Edge Transient Effects on Power LED Switching · effects can be seen at the rise and fall edges of an LED pulse when exposed to certain biased voltage(s). Power LEDs have gone through

Edge Transient Effects on Power LED Switching

Imaobong E. Obot and Richard Binns Physics and Electrical Department, Northumbria University, Newcastle upon Tyne, U.K.

Email: [email protected], [email protected]

Abstract—The variation of high-power LEDs in

performance from manufacturer to manufacturer. However,

Information provided may not be sufficient in determining

concise switching properties of these LEDs at certain biased

mode. This has caused switching problems and performance

limitations depending on the manufacture type. This paper

demonstrates the rising and falling edge transient effects for

a number of different LEDs using MATLAB CAD tool. By

biasing these LEDs with external clock pulse, effects such as

exponential decay were observed at some pulse edges as

output. Therefore, highlights via experiment of the

problems seen in edge transients and proposition as to why

they occur were documented in this paper.

Index Terms—high power LED, rise and fall time, turn-on

and turn-off voltage

I. INTRODUCTION

The commercial availability and less environmental

consequence has paved way for extensive research and

development in the area of optimization of Light Emitting

Diodes (LEDs) as switching devices. Unlike other

illumination devices (e.g. incandescent bulbs) which are

voltage-driven, LEDs are current-driven. They are

capable of handling more current than the rated value

when connected to a power source of higher voltage [1]

and can control large amounts of current passing through

them. Hence, possess the ability to turn -on and –off at a

given time, t. The speed at which LEDs switches is

dependent on factors such as edge transient effects. These

effects can be seen at the rise and fall edges of an LED

pulse when exposed to certain biased voltage(s).

Power LEDs have gone through series of development

since 1999 [1]. This happened after the advent of blue

LED, followed in 1996 by the white version which was

created by adding phosphor coating to the blue LED [2].

They are designed to operate at >100mA forward current

value and power of 1 Watt (W) or more. Super-bright

RGBs or white LEDs from illumination point of view can

be used for fast switching [3].

Ref. [4] pointed out that the performance of LEDs

from different manufacturers can be a complex problem,

especially with a key specification in mind. Typically, the

operating voltage and current limit as well as other

parameters such as wavelength are provided in the

datasheet from the manufacturer of the LEDs. However,

Information provided may not be sufficient in

determining concise switching properties of these LEDs

at certain biased mode.

Manuscript received July 19, 2015; revised January 14, 2016.

This paper seeks to experimentally address the

underlining problems caused by transient at edge-

triggered power LED pulses when forward biased. Thus,

proposed reasons to explain the results seen at the

outcome. By pulse biasing different LEDs, the turn-on

and turn-off voltage(s) were observed and analyzed.

II. EXPERIMENTAL

A. Pulse Biasing Technique – Theory

The aim of this experiment was to observe how the

LEDs behave when a pulse signal was passed through

them. For the test power LEDs (RGB, white and Infrared

Emitting Diode i.e. IED) to be suitable for optical data

transmissions then we need to know exactly how they

respond to signals. Here, a pulse generator was used as a

power supply source (pulse biasing) for the LEDs. A

clock pulse was use in exciting the LEDs by sending the

pulse across the diode’s p-n junction.

+

-PULSE

GENERATOROSCILLOSCOPE

Vin VoutIin

R2

R1

Iout

Figure 1. Experimental setup – schematic circuit diagram.

Fig. 1 shows the experimental setup schematics. The

pulse period (T), delay and transition time were initially

set to 1µs, 1µs and 2ns respectively on the pulse

generator. A resistor, R1 was used to match the

impedance across the TX-line from the pulse generator to

the LED. The R2 resistor served as current limiter in the

circuit. The operating frequency was calculated thus,

frequency, f = 1/T = 1/1e-6 = 1MHz. The output clock

signal from the pulse generator was sent to channel 2 of

the oscilloscope. Channel 1 of the scope was connected

between R2 and LED using probes. R1 and R2 = 51Ω

each.

