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Application Note No. AN040
Comparison of LED circuits
Application Note
Valid for:
all OSRAM Opto Semiconductors LEDs
Abstract
In recent years, Light Emitting Diodes (LEDs) have become a viable alternative to
conventional light sources. The overriding advantages long life, high efficiency, small size
and short reaction time have lead to the displacement, in ever increasing numbers, of
incandescent bulbs. One of the markets where this change has become most evident is
Automotive, where LEDs are used now not only for backlighting dashboards and switches,
but also for exterior illumination in Center High Mounted Stop Lights (CHMSL), Rear
Combination Lamps (RCL), turn signals and puddle lighting.
Despite the long life and low failure rates of LEDs, cars can be found, on occasion, with
failed LEDs in their CHMSL. Most often this is due to a flawed circuit design wherein the
LEDs were allowed to be overdriven. It is with that supposition in mind that this application
note is written: to identify, characterize and comment on LED behavior and failure modes
in serial and matrix circuits.
Authors: Bartling Hanna / Hofman Markus
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Table of contents
A. Failure modes of LEDs ............................................................................................2
B. Electrical characteristics of the LED and forward voltage grouping .......................2
C. Simulation of different LED circuits ........................................................................3
D. Circuit topology 1: matrix circuit .............................................................................4
Matrix circuit with one resistor for the complete circuit .....................................4
Matrix circuit with one resistor for the complete circuit, one LED failed ............4
E. Circuit topology 2: serial circuit ..............................................................................6
Serial circuit ........................................................................................................6
Serial circuit, one LED failed ...............................................................................6
F. Additional thermal considerations ...........................................................................7
G. Conclusion ..............................................................................................................8
A. Failure modes of LEDs
Ultimately there are two possible failure modes for LEDs: light degradation and
total failure. Light degradation occurs when the emitted light falls to 50 % of its’
initial value. This is simply due to aging of the LED. The second failure mode, total
failure, is caused by an open contact between the chip and the lead frame,
between the chip and the bond wire or between the bond wire and the lead
frame. The reason for this failure is an overheating of the LED past the glass point
of the resin. This leads to a softening of the resin, and when the resin material
cools and becomes hard once again, mechanical forces on the bond wire cause
an open contact.
B. Electrical characteristics of the LED and forward voltage grouping
The characteristics of LED forward voltage have similar electrical properties to
that of any other diode, which are:
1. A forward voltage threshold must be reached before the diode will begin
conducting.
2. There is a thermal coefficient for forward voltage .
3. The diode is non-conductive in reverse.
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In order to meet the exacting standards of light-output and consistency typical
in automotive exterior illumination, it is necessary to segregate the forward
voltage of the LEDs used in these applications into groups. By tightly controlling
the parameter of voltage in such a way, uniformity in appearance is better
achieved in the end application.
The voltage groups and values of the Power TOPLED® LA E67B (which is used
in the simulations below) are shown exemplarily:
• 3A: Vf = 1.90 V – 2.05 V, (Vf typ = 1.975 V)
• 3B: Vf = 2.05 V – 2.20 V, (Vf typ = 2.125 V)
• 4A: Vf = 2.20 V – 2.35 V, (Vf typ = 2.275 V)
• 4B: Vf = 2.35 V – 2.50 V, (Vf typ = 2.425 V)
C. Simulation of different LED circuits
To demonstrate LED performance in a circuit, two simulations have been
performed for each circuit topology: one being a typical simulation with all LEDs
performing normally, and the second, a simulation with one failed LED in the
circuit. The failed LED is invariably from a string with typical forward voltage.
For each of the circuit simulations to follow, the proceeding parameters will be
considered constant:
1. Sixteen Power TOPLED® (LA E67B) LEDs with a voltage group 3B
(Vf = 2.125 V @ 50 mA) have been used, wherein four LEDs are in parallel
and four LEDs are in series.
2. The LED strings for each circuit have been arranged from left to right in a
minimum, mean (definition: mid), maximum arrangement of forward
voltage. This equates to a forward voltage of 2.05 V @ 50 mA for the left
most string, a forward voltage of 2.125 V @ 50 mA for the middle two
strings, and a forward voltage of 2.20 V @ 50 mA for the right most string.
3. The LEDs have been driven by a voltage source of 12.8 V DC. (This is
equivalent to a voltage source of 13.5 V DC, minus a 0.7 V drop at a reverse
protection diode.)
