LED speed
LEDs fall into to general methods of light generation – direct and indirect.
- The direct LEDs emit light directly from the diode junction. These include most visible colour LEDs and the IR and UV types too.
- The indirect LEDs have phosphors which convert the LED’s monochromatic light into a broad spectrum of light. These were commonly used in early white LEDs as blue was difficult or expensive to create.
Direct emission LEDs typically have a turn-on time in single-digit nanoseconds, longer for bigger LEDs. Turn-off times for these are in the tens of nanoseconds, a bit slower than turn-on. IR LEDs typically show the fastest transition times.
Special purpose LEDs are available, whose junction and bond-wire geometries are designed specifically to permit 800 picosecond to 2 nanosecond pulses. For even shorter pulses, special purpose laser diodes, in many ways operationally similar to LEDs, work all the way down to 50 picosecond pulses.
Phosphor type LEDs have turn-on and turn-off times in the tens to hundreds of nanoseconds, appreciably slower than direct emission LEDs. This is due to the time taken to excite the phosphors enough to emit light.
The dominant factors for rapid LED switching are not just the LED’s inherent emission transition times:
- Inductance of the traces causes reduced rise and fall times. Longer traces = slower transitions.
- Junction capacitance of the LED itself is a factor(#2). For instance, these 5mm through-hole LEDs have a junction capacitance of 50 pF nominal. Smaller junctions e.g. 0602 SMD LEDs have correspondingly lower junction capacitance, and are in any case more likely to be used for screen backlights.
- Parasitic capacitance (traces and support circuitry) plays an important role in increasing the RC time constant and thus slowing transitions.
- Typical LED driving topologies e.g. low-side MOSFET switching, do not actively pull the voltage across the LED down when turning off, hence turn-off times are typically slower than turn-on.
- As a result of the inductive and capacitive factors above, the higher the forward voltage of the LED, the longer the rise and fall times, due to the power source having to drive current harder to overcome these factors. Thus IR LEDs, with typically the lowest forward voltages, transition fastest.
Thus, in practice the limiting time constants for an implemented design can be in the hundreds of nanoseconds. This is largely due to external factors i.e. the driving circuit. Contrast this with the LED junction’s much shorter transition times.
To get an indication of the dominance of the driving circuit design as opposed to the LEDs themselves, see this recent US government RFI (April 2013), seeking circuit designs that can guarantee LED switching time in the 20 nanosecond range.
Credits: Anindo Ghosh on electronics.stackexchange.com.