B. Equations

Generally, a p-n junction diode has I-V characteristics

for small voltages [5] given by the expression:

I = Is (exp (qV

kT) -1) (1)

International Journal of Electronics and Electrical Engineering Vol. 4, No. 5, October 2016

©2016 Int. J. Electron. Electr. Eng. 392doi: 10.18178/ijeee.4.5.392-397

For normal or actual diodes, forward and reverse

biased modes are described in (2) and (3) respectively as

follows:

𝐼 = 𝐼s (exp (𝑞𝑉

𝑛kT) − 1) (2)

and

𝐼 = 𝐼s (exp (𝑞𝑉

𝑛kT)) (3)

where:

𝐼 = forward current of diode;

𝐼s = saturation current, the diode leakage current density

in the nonappearance of light;

V = applied voltage across the terminals of the diode;

q = absolute value of electron charge;

k = Boltzmann's constant;

T = absolute temperature (K); and

n = numeric factor (ideally, the value of ‘n’ lies between

1 and 2 according to [6]).

Let 𝑟𝑑 be the LED’s resistance. From Fig. 1 applying

voltage divider rule, we can deduce the following:

𝑉out = (rd

R2 + rd) 𝑉in (4)

𝑉out is the voltage across the test power LED. Since 𝑅1 is

in parallel with (𝑅2 + 𝑟𝑑) S.I units in ohms (Ω). The

current flowing through the LED 𝐼 = 𝐼out is given by:

𝐼out = 𝑉in

R2+ rd (5)

𝑉in is the forward bias voltage. Therefore, from (3), (4)

and (5):

𝑉out = 𝐼𝑠 (exp (𝑞𝑉

𝑛kT)) × rd (6)

and

𝐼out = 𝑉out

rd (7)

C. Tools Used

An Agilent oscilloscope was used in displaying the

pulse from the LED and measurement of the rise (10%-

90%) and fall (90%-10%) times for each LED. LEDs

used were green, red, blue and white with their

specifications. The rise and fall time was measured at

different bias level for different LEDs. By adjusting the

amplitude knob of the pulse generator, the different

values for the bias voltage was set and readings were

taken from maximum to minimum adjustments. The bias

levels for the experiment i.e. input voltage were randomly

chosen. Although, for blue and white LED two particular

bias levels were intentionally selected to monitor the

transient behaviour of both LEDs at that bias point.

Offset of ± 2.5V was used in adjusting the LEDs to

forward and reverse biased conditions for measurements.

The forward bias measurements were taken from 0V

(ground) while the reverse measurements were taken

from step of -0.5V to -1V depending on the LED

response. The test procedures described above which

involved linear circuit designs are straight forward and

easily implemented. However, we would expect some

level of inaccuracies in outcomes of the experiments due

to limitation on measuring devices. Analysis of the

relationship between LEDs’ rise/fall times and input or

bias voltage was carried out using the MATLAB R2012a

CAD tool. Appendix A is a list of datasheet information

on the test LEDs.

III. FORWARD PULSE BIASING MODE – EDGE

TRANSIENT EFFECTS

LEDs excitation with an external pulse from a pulse

generator yielded results which are shown in this section.

The pulses highlight the correlation existing between

applied voltages and switching time. At certain time 𝑡1

the bias voltage switches to a positive voltage, such that

the test LED is forward biased (turn-on). When time = 𝑡2

the voltage is reversed thus, turning off the LED. For

example in Fig. 3, 𝑡1 = 0.8ns while 𝑡2 = 0.2ns. The

negative sign on the x-axis is as a result of the scope

being triggered at the falling edge. It was assumed here

that the source resistance (51Ω) was large enough to

permit current flow in the forward biased mode. Due to

noise from the measuring device and environment, to get

a good capture of LEDs pulse, the scope was triggered at

both rising and falling edges. The RGB and white LEDs

were falling edge triggered while IED was triggered at

the rising edges.

Fig. 2-Fig. 6 shows the graphical representation of

results for the pulse bias experiment on test LEDs. Each

graph represents relationship between the bias or input

voltage and time taken for turn-on and turn-off. For the

purpose of this paper, maximum of two readings were

taken (pulse 1 and pulse 2) and recorded for each test

LED. The different in scaling depicts response of the

LEDs to input voltage.

