POWER SUPPLIES LED lights, and how to power them

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LEDs are increasingly becoming the lighting technology of choice, because of their low energy consumption, long life, and other advantages. However, unlike incandescent luminaires which plug directly into the AC mains, they need an AC-DC power supply.This article looks at the factors to be considered in choosing power supplies suitable for lighting LEDs.

According to a new report by Grand View Research Inc., the global LED lighting market is expected to reach USD127.04 billion by 2027*, expanding at a CAGR of 13.4 percent over the 2020-2027 forecast period.

The LED popularity behind this growth rate has arisen for several good reasons. Their most obvious and well-known properties are their high electrical efficiency and long maintenance/replacement intervals. This means that, compared with incandescent types, they allow a much smaller carbon footprint together with a significantly reduced cost of ownership. This is especially true in large buildings with high ceilings, where the labour needed for luminaire replacement, as well as the luminaires themselves, are significant cost factors.

They are also attractive because of their amenity to Internet of Things (IoT) – type lighting control. LEDs light instantly on demand, and are controllable for brightness and colour. Their light output can be directed as required, rather than incandescent radiation, which is omnidirectional and must be directed by reflectors.

However, there’s another key aspect of LED technology that all lighting system designers and installers must allow for. LED stands for Light Emitting Diode; LEDs are semiconductor devices, just like transistors or other diode types, but with the added ability to convert electricity into light energy. This means that they operate on low DC voltages, and cannot be plugged directly into an AC mains supply like an incandescent light bulb.

It is possible to buy LED luminaires that look like incandescent types, complete with bayonet or Edison Screw fittings. These operate from the mains as incandescent replacements to provide longer life, lower energy operation. However, luminaires can be costly as each must include its own inbuilt AC-DC electronics. They are also unsuitable for IoT applications seeking to exert sophisticated control, as they are inaccessibly located beyond the connecting AC power cable.

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Low-voltage DC characteristic

Instead, smart lighting system installers can use LED lights or strips with, say, 12VDC or 24VDC input ratings, and supply them from an AC-DC power supply or driver that also has dimming control. This might comprise a sophisticated control interface, such as the DALI (Digital Addressable Lighting Interface) standard. This allows users to send lighting control commands via a network, addressable to an individual light or a group of lights. Fading sequences and brightness levels can be set. It is also possible to set scenes, where a single broadcast command sets multiple lights to preset levels. In a theatre, for example, there could be a setting for dim lights above the audience and bright lights over the stage.

Irrespective of how sophisticated an LED light power supply’s control input is, the supply’s ultimate role remains the same – to translate the control signal level into a corresponding brightness level for the driven LED (or LEDs). An LED’s brightness is related to the current flowing through it, and is typically controlled using pulse width modulation (PWM). The LED’s peak current in mA is selected, then modulated by varying the PWM duty cycle. An LED's light output in foot lamberts (fL) can be demonstrated as being linearly proportional to this duty cycle.

However, when you browse a manufacturer’s website for suitable LED power supplies, you will see another choice: constant-voltage vs constant-current mode. The following points explain what these modes mean, and how to choose between them.

Constant-voltage vs constant-current lighting power supplies

LEDS are semiconductor devices as mentioned, and do not have a linear forward voltage/current characteristic. A small increase in forward voltage greatly increases current flow. This means that unless some type of current-limiting is used, there is a danger of over-driving the LED. Increasing current heats the LED junction, in turn allowing more current. This can lead to a thermal runaway cycle, which can severely reduce the LED’s life and eventually destroy it.

One industry response to this is to construct LED assemblies comprising a group of LEDs in series with a current-limiting resistor. If used with a power supply having a constant voltage output – which remains fixed irrespective of load – the resistor value in ohms can be set to ensure that current never exceeds the LEDs’ safe limits.

This approach can be attractive to lighting system installers, because of its flexibility. Further LED assemblies can be paralleled, while maintaining consistent brightness across the entire group, providing the power supply has sufficient capacity to maintain its constant voltage output as the load increases.

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However, this solution has a significant drawback in that it is electrically inefficient because the current-limiting resistors dissipate power as heat. With a consistent need to reduce carbon footprint, this is increasingly unacceptable due to both cost and environmental considerations.

This is where constant-current power supplies have a role. LEDs can be connected directly to them without inefficient current-limiting resistors, because the power supply’s internal control circuit handles any necessary current limiting, as the name suggests.

This does mean that LEDs cannot be connected in parallel to a constant current power supply, as they would have to share the fixed current level and would be dimmer accordingly. However, they could be connected in series, where all would operate at the same original brightness level – subject to the power supply’s capacity limits.

_{*Although the report was released in June 2020, it does not make any estimate of the impact of Covid-19 on these figures. It does note that this will be severe for several reasons, although it expects the market to return to positive revenue growth by the end of 2021. However, even if the actual market does turn out to be significantly lower than the forecast, the original figure is still useful in indicating LED technology’s attraction as a lighting solution.}

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