also provide features such as dimming, fault protection and the ability to control multiple strings of LEDs. In this article, we look at the background, the cost/benefit factors and how LED drivers are selected for different example applications.
Dip don’t dazzle!
Back in the 1970s, in the UK, there were regular public service announcements on the TV. They ranged from advice on sheltering under kitchen tables in case of nuclear attack to how to cross roads safely (explained by David Prowse, who would later be cast as Darth Vader in the Star Wars movies) and how to drive safely. One of the campaigns was ‘Dip Don’t Dazzle!’ – reminding that ‘full beam’ could blind the drivers of oncoming traffic. It was hardly a big problem at the time – with the yellowish light, cloudy lenses and tarnishing reflectors …but today, the piercing HBLEDs of cars (and even pedal bikes) are a distinct hazard. HBLED technology has made it possible to replace incandescent bulbs, fluorescent tubes and sodium/mercury vapour lamps with far higher performance products. The market agrees, with a projected $22B global revenues by 2023 at 4.9% CAGR (according to Global Market Insights).
HBLED basics
LEDs exploit the characteristic of a semiconductor p-n diode junction – where photons are emitted by electroluminescence when electrons recombine with electron ‘holes’ crossing the semiconductor band gap as the junction is forward biased. The amount of doping of the semiconductor affects the band gap size which in turn affects the energy and hence frequency or perceived colour of the photons emitted. Modern HBLEDs are typically a high-power blue emitter behind a transparent melding that has been impregnated with a cerium-doped yttrium aluminium garnet phosphor (Ce3+:YAG), which emits yellow light. The combination of blue and yellow then gives white with good colour rendition. Semiconductor material and packaging advances for better thermal performance have now resulted in single HBLEDs that can produce over 100lumens/W. Other LED phosphor combinations can give even better colour characteristics (but with lower efficiency), and white can also be derived from three separate red, green and blue LEDs (though with poorer colour stability in relation to temperature and time). These RGB systems are useful, however, where dynamic colour change is required – in applications such as mood lighting or stage illumination.
Remaining HBLEDs challenges
HBLEDs are not necessarily the obvious solution for every lighting application. They have better efficiency than incandescents, converting 6x as much electrical power into light and have as much as 25x longer lifespans, but are around 20x more expensive. Fluorescents have about the same efficiency and longevity, but are about a quarter of the price of their solid state equivalents. That said, there are other factors that need to be considered – such as fitting/replacement costs, dimmability, mechanical robustness and carbon footprint, as well as disposal costs. Sometimes efficiency isn’t even an issue – if an incandescent is in a room that needs warming, the power dissipated as heat is utilised anyway. If the voltage is reduced a few percent, the lifetime extends dramatically as well. For example, a 10% reduction will reduce luminous intensity by about 30% – the threshold at which it just becomes noticeable, but lifetime increases by about a factor of 4.5 (colour temperature does change as well though).
HBLEDs can win out in most applications, if their prospective lifetime can be achieved – and this comes down to effective thermal management. LEDs rarely fail abruptly under normal conditions, but lose brightness due to propagation of lattice defects or ‘dislocations’. A typical plot of lifetime against HBLED chip temperature is shown in Figure 1.
chip temperatures [adapted from Philips Luxeon K2 data,
when driven at maximum rated current].
An accepted value for dimming with time is the ‘L70’ value, or when the lumen output has reduced to 70% of the initial value. The plot shows that this can be from 20,000 to 70,000 hours with a junction temperature difference of just 20°C.
HBLED driver performance is key
Fundamental to maintaining chip temperature within design limits is the driver IC specified, but the application also dictates the type of driver needed. Many uncritical, lower power applications might use a series resistor from a constant voltage source to set LED current. This is certainly simple, but also inefficient – with power lost in the resistor. If the source voltage is kept close to the LED forward voltage then dissipation in the resistor is lower, but LED current is less accurate – there can easily be an initial variation in forward voltage (VF) of 20%. A typical blue HBLED VF can vary from 3.03V to 4.47V (with manufacturing tolerances), so with a source voltage of say 5V current will vary by a factor of about 3.7 to 1 – clearly not an optimal situation with large changes in LED dissipation and colour rendition. The HBLED voltage itself also changes with junction temperature so the actual current variation could be even greater.