Figure 2. Pulse generated from Luxeon star white LED graph.

-1 -0.5 0 0.5 1 1.5

x 10-6

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

Time (ns)

Bia

s v

olta

ge

(V

)

Falling Edge Triggered Pulses

Pulse 1 biased at 2.41V


Exponential decay


©2016 Int. J. Electron. Electr. Eng. 393

Figure 3. Pulse generated from Luxeon star/o royal blue batwing LED graph.

Figure 4. Pulse generated from N74KG 5mm super-bright green LED graph.

Figure 5. Pulse generated from N73KG 5mm super-bright red LED graph.

Figure 6. Pulse generated from 5mm High IED graph.

-1 -0.5 0 0.5 1 1.5

x 10-6

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

Time (ns)

Bia

s v

olta

ge

(V

)Falling Edge Triggered Pulses



Exponential decay

Spike before LED's turn-off

-1 -0.5 0 0.5 1 1.5

x 10-6

-1

0

1

2

3

4

5

Time (ns)

Bia

s v

olta

ge

(V

)



Pulse 2 biased at 3.26VOscillating effect

Exponential decay

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

x 10-6

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

Time (ns)

Bia

s v

olta

ge

(V

)




Staggered steps before LED's turn-off

Oscillating effect

-1 -0.5 0 0.5 1 1.5 2

x 10-6

-1

-0.5

0

0.5

1

1.5

2

2.5

3

Time (ns)

Bia

s v

olta

ge

(V

)

Rising Edge Triggered Pulses



Energy

region at

constant

voltage

Oscillating effect

Exponential decay



For the red LED, bias voltage values were 1.89V, and

2.01V. Oscillating effects (also known as current ripple)

and were noticeable here. Pronounced oscillating effects

were noticed at 0V after LED’s turn-off.

The respective applied voltages for green LED biasing

were 2.65V and 3.26V. The same effects seen in IED and

red LED is noted here. Ripples with spikes were not seen

at turn-off although, it was observed at the tail-end of the

falling edge.

Applied bias voltages for the blue LED were 2.65V

and 2.93V. Interesting huge spikes were seen here before

diode actual turn-on (exponential rise in voltage). These

spikes were the cause of the negative voltage scale (see

Fig. 3). At 2.65V oscillating effect was absent at turn-on

whereas it appeared at 2.93V. Other huge spikes were

noticeable at the diode’s turn-off before an exponential

decay. The oscillating effect was absent at the tail-end of

the falling edges of the LED’s optical pulses.

The bias voltages for the white LED were 2.41V and

2.65V. Similar effects seen in the blue LED was noticed

here too except for the absent of ripples at the rise edges

of the white LED pulses.

On the other hand, bias voltage values for IED were

1.21V and 1.29V (maximum). IED’s response to voltage

changes was so sensitive that higher voltage readings

reading could not be obtained with the pulse generator.

Some oscillating effects with huge spikes were

observed here. These ripples occurred around the left and

right (just before turn-off event) part of peak voltage

forming sort of an exponential decay.

From the pulses graphs below, a linear rise in voltage

was observed in red and green LEDs while an

exponential rise was witnessed in the blue and white

LEDs. However, almost linear rise in bias voltage was

observed at high bias level for the blue and white LEDs.

Oscillating Effects (Current Ripple) Explanation

When a pulse is loaded down a waveguide, if

impedance is not perfectly matched, some of the energy

gets absorbed while some reflects back as distortion(s) on

the pulse. Oscillations or current ripples were seen as

such distortions on white, green, red LEDs as well as IED.

Although not verified, it was believed to have been

caused by parasitic elements as results of impedance

mismatch along the TX-line. Such elements included

probes’ capacitance, inductance and resistance.

Additionally, parasitic elements could be as a results of

packaging processes.

