Control the Junction Temperature
In considering the factors discussed, the single most important goal for a prudent designer is to remove heat from the LED, in order to keep the junction temperature below the maximum rating, thereby avoiding premature failure.
The electronics used to generate the required LED current can easily incorporate methods of detecting over temperature conditions which serve to reduce the LED drive current in order to maintain a stable operating temperature.
Clearly the light output would reduce, but the LED would survive to enjoy a long and bright future. By way of an example, the circuit shown in Figure 1 incorporates temperature control in a buck converter configuration. The circuit is designed to drive an LED with a drive current up to 1 amp. The supply voltage is from 4 to 6 volts.
Buck Converter Operation
When the switch Q1 is on, current flows through the LED and L1. This current builds up to a point where the voltage across Rsense reaches the threshold of U1. The ZXSC300 controller then removes the drive to Q1 which turns off. The energy stored in L1 is then discharged, flowing through D1 and the LED. The ZXSC300 has a fixed turn off period of 1.7 μs, after which time Q1 turns on and the cycle is repeated. In this application the switch frequency is about 150 kHz.
by Zetex Semiconductors
In considering the factors discussed, the single most important goal for a prudent designer is to remove heat from the LED, in order to keep the junction temperature below the maximum rating, thereby avoiding premature failure.
The electronics used to generate the required LED current can easily incorporate methods of detecting over temperature conditions which serve to reduce the LED drive current in order to maintain a stable operating temperature.
Clearly the light output would reduce, but the LED would survive to enjoy a long and bright future. By way of an example, the circuit shown in Figure 1 incorporates temperature control in a buck converter configuration. The circuit is designed to drive an LED with a drive current up to 1 amp. The supply voltage is from 4 to 6 volts.
Buck Converter Operation
When the switch Q1 is on, current flows through the LED and L1. This current builds up to a point where the voltage across Rsense reaches the threshold of U1. The ZXSC300 controller then removes the drive to Q1 which turns off. The energy stored in L1 is then discharged, flowing through D1 and the LED. The ZXSC300 has a fixed turn off period of 1.7 μs, after which time Q1 turns on and the cycle is repeated. In this application the switch frequency is about 150 kHz.
by Zetex Semiconductors
