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.