delamination between the LED and surrounding
epoxy. Under extreme conditions, the epoxy
surrounding the LED die can be charred and
the gold bond wire and LED die can be
destroyed, resulting in an open circuit. The
AlInGaP LED die can tolerate non-recurring
peak current transients of several hundred
milliamperes for short time periods (t << 1 ms)
with minimal permanent effects. However,
longer transients can cause sufficient localized
heating to cause the various effects listed
earlier. The “Load Dump” transient pulse can be
especially damaging since the pulse duration
can be up to 400 ms.
limiting properties of the LED drive circuit,
determine the maximum peak current through
the LED array. For best results, the breakdown
voltage of the transient suppressor should fall
within the following range:
24 V < VBR < 45 V
Note that the 24 V restriction is determined by
the “Jump Start” voltage condition. The 45 V
restriction is determined by the ability of the LED
array to withstand the peak current imposed by
the transient voltage. Since the 45 V limit
depends on the circuit topography of the LED
array and the maximum “Load Dump” transient
pulse duration, this voltage limit should be
established by reliability testing.
The effects of “Load Dump” transients can be
minimized by putting a surge-suppressor or
silicon transient suppressor in parallel with the
LED array as previously shown in Figure 3.6.
Note that the breakdown voltage of the
transient suppressor, as well as the current-
Table 3.2
Silicon Diodes for EMC Negative Transient Protection
Diode Part
Number
Maximum Continuous
Reverse Breakdown
Forward Current, IO
Voltage, VRRM
Applications
1N4005
1N5396
1N5406
1.0 A
1.5 A
3.0 A
600 V
600 V
600 V
CHMSL
Rear Combination Lamp
Rear Combination lamp,
& Front Turn Signal
Special Considerations for Dual Luminous Intensity Operation
Some applications, such as a Stop/Tail signal
lamp require two discrete levels of light output.
These applications require some additional
design considerations. In most cases, the ratios
in light output at the two signal conditions are
determined by the various signal lamp
ratios for Stop/Tail signals are 7:1 to 15:1.
Generally, the LED emitters should appear
matched in luminous flux at both drive
conditions. This implies that the forward currents
for the LED emitters in the array should be
matched at both drive currents. SuperFlux and
SnapLED 70 emitters are categorized for
specifications or regulations. Typical dimming
29
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