Electrical Characteristics of LEDs: LED Fundamentals

An LED is a device that emits light when electrically biased. Similar to any electronic component, LEDs also have electrical parameters that need to be taken into consideration when designed into a system.

1. Electrical Characteristics of
LEDs

2. Introduction A Light Emitting Diode (LED) is
a device that emits light when
Reflector
Mold
LED-Chip
Wire Bond
a

device

that

emits

light

when

electrically biased.
Similar to any electronic
component LEDs also have
Lead frame
component
,
LEDs

also

have

electrical parameters that need
to be taken into consideration
when designing with LEDs.
Silicone
Bond Wire
LEDs are very similar to
standard diodes and most of the
electrical characteristics of
standard diodes also apply to
Chip
Wire
Cavity
025
standard

diodes

also

apply

to

LEDs.
Here is a simple picture to show
how LEDs are constructed
Cavity
0
.
25
mm
how

LEDs

are

constructed
.
Printed Circuit Board (PCB)

3. How does an LED emit light?
n
Cr stal
p
Crystal
n
-
Cr
y
stal
p
-
Crystal
-
+
+
Epitaxy Layer
Metallic Contact
Depletion zone
-
Substrate
LED chip‘s PN junction is biased in a forward direction;
Electrons
Holes
Free charge is forced (overcome Vf) into the depletion zone;
Electrons recombine with holes, and some of these recombination's emit light;
The color of the light is based on the material selection, which directly affects the
forward voltage of the LED.

4. Electrical parameters of an LED
The following electrical parameters should be taken in to consideration when designing with LEDs.
1. Vf and current: the I-V curve of the LED will have these information
2Pl d t fth LED 2
.
P
u
lse an
d
surge curren
t
o
f

th
e
LED
3. Reverse current and/ or reverse voltage
4. Junction temperature (Tj)
a reduction in Vf due to Tj a
.
reduction

in

Vf

due

to

Tj
b. shift in color due to Tj c. flux degradation due to Tj
5 Recommended PCB foot
print
5
.
Recommended

PCB

foot
-
print

5. Forward Voltage -
Vf
Similar to standard diodes, in LEDs,
nothing happens until a threshold voltage is nothing

happens

until

a

threshold

voltage

is

reached. Once the threshold is reached,
current through the LED rapidly increases
with increasing voltage.
Due to this behavior the preferred method
to drive the LED is with constant current.
As it can be seen from the graph on the
right, nothing happens until the threshold
voltage of ~2.75V. Once the 2.7V is
hd tth hth LED
reac
h
e
d
, curren
t

th
roug
h

th
e
LED

increases exponentially with slight increase
in voltage.

6. LED current (forward current) LED current is one of the key parameters as it determines the amount of light that the
LED puts out, the forward voltage of the LED, and the color or wavelength shift when
the LEDs is driven, in a particular design, at a different current than the binning current.

The Vf of an LED varies slightly depending on the LED current As LEDs are driven

The

Vf

of

an

LED

varies

slightly

depending

on

the

LED

current
.
As

LEDs

are

driven

using constant current, if the system has a re sistor type current regulation, an accurate
Vf should be used to calculate the resistor value.
The color shift due to different LED current also determines what dimming methodology
to be utilized in a system, if the system require s some kind of dimming. If color shift is
due to analog dimming, (where LED DC current is varied to achieve different dimming
levels), is not acceptable, PWM (Pulse Width Modulation) dimming should be utilized.
LED current also determines the efficacy of the LED as well as the system efficacy.

7. LED current (forward current) … Shown on the right is the relative flux vs
LED current.

Since the binning current for this LED is

Since

the

binning

current

for

this

LED

is

350mA, the flux at 350mA is x 1 in a
relative graph.

When the LED current is 700mA the flux

When

the

LED

current

is

700mA
,
the

flux

will be ~1.74 times that of the flux at
350mA.
Wh d i i LED t th LED

Wh
en
d
es
ign
ing an
LED
sys
t
em,
th
e
LED

current will determine the total flux/ light
output of the system, along with some
other key parameters of the system.

8. LED current (forward current) … Efficacy of an LED with respect to
LED current is shown on the right.
Th ffi f LED d

Th
e e
ffi
cacy o
f
an
LED

d
ecreases as
LED current is increased.

It is required to consider this

It

is

required

to

consider

this

phenomenon when designing an LED
system as this will impact the overall
system efficacy.

