Simple guide to LED luminaire design

So, your company has decided that it has either found a niche market or an industrial customer(s) with a sufficiently large volume to justify a business case for some kind of LED lighting. Now what?

Perhaps you are the one who has been chosen to make it happen. A lot of what you decide to do will be determined by what kind of company you work for. Are you a fixture company? A ballast company? An entrepreneurial start-up company? In any case, you probably know what it is you're planning on lighting up, and how much light is needed to do the job. That's your starting point. Or is it?

In the industry, I have found that an LED ballast circuit is really more like an AC/DC power supply than like a traditional fluorescent ballast circuit. Today in fluorescent lighting, one need only know the type and number of lamps to specify an appropriate off-the-shelf ballast. In contrast, there is no such thing as a standard off-the-shelf ballast in the LED lighting world.

In the AC/DC power supply arena, many companies, even smaller ones, have their own power supply design group. This is because most AC/DC power supplies are custom designed both for the load they are intended to power and for the source from which they draw their energy. The design always begins with the load and then works its way backward to the source.

The problem today is that many people are trying to choose off-the-shelf LED ballasts and then design the LEDs to match the ballasts. So, how do we bridge the gap from an off-the-shelf lamp approach to a form of lighting that usually demands lots of customization? Who is going to do the customization? Is an off-the-shelf approach even feasible?

LED lighting is more of an art than a science. There are myriad of LED types from which to choose and varying ways to arrange them. This gives room for lots of creativity.

To Isolate or not

Probably the most important question designers need to ask is whether or not the application will need isolation. The driving factor in this decision is safety and regulatory compliance. There are two solutions to the safety issue. One is mechanical double insulation and the other is galvanic isolation (a transformer). The main reason galvanic isolation is used is so the mechanical design is simpler and less expensive.

Somewhat related to this question is whether you will be putting all the LEDs in one series string or using several branches of LEDs in series. We'll start with the simplest from a circuit point of view. Suppose you plan to put all the LEDs in one series string and the voltage required to drive the string exceeds 60 V. Then isolation is almost a moot point. If the secondary voltage exceeds 60 V, then you will need to provide the mechanical double insulation anyway, regardless of isolation. Use of isolation gives no advantages, except perhaps for lightning surge protection.

In general however, the efficiency of a non-isolated design is much higher and the cost is much less than for an isolated design. Suppose, however, you decide you really do need isolation and the LED voltage of a single branch exceeds 60 V. Then you would probably do well to divide the LEDs into two or three equal branches such that the maximum voltage any branch needs is below 60 V. Then you will not need to provide mechanical double insulation in addition to the isolation.

This configuration requires additional circuitry to regulate the current in each branch, but the additional complexity is usually worth the effort as a way to avoid mechanical double insulation. If all your LEDs in single series are less than 60 V, then isolation is easier to implement.

The next decision is whether the application requires power factor correction (PFC). PFC puts the current drawn from the ac line in phase with the ac line voltage without excessive harmonics. The result is that all power delivered to the circuit is real power.

Without PFC, there is reactive (non-dissipative) power that circulates into and out of the system. This results in currents circulating through the distribution wires that are higher than what would normally be the case. PFC lets a circuit draw more power from a source in that without PFC, the system circuit breakers will trip sooner. This phenomenon brings a misconception that PFC makes a power circuit more efficient. The opposite is true. PFC usually adds a second stage to the power system, which reduces the efficiency.

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In general, a PFC circuit will dissipate more heat and be less efficient than a non-PFC circuit. It will probably cost more, too. So, why would anyone want PFC?

The answer lies in the distribution system. PFC is easier on the wires and generation equipment in the power grid. If your luminaire is over 50 W or so, PFC may be necessary. It may also be advisable even at lower power levels if your customer is an industrial firm that will be installing many low-power luminaries in a single installation. In this case PFC would make sense if the sum-total power drawn by all the lighting is high.