4. The resistors have been chosen so that for the typical voltage bin of 3B
(Vf = 2.125 V @ 50 mA), a current of 50 mA flows for every LED. (The
resistor values are theoretically calculated.)
5. The simulations have been carried out at an ambient temperature of 25 °C.
6. The simulation results were recorded instantaneously, after having had
current applied directly from a power supply.
Additional note: Thermal effects, though not been taken into consideration for
the individual simulations, are discussed, generally, at the end of this application
note.
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D. Circuit topology 1: matrix circuit
Matrix circuit with one resistor for the complete circuit
As calculated, the forward current for the LEDs with the typical forward voltage
of group 3B is 49.46 mA (~ 50 mA). For the LEDs from the lower forward voltage
group, the forward current is 60.70 mA. For the LEDs from the upper limit of the
voltage group, the forward current is 40.19 mA. In the worst case, the overall
current variation in this circuit is 50 mA ± ~ 20 %. This leads to a variation of the
brightness which can be seen by the customer.
The forward voltages of the LEDs have a negative temperature coefficient
(Tk = - 3.7 mV/K). Accordingly, as the temperature increases, the forward
voltage decreases while the forward current increases. In the case of this
simulation, the current for the LEDs with 60.70 mA would increase more than for
the LEDs with 41.19 mA. The variation of current within the complete circuit
would therefore increase.
Figure 1: Simulation of a matrix circuit with one resistor for the complete circuit
VCC
200.8 mA
R1
21.5
60.70 mA 49.46 mA 49.46 mA 41.19 mA
D1 D5 D9 D13
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
60.70 mA 49.46 mA 49.46 mA 41.19 mA
D2 D6 D10 D14
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
60.70 mA 49.46 mA 49.46 mA 41.19 mA
D3 D7 D11 D15
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
60.70 mA 49.46 mA 49.46 mA 41.19 mA
D4 D8 D12 D16
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
Matrix circuit with one resistor for the complete circuit, one LED failed
When one LED fails, two effects can be observed: first, the total current flowing
through the complete matrix drops slightly as the equivalent resistance of the
circuit increases. Second, and more significantly, the three LEDs that are parallel
to the failed LED pull more current. In the worst case this means that a LED from
the lower limit of the voltage group will pull 76.22 mA. This current exceeds the
maximum specified value of 70 mA for the LA E67B.
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Advantages of the matrix circuit with one resistor for the complete circuit:
• If one LED fails, the remaining LEDs still operate.
• Simple circuit design, comparatively low cost for the resistors.
Disadvantages of the matrix circuit with one resistor for the complete circuit:
• In the worst case, as illustrated in the simulation (Figure 2), the current
distribution can be very unsymmetrical. Because of the differences in
current, the LEDs do not experience a consistent rise in temperature across
the circuit.
• The failure of one LED leads to an overdriving of the remaining LEDs to
which it was in parallel. This effect is increased when fewer diodes are in
parallel, and, when combined with the effects of temperature reference
above, will compromise uniformity to the rest of the circuit and shortened
overall life.
Due to the small change in current registered by the failure of a single LED, the
failure can not be easily or economically detected by current sense; only the
failure of the complete circuit can be detected.
Figure 2: Simulation of a matrix circuit with one resistor for the complete circuit, one
LED failed
VCC
196.6 mA
R1
21.5
76.22 mA 64.36 mA 56.03 mA
D1 D5 D13
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
59.61 mA 48.42 mA 48.42 mA 40.16 mA
D2 D6 D10 D14
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
59.61 mA 48.42 mA 48.42 mA 40.16 mA
D3 D7 D11 D15
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
59.61 mA 48.42 mA 48.42 mA 40.16 mA
D4 D8 D12 D16
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
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E. Circuit topology 2: serial circuit
Serial circuit
The forward current of the LEDs from the typical forward voltage of group 3B is
50.01 mA (~ 50 mA). For the LEDs from the lower forward voltage group, the
forward current is 52.94 mA. For the LEDs from the upper limit of the forward
voltage group, the forward current is 47.80 mA. In the worst case, the overall
current variation in this circuit is 50 mA ± ~ 5 %.
The subsequent effect of temperature on appearance is less profound in this
circuit than in circuit topology 1 due to less variation in the forward current of the
LEDs.
