Keeping your LED cool with PWM

Question

SO I need to use some 1W and 3W leds for the brightnes, but my application will not allow the start heat sink. I have come across a few articles talking about using PWM to drive LEDs Does pulsing an LED at higher current yield greater apparent brightness?

What got me in the link above is

as you reduce your duty cycle you can increase your current because there will be less time for heating affects to kick in. I have done this with a very very short duty cycle using a pulsed laser power supply at 2kV on a normal 1.9V diode. This was pulsing so often that they eye cannot tell. the brightness of the diode increased significantly. My professor and I built a power supply that had a short duty cycle and only overvoltaged the diode to a little over an amp of current and successfully made a normal diode look brighter then a 1W LED when it was only a cheap LED. – Kortuk Jul 28 '11 at 16:09 Blockquote

What I'm needing to achieve is an LED that can be seen in day. I'm putting them on a RC Airplane.

Answer 1

You can indeed achieve higher brightness by PWMing an LED. You can also (sometimes) achieve better power efficiency. But those things are usually mutually exclusive.

If you turn an LED on for 50% of the time, and off for the other 50%, you will decrease its perceived brightness by about half. This creates a lower brightness with less power consumption (provided you are providing it the same current).

The same goes for the other way. You can provide a higher current in a short PWM cycle to achieve higher brightness. Keep in mind that this usually uses more power and can shorten the life of the LED. The extent of these drawbacks depends on how much current you are applying to the LED and for how long (depends on the PWM cycle you decide to use).

If you look at a data sheet for a 1 Watt LED you will see there is a DC forward current and a Pulsed Current.

Your specifications will likely differ but for this LED:

• DC Forward Current: 350ma
• Pulsed Forward Current: 700ma

The manufacturer also specifies a Typical Period (TP) of 100uS and a duty cycle of 0.005. This means that 1 cycle takes 100 microseconds and the LED is on for 0.5% (0.005 x 100). That means that if implemented as the specification says the LED will be on for 0.5us then off for 99.5us.

The most useful thing is that it keeps the LED cool. The LED is turned on for a small period and off for a larger period while it cools down.

To achieve what you want, higher brightness, you can certainly over power the LED at small duty cycle. Just be aware of the consequences of more heat and potentially less life time.

To tell how much current you should apply to achieve your desired results, you should be able to look at your LED Spec sheet. It should have a graph of current (ma) against luminosity (lumens). In general you can see what the apparent brightness is with: Apparent Brightness = Luminosity x Duty Cycle

Answer 2

You want to make your LED more visible.

The problem is, with higher current you reduce the efficiency of the LED:

• If you have a LED and you drive it with 1A, then you have the light intensity of 1. (as an example)
• If you want to double the brightness, then you have to drive this LED with 3 Ampere.

But there is an other effect:

• If you use PWM with 50% on and 50% off (create bright flashes with dark-phase between them), then the eye of humans would recognize this better as a constant brightness.

As an example:

1. LED flashing with 50% on and 50% off. 500ms with 3 Ampere @3V, 500ms with 0A. This are 4.5Ws
2. LED is constant on with 100% on at 1A @3V for 1 second. This are 3Ws.

You could see case 1 better. It is more bright and it is blinking, this leads to better recognition. Everything what is changing increase the attention of the human.

I think the position lights of normal planes are blinking with 0.5Hz PWM with 200ms on and 1800ms off.