For output power levels of up to 25 W, passive power factor correction (using just a capacitor/inductor network) is probably most cost effective. It isn't as efficient as active power factor correction with a specialized IC, and thus generates heat. But at lower power levels, the additional heat is usually not a problem. For power levels above 30 W, active power factor correction - such as a PFC flyback or PFC boost topologies - will most likely be needed.

And for residential customers replacing Edison lamps, PFC probably doesn't make sense.

To dim or not

Dimming capability seems to be an increasing requirement for LED lamps and luminaries. There are two ways to dim an LED at the output. The first is by reducing the dc current to the LED string. The second is to turn the string on and off at a fast rate (usually on the order of 1 kHz), modulating the light on and off times through a technique known as PWM (pulse-width modulated) dimming.

A point to note is that the light from some LEDs changes with fluctuations in their dc current. These LEDs require the latter dimming method. LEDs whose color content is relatively constant with dc current can use either method.

The big question is what to do with the input dimming method. Wherever possible, it makes sense to use 0-to-10-V linear dimming, a digital addressable lighting (Dali) type interface, or a wireless control, such as Zigbee or equivalent. These techniques are preferable to the dimming method for ordinary incandescent lights. It employs a triac (also known as phase-cut) dimming, which notches out sections of the ac waveform to lower the power delivered to a bulb. But this method has issues when used with the ac/dc conversion circuits normally comprising fluorescent and solid-state lighting ballasts.

The main problem is that triacs, by their very nature, require a minimum amount of current to stay on once they are triggered. For ac/dc converters without PFC, current only flows from the input when the voltage reaches a value close to its peak value. Therefore, such circuits require a holding current. If the holding current used is dissipative (that is, it is dissipated through a resistive element rather than moved to the input or output), which is usually the case, it could require as much as 5 W of extra power just to keep the triac on. This is wasted power and will reduce the efficiency of your system.

It is also a mispreception that adding triac dimmers to a house fitted with LED lighting will reduce the building's power consumption. Dimmers only reduce energy when they are used, dimming lights where needed so that less overall power is used. It would make more sense to install dimmers only in areas that need adjustable light.

If the application doesn't need adjustable light, simply install enough lumen output to meet application needs and leave stop there. The exception to this practice is when active PFC is used. Active PFC makes the load appear to be a resistor (i.e. the ac input current is in phase with the ac input voltage), which will draw a uniform holding current. For higher power levels, PFC lets the lamp power also act as the holding current. It could almost be said that if you need triac dimming, you would do well to provide PFC as well.

Voltage or current mode

Although most of us are accustomed to thinking in terms of voltage, LEDs are current-fed devices by nature. Even if you choose a voltage mode (constant voltage output) ballast, it will still need some additional circuitry to control the current delivered to the LEDs. This added circuit is most likely provided by extra circuitry inside a lamp. If you are working with bare LEDs, or if you really want the lowest cost, then a current mode (constant current output) ballast makes the most sense.

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The nearby flow chart will help discern the best topology for ac-input LED ballast circuit needs that are in wide use. This still leads to the question of whether an off-the-shelf ballast is preferable to a custom design. The answer is not clear-cut; it is always a trade off.

For the best performance, a power supply/ballast is designed to provide rated power at rated output current and no more. Future off-the-shelf ballasts may provide configurable output currents and output voltages. But suppose the ballast is designed to provide a 750 mA constant-current output at maximum, and you configure it for 450 mA constant current output. There is wasted size and needless extra expense.

A ballast designed to provide 450 mA would use smaller transistors optimized for 450 mA output rather than for 750 mA. Configurability may be the best trade-off, nonetheless. Producing a large volume of configurable ballasts will most likely cost less than using multiple custom ballasts.

Many companies lack the expertise to design a ballast but do have the capability of manufacturing one (or of having a contract manufacturer build one). Here help is available from the applications engineers of semiconductor companies that make offline LED controller ICs and devices. International Rectifier, for instance, has a dedicated LED applications group working on evaluation boards for end-point circuits.

The local field application engineer often has the expertise to customize an evaluation board design for specific applications. Some may be able to provide everything needed for a circuit to pass UL certification - or even provide example ballast circuits that have already been UL-approved.