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What makes an LED driver different from a standard power supply?

12 Apr 2011

LStacey

United Kingdom

What makes an LED driver different from a standard power supply?

Replies

Harbi

France
1 year ago

At this time 2011, you can buy for small price a dedicated device to a usual and very popular fonction.

You have to believe that a lot of engineers (so good) have worked very hard to do the best electronic for the best price.

You can play with a power supply to deliver some power to a LED, but after many problems of for an economic design you will make a "small" "LED driver" with 10% of the official focnctions.

iainmosely

United Kingdom
1 year ago

Good question!

The power converter toplogies used as a basis to create an LED driver or standard constant voltage power supply are the same. The fundamental difference with an LED driver is that it is required to generate a constant current output rather than a constant voltage.

As a load, an LED looks rather like a constant voltage sink (e.g. like a zener diode) in series with a small resistance. The light output (or lumen output) is largely proportional to the LED drive current so, to ensure a consistant light output level, the current being driven through the LED must be controlled. 

So, if we have a constant voltage sink and resistance in series, why can't we use a constant voltage supply? Well, the issue is that the 'constant voltage sink' part of the model is far from being constant and will vary with die junction temperature. In addition, process variations mean that the different batches of LEDs will have slightly differing voltage drop. If we were to produce a driver that just provided a regulated output voltage then our population of LED lights would show vastly differing Lumen output levels and a high proportion would fail through being driven too hard and overheated.

The good news is that it is quite easy to turn a constant voltage supply into an LED driver. In a constant voltage supply, the output voltage is measured and compared to a reference level from which a feedback error signal is generated. The closed loop system then acts to control the output voltage to the pre-determined fixed level. By measuring the LED current directly and then comparing to a reference to create a feeback signal, the power system will then do everything it can to maintain the output current at a constant level.

Sometimes, measurement of the LED current directly can be rather inefficient as it is often achieved using a sense ressitor. Some of the 'LED Specific' driver ICs used at the heart of LED drivers actually infer the LED current by monitoring other more easily measured system operating parameters.

There is an interesting design example on our DesignSpark partner portal which shows how to use power over ethernet to create a 10W LED driver. Converter Technology will also be presenting at an IET event at York University on the evening of the 29th of April where we will be discussing some of the latest techniques which can be used to create low cost offline LED driver systems. We will also be showcasing some of the latest high brightness LEDs in practice. See the events section on DesignSpark for more details.