Observed Spikes Explanation

An explanation for the spikes before turn-on was not

concluded at the time of this report. But, possible reasons

could be linked to the dopants and other materials added

by manufacturers to increase efficiency and reduce

‘droop effect’ LED’s internal structure [7], [8]. Droop

can be simply defined as the loss of efficiency of LED

when operating at high power [7]. Also, an attempt could

have been made to charge the parasitic capacitor(s) (from

breadboard or probes), before charging the actual LED’s

inherent capacitor.

Rise and Fall Times of LEDs

The rise time and fall time of a pulse is simply defined

as the change in time from the 10% to 90% and 90% to

10% points of the voltage respectively [9]. As displayed

in the results, at different bias levels, the rise and fall

times of output pulse was greatly affected, resulting in

different values. This is explained as follows:

Assuming a step-function input pulse was asserted into

the LEDs.

From Fig. 7 mathematically, there was an exponential

change in the rising and falling edges of the pulse. The

output voltage, 𝑉out as a function of time, t, using the first

order of differential equation is summarized below:

𝑑𝑉out

𝑑𝑡= −∝ 𝑉out ⇒ 𝑉out (𝑡) = 𝑉o [exp(−𝑡

𝜏⁄ )] (8)

where:

𝜏 = 1∝⁄ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 = RC time constant;

𝑉out = output voltage;

𝑉o = bias voltage.

Figure 7. Rise time measurement for white LED.

Fig. 7 depicts a pulse from an RC circuit. It is believed

that when a diode is turned on, charges are built up. Let τ1

and τ2 be the time taken to charge and discharge the

capacitor respectively. Applying (8) with appropriate

substitution, an increase in the bias voltage causes LED

to turn on (i.e. charge). Therefore:

𝑉out (𝑡) = 𝑉o[1 − exp ( −𝑡𝜏1

⁄ )] (9)

The reverse is true at turn-off (i.e. discharge) resulting

in:

𝑉out (𝑡) = 𝑉o [exp(−𝑡𝜏2

⁄ )] (10)

Equation (9) occurred at 0V bias voltage. According to

[9] τ1 and τ2 are time constant of an RC circuit and are

related to:

rise time, 𝜏r = (ln 9)𝜏1 (11)

and

fall time, 𝜏f = (ln 9)𝜏2 (12)

The fall time determines how fast an LED can switch

off. Theoretically, from equation (5): 𝜏1 = 𝜏2 for RC

circuits. This means that, for an LED with RC step

function, the rise and fall times are meant to be equal.

Apparently from data obtained, this is not so. The reasons

for this behaviour were as follows:

1) The LEDs p-n junction does not emit light when

the bias voltage is not sufficient to turn it on.

-1 -0.5 0 0.5 1 1.5

x 10-6

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

Time (ns)

Bia

s v

oltage (

V)

100%

90%

10%



A.

B.

C.

2) Due to the rapid discharge effect, the fall time of

LEDs decreases as the voltage increases although,

at a slow decay, the fall time increases. This can

be linked to the carriers sweep-out from the active

region [9].

The falling edges of the LEDs showed an exponential

decay due to abrupt discharge of charges. For a decay

constant 𝜆, the decay equation is:

𝑉out (𝑡) = 𝑉0𝑒−𝜆𝑡 (13)

Carefully considering Fig. 8, the falling edges reflects

a constant voltage right after the current ripples and

before turn-off. The IED seems to have two responses

just before discharge i.e. at 8ns and 12.5ns. This region

appears to store energy at a constant voltage (also refer to

Fig. 6). Energy stored is based on the principle that when

a diode is turned-off, the charges generated by previously

applied bias voltage are stored as the diode’s internal

capacitor discharges. The same effect was noticed as

‘staggered step’ behaviour in the red LED (Fig. 5).

Figure 8. Fall time measurement for IED.

As the bias voltage was increased, the extra energy

region increased as well. From Fig. 8 the falling edge of

IED was non-linear although, a linear calculation for fall

times of these LEDs was assumed in this work. The

constant or linear portion of the stepped edge was the

charge decaying linearly from the capacitance in the IED

of a more regulated bandgap drop. Consequently,

constant current decay then the exponential decay occurs

at the end following a capacitance general discharge.