Figure 3: Simulation of a serial circuit
VCC
52.94 mA R1 50.01 mA R2 50.01 mA R3 47.80 mA R4
86 86 86 86
52.94 mA 50.01 mA 50.01 mA 47.80 mA
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
52.94 mA 50.01 mA 50.01 mA 47.80 mA
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
52.94 mA 50.01 mA 50.01 mA 47.80 mA
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
52.94 mA 50.01 mA 50.01 mA 47.80 mA
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
Serial circuit, one LED failed
The failure of one LED causes the remaining LEDs in that string to fail. As a result,
the total current drops from 200 mA to 150 mA, approximately. The current of
the LEDs in the remaining strings is unaffected.
Advantages of the serial circuit:
• The current for each string can be adjusted very accurately by the resistors.
• Simple circuit design, comparatively low cost for resistors.
• The failure of one LED string will not affect the current of the remaining LED
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strings.
• Due to the significant change in current registered by the failure of a
complete string of LEDs, complex failure detection using current sense is
made possible.
Disadvantages of the serial circuit:
• The failure of a single LED will cause the remaining LEDs in that string to
fail.
Figure 4: Simulation of a serial circuit, one LED failed
VCC
52.94 mA R1 50.01 mA R2 0A R3 47.80 mA R4
86 86 86 86
52.94 mA 50.01 mA 47.80 mA
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
52.94 mA 50.01 mA 0A 47.80 mA
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
52.94 mA 50.01 mA 0A 47.80 mA
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
52.94 mA 50.01 mA 0A 47.80 mA
LA_E67B-typ-3B-min LA_E67B-typ-3B-mid LA_E67B-typ-3B-mid LA_E67B-typ-3B-max
F. Additional thermal considerations
In automotive applications, it is understood that the ambient temperature (Ta) is
specified up to 85 °C. For the LA E67B the temperature coefficient of the forward
voltage (TCV) equates to - 3.7 mV/K. Thus the forward voltage of a LA E67B
working in an ambient temperature of 85 °C drops. This voltage drop (VD) can be
calculated as the product of the temperature rise (Tr) and the temperature
coefficient as shown below:
VD = Tr * TCV = 60 K * (- 3.7 mV/K) = 0.22 V
Calculations and measurements show that this voltage drop (VD) leads to an
increase in forward current of between 10 – 20 % of the value at 25 °C for every
LED. Thus, an LED that draws a current of 50 mA at 25 °C would draw a current
of 55 – 60 mA at 85 °C.
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G. Conclusion
Essentially, there are two ways to design a cluster of LEDs: a serial circuit or a
matrix circuit with one resistor for the entire circuit. Each of these possibilities
has advantages and disadvantages (see Table 1) that will be dependent upon the
end application and the respective requirements therein.
For each circuit topology, especially so for circuit topology 1 (Figures 1 and 2),
the distribution of current within the circuit is critical. Care must be taken in the
design of the circuit so that the LEDs do not get overdriven, for as current
increases, so to does temperature. This self heating effect: increasing current
resulting in increasing temperature, resulting in increasing current until such a
point as equilibrium is reached, is exacerbated in circuit topology 1 by having
just the single resistor for the entire circuit.
Table 1: Advantages and disadvantages of the different circuit topologies
Matrix circuit with one resistor
Serial circuit
for the complete circuit
Advantages The current for each string If one LED fails, the remaining
can be adjusted very LEDs still operate
accurately by the resistors
Simple circuit design, Simple circuit design,
comparatively low cost for comparatively low cost for the
resistors resistors
The failure of one LED string
will not affect the current of
the remaining LED strings
Due to the significant change
in current registered by the
failure of a complete string of
LEDs, complex failure
detection using current sense
is made possible
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Table 1: Advantages and disadvantages of the different circuit topologies
Matrix circuit with one resistor
Serial circuit
for the complete circuit
Disadvantages The failure of a single LED In the worst case the current dis-
will cause the remaining tribution can be very unsymmetri-
LEDs in that string to fail cal. Because of the differences in
current, the LEDs do not experi-
ence a consistent rise in tempera-
ture across the circuit
The failure of one LED leads to an
overdriving of the remaining LEDs
to which it was in parallel. This
effect is increased when fewer
diodes are in parallel, and, when
combined with the effects of tem-
perature referenced above, will
compromise uniformity to the rest
of the circuit and shortened over-
all life
Due to the small change in current
registered by the failure of a sin-
gle LED, the failure can not be
easily or economically detected
by current sense; only the failure
of the complete circuit can be
detected
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