A controlled current source is the best solution and again there are various options, the simplest being a linear constant current regulator. This type of driver can maintain LED current at elevated degrees of accuracy with no generated noise, but dissipation is relatively high unless HBLEDs are selected (or ‘binned’) to be within a small VF band and the source voltage minimised. Analog dimming is possible but colour temperature varies with intensity. Figure 2 shows a circuit using the PAM2808 from Diodes Inc. which is suitable for a single emitter. Pulse width modulated (PWM) dimming is possible if a signal is applied to the enable pin, but in simple applications a signal may not be available. RS is a low value, typically 100mΩ for a 1A LED. Quiescent current is also low, which is of greater importance in battery-powered applications.
Switched-mode regulators are the favoured solution for high efficiency and when sophisticated control is needed. Source voltage can be higher or lower than the total LED VF and a multitude of features can be included (such as remote control and protection against over-temperature or other fault conditions).
Application example – architectural lighting
Architectural lighting is all about mixing colours to enhance appearance and create impact. The power source will be AC mains, so an AC/DC converter can be used – providing a convenient constant source voltage to the LED driver at some low, safe value for distribution within the lighting fixtures. HBLEDs will typically be in series strings with multiple channels for increased light intensity and different colours. An example driver circuit is shown in Figure 3, based on the Micrel MIC3201 driver IC.
The input voltage is up to 20V, allowing 1A to be delivered to a string of four LEDs. The circuit topology takes the form of a hysteretic buck converter, which is a variable-frequency topology maintaining better than 90% efficiency and ±5% LED current with only a few external components required. The IC has a dim control, which can be driven with a low-frequency PWM signal to vary the LED high-frequency PWM signal from 1% to 99%. Selective dimming of drivers with red, green and blue LEDs can then combine their projected light to give any colour. Current is sensed with a convenient high-side resistor, which drops just 0.2V at the maximum rated 1A current.
Application example – horticultural lighting
In horticulture, artificial lighting has long been utilised but cost has been a major concern in an industry that works within tight margins. Also, heat produced by incandescent lights has proven to be problematic meaning that light sources have to be kept a distance away from sensitive plants. LED lights, with their greater efficiency, have opened up the market, and growers have realised that the different colours of LEDs available can be employed to promote different stages of plant growth.
For example, ‘deep blue’ and ‘hyper red’ are optimum for photosynthesis and ‘far red’ controls germination, vegetative growth and flowering. A mix of colours, along with white, for human comfort is easily programmed to change a plant’s qualities. LED manufacturers have also responded by providing specific LED colours, optimised for horticulture. The driver arrangement shown in Figure 3 would also be suitable in this application with multiple channels for the selected colours.
Application example – automotive displays
An automotive application for either interior lighting or displays can use a similar circuit to that used in architectural lighting, except it is useful to be able to operate to higher and lower voltages so as to at least partly meet the automotive specifications for transients and dips. Typically, an initial stage of protection is needed for high energy ‘load dump’ transients, but this may already be present for protecting other electronics. A buck converter as in the circuit of Figure 3 can only reduce the voltage, so with a nominal 12V system in a car, practically only one or two white HBLEDs can be driven in series, with their worst-case voltage drop being about 4.5V each. The Maxim MAX16832A/C drivers fit the bill here, with their -40°C to +125°C automotive temperature range and operation from an input of 6.5V through to 65V. These ICs feature PWM dimming for constant colour rendition and analogue reduction of LED current in case of over-temperature conditions.
For precision backlighting applications where flexibility is called for, a boost converter can be used – so that for a nominal battery range, a high voltage will be generated (thereby allowing multiple series LEDs to be driven). The Semtech SC5012 is an example which operates from an input of 4.5V to 45V and can drive four strings simultaneously at up to 65V/150mA, each matched within ±1%. This device has an I2C interface – which can be used for fault monitoring, as well as detecting open/short LEDs and over-temperature, while also providing external frequency synchronisation.
5012 LED driver.
Analog and PWM dimming is available with selectable 9/10-bit resolution. A particular feature is ‘phase spreading’ where each of the four strings is driven with pulses separated by 90° in the switching cycle. This reduces overall ripple current (requiring less input/output capacitance) and improves dimming linearity. The SC5012 is available AEC-Q100 (Grade 2) qualified in a 4mm x 4mm 24-lead QFN package. Figure 4 shows a typical application circuit for it.
Summary
There is increasing support for HBLED technology, with an expansive range of drivers now available and an array of sophisticated features offered. Whether it is for the simplest of lighting systems or ones for the most demanding of applications, there are both high performance and cost-optimised solutions out there that can meet whatever the required criteria may be.
About the author:
Mark Patrick is Technical Marketing Manager, EMEA at Mouser Electronics – www.mouser.com
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