9. LED current (forward current) … 
There will be a slight color shift due to

There

will

be

a

slight

color

shift

due

to

LED current, if the LED current is different
from the binning current.

As it can be seen in the graph on the

As

it

can

be

seen

in

the

graph

on

the

right, there will be no shift at 350mA
because that is the binning current.
At 700 A h ld t t C

At

700
m
A
, one s
h
ou
ld
expec
t

t
o see a
C
y
shift of ~0.0075 and a Cx shift of ~0.003
on the CIE 1931 diagram.
This particular characteristics of an LED
will eventually determine the dimming
methodology, if the system requires some
kind of dimming.

10. Pulse and Surge current Surge current is the absolute maximum non-
DC t th t th LED h dl Th DC
curren
t

th
a
t

th
e
LED
can
h
an
dl
e.
Th
e
maximum surge current and the definition of
it should be taken in to consideration when
designing with LEDs.
The definition of surge can be represented
as:
t < 50mS, D=0.016, and Ts=25°C
where Ts is the solder point temperature.
The frequency and the duty cycle of the
pulse current is very important and should be pulse

current

is

very

important

and

should

be

considered during system design.
Also, note that the definition of pulse can
vary at different solder point temperatures vary

at

different

solder

point

temperatures
.

11. Reverse current/ voltage –I
R
/ V
R
Reverse current and/ or the reverse voltage of an LED is one of the critical parameters
to be considered when designing with LEDs to

be

considered

when

designing

with

LEDs
.
Most LEDs are not designed to be operated in the reverse direction.
Also, because of how the protection device wi thin an LED is oriented (see below), care
should be taken when the LEDs are placed in anti-parallel manner.
Since LEDs are not designed for reverse operation, negative spikes within the circuit
should be taken into account to ensure the LEDs are properly operated.

12. Junction temperature -Tj 
The junction temperature of the LED is a

The

junction

temperature

of

the

LED

is

a

key factor of the life of an LED.
In terms of electrical characteristics of an
LED, junction temperature plays a role on
the forward voltage of the LED (Vf), pulsed
current, flux reduction, and color shift.
As demonstrated in the graph, Vf reduces
when Tj increases. This should be
considered when using a resistor to regulate
LED the current LED

the

current
.

13. Junction temperature –Tj … The graph on the right shows the flux
reduction when Tj increases. Even though
this may not be considered an electrical
parameter, it will impact the electrical
parameters indirectly.
Flux degradation at higher Tj can be
compensated with LED current and when
the LED current is changed many of the the

LED

current

is

changed
,
many

of

the

other electrical parameters of an LED are
impacted.
For this and other reasons such as Vf drop
and color shift, Tj should be taken into
consideration when finalizing other
electrical parameters.

14. Junction temperature –Tj … As demonstrated in the chart, the
color shift due to Tj may be significant
and needs to be taken in to
consideration durin
g
the desi
g
n
gg
process.

15. PCB footprint
The PCB footprint may not be
considered an electrical
parameter, but is included here
because it can impact the
electrical characteristics.
Shown on the right is the
recommended footprint for
OSRAM
’s OSLON package
OSRAM s

OSLON

package
.
Proper footprint is required for
proper thermal management of
the LED and ease of assembly, including correct placement and reflow of the LED.

16. Disclaimer 
All information contained in this document has been checked with the greatest care

All

information

contained

in

this

document

has

been

checked

with

the

greatest

care
.
OSRAM Opto Semiconductors GmbH and its affiliates and subsidiaries can however,
not be made liable for any damage that occurs in connection with the use of these
contents.
OSRAM Opto Semiconductor GmbH and its affiliates and subsidiaries makes no
representations and warranties as to a po ssible interference with third parties'
intellectual
p
ro
p
ert
y
ri
g
hts in view of
p
roducts ori
g
inatin
g
from one of OSRAM O
p
to
pp yg p g g p
Semiconductor GmbH's partners, or in view of products being a combination of an
OSRAM Opto Semiconductor GmbH's product and a product of one of OSRAM Opto
Semiconductor GmbH's partners. Furthermore, OSRAM Opto Semiconductors GmbH
and its affiliates and subsidiaries cannot be made liable for an
y
dama
g
e that occurs in
yg
connection with the use of a product of one of OSRAM Opto Semiconductor GmbH's
partners, or with the use of a combinat ion of an OSRAM Opto Semiconductor GmbH's
product and a product of one of OSRAM Opto Semiconductor GmbH's partners.

17. Thank you for your attention.