Pulsed Operation of LED Illuminators

- Application Note 001 -

Introduction

There are a variety of reasons why it can be beneficial to use LEDs in pulsed conditions rather than CW. In this application note we aim at providing some background information to users intending to do this, and answering some of the questions we are most frequently asked on the topic.

Why use pulsed operation?

There are two main reasons for the use of pulsed or strobed operation with LED illuminators intended for use in machine vision and similar applications. The first is to freeze action to acquire an image with a shutterless camera or detector. The second reason is to increase the effective brightness of the illuminator during the pulse by using a higher pulse current than the CW rating, since the luminance is proportional to current.

Some rules of thumb

For non-critical applications, some simple rules of thumb can be applied with LED illuminators. In general, brightness can be increased by a factor of up to 15X in relative safety. It is necessary to drive the LED illuminator at an increased voltage during the current pulse, and to use a short on-time and a low duty cycle in order to protect the LEDs. For example, if the normal operating voltage is doubled, then, provided there are no voltage-dropping resistors in the circuit, the current through the LEDs increases by approximately 15X due to the exponential IV characteristics of the diodes. This increases the brightness by 15X. In order to prevent over-heating of the LEDs the duty-cycle of the pulses ( on-time divided by the pulse period) should be kept below 5%. In addition, to prevent reduction in luminosity during the pulse, the pulse on-time should be kept below 5 ms. Since series resistors are usually included in the LED-circuit in illuminators, the full benefit of brightness increase will not normally be available due to increased power dissipation across the resistors at higher voltages

Pulsing white LED illuminators

White LED illuminators can be operated pulsed in the same way as single colour illuminators, but one must keep in mind some frequency limitations. The white LED as usually incorporated in an illuminator actually consists of a blue LED chip surrounded by a yellow phosphor, so that the combined light from both chip and phosphor looks white to the human eye. Although the LED chips can respond effectively to frequencies up to about 10 MHz, the response time of the phosphor is quite a bit slower. If the LED is pulsed at frequencies above about 5 kHz, the colour balance of the light starts to change during the pulse.

LEDs and pulsed operation

In this section we give some of the relevant background technical detail which supports the guidelines discussed above. The remarks apply to typical LED chips, size in the range 0.2 mm to 0.3 mm square and 0.1 mm to 0.2 mm thick. Because of variations between chips and manufacturers, the following comments are guideline only, for specific help on a customer application please contact StockerYale for technical assistance.

Figure 1 shows the current-voltage characteristic of a typical light emitting diode. Since it is a diode, it has an exponential current-voltage characteristic, which under forward bias conditions may be expressed by the following equation:

This approximates to a straight line on a plot of ln(I) versus V, as shown in Figure 1, except at higher currents where the series resistance Rs causes it to deviate.

Useful facts about LEDs are the following:

IV characteristics are almost exponential
Luminosity is directly proportional to operating current
Luminosity decreases with temperature, by about 1% per °C.
Maximum forward currents must be derated for increased ambient temperature
LED response times are typically in the range 10 ns to 100 ns.
LED efficiency peaks in the 10 mA to 30 mA range.
Typical thermal response times with LED chips are about 1 ms.

Figure 2 shows a typical derating curve for an LED. In general, the fact that luminosity is proportional to current can be used to increase pulse brightness, subject to the limitations encompassed on this diagram.

You will notice that

The maximum dissipated power can be derived by taking the DC rating multiplied by the operating voltage
Using this maximum power as a limit, along with the exponential characteristics, means that the allowed pulse current does not increase in strict inverse proportion to duty cycle.
As pulse length increases, especially beyond 1 ms, derating must be used since the peak junction temperature is higher than the average junction temperature.
The maximum recommended current density caps performance for short on-times, even for low duty cycles.
Additionally, not shown on this graph, there are other forms of limitation which can result in the derating curves showing other forms of limitation at short pulses and short duty cycles.

Lifetime and Pulsing

In broad terms, the limitations on LED lifetime are mostly thermally-induced. Provided that LED illuminators are strobed within the recommend manufacturer's guidelines for the specific chip type, then the normal LED lifetime (up to 100,000 hours for monochromatic chips) should be achieved, corresponding to the operating temperature and the average dissipated current.

If low duty cycles are combined with short on-times, so that the junction temperature of the LED is kept close to ambient, then effective operating lifetimes can be extended.