LED Characteristics
LED datasheets commonly provide:
- \$ I_{v}\ [cd] \$ Light Intensity vs \$ I_{f}\ [A] \$ Forward Current
- \$ V_{f}\ [V] \$ Forward Voltage vs \$ I_{f}\ [A] \$ Forward Current
- (Often, the inversion of these axes is depicted)
As an LED is driven at higher forward currents, its light intensity generally increases (non-linearly).
As an LED is driven at higher currents, its forward voltage increases (non-linearly).
LED efficiency
[ \$ I_{v} \$ ] vs [ \$ I_{f} \$ ]
LED efficiency is often described in terms of:
Output \$ I_{v} \$ Light Intensity \$ [cd] \$ vs
Input \$ I_{f} \$ Forward Current \$ [A] \$.
[ \$ \frac{ I_{v} }{ I_{f} } \$ ] vs [ \$ I_{f} \$ ]
Datasheets often suggest a tested operating state, which incidentally often corresponds to the peak \$ \frac{ I_{v} }{ I_{f} } \$ vs \$ I_{f} \$ efficiency.
Designers may utilize a forward current above the suggested tested operating state, typically to achieve increased maximum light intensity at the expense of direct [ \$ \frac{ I_{v} }{ I_{f} } \$ ] efficiency.
PWM methodologies, for example, reduce actual light intensity in proportion to the ratio of the duty cycle, but may achieve gains elsewhere, (such as through reduced temperature cycling, increased perceived light due effects of the Weber-Fechner Law), and simple dynamic control of intensity during operation).
It is common for designers to partly compensate for the reduction in maximum actual light intensity due to duty cycle by driving the LED at a higher forward current.
Issue
The level of change to [ \$ \frac{ I_{v} }{ I_{f} } \$ ] efficiency is not always easily distinguished on a datasheet, however, as it is depicted as the slope of the non-linear [ \$ I_{v} \$ ] vs [ \$ I_{f} \$ ] graph.
[ \$ \frac{ I_{v} }{ I_{f}\ •\ V_{f} } \$ ] vs [ \$ I_{f} \$ ]
As stated before, as an LED is driven at higher currents, its forward voltage increases (non-linearly). Increases to forward voltage are relevant, as they will increase the minimum bus voltage necessary to supply the LED. [This is especially relevant in limited power storage (battery) applications.]
This thus has an impact on a more robust consideration of efficiency:
Output \$ I_{v} \$ Light Intensity \$ [cd] \$ vs
Input \$ ( I_{f}\ •\ V_{f} ) \$ Forward Power \$ [W] \$.
Output light intensity and forward voltage are both relational to input forward current, and both of those relationships are provided on LED datasheets already.
Issue
Where [ \$ \frac{ I_{v} }{ I_{f} } \$ ] efficiency requires eyeballing the slope of the non-linear [ \$ I_{v} \$ ] vs [ \$ I_{f} \$ ] graph, [ \$ \frac{ I_{v} }{ P_{f} } \$ ] efficiency requires mentally dividing the non-linear [ \$ I_{v} \$ ] vs [ \$ I_{f} \$ ] graph with the non-linear [ \$ V_{f} \$ ] vs [ \$ I_{f} \$ ] graph. This makes part and operating state selection with respect to [ \$ \frac{ I_{v} }{ P_{f} } \$ ] efficiency especially challenging.
Question
Why aren't Efficiency vs Forward Current graphs depicted in datasheets, such that the nonlinear relationships and salient operating points are much more transparent:
- \$ \frac{ I_{v} }{ I_{f} }\ [ \frac{cd}{A} ] \$ vs \$ I_{f}\ [A] \$
- \$ \frac{ I_{v} }{ I_{f}\ •\ V_{f} }\ [ \frac{cd}{W} ] \$ vs \$ I_{f}\ [A] \$