Interestingly, the green LEDs showed less of this

behaviour as seen in Fig. 4. This was attributed to the

GaP (Gallium Phosphate) structure of green to the

GaAlAs (Gallium Aluminium Arsenide) structure of red

LEDs [1]. The p-n junction of the green LEDs is nitrogen

doped so at high voltage; there was colour shift where

yellow instead of green light was emitted. This colour-

shifting could have affected the falling edge, thus

reducing this behaviour in the green LEDs.

Moreover, the behaviour was not exhibited by the blue

and white LEDs since they both have InGaN (Indium

Gallium Nitride) structure [10].

IV. ENERGY STORED IN HIGH POWER LEDS

In LEDs, capacitor stores energy and gives it back to

the device when needed. For more energy to be stored

more bias voltage is required thus, more energy stored in

an LED implies that the LED can stay on for a longer

period. Energy stored is expressed as:

Ec = 12⁄ 𝐶𝑉2 S.I unit is Joules (J) (14)

Considering the capacitance value for blue LED at

2.65V, with current equal to 52mA (quite small), from

equation (16), the energy stored by the 14nF capacitor

will then be ~49E-9 J. The same value would be true for

white LED at the same input voltage. At a higher voltage

say 2.93V, more energy (~34.1E-9 J) was pushed into the

LED. Therefore, a greater voltage is required to push

more energy thus, increasing the switching speed of

LEDs. Another interesting point noted here about the

green LED is that, at 2.65V, the energy (~1.8E-9 J)

served to the green LED was much higher than that of the

blue and white LEDs at the same bias.

V. CONCLUSIONS

Although different manufacturers provide

specifications for a LED, it was noticed that these

information was insufficient. Hence, this work followed

the fundamental principles guiding LEDs via

experimentation. Thus, biasing the LEDs with pulses was

carried out to study the response time of LEDs to applied

voltage or current. For instance, we would expect high

fall time values in red and green LEDs as they were

labeled High Power LEDs. From results, the reverse was

seen implying that, it takes a short time for these LEDs to

turn off.

The current-voltage relationship is paramount in

choosing the right bias level thus, determining right

response time(s) for LEDs. This response time in effects

creates the rise and fall times needed for LEDs fast

switching. In general, this tells us that as the bias voltage

is increased, the energy level also increases.

TABLE I. DATASHEET SPECIFICATIONS [10]

Colour of

LEDs

Red Green Blue White Infrared

Forward

voltage (V)

1.85 at IF = 20mA

(max)

3.3 at IF = 20mA

(max)

3.99 at IF = 350mA

(max)

3.99 at IF = 350mA

(max)

1.2 at IF = 20mA

(max)

DC forward

current

(mA)

30 25 350 350 100

Average

forward

current

(mA)

- - 350 350

Reverse

voltage

5 5 - - 5

Peak

wavelength

(nm)

660 515 460 (max) 10000

(max)

940

Viewing

angle

34 34 10 110

Typical

intensity

3390 at IF = 20mA

(mcd)

8990 at IF = 20mA

(mcd)

120 (cd)

LED

junction

temperature

(°C)

- - 135 135

-2 0 2 4 6 8 10 12 14 16 18

x 10-7

-0.5

0

0.5

1

1.5

2

2.5

Time (ns)

Bia

s V

olta

ge

(V

)

Not to scale

90%

100%

10%



Analysis of results of the experiment using CAD tools,

unusual transient behaviours were noticed in the tests

LEDs. Biasing the LEDs with external clock pulse, the

output optical pulses exhibited some current ripples or

oscillating effects which was linked to impedance

mismatch along the TX-line. In addition, huge spikes

were observed before actual turn-on and –off in the blue

and white LEDs. These spikes, although not verified were

linked to the effects of pre-charging and discharging of

parasitic capacitors along the transmission line before the

actual LED turn-on or -off respectively. These effects

could be a major problem in switching of the LEDs to

obtain desired output(s). Therefore, it is advised that

before an LED is used for any application a test should be

carried out to ascertain LED’s specification to real time

application. For instance, Fig. 2 shows a turn-on at 2.5V

contrary to specification from manufacturer (see Table I).

APPENDIX DATASHEET SPECIFICATIONS

The test LEDs were LEDs, 5mm infrared emitting

diode, Luxeon star/o royal blue batwing, star white, 5mm

Super-bright red and green LEDs.

ACKNOWLEDGMENT

The authors wish to thank Northumbria University,

U.K, staff of the Physics and Electrical department and

Redeemer’s University, Nigeria.

This work was in partial fulfilment for Master’s

degree in Microelectronic and Communications

Engineering.

REFERENCES

[1] C. M. Bourget, “An introduction to light-emitting diodes,”

Hortscience, vol. 43, no. 7, pp. 1944-1946, Dec. 2008. [2] R. Forster, Light-Emitting Diodes: A Guide to the Technology and

Its Applications, Berkshire: BSRIA, 2005.

[3] C. G. Lee, “Visible light communication,” in Advanced Trends in Wireless Communications, M. Khatib, Ed., Croatia: INTECH,

2011, pp. 327-338.

[4]

Semiconductor Thermal Measurement and Management

Symposium, April 2009, pp. 151-158.

[5] D. Mynbaev and L. Scheiner, Fiber-Optic Communications Technology, New Jersey: Prentice Hall, 2006.

[6] Light Emitting Diodes: An

Introduction, Cambridge: Prentice-Hall, 1987. [7] U. S. Barbara. (April 19, 2011). LED efficiency puzzle solved by

theorists using quantum-mechanical calculations. [Online].

Available: http://www.sciencedaily.com/releases/2011/04/110419164211.htm

[8] R. Stevenson. (August 1, 2009). The LED’s dark secret. IEEE

Spectrum. [Online]. Available: http://spectrum.ieee.org/semiconductors/optoelectronics/the-leds-

dark-secret

[9]

[10] Quadica Developments Inc. (2013). Luxeon Star/O LED - Royal

Blue Batwing, 220mW. Technical Datasheet DS23. Power light source LUXEON® Star. [Online]. Available:

http://www.luxeonstar.com/Luxeon-Star-O-LED-Royal-Blue-

Batwing-220-mW-p/lxhl-nrr8.htm.http://www.luxeonstar.com/v/vspfiles/downloadables/DS

23.pdf

Imaobong E. Obot was born in Akwa Ibom State, Nigeria. She received the B.Sc. degree

from the Redeemer’s University (RUN), Ede,

2009 and M.Sc. degree from Northumbria University (NU), Newcastle upon Tyne, 2013,

in Physics with Electronics and

Microelectronic and Communications Engineering respectively.

Her research interests include optical sensors,

Bulk Recombination on Switching Characteristics of AlGaAs/GaAs pnpn Bistable Device, Nano-optical

devices and information theory.

Ms. Obot is a member of the IEEE and IET. She was a recipient of the Niger Delta Development Commission (NDDC) 2012/13 postgraduate

scholarship award.

Richard Binns attended Huddersfield University to study Electronics

and Information Engineering as an undergraduate and then did a PhD in Analogue Test Strategies. This involved the design of circuitry for an IC

and then deriving methods for testing the design built without direct

connection to parts of the circuits. The work basically centered on the clever application of current monitoring techniques and the simulation

of circuitry using pulses.

His interests include rebuilding servers, networks and operating systems. Dr. Binns joined Northumbria University in 1997 under a Post-Doctoral

research programme lead by Professor Phil Hallam. His research work

was on Analogue Synthesis techniques.



K. Gillessen and W. Schairer,

A. Poppe and C. J. M. Lasance, “On the standardization of thermal characterisation of LEDs,” in Proc. 25th Annual IEEE

E. F. Schubert, Light-Emitting Diodes, second ed., New york: Canbridge University Press, 2006, pp. 393-396.

Documents

Edge Transient Effects on Power LED Switching · effects can be seen at the rise and fall edges of an LED pulse when exposed to certain biased voltage(s). Power LEDs have gone through